/IDemoirs of tbe 36oston Societv of Iftatural Tbtatocy. Volume 7. vVi ^ PHYLOGENY OF THE ECHINI, WITH A REVISION OF PALAEOZOIC SPECIES. BY ROBERT TRACY JACKSON. WITH SEVENTY-SIX PLATES. TEXT BOSTON: printed for the society with aid from the gurdon saltonstall fund. January, 1912. ^^ MEMOIRS READ BEFORE THE BOSTON SOCIETY OF NATURAL HISTOKY; BEING A NEW SERIES OF THE BOSTON JOURNAL OF NATURAL HISTORY. VOLUME VII. BOSTON: PUBLISHED BY THE SOCIETY. 1912. PUBLISHING COMMITTEE. REGINALD A. DALY CHARLES S. MINOT WILLIAM G. FARLOW WILLIAM M. WHEELER GLOVER M. ALLEN ^^7 t CONTENTS. Phylogeny of the Echini, with a Revision of Palaeozoic Species. 491 pages, 258 text-figures, 76 plates. By Robert Tracy Jackson. January, 1912. TO THE MEMORY OF THE LATE ALEXANDER AGASSIZ, TO WHOM THE SOCIETY MADE ITS FIRST AWARD OF THE WALKER GRAND PRIZE IN RECOGNITION OF HIS 'REVISION OF THE ECHINI. AND OF ALPHEUS HYATT, LATE CURATOR OF THE SOCIETY, MY BELOVED MASTER AND FRIEND. WHOSE PRINCIPLES OF RESEARCH ARE THE KEYNOTE OF THIS MEMOIR. TABLE OF CONTENTS. Page. PREFACE 9 INTRODUCTION •. 15 TERMINOLOGY 25 PART I. COMPARATIVE MORPHOLOGY OF ECHINI 31 Form of the Test 31 Orientation 32 The Pentamerous System and Variation 35 Structure of the Skeleton and Growth 51 The Ambulacrum of the Corona 53 The Interambulacrum 62 Characters op Basicoronal Plates 69 Imbrication of Coronal Plates 73 Spines 77 The Peristome 79 Ocular and Genital Plates 86 Special Characters of Genital Plates 165 The Periproct 173 The Aristotle's Lantern and Perignathic Girdle 177 PART II. SYSTEMATIC CLASSIFICATION OF ECHINI . 199 PART III. PALAEOZOIC ECHINI 235 Geological Distribution 235 Systematic Descriptions 238 (5) 6 Order BOTHRIOCIDAROIDA Jackson Family Bothriocidaridae Klem Bothriocidaris Eichwald . B. archaica sp. nov. B. pahleni Schmidt . B. globulus Eichwald. Genotype Order CIDAROIDA Duncan Family Cidaridae Gray Miocidaris Doderlein M. keyserlingi (Geinitz) M. camioni sp. nov. Order ECHINOCYSTOIDA nora. nov. Family Palaeodiscidae Gregory . Palaeodiscus Salter P. ferox Salter. Genotype Family Echinocystidae Gregory . Echinocystites Wyville Thomson E. pomum Wyville Thomson. Genotype Order PERISCHOECHINOIDA M'Coy Family Archaeocidaridae M'Coy Eocidaris Desor E. laevispina (Sandberger) . Genot3rpe Archaeocidaris M'Coy A. worthcni Hall A. legrandensis Miller and Gurley A. longispina Newberry . A. glabrispina (Phillips) . A. nerei (Miinster) . A. rossica (Buch) A. agassizi Hall A. illinoisensis Worthen and Miller A. coloradensis nom. nov. A. keokuk Hall A. gracilis Newberry A. aculeata Shumard and Swallow A. shumardana Hall A. edgarensis Worthen and Miller A. newberryi Hambach A. Irudifer White A. norwoodi Hall A. paradoxa (Eichwald) . A. mucronata Meek and Worthen A. dininnii White A . oralis White A. acanthifera Trail tschold A. pizzulana Gortani A. megastyla Shumard and Swallow A. biangulata Shumard and Swallow A. ornata (Eichwald) A. ourayensis Girty CONTENTS 238 A. triplex White .... 238 A. triserialis (M'Coy) 238 A . iriserraia Meek .... 239 A. rankini Young .... 242 .4. prisca (Miinster) .... 243 A . wervekei Tornquist 244 .1. urii (Fleming). Genotype 245 A. halliana (Geinitz) 245 A. muensteriana (Koninek) 245 ^. /orbesiana (Koninek) . 247 A. spinoclavata Worthen and Miller 250 ^4. sp. b. Girty 250 A. clarata (Eichwald) 250 Lepidocidaris Meek and Worthen 250 L. squaniosa Meek and Worthen. Gene 252 type 252 Family Lepidocentridae Loven 252 Koninckocidaris DoUo and Buisserct 253 A'. colUaui Dollo and Buisseret. Geno 254 type 254 A', silurica sp. nov. . . . .■ 255 Lepidocentrus Muller 256 L. rhenanus (Beyrich) 259 L. drydenensis (Vanuxem) 260 L. miilleri Schultze .... 261 L. whitfieldi sp. nov. 261 L. eifelianits Miillcr. Genotype 262 Tornquistellus Berg 263 T. gracilis (Tornquist). Genotype 266 Hyattechinus gen. nov. 266 H. rarispinus (Hall) 267 H. pentagonus sp. nov. 267 H. beecheri sp. nov. Genotype 267 Pholidechinus gen. nov. 268 P. brauni sp. nov. Genotype 268 Family Pai.aeechinidae M'Coy 269 Palaeechinus M 'Coy 269 P. quadriserialis Wright . 269 P. ellipticus M 'Coy. Genotype 270 P. elegatis M 'Coy .... 270 P. (?) minor sp. nov. 271 Maccoya Pomel 271 M. burlinglonensis (Meek and Worthen) 272 M. intermedia (Keeping) . 272 M. phillipsiac (Forbes) 272 M. sphaerica (M 'Coy) 273 M. gigas (M'Coy). Genotype 273 M. gracilis (Meek and Worthen) 274 Lovenechinus gen. nov. 274 L. lacazei (Julian) .... 274 275 275 276 276 276 276 279 280 280 281 281 282 282 282 284 284 285 285 286 288 288 289 290 291 291 291 291 292 295 297 299 299 302 304 305 307 308 310 311 312 314 316 317 321 323 324 326 CONTENTS L. mutalus (Keyes) .... 335 L. nobitia (Meek ami Worlhen) 335 L. missouriensis (Jar'kson). Genotype 337 L. anglicus sp. nov 340 L. seplies sp. nov. .... 348 Oligoporus Meek and Wort hen . 350 U. blairt Miller and Gurley 351 0. coreyi Meek and Worthen 353 0. sulcalus Miller and Gurley . 354 0. halli sp. nov. .... 355 0. danae (Meek antl Wort lien ) . Genotjrpe 350 Melonechinus Meek and Wortlien 359 M. dispar (Fischer von WaUlheini) . 365 M. parvus (Hambach) 365 M. springeri sp. now 366 M. crassun (Hambach) 367 M. indianensis (Miller and Gurle.\) . 309 M. slewarlii (Safford) 371 M. seplenarius (Jackson) . 373 M. obovalus sp. nov. 373 M. liratus sp. nov. .... 374 M. muliiporxis (Norwood and Owen) Genotype 375 M . keepringi sp. nov. 384 M. etheridgii (Keeping) 385 M. vanderbiUi sp. nov. 388 M. giganleus (Jackson) 389 ^amily Lepidesthid.\e Jackscni 393 Lepidechinus Hall .... 394 L. irregularis (Keeping) . 390 L. iowensis sp. nov. L. tessellatus .sp. nov. L. imbrioiins Hall. Genotype Perischodomus M 'Coy P. biserialis M 'Coy. Genotype /■*. itiuKiiscnsis Worthen and Miller Perischocidaris Xeunia>i' P. harteiana (Baily). Genotype Proterocidaris Koniiuk /'. i/ii/iinlcii.-i Koninck. Genotype Lepidesthes Meek and Worthen L. u'orlheni Jackson . L. laens Trautschold L. formosa Miller L. devonicans Whidborne . L. spectabilis (Worthen and Miller) L. coreyi Meek and Worthen. Genotype L. carinaia sp. nov. .... L. colleUi White .... L. extremis sp. nov. .... h. caledonica sp. no\'. Pholidocidaris Meek and Worthen P. intgiilari.s (Meek and Worthen) Genotype P. sp P. tenuis Tomquist . P. ncauria (Whidborne) . /'. gnuiln/i (Julien) . Meekechinus gen. no\-. M . elegans sp. no\'. Genotype 397 397 399 401 401 400 407 408 409 410 412 410 418 418 420 421 423 424 425 430 432 432 434 440 440 441 442 442 443 Incertae Sedis .... Archaeocidaris konincki Desor A. ladina Stache A. scotica Young A. selwyni R. Etheridgc, Jr. A. sin Barrois A. Iraulscholdi Tornfiuist A. sp. Meek and Hayden A. sp. Meek and Hayden A. sp. Stache . A. (?) sp. Worthen and Miller A. sp. R. Etheridge, Jr. . A. sp. Julien A. sp. Julien A. sp. Julien Cidarites tennesseae Troost Echinocrinus anceps T. and T. Austin E. spinosus T. and T. Austin . 440 446 447 447 447 448 448 448 448 448 448 449 449 449 449 449 449 E. slriiilus Eichwald E. sp. Austin Melonitiden Tomquist Oligoporus (?) minutus Beede Palaechinus (?) konigii M 'Coy Palaeechinus regnyi nom. nov. Palaechinus rnbineti Julien Palechinus sp. Tomquist . Palaeocidaris exilis Eichwald Palaeodiscus gothicus Wyville Thoni; Palaeospatangus skiptoni Harte Perischodomus magnus Tomquist Protoechinus T. Austin P. ancips T. Austin. Genotype Rhoechinus sp. Duncan Rhoechinus (?) sp. Tomquist . 440 450 450 450 450 450 451 452 452 453 453 453 453 453 4.54 454 454 8 CONTENTS. Incertae Sedis (conliniml). Xenocidaris Schultzc . A', clavigera Schultzc. Genotype 454 455 X. conifera Schliiter . X. cylindrica Schultze 455 456 Nomina Nuda Archaeocidaris tirolensis Staclie Echiiwcnmis cidariformis (?) T. and T, Austin Echinocrinu/i immum T. and T. Aust Heterocidaris Hall . Hetcrocidnns keokuk Hall Heterocidaris laevispiiia Hall 450 45(3 456 456 456 456 456 Lepidocentrus desnri DoUo and Bnisseret . 457 Lepidocentrus duponli Dollo and IJuisseret . 457 Lepidoccntrus gaudryi Dollo and Buisseret . 457 Meloniles youngi Young .... 457 Palacchinus agassizi Dollo and Buisseret . 457 Palaechinus carpenleri Dollo and Buisseret 458 Palaechinus loveni Dollo and Buisseret . 458 Palaeozok' Fokms Incourkctly Refeiired to Echim Discocystis Gregory 458 D. kaxkaskiensis (Hall) .... 4.58 D. op/alus (Worthen and Miller) . . 4.58 D. sampsoni (Miller) .... 485 Myriastiches Solla.s M. gigas SoUas Palaechinus sp. Mitchell Spatangopsis Torcll . S. costata Torell 458 458 4.58 458 458 458 BIBLIOGRAPHY 459 INDEX 475 ERRATA AND ADDENDA 491 PREFACE. In 1896, I published with Dr. (now Professor) T. A. Jaggar, Jr., a detailed study of Melon- echinus multiporus and at the same time, independently, a general study of Palaeozoic Echini. These papers, which were published together in the Bulletin of the Geological Society of America, were the first attempt to make a comprehensive study of this interesting group of fossil animals. During a year recently spent in Europe, opportunity offered to study most of the European species of Palaeozoic Echini and in many cases the types. In this country similar opportuni- ties have been enjoyed with American species. The great liberality and kindness of all to whom I applied for permission to study material is later acknowledged in detail, but I cannot refrain from saying here that the freedom and liberality with which the choicest and rarest material was placed in my hands and every facility offered for study at home and abroad, was one of the pleasantest experiences of my life. The present memoir is naturally divided into three parts: first, a study of the development and comparative morphology of Echini, based on the young, adult, fossil, and living types; second, a proposed systematic classification of Echini based on the preceding studies; third, a revision of Palaeozoic Echini comprising the systematic description of all known forms. The late Professor Alpheus Hyatt originated most important and novel methods of studj' in Palaeontology. It is both an obligation and a privilege to say that during^ many years, when closely associated witli him as student and assistant, he impressed upon me the important principles of stages in development, senescence, acceleration, and parallelism. During my first sea-urchin studies I had the privilege of his ever ready sympathy and personal help. I have tried to carry out this work on his lines and as he would have done it, so far as in me lay. The Hyatt principles are the pith and center of the work, and I take pleasure in feeling that his views worked out largely on the Mollusca are supported and strengthened by their applica- tion to another group of animals. In revising a group, it is most important to see the types or original specimens on which species have been based. Omitting Incertae Sedis, the types have been studied of 69 of the 119 species of Palaeozoic Echini described in this memoir. Specimens of 96 of the species have been studied. These include almost all of the important forms. The genera and species of Palaeozoic Echini have for the most part been described in the publications of geological surveys as parts of the fauna of a horizon or locality, or as descrip- tions of a limited number of species in the proceedings of learned societies. ■ (9) 10 ROBERT TRACY JACKSON OxV ECHINI. Of those who have published on Palaeozoic Echini, the most important are as follows. M'Coy did pioneer work; in 1844 he published several species of Palaeechinus, the first known, also species of Archaeocidaris, and later, Perischodomus. James Hall described several new species of Archaeocidaris, also Lepidechinus. Meek and Worthen, or Meek independently in a few cases, did most accurate and painstaking work, describing many genera and species with a care and faithfulness that have rarely been equalled by investigators of these ancient and difficult fossils. Sir Wyville Thomson described the remarkable Echinocystites. Bailj- published observations on Palaeechinus and described Perischocidaris harteiana. Schmidt made important studies of Bothriocidaris, describing a new species. Keeping described new species and pubUshed good figures of old species. Duncan made studies of the ambulacrum in species of the Palaeechinidae. C. A. White described a number of new species of .\rchaeo- cidaris and a Lepidesthes. Keyes described a new species of Oligoporus, published a number of new figures, and gave a revision of the American species. Jaekel published a careful study of a species of Bothriocidaris. Jackson and Jaggar published a detailed study of Melonechinus multiporus, and Jackson a general study of Palaeozoic Echini, described a few new species, and made an attempt at a natural classification. Julien described new species and figured others from the Carboniferous of France; his material was evidently not very good as shown by the photographic illustrations. Tornquist made a careful study of German species with most excellent figures; he described a number of new species and genera and gave a classification. Miss Klem published a revision of Palaeozoic Echini with a synopsis of known species. Her bibUography and synonymies are very full and most helpful, but unfortunately there are many errors in the text, so that statements need verification. Fraipont published an interesting series of species from the Carboniferous of Belgium. Besides the above, new species or genera of Palaeozoic Echini or observations on specimens have been published by many authors. Aldrovandus, in 1618, published the first Palaeozoic echinoid that has been discovered in the literature. He figured at this early date a specimen of Bothriocidaris globulus under the caption, "Echinus lapis spoliatus a spinis." It is extraordinary that this especially rare type should have been so early discovered. De Koninck, Miinster, L. Agassiz, Norwood and Owen, Fischer von Waldheim, Forbes, Geinitz, King, Roemer, J. Miiller, and Sandberger published species and genera with more or less full descriptions of the same. Desor published some new forms and gave a revision of the then known Palaeozoic species. Young, Wright, Shumard, Schultze, and Safford published a few types, some of much importance. Lovcn in his "Etudes" gave a revision of the then known species, but apparently without observations on specimens. A. Agassiz in the "Revision" (1872-'74, p. 644-650) gives a general discussion of Palaeozoic Echini and in several other monographs has published observations on the group and some discussions of the observations of others. Stache, Miller, and Schliiter published new species, Zittel in his text-book introduced new PREFACE. 11 generic names and higher divisions of the group. Pomel gave a revision of the genera and published a number of new ones. This part of his work was apparently based wholly on the observations of others, to whom he gives neither credit nor reference. Waagen, and Miller and Gurley published new species. Duncan revised the genera in his revision of the genera of the class. Sollas, Gregory, Whidborne, DoUo and Buisseret, Girty, Bather, and Spencer pubhshed new species or observations on older species. In numerous text-books, figures and more or less full descriptions are of course given, wholly or mainly from the observations of others. The important monographs of A. Agassiz, Loven, and Mortensen have been constantly in hand for comparative studies of recent types. The late lamented Alexander Agassiz stands unique in the history of echinology, for his work on the group extended over a period of nearly fifty years, during which time he published numerous extensive monographs besides many smaller papers. The attempt has been made to give full credit to the above authors for their observations. While I have not always been able to agree with the observations or conclusions of others, it is to be remembered that these fossils have a complex structure and are often very imperfectly preserved, so that they are exceptionally difficult to study. Also, I have had the privilege of a fuller opportunity than probably any one before to study a large range of Palaeozoic species. Those who are most familiar with the group can best appreciate the difficulty of avoiding errors, as I hope may be remembered in my own case. Barring possible oversights, the systematic part of this paper includes every pubhshed species, practically all of which are figured. A figure is given of all recognized species excepting Archaeocidaris rankini, A. prisca, and Konincko- cidaris cotteaui of which no specimen was available and no figure has been published. Also excepting Pholidocidaris gaudryi of which the published photographic figures are too indistinct to be safely copied. The location in museums of types and rare species is given as an aid to investigators. For their kindness and liberality in allowing me freest access to material in collections and for every facility in the use of the same, as well as other kindnesses, I would return grateful thanks to the following curators and private collectors. My friend, Dr. F. A. Bather, of the British Museum, to whom I am under special obligations, most kindly helped me by corre- spondence and the loan of rare specimens, as well as by access to the rich material in the British Museum. He also had the great kindness to borrow, study, and send me drawings and notes on the specimen of Lovenechinus lacazei described from the York Museum. During a visit at the British Museum I enjoyed the privileges of the rich library facilities there available. Dr. F. L. Kitchin and Mr. H. A. Allen, of the Geological Museum in Jermyn Street, London, helped in a similar way, and most kindly had photographs taken of some of their important specimens as here reproduced. Professor Henry Woods, of Cambridge, England, and Pro- fessor John Joly, of Trinity College, Dublin, gave access to their choice material. Dr. R. F, 12 ROBERT TRACY JACKSON ON ECHINI. Scharff, of the Science and Arts Museum, Dublin, gave me access to invaluable material, had casts and photographs of M'Coy's types made, and most generously brought over to this country for studj" one of their most valuable specimens. Dr. Otto Jaekel, recently of Berlin, kindly gave me the opportunity to studj^ his most precious specimen of Bothriocidaris, which I feel is the backbone of this work. Professor A. Rothpletz and Dr. F. Broili gave me every oppor- tunity to study the rich collections of the Munich Museum. At Geneva, Dr. Bedot, Director of the Musee d'Histoire Naturelle, and Dr. Jules Favre, of the same institution, kindly gave me access to their rich collection of Recent and Mesozoic Echini, including the very valuable material of the late M. de Loriol. Professor G. Steinmann and Professor G. Boehm, of Freiburg i. Br., and Professor A. Tornquist, of Strassburg, placed much fine material at my disposal. In this country, for opportunities to study fossil Echini, I am under deep obligations to many curators of museums and to private collectors. Mr. Samuel Henshaw, Curator of the Museum of Comparative Zoology, kindly gave me free access to the rich collections in that institution. From Professor Charles Schuchert, of Yale University Museum, besides other specimens, was borrowed the superb material of HyaUechinus beecheri and rarispinus collected in Warren, Pennsylvania, mostly by the late Professor C. E. Beecher. I am under similar obligations to the late Professor R. P. Whitfield, of the American Museum of Natural History; Dr. C. D. Walcott and Dr. R. S. Bassler, of the United States National Museum; Dr. George H. Girty, of the United States Geological Survey; Professor Stuart Weller, of the University of Chicago; Professor John M. Clarke, of the New York State Museum; Professor William B. Clark, of Johns Hopkins University; Professor J. W. Beede, of Indiana University, who sent me the remarkable new type, Meekechinus elegans and other material ; Dr. A. G. Ruthven and Professor E. C. Case, of the University of Michigan; Professor R. S. Breed, of Alleghanj^ Col- lege, Meadville, Pennsylvania; Professor L. C. Glenn, of Vanderbilt University; Professor A. W. Grabau, of Columbia University; and Mr. C. W. Johnson, Curator of the Boston Society of Natural History. Of private collectors, Mr. Frank Springer generously loaned me his whole collection of about 125 Palaeozoic Echini, including types and other material recently purchased from Pro- fessor G. Hambach, of St. Louis. Mr. Frederick Braun, of Brooklyn, lent very choice material from his private collection; and Mr. E. Kirk, now of the United States Geological Survey, lent several specimens. Mr. J. E. Hj^de, of Columbia University, loaned the remarkable new Lepi- desthes extremis. To all these gentlemen I would render warmest appreciation for their gen- erosity. The actual specimens loaned or used will be referred to as from the institution or collection to which they belong under consideration of the species. For the opportunity to study the great collection of Recent Echini in the Museum of Comparative Zoology, I am indebted to the Curator, Mr. Samuel Henshaw, and to Dr. Hubert Lyman Clark, who is in charge of that department. For similar opportunities in the United PREFACE. 13 States National Museum I am indebted to Mr. Austin H. Clark and Miss Mary J. Rathbun. Professor Verrill kindly gave me the opportunity to study the Recent Echini in Yale University Museum. The authorities at Ward's Natural Science Establishment at Rochester liberally allowed me to study their large trade collection. For opportunities to study Recent material in their charge, I am indebted also to Mr. Roy Miner, of the American Museum of Natural History; Dr. F. A. Lucas, lately of the Brooklyn Institute of Arts and Sciences; Dr. A. S. Pearse, of the University of Michigan; and Professor E. S. Morse, of the Peabody Museum at Salem. Professor Morse also kindly gave me opportunity to study many Japanese Echini that he collected in that country. Several gentlemen have most kindly supplied me with fine series of specimens ; they are : Mr. A. P. Romine, of Bellingham, Washington ; Dr. W. K. Fisher, of Stanford University; Professor W. E. Ritter, of the University of California; Professor E. L. Mark, of Harvard University; Dr. H. C. Chadwick, of the Port Erin Biological Station; Dr. F. D. Lambert, of Tufts College; Dr. Thomas Barbour, of Cambridge, Massachusetts; Mr. Dwight Blaney and Mr. Owen Bryant, of Boston. Dr. Theodor Mortensen kindly loaned me valuable material and helped me by correspondence. In addition to material studied in various institutions, my work has been based on my own collection of some 40,000 Recent and Mesozoic Echini. These I recently gave to the Museum of Comparative Zoology. My Palaeozoic Echini, accumulated during many years and including 100 specimens, are now also in the collection of the same Museum. I would express my warm obligations to my friend, Mr. Samuel Henshaw, Curator of the Museum of Comparative Zoology, for the use of a room and for the use of the great library facilities there afforded. I would express mj^ deep appreciation of the laborious painstaking care and skill with which IVIr. J. Henry Blake made the larger part of the drawings for the plates and text-figures. It is very difficult to get an artist who can draw a sea-urchin correctly, and without Mr. Blake's skill and patience I should have been badly handicapped. Any sea-urchin is difficult to draw accurately, but in the Palaeozoic, one has to contend with the difficulties of imperfections, often the fusion or indistinctness of sutures; the fossils are frequently external or internal molds or siliceous replacements, and these conditions add much to the difficulties of critical stud.y and correct interpretation. The drawings were all made under my eye, and are as faith- ful to the originals as it was possible to make them. No restorations were permitted, except as indicated by dotted lines. Of the text-figures, Mr. Blake drew figures 1 to 239 and 245 to 253, while figure 244 is the work of Mr. J. H. Emerton; figures 240 to 243 were done by Mr. A. H. Searle, and figures 254 to 256 by Mr. William M. Barrows. I would also express obliga- tions to Mr. F. A. Saunderson, of Boston, for the remarkable set of photographs of Echini which are reproduced as heliotypes. Some of the drawings and photograjjhs were made by other artists in Europe or this country, as mentioned in the description of plates. The illustra- 14 ROBERT TRACY JACKSON ON ECHINI. tions in the plates are arranged as nearly as practicable in systematic sequence following the classification here presented. I am under great obligations to my friend, Dr. Hubert Lyman Clark, for his help in veri- fying references, and help with my manuscript and other matters during my absence from home. Miss H. I. Cowdery rendered valuable service as a typewriter by her careful painstaking copy- ing of the manuscript for the printer. The requisite research work and the preparation of this memoir have taken my close attention in available time for some seven years. That it has taken so long has been a source of regret for more than one reason, but careful work cannot be done in a hurry. I adopt the title "Phylogeny of the Echini" for this memior, because the phylogenetic rela- tions expressed by stages in development and by variation are considered throughout the work. As I have shown previously in my "Phylogeny of the Pelecypoda" (1890), and also in studies of Echini (1896), and of Plants (1899), I believe that by following the Hyatt methods of a comparative study of young, adult, fossil, and living forms we can arrive at a series of facts which justify us in accepting them as expressing an approximation to the real genetic relationships of the forms under consideration. The genealogical relations of the Echini, as understood from the results of my studies, are shown in tabular form on p. 209. INTRODUCTION. The principles made use of and the methods of work employed in studying Echini are here treated before starting into the detailed considerations. The one essential principle that has been applied consistently throughout is the principle of stages in development and the constant comparison of these stages with the characters of more or less closely associated types. This principle applied conjointly to fossil and living forms was urged by Professor Louis Agassiz, and was the center of a large part of Professor Hyatt's work. An attempt has been made to bear in mind the characters of the young, adult, and old age, and to compare these with the young or with the adult of simpler or more specialized types, both living and fossil. As far as material permitted, the entirety of the organism has been taken into account. If Echini have taught me anything, they have impressed upon me that classification should be based on the sum of the characters and not on single characters. Herbert Spencer pointed out the distinction that growth is mere increase in size without the addition of differential characters; development is the addition of differential characters. Development proceeds rapidly during the early growth of an organism, but in later life we may get extensive growth without any development. In Palaeozoic Echini, development, that is, the addition of new characters, is often long continued, so that new characters, especially additional columns of interambulacral plates, may be taken on up to or even dorsal to the mid-zone. Then in the dorsal region we come into the zones of localized stages in development, where the young, last added plates are progressively taking on their full characters as they are pushed ventrally by the intercalation of still younger plates on their dorsal border, between them and the apical disc. Dorsally, we also often find senescent stages or characters of regres- sive development, especially marked by the dropping out of columns of interambulacral plates, as in Melonechinus indianensis (Plate 53, fig. 1). In the Palaeozoic Echini, as elsewhere, specimens of a species vary in size, and doubtless this usually means age, but really young specimens are almost unknown. The only very young one seen is Lovenechinus missouriensis (Plate 39, fig. 1; compare with adult, Plate 39, fig. 5). In the absence of the young, youthful stages are gathered from the characters of the plates at the ventral area of the corona, which, as shown in my earlier paper, is an area that preserves the early stages of development in a remarkable degree. Except when destroyed by resorption, this area in a perfect specimen represents the first interambulacral plates built in any corona, and the first ambulacral as well, unless they have passed on to the buccal membrane. Wliile these ventral plates have increased in size since early youth, their presence, number, angles, (15) 16 ROBERT TRACY JACKSON ON ECHINI. and mutual relations show developmental stages which it is felt, from numerous and careful comparisons, can be trusted as stages in development. Stages in development are not limited to the ventral border, but are seen in many types appearing progressively up to or even dorsal to the mid-zone. This is especially seen in the development of the interambulacrum, where new columns come in often very late in the growth of the individual, as shown in numerous cases, for example, Melonechinus multi'porus (Plate 57, fig. 1). Localized stages in development is the principle that throughout the life of the individual, stages may be found in definite parts that are comparable to the condition in the young and to the adults of simpler types of the group. This principle was discovered in and applied to the young plates at the dorsal portion of the interambulacrum in Palaeozoic Echini in my paper, "Studies of Palaeechinoidea, " published in 1896, p. 228. It was applied to Palaeozoic and Recent Echini in my paper (1899) on this principle as a new law in evolution published in the Memoirs of the Boston Societj^ of Natural History, under the title, " Localized Stages in Development in Plants and Animals." In this memoir the principle of localized stages in development is applied to the interambulacrum, but especially to the ambulacrum, where it is found that primitive characters, seen in the nascent plates at the dorsal border of the area, are striking and most valuable for a comparison with the condition in the young and with simpler types in the group. This principle is shown well in the development of young ambulacral plates dorsally in Centrechinus (text-figs. 92, 94, pp. 106, 107) and is seen especially in the family Palaeechinidae (text-fig. 237, p. 231). This principle was found to have a broad applica- tion in various groups of organisms, as ophiurans, crinoids, corals, cephalopods, also plants; and it has been taken up by the late lamented Professors Hyatt and Beecher, by Grabau, Cushman, Ruedemann, Buckman, Hubert Lyman Clark, and Jeffrey, who found it a valuable aid in studying many different groups of animals and plants. Senescence, which was made so much of by Professor Hyatt in his studies of cephalopods, is best shown in Palaeozoic Echini by the dropping out of columns of interambulacral plates at the dorsal or last built portion of the test. In the old individual of Lovenechinus 7nissouriensis (Plate 42, fig. 6) columns 1 and 2 have in part dropped out dorsally, whereas in a younger specimen (Plate 41, fig. 2) they continue directly to the apical disc. Melonechinus giganieus (Plate 59, fig. 14) shows the dropping out of the eleventh or last added column at the dorsal border of the area. Dropping out of columns is shown graphically in the old-age Melonechinus indianensis (Plate 53, fig. 1) where columns 1 and 2 or even more drop out before reaching the apical disc. Hyattechinus beecheri (Plate 26) shows the dropping out of columns 1 and 2 toward the dorsal portion of the areas. Progressive types are those which show in their development to maturity the addition of differential characters only, without the dropping out or disappearing of such characters, unless in senescence. A good case of a purely progressive type is Palaeechinus quadriserialis (Plate 30, INTRODUCTION. 17 fig. 3), which progressively adds characters, but as far as known does not lose any of these additions. It may be said that in general most Palaeozoic Echini arc progressive types. Regressive types are those whicih, after attaining a degree of specialized characters, in later development, and before old age, lose some of these characters, so Ihat what we call the adult, as gathered especially from the characters of the mid-zone, is simpler than its own young. An excellent case of this is Lepidesthes wortheni (Plate 67, fig. S) which in the young has four columns of interambulacral plates, but the fourth column drops out earlj^, and in later life it has three columns only. The sixth column represented by only a few plates in an area seems to mark Lnvenechimis missouriensis (Plate 41, fig. 1) as a regressive type, at least in this char- acter. To go outside of Palaeozoic Echini, Homicidaris has compound ambulacral plates in the lower or youthful half of the test, and above this point has only simple plates, showing a complete reversion to Cidaris, a more primitive type of Echini. Such an extreme case ma}' be compared to Lituites in cephalopods (Zittel, Handbuch der Palaeontologie, vol. 2, text- fig. 519), which, after an early coiled stage, takes on a straight stage directly comparable to the early straight forms of nautiloid cephalopods. Cases could be multiplied and some will be mentioned later, but here it is the object simply to point out examples that illustrate the principles made use of. Acceleration of development, one of Professor Hyatt's most important principles, is abun- dantly shown in Palaeozoic Echini. U.sually columns of interambulacral plates, after the first four columns, are added at considerable intervals, as in Melonechinus muUiporus (Plate 57, fig. 1), l)ut M. giganteus (Plate 59, fig. 14), which is a higher species in the series, adds the fifth, sixth, seventh, and eighth columns earlier than does M. muUiporus, as shown in a detailed study in my earlier paper (189(5, p. 179). Hyattechinus beecheri (Plate 26), a most specialized type, has a very accelerated development, and new columns of interambulacral plates are added so rapidly that the fourth to the tenth are added in succeeding rows, or even two columns may be added in a single row. The same character of acceleration is shown well in Hyattechinus rari- spinus (Plate 23, fig. 1) and H. pentagonus (Plate 25, fig. 1). Acceleration is shown well in the •ambulacrum in Melonechinus (text-fig. 237, p. 231), in which at the ventral border we find four plates in each ambulacrum, while the lower genera of its family have only two plates. Parallelism is an important feature which was much studied by Professor Hyatt, and is of great value in studying Echini. Parallelism is the taking on of a similar character by inde- pendent lines, and is sometimes difficult to distinguish from real genetic connection. A case of parallelism is .seen in the imbrication of plates of Echini. It has been thought that imbri- cation was a sufficiently important character to group together those forms that possessed it, but it is assumed independentlj' in several distinct groups in the Palaeozoic, as well as in the post-Palaeozoic Echinothuriidae. In the family of the Palaeechinidae (Melonechinus, etc.) ambulacral plates on the adradial suture are beveled over the adambulacrals (Plate 45, fig. 5). 18 ROBERT TRACY JACKSON ON ECHINI. A similar beveling is seen in the petaloid area in some clypeastroids, Clypeaster, surely a parallelism only. Accessory pores in genital plates in modern Echini exist as an individual variation occasionally (Arbacia, text-figs. 197, 198, p. 171; Echinus and others). This appar- ently' represents a parallelism rather than a genetic connection with the several pores charac- teristic of these plates in most Palaeozoic genera (Plate 56, fig. 6). Increase in the number of columns of interambulacral plates is taken on independently in many genera. The absence of resorption of the base of the corona, which permits of the retention of the single pri- mordial interambulacral plate in each area, appears as a parallelism in many independent groups, as represented by Hyattechinus (Plate 26), Perischodomus (Plate 64, fig. 2), Pholi- docidaris (Plate 73, fig. 6), Phormosoma, clypeastroids (text-figs. 43, 52, p. 80), and spatan- goids (Plate 3, fig. 15). Resorption is an important process to bear in mind; it occurs continuously in the growth of each individual plate, where there is a constant resorption within the plate concurrently with increase in external dimensions. Resorption of the base of the corona by the encroach- ment of the actinostome is a very important factor, as shown by Loven (1892). In Palaeozoic types, as I showed in my earlier paper (1893), there is either no resorption (Hyattechinus, Plate 26), resorption of apparently one plate only (Palaeechinus, Plate 30, fig. 3), or resorption of several rows of plates (Archaeocidaris, Plate 9, figs. 6-8). Resorption may cut holes directly through the test, as shown by Mr. Agassiz (1872) in the development of the lunules in the recent Mellita sexiesperforatus (Leske). See text-figs. 22-31, p. 70. Variation is an extremely important subject and is graphically shown in sea-urchins. Here we find the striking character of radial variation well developed, as seen especially in the corona and ocular and genital plates. One radius may be more or less accelerated, or may reach a greater or lesser degree of differential development than other radii in the same individual. This is especially marked in the variation of rate of development and number of columns of interambu- lacral plates attained in Palaeozoic Echini, and in the number of ocular plates reaching the peri- proct in Recent Echini. The range of radial variation in one individual may equal the range of variation of different individuals within the limits of the species. In IVIelonechinus typicallj' there are four plates at the ventral border of the ambulacrum, but in one specimen of Melon- echinus muliiporus (Plate 57, fig. 3) it is seen that in area B there arc two plates only. This is the normal character of the ventral border of Oligoporus (Plate 50, fig. 8) and Lovenechinus (Plate 42, fig. 1), the next lower genera of the family. In order to appreciate variation it is of fundamental importance to be familiar with the characters of the associated species and genera of a case in hand, and also the developmental characters of the same. Variation may be fairly classified under five more or less distinct heads : 1. Arrested variation, in which the variant retains characters seen in its own young and INTRODUCTION. 19 typical of the adults of more primitive allies, but characters which are usually eliminated in development. Arrested variants are shown abundantly in the consideration of ocular plates, when fewer plates may become insert than is characteristic of the species (Tripneustes, text- figs. 123-125, p. 124). 2. Progressive variation, in which the variant has characters not typical of the species, but which are further evolved on the direct line of differential development, and are seen typi- cally in more evolved nearly allied species or genera. Such progressive variants are also shown abundantly in the consideration of ocular plates, where more plates become insert than is typical in the species (Centrechinus, text-figs. 93-95, p. 107 and 176, p. 153). 3. Regressive variation, in which the variant takes on characters of the adult of some simple and more primitive type of the group. Such characters are not necessarily a repetition of youth- ful characters but may go back to a remote ancestry. An arrested variant in a sense is one form of regressive variation, but a regressive variant includes much more than arrested variation. To distinguish them, an arrested variant is one that has developed to a certain point as usual, and then failed to take on the later added characters typical of the species, so that, although an adult, it has immature characters. A regressive variant is one that has attained full char- acters and then in later life has reverted to youthful or primitive characters as an individual variation, or it is a variant that from youth has primitive characters not normally seen in the development of the species. A modern horse with extra digits as in Tertiary times could be considered a regressive variant, but could not be considered an arrested variant. In Echini, cases of regressive variation are shown in the simple ambulacrum in Melonechinus (Plate 57, fig. 3), and in the single column of interambulacral plates in Arbacia (Plate 4, fig. 11) and Trip- neustes (Plate 6, fig. 4). 4. Parallel variation is where a character is taken on exceptionally which may be com- pared with characters normally occurring in some type of the group not closely connected, so that it cannot be genetically compared. A case apparently is the extra genital pores fre- quently developed in Recent Echini (Echinus, text-fig. 115, p. 117). 5. Aberrant variation is where a character is taken on which is quite abnormal, not to be correlated with the typical condition in associated forms. Cases of aberrant variation are sea-urchins which have four or six areas developed (Plate 6, figs. 1-4). All the evidence goes to show that aberrant variation is rare, and most variants can be considered as arrested, progressive, regressive, or parallel variants, and as such can be correlated with species more or less nearly allied which typically possess the character which is a variant of the case in hand. This holds true as far as known in both animals and plants, as seen in the present paper, and as I showed in detailed studies of plants (1899). Next to stages in development, variation is the most suggestive line of study in attempt- ing to work out the genetic relations of plants and animals. When a specimen of one species 20 ROBERT TRACY JACKSON ON ECHINI. has characters approaching those of another, hybriditj' is often assumed as a cause. I have found numerous cases of variation to the character of another species where hybridity cannot be considered on account of geographical separation, and it is feU that we should be extremely cautious in assuming hybridity when the same results might be attained by variation. From my studies on Echini, all the evidence goes to show that variation is in perfectly definite lines, mostly arrested or progressive in character. When variatTon is aberrant, it still follows defi- nite lines of aberration, and sporadic variation is very rare. This opinion is based largeh' on a detailed study of genital and ocular plates which were carefully examined in over 50,000 specimens of Echini (text-fig. 176, p. 153; p. 164). To the late Professor Sven Loven all students of the Echini owe a great debt for the keen ej^esight and insight with which he studied these fascinating animals. His method was largely a critical detailed study of plate structure and relations, and this method I have attempted to follow in the study of Palaeozoic forms. Frequently sutures are so difficult to see and speci- mens are so contorted or badly preserved in these ancient forms, that I can hardly hope to have escaped mistakes; but this method has been followed out consistently and with every effort faithfullj' to describe and figure the specimens. Loven worked with Recent or post-Palaeozoic Echini, in which the axes are known from the presence of the madreporite as well as in many t^ypes by the bilateral symmetry. With •known axes he devised a nomenclature of areas which is of very great value and convenience in brevity and clearness of description. He numbered the ambulacral areas from I to V, Roman, and the interambulacra from 1 to 5, Arabic. The enumeration passed from left to right, re- volving like the hands of a watch, with the specimen viewed from below and the odd anterior ambulacrum being III. This nomenclature is considered more fully under Morphology. It is readily seen in its application to Goniocidaris (Plate 2, figs. 1-3). In dealing with Palaeo- zoic types, there was usually no means of ascertaining the axes, for the madreporite was seen in only four species. Since, in the absence of known axes, Loven's system could not be used, a system of nomenclature like that given in mj^ earlier paper is followed. Taking any inter- ambulacrum as A, the several areas are lettered from A to J inclusive, revolving like the hands of a watch, with the specimen viewed from the dorsal side. When a specimen is viewed from below, the letters naturally revolve in the reverse order. This is shown in its application to Lovenechinus (Plate 39, figs. 4, 5). It may be well to call attention to the application of this nomenclature of areas to speci- mens viewed from different points so that it may be clearly in mind. External views, or in- ternal molds, viewed dorsally: the axes revolve clockwise, from left to right (Maccoya, Plate 34, fig. 4; Lovenechinus, Plate 39, fig. 5). External views, or internal molds, viewed ventrally: the axes revolve anticlockwise, or from right to left (Melonechinus, Plate 55, fig. 1; Lovene- chinus, Plate 39, fig. 4). In molds of the exterior, if dorsal: the axes revolve anticlockwise, from INTRODUCTION. 21 right to left, as Hyatkchinus rarispinus (Plate 23, fig. 3) and H. pentagonus (Plate 25, fig. 3). In molds of the exterior, if ventral : the axes revolve clockwise, from left to right, as in Hrjatt- echinus rarispinus (Plate 23, fig. 1) and H. pentagonus (Plate 25, fig. 1). Orientation, not only for the sake of the axes, but also for the correct position and introduction of columns of plates and direction of imbrication, must be borne in mind in a reversed specimen; for naturally confusion results unless this is taken into consideration. It was a strong temptation to reverse the drawings, so that external molds seen from the interior might appear as if seen from the exterior. It would have simplified matters as regards orientation, but then the drawings would not have corresponded with photographic figures of the same specimen ; also the com- plication is so great that I feared to make errors by reversing drawings. The terms molds and casts are often used indifferently, but it is necessary to distinguii^h them. A mold, either internal or external, is an impression of a fossil and represents the object in reverse. A cast, either natural or artificial, is a mechanical filling of the mold and represents therefore a replica of the original. A pseudomorph differs from a cast simply in that it is a molecular chemical replacement of the original, as in silicification, instead of a mechanical filling of a mold, as in the case of mud, sand, or plaster. Most fossils, when they are not the original skeleton, are either pseudomorphs or molds; natural casts, strictly speaking, are com- paratively infrequent. In the figure of H yattechinus rarispinus (Plate 23, fig. 1) the test is wanting, and the view represents an external sandstone mold of the ventral side, together with an internal mold of the dorsal side, seen from above. The mold of the ventral side, being external, shows the tubercles and peripodia of the ambulacral pores, but the mold of the dorsal side, being an impression of the interior, has no tubercles, and the ambulacral pores are represented by vertical plugs. This shows that the point of view must be constantly borne in mind and orientation carefully considered. As an aid in orientation, a rubber ball was marked with ambulacral and inter- ambulacral areas, then cut down on these lines, and marked within. Also dorsal and ventral plaster molds of a Cidaris were made and lettered as models, for it was found difficult to keep orientation clearly in mind in reversed views, as are external molds. An interesting matter is the condition of preservation of Palaeozoic or other fossil Echini. I have never seen a fossil sea-urchin in which the test was preserved in its original condition, but rather some chemical change seems always to have taken place even in late Tertiary speci- mens. In other groups of animals, as fossil molluscs, corals, etc., the original skeleton is often preserved without any change except the leaching out of organic matter. The sea-urchin skeleton is composed of such loose network-like structure, that it seems to be peculiarly open to chemical readjustment and change. When the skeleton is calcified, the original microscopic ' structure is, at least usually, quite destroyed. The skeleton is often replaced by silica, when we may get very beautiful pseudomorphs preserving the finest details; an example of this is Melonechinus giganteus (Plate 60, fig. 3). 22 ROBERT TRACY JACKSON ON ECHINI. One of the most interesting and instructive methods of preservation is when the surround- ing matrix, both internal and external, is completely silicified and the original skeleton is entirely gone, but is represented in finest details by molds in silica. Such an example is shown in Loven- echinus missouriensis (Plate 39, figs. 4, 5). Here it is seen that everything that was not, is represented bj^ the silica. This is rather a paradoxical statement, but fairly represents the facts. The plates are not preserved, but the sutures between the plates are represented by vertical siliceous ridges; the pores, both ambulacral and genital, are represented by vertical tubes, which can be actually seen (Plate 43, fig. 5) passing from the proximal to the distal side through the space which the plates originally occupied, but now simply a hollow. When the external as well as the internal matrix is in hand (Plate 44, figs. 3, 4), we find on one side, the internal matrix, a mold of the proximal side of the plates; on the other or external matrix we get the distal mold of the plates showing spine tubercles and other external features. Sili- ceous molds of the interior or exterior may be found in which there are no ridges representing spaces between the plates, but the difference of conditions between these two types is not understood. Molds, either internal or external, in fine sand or calcareous clay, are often ex- tremely clear and may be accompanied by more or less of the shell of the test, or this may be wanting. In a specimen of Palaeechinus quadriserialis in the British Museum, the test is crystalline calcite, but only part of the plates are preserved; most of the rest are, however, represented by impressions of their proximal faces on the matrix, as shown in Plate 29, fig. 1, and Plate 30, figs. 1, 3. Sandstone molds may be so fine in details of preservation as to show the outline of plates, tubercles, pores, and even peripodia. Internal sandstone molds show only impressions of the proximal sides of plates with pores in relief as slight plugs. An excellent example is Hyatt- echinus beecheri, a unique specimen (Plate 24, figs. 5-8), which shows the details of outline of the proximal faces of the plates so clearly that I am enabled in Plate 2G to represent the sea- urchin spread out, in which every plate was measured and the angles counted. External sandstone molds are even better, for, on the exterior, details of structure are more strongly marked; tubercles and peripodia exist as depressions (the impression being in reverse), and ambulacral pores as slight elevated plugs. Examples are shown in Hyattechinus rarispinus (Plate 23, fig. 1) and H. pentagonus (Plate 25, figs. 1-4). A curiously complex case is the Hyattechinus rarispinus previously mentioned (Plate 23, fig. 1), in which is seen an external mold of the ventral side, showing tubercles and peripodia, and in the same view an internal mold of the dorsal side. The counterpart of this remarkable specimen (Plate 22, figs. 1, 2) shows just the reverse condition; that is, an internal mold of the ventral side and an external mold of the dorsal side. The test, which was extremely thin, is mostly wanting, but is existent on the margins. The dorsal and ventral sides are brought almost in contact, yet the sandstone did not close up, but retained as a thin interspace the vertical space originally occupied by INTRODUCTION. 23 the thickness of the test with some sand which filled the interior of the test, thus making an internal mold possible. Loven devised the incomparable method of representing the structure of a sea-urchin by a figure drawn from the ventral view and with the several ambulacral and interambulacral areas spread out flat in one plane in a star-like fashion. Of course in the Palaeozoic it is only rarely that one finds a fossil sufficiently perfect to admit of such treatment; but I give figures drawn by this method of Bothriocidaris (Plate 1), Hyattechinus (Plate 26), Archaeocidaris (Plate 10, fig. 10), Palaeechinus (Plate 30, fig. 3), Lovenechinus missouriensis (Plates 40, 41), Lovenechinus sepiies (Plate 45), and Melonechinus (Plate 57). These genera are fairly repre- sentative of the several groups of Palaeozoic Echini, and the figures show the characters of the plates and the method of introduction of columns better than by any other method. Wliile perfect specimens are desirable and most highly prized, yet a great deal can be made out of fragments when studied carefully. A left or right half of an ambulacrum repre- sents the character of the whole. A piece out of the mid-zone, that is, halfway between the mouth and middle of the periproct, represents the area where full specific characters are devel- oped in both the ambulacrum and interambulacrum, and many species are represented by a figure taken from the mid-zone (Plate 47). Such a figure does not show the development as seen ventrally, or the apical disc, but these features are often wanting, or shown in related species, so that a figure of an ambulacrum and interambulacnmi from the mid-zone gives for these areas the essential specific criteria. The skeletons of Recent sea-urchins are often very fragile, and even if not so, the peristomal and periproctal plates are easily injured and lost. It has been found a great help to diji the specimens in, or brush them over with, a dilute solution of shellac in alcohol, or gelatine dissolved in water. Specimens so treated are firm and solid and will stand even rough treat- ment. A specimen with the spines all in place may be soaked in this manner by dipping, and on account of the porosity of the skeleton, there will be no external evidence of the treat- ment; the spines will be firmly fixed in place, and the specimen is in much safer condition for storage or study. Where sutures are difficult to see, as in minute recent material, or ambu- lacral details, it has been found a great help to wet them with benzole. When drying out, sutures stand out clearly that are nearly or quite unrecognizable in the dry state. In prepar- ing fresh material of Echini, I have found that a satisfactory method is to soak the specimens over night in a considerable volume of fresh water to remove the salt, then immerse for a few minutes in boiling water containing a liberal amount of corrosive sublimate in solution. This treatment coagulates the albumen, poisons the specimens, and does not affect the color. They will dry out without odor, and in excellent condition as museum specimens. The method has the advantage of cheapness and quickness. When specimens are to be preserved in alcohol, the treatment with fresh water is also desirable, as it kills the animal nicely relaxed, besides removing much of the salt. 24 ROBERT TRACY JACKSON ON ECHINI. The theoretical views expressed in this paper are supported by a sufficient basis in facts, so that they appear reasonable to me; Init I have endeavored to keep the descriptions of struc- tural detail and the conclusions drawn from them distinct, so that the facts will stand for them- selves, whether the deductions are accepted or not. TERMINOLOGY. Some new terms are made use of and old terms are not used uniformly by all investigators, so that the principal terms employed are given here in brief. Test: the whole skeleton including the spines; or without spines, denuded test. Fre- quently test alone is applied to the skeleton in which spines are more or less wanting, the com- mon condition in fossils. Corona: the portion of the test, including the ambulacral and interambulacral plates, extending from the periphery of the peristome to the ventral margin of the apical disc. In describing an ambulacrum or interambulacrum, the terms right or left are frequently used. They mean the right or left side as seen facing the given area, viewed from the exterior and with the dorsal portion uppermost. Ambitus: the zone of greatest circumference as viewed from above. The ambitus may coincide with the mid-zone, Maccoya sphaerica (Plate 32, fig. 5), or it may lie ventral to the mid- zone, Hyattechinus beecheri (Plate 24, fig. 7), or Clypeaster; or rarely dorsal to the median zone, Lepidesthcs coreyi (Plate 66, figs. 8-10). A sea-urchin is divisible into a series of zones which theoretically are as many as there are horizontal rows of plates, and each zone, at least in earlier life, differs or may differ in char- acter from the preceding or succeeding zone. This is shown graphically in Hyattechinus beecheri (Plate 26) ; here in the first zone of interambulacral plates there is one plate in each area, in the second zone two plates, in the third three plates, and so on up to the introduction of the full number of columns of plates, above which we reach zones of senescence, where plates begin to drop out. In practice, radial variation interferes somewhat with the perfect zonal expression, as all the areas do not develop with exactly the same degree of rapidity. For example, in Lovenechinus missouricnsis (Plate 41, tig. 1), the fifth column originates in the fifth row above the initial plate of the fourth column in area A, whereas in (he other areas it origi- nates in the third or fourth row. Similar variations in rate of development are frequent in later added columns, as shown (Jackson and Jaggar, 1896) in detailed stiulies of Melonechinus multiporus, and as shown here in many figures. A column of plates may be quite wanting in one area, though developed in other areas of the same specimen. In Hijattechimis rarispinus (Plate 23, fig. 3) there are only eleven columns of plates in areas A and G, while there are twelve in area I and thirteen in C and E. In Lovenechinus missouriensis (Plate 41, fig. 1) the sixth column is represented by one or two plates in areas E, G, and I, i)ut is wanting in A and C. A single zone is of course greatly modified in certain areas in bilateral types, as in ambu- (25) 26 ROBERT TRACY JACKSON ON ECHINI. lacrum III in many spatangoids. Such extreme zonal modification does not occur in normal regular Echini. The term zones has been sometimes applied as "ambulacral zones," but this seems an unfortunate use of the word, for such areas do not surround the test, as a zone should, and moreover are not parallel to the equator, which also the term commonly implies. Three main zones may be recognized in any sea-urchin, which I would designate as the basicoronal zone, mid-zone, and placogenous zone. In the basicoronal zone occur the basicoronal plates, the first row of ambulacral and interambulacral plates of the corona bordering on the peristome or buccal membrane. Loven called these plates peristomal plates, but it seems an undesirable use of the word, which is usually applied to the buccal membrane and plates on the same. I therefore introduce the term basicoronal as meaning exactly what is intended, and not to be confused with terms applied to other parts. The basicoronal plates are of special interest as being the oldest or first formed plates occurring in any given corona, and also they are the plates giving support directly or through the perignathic girdle to important muscles of the lantern (text-figs. 22-31, p. 70; and 221-230, p. 193). The median zone, or mid-zone for brevity, is that horizontal zone measured halfway between the poles; it may coincide with the ambitus or lie dorsal to it, or rarely ventral to it. The mid- zone is important in Palaeozoic and most other Echini, as the area in which in the ambulacra and interambulacra the differential specific characters are typically most fully developed. Ventral to it the full features may not have been attained, and dorsal to it we soon get into the area of localized development, where the young plates have not attained the full characters of the species. In clypeastroids and spatangoids that have petaloid areas, these lie dorsal to the mid-zone, so that in such types special consideration of the dorsal area is necessary, but it does not invalidate the mid-zone as an area of importance in the consideration of structure. The placogenous zone (■"'^af = plate, + yeved = birth) is that portion of the corona next to the apical disc in which the newly added coronal plates occur both in the ambulacrum and the interambulacrum. This zone is of importance as the plates in it quite generally present youthful characters, which, as localized stages in development, can be compared with the characters seen in the young, or in the adults of simpler tj'pes in the group (text-fig. 237, p. 231). Roiv, as applied to ambulacral and interambulacral plates, refers to plates lying in one horizontal plane or zone, in contradistinction to columns. Column, as applied to ambulacral and interambulacral plates, refers to plates lying super- posed in a vertical series. The column is of course made up of nothing other than plates of a succession of rows, but is a convenient distinction to maintain in Palaeozoic types, where we find in various forms from two to twenty columns of ambulacral or one to fourteen columns of interambulacral plates. The column, especially in the interambulacrum, usually consists of a very definite succession of plates forced into a continuous series by the mechanical impact TERMINOLOGY. 27 of adjacent plates in the several succeeding rows. In the figures, the columns of interambula- cral plates are numbered from one upward (Plate 57, fig. 1; text-fig. 246). Adradial suture, a term introduced by Bather (1909a), is the suture between the interam- bulacrum and the ambulacrum. As a corollary, in Palaeozoic Echini, when there are more than two columns of interambulacral plates, those lying next the ambulacra are called the adradial plates. The term adambulacral plates or columns has been used, but the term adradial has the advantage of brevity. Median suture is the suture in the median line of the ambulacrum. When there are only two vertical columns of plates, it is perfectly obvious, but when more, it is distinguishable by the size of the two medial columns, as in Melonechinus (Plate 56, fig. 4) ; or if the plates do not differ in size, it is distinguished by the fact that the pores in the plates are set on the side toward the next adjacent interambulacrum and therefore are set on opposite sides of the plates on either side of the median sulure, Lepidesthes wortheni (Plate 67, fig. 8). The term median suture in Recent Echini is ordinarily applied also to the interambulacrum, and in Recent foims it is applicable, as there are two columns of plates; but, taking sea-urchins as a whole, it is not applicable to the interambulacrum; for it is only a coincidence of ordinal or specific character, if there is an even number of columns, and when there is, as in Palaeechinus quadri- seriaHs (Plate 30, fig. 3), the suture in the median line is not distinguishable from any other interambulacral suture except the adradial. In considering ambulacral plates, a certain degree of nomenclature is convenient. Pri- mary plates extend from the middle of the ambulacrum to the interambulacral suture (Palae- echinus, text-fig. 9). Demi-plates extend from the interambulacrum inward, but do not reach the middle of the ambulacrum. Occluded plates extend outward from the middle of the am- bulacrum, but do not reach the interambulacrum. Demi- and occluded plates are typically shown in Lovenechinus (text-fig. 11). Isolated plates do not reach either the interambulacral suture or the middle of the ambulacrum; such plates are extensively developed in Melonechinus and Lepidesthes (text-figs. 13, 14, p. 54). This nomenclature was based on the elements of compound plates in the Centrechinoida,' but it seems that it may equally be applied to the complex arrangement of simple plates in Palaeozoic Echini. ' The name Diadema, given by Schynvoet in 1711, cannot stand for a genus of echinoderms. The first post-Linnaean use of the generic name Diadema is in the "Museum Colonnianum," a dealer's catalogue published in London, in 1797; of this work, the author's name does not appear on the title page, but in the British Museum copy it is stated on two author- ities that the author was G. Humphreys. There are 12 species listed under Diadema, but of these only one is recognizable, as it is stated to be the same as Echinus esculentus Linne. On this evidence, if this work should be accepted, which is very doubtful, the genus Diadema would become a synonym of Echinus, as esculentus is the type of that genus. The next post- Linnaean use of the name Diadema was by Schumacher, 1817, who gave this as a generic name for a cirriped, basing it on Lepas diadvma Linn6, which Schumacher (1817, p. 90-91) changed to Diadema vulgaris. This name is a synonym of Coronula diadema Oken (Lehrb. Naturg., 1815, part I, section 1, p. 3G0). Ranzani, in 1820, also used the generic name Diadema for a cirriped, adopting, as did Schumacher, Lepas diadi-imi Linn6, as the type and giving it the new name Diadema candidum. 28 ROBERT TRACY JACKSON ON ECHINI. A simple plate is one which has held its original character of one undivided and independent piece. All plates in Palaeozoic Echini are of this character. , A compound plate is one made up by the coalescence of adjacent plates, seen in the ambula- crum in certain groups, in which area alone is it known to exist. It is a character of the ambu- lacral plates in the Centrechinoida and some Holectypina, but is unknown in the Palaeozoic. The statement that compound ambulacral plates occur in Palaeozoic types has been made (Duncan, 1889a, p. 13), but it is an error; no such plates occur in any known type. Occasion- ally in regular Echini two genital plates, or a genital and an ocular, may fuse when the resultant may in a sense be considered a compound plate (text-figs. 186, 195, 196, pp. 168, 169). Divided plates are those derived by the splitting on lines of solution of a previously continuous plate. This has been shown by Mr. Agassiz (1904) in the genital plates of Phor- mosoma. According to him (1904, p. 96), splitting occurs in interambulacral and ambulacral plates in Phormosoma. Mr. Agassiz (1874, p. 642) attributed the formation of compound ambulacral plates in the Centrechinoida, etc., to the splitting up of original plates. He also says (1883, p. 17) that anal plates in Palaeozoic Echini are formed by splitting. I have had this possible origin of plates in mind, but in my studies have seen no evidence of the origin of plates by splitting in Palaeozoic or other Echini excepting genital plates in echinothuriids, and rarely the same or ocular plates as variants (text-figs. 190-194, p. 169). When the antero-posterior axis is known, Loven's nomenclature of areas is adopted, num- bering the ambulacra I-V and the interambulacra 1-5. In Palaeozoic species the axes are rarely known on account of the usual absence of madreporic pores. The axes are known from the presence of madreporic pores in Lovenechinus lacazei (text-fig. 240), Lepidesthes formosa (Plate 68, fig. 5), L. colletti (Plate 71, fig. 1), and Meekechinus elegans (Plate 76, fig. 1). Axes are assumed from bilateral symmetry in Hyattechinus beecheri (Plate 24, figs. 5, 8), from the orientation of primordial ambulacral plates, Bothriocidaris archaica (Plate 1, fig. 1), and from the apparently eccentric anal area, Echinocystites (Plate 20, fig. 1). In other Palaeozoic types the axes are oriented arbitrarily by letters A to J, as discussed in the Introduction. Dr. Bather called my attention to a paper by M'Clelland (1840) in which he recognized that the name Diadema is not available for a sea-urchin, but he did not propose a new name. To quote his words, p. 170, "We luckily get rid of the genus Diadema [for Echini], from the term having been previously applied to a genus of Girrhopoda by Ranzani." High authorities reject Muerchen's (1774) names, therefore his Anademais unavailable. The name Calmarius annellaia is given by A. Agassiz (1872, p. 104) as a manuscript name of Gray's, and is treated by Agassiz as a synonym of Diadema selosum. As there is a closely related species, Echinolhrix calamaris {Diadema calamaris Gray), it seems probable that Gray's generic name was Calamarius, taken from that species. Calmarius is evidently the same name as Calaniarius with the letter a omitted by error; on this basis it is preoccupied by Calamaria Boie, 1827 (reptiles). As a new name is thus necessary for this genus, I propose Centrechinus (Ke'vrpov, a sting, and e'xivos, sea-urchin) with C. selosus (Leske), from Key West, Florida, as the type. The Florida and \\est hulian Diademas are considered as Diadema anlillarum Philippi by \. Agassiz and Clark (190S), but here antillarum is considered a synonym of selosum. As the name Diadema cannot stand for an cchinoderm, being preoccupied for a crustacean, the family and ordinal names derived from that genus cannot by the rules of nomenclature be retained. They can be replaced by names from the typical genus and may be called Centrechinidae and Centrechinoida. TERMINOLOGY. " 29 The term peristome is not used in Lov6n's sense, l)ut in the more usual sense of the buccal membrane with its plates extending from the mouth to the basicoronal plates. The peristome may be more or less plated with ambulacral plates only, with ambulacral and non-ambulacral plates, with non-ambulacral plates only, or may he a naked membrane (text-figs. 40-54, p. 80). Primordial ambulacral plates: these are the first formed ambulacral plates, ten in number, primarily situated around the mouth in the buccal membrane (Plate 3, fig. 11), or secondarily as basicoronal plates of the corona, as is usual in clypeastroids and spatangoids (Plate 3, fig. 15). Primordial interambulacral plates: these are the first formed interambulacral plates, five in number, one in each area, which may be retained at the base of the corona in the adult, or may disappear by resorption (text-figs. 22-31, p. 70). Ocular and genital plates are used in preference to radial and basal, as not implying an homology with those plates in crinoids. The sum of these plates may conveniently be termed the oculo-genital ring. Gills are external and interradial in position, situated on the border of the peristome, where they may well be called peristomal gills (text-figs. 55, 56, p. 83) ; or they are internal and radial in position, as Stewart's organs, Cidaris; or they may be external as modified tube- feet, when they may be called ambulacral gills to distinguish them from the interradial peristomal gills. The terminology applied to spines is necessarily rather arbitrary, as such great difference in size exists, but it is convenient to have some basis for description. Primary spines are the large spines occurring in such types as Cidaris or Arbacia. Secondary spines are best represented by the smaller spines clasping about the base of the large ones in Cidaris. Miliaries are minute spines seen between the smaller ones in Cidaris, or all the spines of Echinarachnius could be classed under this head. These are spines so small that it requires a microscope to see their structure. In the Palaeozoic we find primary spines and secondary spines in Archaeo- cidaris (Plate 9, fig. 13) and Lepidocidaris (Plate 17, figs. 10-14). In some types secondary spines only are known, as in Melonechinus (Plate 52, figs. 10, 11) and allies. Mihary spines are so far known in Archaeocidaris only (Plate 11, fig. 4). The points of attachment of the spines are the tubercles which may be primary, secondary', etc., according to their size. The consideration of nomenclature of the parts of spines, plates, etc., of Echini is very clearly and definitely set forth by Bather (1909a). The Aristotle's lantern is described as inclined, when in side view it subtends an angle of about 90°, the character of the Echinocystoida and Perischoechinoida (Plate 27, fig. 6) ; erect, when the faces of opposite sides are approximately parallel, the usual character of Recent regular Echini (Plate 5, fig. 2) ; and procumbent, when the flaring lantern rests almost on the base of the test, as in the Clypeastrina. Terms applied to the parts of the lantern are as follows. There are five teeth which are grooved or keeled (text-figs. 210, 212, p. 184). Each tooth is 30 ROBERT TRACY JACKSON ON ECHINI. embraced and held tightl}^ in place by one of the pyramids, and proximallj' the soft growing end is enveloped by the dental capsule (Plate 5, figs. 1, 6). The pyramid dorsally includes a triangular space, the foramen magnum, which is shalloiv, or deep, and dorsally open, or closed by the lateral extension of the epiphyses (text-figs. 207-21G, p. 184). The two half -pyramids, forming one pyramid and joined by suture, are inierradial in position (text-fig. 211, p. 184); or the two half-pyramids joined by an interpyramidal muscle are radial in position (text-fig. 220, p. 191). These radial half-pyramids are a feature connected with the view of considering a sea-urchin as composed of five radial parts (pp. 62, 190, text-figs. 217-220). Each half-pyramid bears on its inner face an elevated ridge, the dental slide, which supports the tooth (Plate 2, fig. 10); and has a lateral wing that extends to the oesophageal cavity; on the outer face of the wing in regular Echini are ridges for the attachment of interpyramidal muscles (Plate 5, figs. 3, 5, 9). The dorsal face of a half-pyramid, seen when the epiphysis is removed, is described as smooth when no pits exist, Perischoechinoida, Cidaroida, or is described as having pits when such structures exist, Centrechinoida (text-figs. 208-213). Each half-pyramid is surmounted by an epiphysis, which is narrow, when it extends only slightly beyond the half-pyramid, or wide, when it extends across the foramen magnum uniting in median suture with its fellow of the opposite side (text-figs. 207-216, p. 184). Each epiphysis presents a glenoid cavity and tubercles for articulation with the brace, and in the Camarodonta crests for support of the tooth (Plate 2, fig. 14; Plate 5, figs. 2, 5, 9). The brace is a block-shaped plate and presents cond;//fs and foramina which interlock with the glenoid cavities and tubercles of the epiphyses (Plate 2, figs. 9, 13, 15). In all regular Echini there are five compasses, each composed of two pieces joined by a transverse suture (Plate 2, fig. 12). The lantern muscles are termed protractors, retractors, interpyramidal, brace muscles, circular compass and radial compass muscles (Plate 5, figs. 1, 2, 4, 12; Plate 4, fig. 4; text-figs. 218-236, pp. 191, 193, 197). The perignathic girdle, when existent, consists of processes arising from the basicoronal plates. Apophyses are upward growths of basicoronal interambulacral plates; auricles are here limited to separate pieces arising from the basicoronal ambulacral plates, to which they are joined by close suture, not being continuations of these plates as apophj'ses are of inter- ambulacral plates (text-figs. 218-230, pp. 191, 193). The auricles in Phormosoma give rise to radial somatic and radial peristomal muscles, which are known only in the Echinothuriidae (text-fig. 226). The diameter of the test in regular circular Echini, where the axes are known, is measured on the plane of the ambitus and passing through ambulacrum III and interambulacrum 5. In regular Echini, which are elliptical in horizontal outline (Echinometridae), or elongate irregular Echini, the length is taken in the longer axis and includes the over-all measurement of the corona. PART I. COMPARATIVE MORPHOLOGY OF ECHINI. Having enjoyed the opportunity of studying most of the genera and species of Palaeozoic Echini, it is of importance to sum up the comparative structure and morphology of these types and to show their relations to living forms. Palaeozoology is only one aspect of zoology, distinguished from the study of living animals about as embryology, a study of the young, is in so far separated from a study of adult organ- isms. As Huxley said, the only difference between a collection of fossils and a collection of recent animals is that one has been dead longer than the other. For an intelligent study of fossils their living representatives must be constantly borne in mind, as their details of struc- ture throw a flood of light on fossil forms. Conversely a study of fossils in relation to the living gives a knowledge of the earlier and often primitive representatives of a group, throws light on the structure of the adult, and especially of the young of living types, and gives a rounding out of the knowledge of a group that cannot be attained by a si-udy of the living forms alone. This is my excuse, if such is necessary, for including in this discussion representatives of living and other post-Palaeozoic Echini in the summing up of the relations and structure of the Pal- aeozoic types, with which this memoir is primarily concerned. Under the consideration of Morphology are taken in order the form of the test, orienta- tion, the pentamerous system and variation, the structure of the skeleton, and growth. In the sea-urchin there are many anatomical parts for consideration, and they are taken up in the following order: the ambulacrum and interambulacrum of the corona, basicoronal plates, imbrication, spines, peristome, ocular and genital plates, periproct, Aristotle's lantern, and perignathic girdle. Form of the Test. The general shape of the test of a sea-urchin varies greatly, but is more constant in Palaeo- zoic than in post-Palaeozoic types. The test may be elliptical with the vertical axis somewhat longer than the horizontal through the mid-zone, Bothriocidaris, Palaeechinus ellipticus (Plate 29, fig. 2), Lepidesthes colletti (Plate 69, figs. 2-6). In Recent Echini the test is occasionally elliptical, as in Amblypneustes formosus Valentin, a species in which the height may exceed the diameter, though not always (A. Agassiz, 1873, p. 479). The test may be nearly or (31) 32 ROBERT TRACY JACKSON OX ECHIXT. quite spherical, Maccoya sphaerica (Plate 32, fig. 5), most species of Lovenechinus and Melon- echinus (Plate 55); or depressed spheroidal, as in Archaeocidaris and Cidaris. In all the above types the ambitus coincides very nearly with the mid-zone. The hori- zontal section in any zone may be nearly or quite circular, or modified by more or less pro- nounced melon-like ribs or elevations of both the ambulacra and the interambulacra, as in Melonechinus (Plate 60, fig. 3). The test may be depressed spheroidal with ambitus below the mid-zone, as in Lepidocentrotus ivhitfieldi (Plate 19, figs. 6, 7), or recent Strongylocentrotus. Rarely the ambitus is above the mid-zone, as in the high obovoid Lepidesthes corcyi (Plate 66, figs. 8-10) and Melonechinus obovatus (Plate 54, fig. 2). As far as I am aware, this character of ambitus above the mid-zone is known otherwise typically only in the recent depressed Echino- strephus molare (Blainville), in which Mr. Agassiz (1873, p. 457, Plate 5a, fig. 11) showed that the greatest diameter is near the abactinal surface. It may occur occasionally as a variation, as I found in three specimens of Strongylocentrotus purpuratus out of 120, and in one specimen of Strongylocentrotus drohachiensis in 33,000. It is undoubtedly an exceptional character for the ambitus to be above the mid-zone in fossil or Recent Echini. The test may be depressed with strongly pentagonal outline, as in Hyattechinus pentagonus (Plate 24, figs. 1-4). It may be circular but greatly flattened dorso-ventrally, as in Hyattechinus rarispinus (Plate 22), which apparently closely approached Echinarachnius in form. Or finally, in Palaeozoic types, the test may be flat on the base with rounded dorsal side, of moderate bilaterality, almost Cly- peaster-like in form, as in Hyattechinus beecheri (Plate 24, figs. 5-8). This includes all the forms of tests known in the Palaeozoic species. The test may be radially equal with central periproct, as are most Palaeozoic Echini and most post-Palaeozoic regular Echini except Echinometra and allies. Or it may be bilateral with central periproct, Hyattechinus beecheri (Plate 24, figs. 5-8; Plate 25, fig. 5), a character not known to me in any other Echini. Or it may be more or less regular in form but with the periproct eccentric in an interambulacrum, which is probably the odd posterior interambu- lacrum, Echinocystites. Orientation. The matter of orientation of a sea-urchin in relation to its true axis is important, and has been briefly touched upon in the Introduction. Loven (1874) urged that the proper orienta- tion of an echinoid is to take as the antero-posterior axis a line drawn through an ambulacrum and opposite interambulacrum, in such a plane that the madreporite lies in the right anterior interambulacrum. It seems rather arbitrary in the regular modern Echini at first sight, but there are good reasons for the acceptance of this orientation here. The most obvious reason, as pointed out by Loven, is that it is morphologically the same line on which bilaterality is ORIENTATION. 33 developed in all the irregular Echini. The ambulacra fall into a posterior pair, the bivium of Loven, and an anterior set of three, consisting of the right and left anterior ambulacra and the odd anterior ambulacrum, the trivium of Loven. It has been claimed that in regular Echini the madreporite is the only structure on which orientation could be based (A. Agassiz, 1881, p. 7). Wliile this is apparently true in many Echini, I have attempted to show under the discussion of ocular plates that in many regular modern Echini a bilateral symmetry is expressed and orientatioii obtained by the order in which ocular plates reach the periproct. To state the case in brief: in the young of apparently all modern regular Echini, the ocular plates are exsert." The same condition exists in the adult of many species. On the other hand, many species have in the adult one or more plates insert, or reaching the periproct, either as a typical character or as a variation. On the basis of ob- servations on 50,000 specimens, the evidence is that the first oculars to become insert are the plates of the bivium, next the plates of the posterior pair of the trivium, and last, if at all, the anterior odd plate of the trivium. The order of reaching the periproct is I, V, or V, I, then IV, II, III. This order is very closely adhered to, as shown later in tabulated form. The ocular plates therefore in many regular Echini express a bilateral symmetry in this group, and an orientation passing through ambulacrum III and interambulacrum 5, the plane of symmetry adopted by Loven. As studied from the ventral view, Loven (1874) showed graphically that the size and character of the primordial ambulacral plates give .excellent data by which to orient a sea- urchin, at least in the Irregulares and often in the Regulares as well. He showed that of the ten basicoronal ambulacral plates in (typical) clypeastroids and spatangoids, the la, Ila, 1116, IVa, \b are larger, and in spatangoids possess two separate pairs of pores, or two separate single pores, indicating two tube-feet. On the contrary, the 16, 116, Ilia, IV6, Va are smaller, and in the spatangoids bear only a single tube-foot, Collyrites (Plate 3, fig. 15). This system of alternation has but this one combination, a sort of key, by which the axes can be ascer- tained with entire certainty, as shown in numerous genera and species (Loven, 1874). Mr. A. Agassiz shows the same characters in his splendid work on Panamic Echini in numerous spatangoids as well as in many regular Echini. Loven (1892) showed that in the primordial ambulacral plates on the peristome in young Goniocidaris (Plate 2, fig. 1) the same system of alternation of large and small plates prevails. He (1874) also showed that it exists in the primordial plates surrounding the mouth in adult 1 Mr. Agassiz (1881, p. 7) objected to Loven's orientation as it does not take account of the madreporite in relation to the spiral system of development of plates. The spiral system was first suggested by Professor L. Agassiz (1834) and is considered by A. Agassiz in several publications (1864, p. 12; 1874, pp. 640, 722, 724; 1881, p. 7; 1892, p. 95). I regret that I have not succeeded in recognizing the spiral arrangement of plates in Echini. 2 It is possible that in Aspidodiadema and close allies the oculars are insert from their earliest stages. 34 ROBERT TRACY JACKSON ON ECHINI. Cidaris. Mr. A. Agassiz (1904) in the Panainic Echini shows the same system in young Poro- cidaris cobosi A. Ag. (his Plate 12, fig. 2), young Salenocidaris miliaris A. Ag. (his Plate 16, fig. 1), young Phormosoma placenta A. Ag. (my Plate 3, fig. 10). He also shows the interesting fact that the same order of ambulacral plates exists around the mouth in the adults of Cidaris (Dorocidaris) panamensis (A. Ag.) (his Plate 3, fig. 4), Porocidaris. milleri A. Ag. (his Plate 7, fig. 6), Echinosonia hispidum (A. Ag.) (his Plate 41), Phormosoma zeylandiae A. Ag. (his Plate 50, fig. 1), and Kamptosoma indistinctum A. Ag. (his Plate 50, fig. 1). It is true that in the adult of most of the regular Echini, where the primordial ambulacral plates surround the mouth, usually more or less isolated, they have not retained the relative proportion of the Loven system seen in the young, and from them one could not orient a sea- urchin. This has been dwelt upon at some length because in my studies of Bothriocidaris archaica (Plate 1, fig. 1) it was found that the ambulacral plates around the mouth are arranged according to the Loven law, which shows that it already was at work in the oldest known sea- urchin. It is worth mentioning that I was ignorant of the law at the time the drawing was made so that no prejudice came in to influence the making of the drawing. Loven maintained that the sea-urchin could also be oriented by the relative size and posi- tion of the ten interambulacral plates found at the base of the corona in regular Echini, the la, 2a, 36, 4a, 5b plates being smaller, the lb, 2b, 3a, 46, 5a larger. This is often true, but often not true, and it seems that the law has no general application. Loven's laws of orientation were carefully studied to see if correct orientation could thus be obtained in the Palaeozoic types, but with the exception of Bothriocidaris without success. In Palaeozoic genera, as far as known, ambulacral plates always extend on to the actinostome as far as the mouth, as shown in Bothriocidaris, Hyattechinus, Pholidechinus, Melonechinus, and Lepidesthes, and if suffi- ciently well preserved, the orientation by primordial ambulacral plates might be applied, as in Bothriocidaris. This, however, must await future study. In the cases of Lepidesthes formosa (Plate 68, fig. 5), L. colletti (Plate 71, fig. 1), Meek- echinus elegans (Plate 76, figs. 1, 6), and Lovenechinus lacazei (text-fig. 240), the specimens are correctly oriented by the presence of a madreporite. Bothriocidaris archaica (Plate 1, fig. 1) is oriented by the relation of the primordial ambulacral plates; Bothriocidaris pahleni and B. globulus (Plate 1, figs. 6, 9) by the large ocular plate which, as shown in B. archaica, overlies ambulacrum III. Hyattechinus beecheri (Plate 24, figs. 5-8) is oriented bj^ the plane of bilateral symmetry through an ambulacrum and jjosterior interambulacrum. Echinocystites pomum (Plate 20, fig. 1) is oriented by the eccentric anal area situated in an interambu- lacrum, presumably the odd posterior. The other species of Palaeozoic Echini figured cannot strictly be said to be oriented, as the letters designating areas are selected arbitrarily in each specimen without regard to a definite axis, which is an unknown quantity. The letters are used simply as a convenience in description for reference to specific areas. THE PENTAMEROUS SYSTEM AND VARIATION. 35 The Pentamerous System and Variation. In echinoderms as a whole the pentamerous system is remarkably constant as a character. In cystoids, however, we may have less than five ambulacra, as in Echinosphaerites, which has three ambulacra, or more than five, as in Caryocrinus, which has many ambulacral parts. In starfishes many genera, as Heliaster and Solaster, have typically more than five areas, and in some that have five, variation is common. In brittlestars more than five areas occur rarely as a specific, and in one instance as a sexual character. In all other groups of echinoderms the pentamerous system is very constant in all genera, and is departed from only as an indi- vidual variation. This variation is quite common in blastoids and crinoids. In Echini amongst Palaeozoic species, no case is known of departure from the pentamerous system, and it is apparently rare in living Echini. Bateson (1894) gives thirteen cases of complete or partial reduction to four-rayed sea-urchins in several genera of post-Palaeozoic fossil or Recent Echini. He also gives two cases of six-rayed urchins, a Galerites and an Amblypneustes, and three cases in which there is a sixth ambulacrum only. Chadwick (1898) described a four- rayed Echinus esculentus Linne, in which, however, there were five teeth. Ritchie and Mcintosh (1908) describe, very carefully, an Echinus esculentus with a partial reduction to four areas. Tower (1901) records an Echinarachnius parma from Woods Hole with one of the ambulacra imperfectly developed, and Hawkins (1909a) an Amblypneustes with two ambu- lacra incomplete dorsally. Osborn (1898) described a four-rayed Arbacia pun£tulata, de Loriol (1883) a partially six-rayed specimen of Stomopneustes, and Ribaucourt (1908) a completely hexamerous Strongylocentrotus lividus. I have been fortunate enough to study 71 specimens of Echini with a complete or partial departure from the pentamerous system. Besides being curiosities, these show very interest- ing morphological characters. Sixty were discovered in an examination of about 50,000 specimens for the characters of the apical disc, so that the variants from the pentamerous system averaged a httle more than one to a thousand. The variants are partially or completely trimerous, tetramerous, and hexamerous. They are numerically as follows: one Eucidaris tribuloides in 849 specimens, six Arbacia punctulata in 2,329 specimens, two Echinus magel- lanicus in 200 specimens, 38 Strongyloeentrotus drobachiensis in 33,000 specimens, one Strongy- locentrotus eurythrogrammus in 56 specimens, two Toxopneustes variegatus in 1,043 specimens, five Toxopneustes atlanticus in 2,643 specimens, three Tripneustes esculentus in 703 specimens, one Echinometra lucunter in 754 specimens, and one Colobocentrotus atratus in 82 specimens. As later discussed, the ocular plates seem to exert a controlling influence in the building up of the corona, as below and in immediate contact with the oculars originate the coronal plates, both ambulacral and interambulacral. In connection with each ocular is developed a whole ambulacrum, and, in addition, a half-interambulacrum on either side. That is, while 36 ROBERT TRACY JACKSON ON ECHINI. an ambulacrum originates on the ventral border of an ocular, each interambulacrum may be considered as composed of two halves, the plates of which originated on the left or right of the area in contact with the adjacent oculars (text-figs. 217, 218, p. 191). If this is true, then the loss of an ocular would cause a failure to develop of the plates that normalh' went with it, also an abnormal position of an ocular (Plate 7, fig. 2) should cause an abnormal distribution of the associated coronal plates. The cases of departure from the pentamerous system are taken up in the order of their structural characters. They are described in at least one case of each class and other cases are noted. The cases fall under twenty more or less distinct combinations of characters, and to facilitate the distinctions, they are numbered under so many distinct heads. Trimerous. 1. Three ambulacra, inter ambulacra, oculars, and genitals, four teeth. — A specimen of Strongylocentrotus drobachiensis collected at Dumpling Islands, North Haven, Maine, measur- ing 5.5 mm. in diameter (R. T. J. Coll., 812), is the nearest approach to a completely trimerous sea-urchin known. There are three ambulacra and interamlaulacra throughout the corona. There are also three oculars and genitals. The oculars all reach the periproct, an extraordinary character in the species, but no rules can be expected to apply in such a freak. There are four teeth and eight primordial ambulacral plates in the peristome. 2. Pentamerous ventrally, three ambulacra and five interambulacra dorsaUy, five oculars and genitals. — It is quite a frequent occurrence for one ambulacrum to fail to reach the apical disc, but rarer for two ambulacral areas to drop out. In a specimen of Strongylocentrotus drobach- iensis from Dumpling Islands, 43 mm. in diameter (R. T. J. Coll., 813), the test is quite normal ventrally, but dorsally ambulacra III and IV drop out (similarly, but later than in Plate 7, fig. 4), so that interambulacra 2, 3, 4 come in contact dorsally and make a continuous series of plates. All five interambulacra continue to the apical disc. There are apparently five oculars, but III and IV are imperforate and merged in a series of split genitals. There are five genitals, but genital 3 is imperforate and much split up by secondary sutures. A Strongy- locentrotus drobachiensis from Frenchman's Bay, Maine (R. T. J. Coll., 907), which measures 53 mm. in diameter, is pentamerous ventrally but ambulacra I and V drop out below the mid- zone. All oculars are in place but I and V are imperforate. As a result of this structure interambulacra 1, 5, 4 are confluent dorsally and the six columns of plates of these three areas extend to oculars II, I, V, IV as usual. A specimen of Toxopneustes atlanticus from Bermuda (R. T. J. Coll., 814) is quite similar. It is 52 mm. in diameter and ventrally normally pentamerous. Ambulacra III and IV fail to reach the apical disc by a distance of 10 mm., so that interambulacra 2, 3, 4 are in contact THE PENTAMEROUS SYSTEM AND VARIATION. 37 dorsally. There are five oculars and genitals, but oculars III and IV are imperforate. An imperforate ocular is usual when an ambulacrum drops out (Plate 7, fig. 4) and evidently is a correlated structure. The same structure is shown also in a specimen of Arbacia pundulata from Woods Hole, Massachusetts (R. T. J. Coll., 815). It is a small individual, 22 mm. in diameter, and distorted. Pentamerous ventrally, the ambulacra I and IV drop out dorsally so that interambulacra 5, 1, and 3, 4 are in contact dorsally. Five oculars and genitals are in place dorsally but somewhat distorted. There are as an independent variation eight plates in the periproct of this specimen. 3. Pentamerous ventrally, three ambulacra and interamhulacra dorsally, three oculars and five genitals in place, two oculars out of place. — An ocular or a genital may be wanting, but when present, they are almost universally in place in the apical disc. No case of a disjunct genital Text-fig. 1.— Slrongijlocentroliis drobachiemis (O. F. Miiller). Smllivan, Maine. Diam. 42 mm, R. T. J. Coll., 710. X 3.5. Oculars I and IV with associated coronal plates displaced. 38 ROBERT TRACY JACKSON ON ECHINI. has been seen, but in two cases oculars are separated more or less widely from the apical disc (text-fig. 1, and Plate 7, fig. 2). In a Slrongylocentrotus drobachiensis (text-fig. 1) a curiously abnormal and instructive condition occurs. The test is pentamerous ventrally, but dorsally the terminations of two ambulacra and four half-interambulacra are disjunct from the apical disc. There are five genitals and three oculars in the apical disc, but two oculars, I and IV, have become separated from the oculo-genital ring. The system is the same as that described in Toxopneustes (p. 42; Plate 7, fig. 2), only, as the separation occurred later in life, the distance of the oculars from the apical disc is not so great as in the Toxopneustes. Ocular IV (text-fig. 1), is furthest removed. .Ambulacrum IV is twisted dorsally, but extends ventrally as usual. The right half of interambulacrum 3 abuts against ocular IV, and the left half of interambulacrum 4 curls around dorsally so as to come in contact with the opposite side of ocular lY. The left half of interambulacrum 3 and right half of 4 extend to oculars III and V respectively as usual. A similar condition exists in the relation of the ambulacrum and half interambulacra to the misplaced ocular I. If the abnormality had occurred earlier in growth, this would have made a very striking specimen, but yet it elucidates the principles involved quite clearly. In this specimen it is clear that ambulacra I and IV, with the associated half interambulacra follow the oculars I, IV in their displaced position. Teframerous. 4. Four ambulacra, interambulacra, oculars, genitals, and teeth. — More or less completely four-rayed specimens are the commonest departure from the pentamerous system, yet com- pletely four-rayed specimens were seen in only nine cases, all in three species. In a specimen of Strongylocentrotus drobachiensis (Plate 6, fig. 9) there are four ambulacra, interambulacra, oculars, genitals, and teeth, and eight primordial ambulacral plates in the peristome. It is oriented by the madreporite and character of the bivium reaching the periproct, or insert, as it is here called, which is a strongly dominant feature of the species. Accepting this orienta- tion, the absent parts are ocular IV and its associated parts, which are ambulacrum IV, the right half of interambulacrum 3 and the left half of interambulacrum 4. The missing genital may be either 3 or 4, which, it is perhaps impossible to say. Four other specimens of Strongylo- centrotus drobachiensis show the same structure. In all, the bivium is insert and the missing parts are the same as in the specimen just described. One of these specimens is from Dumpling Islands, Maine (R. T. J. Coll., 816), and measures 18 mm. in diameter. Two are from French- man's Bay, Maine (R. T. J. Coll., 908, 909), and measure 42 and 52 mm. in diameter respectively. The fourth specimen is from Truro, Massachusetts (R. T. J. Coll., 817), and measures 26 mm. in diameter. In this specimen the lantern is wanting, but there are eight auricles which indicate a four-parted lantern. Four specimens of Microcyphus maculatus, THE PENTAMEROUS SYSTEM AND VARIATION. 39 from Mauritius, with complete or partially tetramerous tests, were found in 343 specimens examined in the Museum of Comparative Zoology by Dr. H. L. Clark. Three of them fall under the present head, with four teeth, eight primordial ambulacral plates, four ambulacral and interambulacral areas in the corona, four oculars and genitals. In one of the specimens two of the genitals are fused, and in one of them a genital is split into three parts. An Echinometm lucunter from Jamaica, in the Museum of Comparative Zoology, is com- pletely four-rayed, with four teeth, eight primordial ambulacral plates, four ambulacra and interambulacra in the corona, four oculars and genitals. The absent parts are ocular and ambulacrum I and genital 1, the left half of interambulacrum 1 and the right half of interambulacrum 5. 5. Four ambulacra, interambulacra, and oculars, five genitals, four teeth. — This structure differs from the last described only in that there are five instead of four genital plates. An example is seen in Arbacia punctulata (Plate 8, fig. 2; Plate 6, fig. 1). The specimen is- small, 26 mm. in diameter, but is higher than usual proportionately, 19 mm. It is perfectly shaped, not abnormal in appearance. There are five genitals, the only pentamerous part of the animal observed, but there are only four genital pores, one being absent in the madreporite. The specimen is oriented by the madreporite and planes of arrangement of the periproctal plates. Only four oculars are present. The absent parts are ocular and ambulacrum II completely, also the right half of interambulacrum 1 and the left half of interambulacrum 2. The corona consists, therefore, of four ambulacra with three interambulacra and two half-interambulacra. There are four teeth, four pairs of auricles, and eight primordial ambulacral plates. A second almost completely four-rayed Arbacia punctulata, from Woods Hole, Massachu- setts, is in the American Museum of Natural History. This like the above is small, perfectly formed, not at all distorted. It measures 25 mm. in diameter through the ambitus and 18 mm. in height. There are five genital plates, but the madreporite has no genital pore, and lies in close contact with genital 3 just as the madreporite lies in contact with genital 1 in Plate 6, fig. 1. There are four oculars, ambulacra, interambulacra, and teeth throughout. The absent parts are ocular and ambulacrum III, the right half of interambulacrum 2, and the left of interambula- crum 3 (compare Plate 6, fig. 1). A third specimen of Arbacia punctulata, from Woods Hole, in the Museum of Comparative Zoology, has a similar structure. It has four ambulacra, interambulacra, and oculars, but five genitals, one depauperate and imperforate. The wanting parts are ocular and ambulacrum I, the right half of interambulacrum 5, and the left half of interambulacrum 1. The teeth are wanting. The Arbacia punctulata described by Osborn (1898) is similar to the above three, the five genitals being the only pentamerous portion. In his specimen as gathered from the description and figure, the absent parts are ocular and ambulacrum I, the right half of inter- ambulacrum 5, and the left half of interambulacrum 1. 40 ROBERT TRACY JACKSON ON ECHINI. In Yale University Museum a specimen of Arbacia lixula from the Mediterranean has four areas. It is one of a lot of four specimens (cat. no. 199). There are five genital plates, but only four oculars and four ambulacra and interambulacra which extend from the basi- coronal row to the apical disc. The lantern is wanting. The absent parts are ocular and ambulacrum I, the right half of interambulacrum 5, and the left half of interambulacrum 1; therefore the left half of interambulacrum 5 and the right half of interambulacrum 1 are in contact and form a single area (compare Plate 6, fig. 1). This area is wider than usual for an interambulacrum, and the plates bear more primary tubercles than usual, but the tubercles are spaced about as in other areas, and in normal specimens. This indicates, as shown in the reverse condition in Tripneustes (p. 47), that tubercles and spines are distributed at a given distance apart, rather than a given number to a plate, as a species character. Genital 1 is somewhat distorted. Professor Verrill (1909) has given an excellent photographic figure of this specimen. A small specimen of Strongyloccntrotus drobachiensis (Plate 7, fig. 3) has much the same character as the above described Arbacias. The specimen is normally shaped except that it is unusually high, dome-shaped. It is 35 mm. in diameter and 24 mm. high. There are four ordinary genitals and a small plate between the madreporite and ocular I may be considered a depauperate and imperforate fifth genital. There are four oculars. The specimen is oriented by the madreporite and the two insert ocular plates which in this species mark the bivium. The absent ocular on the basis of this orientation is evidently that one which should lie l^etween the madreporite and the depauperate genital next to it. The absent part of the corona is ambulacrum II, also the right half of interambulacrum 1 and the left half of interambulacrum 2. The lantern is wanting, but there are four pairs of auricles indicating a four-parted lan- tern. A similar structure occurs in a specimen of the same species from Dumpling Islands (R. T. J. Coll., 818). It is small, 15 mm. in diameter, not distorted, and completely four-rayed except for the five genitals. The madreporite has no genital pore. The missing parts are ocular and ambulacrum II, the right half of interambulacrum 1 , and the left half of interambula- crum 2. 6. Four ambulacra {bui ten primordial ambulacral plates), four interambulacra and oculars, but five genitals and teeth. — This combination was found in a specimen of Strongylocentrotus drobachiensis from Dumpling Islands, 5.5 mm. diameter (R. T. J. Coll., 819). Being very young, all the genitals are imperforate except for the madreporic pores. Ocular I only is insert. The missing parts are ambulacrum and ocular II, the right half of interambulacrum 1, and the left half of interambulacrum 2. Surrounding the five teeth are ten primordial ambulacral plates. This indicates that it began life as a pentamerous individual, but no trace of a fifth ambulacrum is seen in the corona. A second specimen with these characters occurs in an Echinus magellanicus, from Port THE PENTAMEROITS SYSTEM AND VARIATION. 41 Gallant, Patagonia, in the Museum of Comparative Zoology. It has five teeth and ten pri- mordial ambulacral plates, four ambulacra and interambulacra in the corona, four oculars, but five genitals. The wanting parts are ocular and ambulacrum V, the right half of inter- ambulacrum 4 and the left half of interambulacrum 5. 7. Four ambulacra, hut five interambulacra, oculars, genitals, and teeth. — In Arbacia ■punctulata (Plate 7, fig. 1) an ambulacrum is entirely absent, but otherwise the specimen is pentamerous. It is a small specimen, 22 mm. in diameter. Dorsally there are five genitals and oculars, ventrally five teeth and ten primordial ambulacral plates. There is no trace of ambulacrum IV in the corona. Interambulacra 3 and 4 have each two columns of plates which extend dorsally to oculars III, IV, V, as usual in columns of these two areas. In addition to the four columns, there are ventrally several plates representing a fifth column and one plate a sixth column. These extra columns cannot be referred with a,ssurance to either areas 3 or 4, but were in all probability developed below ocular IV and serve as space fillers in the corona to occupy the place that would have been filled by ambulacrum IV if it had developed. This specimen superficially is suggestively like Palaeozoic Echini where four or more columns exist in an interambulacrum, the median columns being made up of plates which in general are hexagonal, and the adambulacrals pentagonal. Here, however, the many columns are the result of the union of two areas, not one area as in the Palaeozoic. 8. Five ambulacra, four interambulacra, four oculars {one with two pores), four genitals, five teeth. — A very peculiar case is the Strongylocentrotus drobachiensis, Plate 7, fig. 5. The specimen is good sized, 50 mm. in diameter and 30 mm. high. It has a pronounced bulging on the median suture line of ambulacra I, II. There are four genitals and four oculars. One of the oculars has two pores, indicating the termini of the radial water tubes of the two sub- jacent areas. There are five ambulacra, but interambulacrum 1 is entirely wanting. The ambulacra I, II have each two columns of plates, but the 16 and Ila are very narrow columns. It is worth noting that on the line of sutural contact of these two areas the outer pore of each pore-pair lies above the inner one, the usual condition where an ambulacrum abuts on an interambulacrum. There are five teeth and five pairs of auricles. A second specimen showing this same structure is a Toxopneustes atlanticus, from Bermuda, R. T. J. Coll., 895. It measures 62 mm. in diameter, and is not distorted. There are four genitals and oculars, but one of the oculars has two pores, as in Plate 7, fig. 5, indicating that the pores connected with the two associated ambulacral areas. There are five ambulacra, IV and V being in contact throughout, as interambulacrum 4, also genital 4, are entirely wanting. There are five teeth, but only eight gill cuts. 9. Five ambulacra, four interambulacra, five oculars, four genitals, five teeth. — • A Toxo- pneustes variegatus from Boca Ciega Bay, Florida (R. T. J. Coll., 820), 52 mm. in diameter, has five ambulacra, but interambulacrum 5 and genital 5 are wanting. A Strongylocentrotus 42 ROBERT TRACY JACKSON ON ECHINI. drobachiensis from Truro, Massachusetts (R. T. J. Coll., 821), 32 mm. in diameter, is similar, with five ambulacra, but interambulacrum and genital 5 are entirely wanting. Both speci- mens have five teeth. The condition in both is similar to Plate 7, fig. 5, excepting that there are two oculars covering the adjacent ambulacra V and I instead of one ocular with two pores. A third specimen showing this combination is a Microcyphus maculatus from Mauritius, in the Museum of Comparative Zoology. In this there are five teeth, ten primordial ambulacral plates, five ambulacra and oculars, but four interambulacra and genitals. Interambulacrum and genital 5 are absent, so that oculars and ambulacra V, I are in contact throughout. 10. Five ambulacra, four interambulacra, five oculars, genitals, and teeth. — This typo like the last has an interambulacrum wanting, but differs in that there are five instead of four geni- tals. A Strongylocenlrotus drobachiensis from Calderwood Island, Fox Island Thoroughfare, Maine, measuring 34 mm. in diameter (Plate 5, fig. 16), has five complete ambulacra but only four interambulacra. Interambulacrum 1 is absent and ambulacra I and II are in contact throughout their length. There are five oculars and genitals, but oculars I and II are in contact and shut out genital 1 from contact with the corona. This is the only case of a genital dorsal to the oculars seen excepting in Bothriocidaris archaica where it is a species character. Genital 1 in this case is very small and imperforate. There are five teeth, ten auricles, and ten primordial ambulacral plates. 11. Five ambulacra ventraUy, four interambulacra throughout, four ambulacra above the ventral border, four oculars and genitals, five teeth. — This peculiar structure occurs in one speci- men only, an Arbacia punctulata (R. T. J. Coll., 881), kindly sent me from Johns Hopkins Uni- versity through Dr. G. T. Hargitt. The specimen is 43 mm. in diameter, 31 mm. high, and quite symmetrical. There are five teeth and ten primordial ambulacral plates, but only eight peristomal gills. At the ventral border of the corona there are five ambulacra, but ambulacrum II exists for only a distance of 2 mm. in which there are three pore-pairs. Above this point the ambulacrum is entirely wanting. There are four interambulacra only, the area lying above ambulacrum II being a complete area, but consisting of the left half of interambula- crum 1 and the right half of interambulacrum 2 (compare Plate 7, fig. 3). Dorsally there are four oculars, genitals, and periproctal plates. Ocular II has quite disappeared. 12. Pentamerous ventraUy, four ambulacra and interambulacra dorsally, four oculars in place {one ventral), five genitals. — One of the most suggestive and interesting cases observed is the Toxopneustes variegatus, Plate 7, fig. 2. The specimen is full grown and has a strong bulge on the abnormal side. It is 61 mm. in diameter through III, 5; 67 mm. through 4, II; and 39 mm. high. Dorsally in place there are five genitals and four oculars, but just below the ambitus lies a fifth ocular which by some chance became separated and left behind during the dorsal growth of the sea-urchin. As there are the usual five areas ventraUy, the specimen is readily oriented by the position of the madreporite. Ocular II, which should be between THE PExNTAMEROUS SYSTEM AND VARIATION. 43 genitals 1 and 2, has in its abnormal position grown to a large and somewhat irregularly shaped plate (compare oculars in text-fig. 1, p. 37). Below this ocular extends ambulacrum II, which finds its dorsal limit in this plate. Also extending ventrally from ocular II there are two half-interambulacra, the right half of interambulacrum 1 and the left half of interambula- crum 2. Proceeding dorsally from ocular II, the coronal plates that are typically associated with this plate cease, but the left half of interambulacrum 1 and the right half of interambula- crum 2 fill the space and extend respectively to ocular I and ocular III, their normal point of origin. There are five teeth. This specimen owes its peculiarities to relatively late influences, not congenital. It is similar in principle to text-fig. 1, p. 37, but is more striking as the ocular is farther removed from its normal position in the apical disc. It lends weight to the view held that ocular plates are a controlling influence in the development of the corona, and that from each ocular originates an ambulacrum and two half-interambulacra (compare text- fig. 218, p. 191). 13. Pentamerous ventrally , four ambulacra and interambulacra dorsally, four oculars, and five genitals. — kStrongylocentrotus drdbachiensis from York, Maine (R. T. J. Coll., 823), 40 mm. in diameter, is pentamerous ventrally, but ambulacrum II consists of only a pair of plates in the basicoronal row. The associated two half-interambulacra of areas 1 and 2 are similarly highly reduced. Above this extreme ventral area there are only four ambulacra and inter- ambulacra. The ocular associated with ambulacrum II has quite disappeared, probably by resorption. Four oculars and five genitals are in place in the apical disc. Genital 1 is depauperate and lies against the madreporite. There are five pairs of auricles. A similar structure is shown in an Echinus magellanicus from the Straits of Magellan in the collections of the United States National Museum. There are five teeth and ten primordial ambu- lacral plates. In the corona there are five ambulacra and interambulacra ventrally, but ambulacrum I and the associated interambulacral plates exist for only a short distance, above which there are only four areas. There are four oculars and five genitals of which genitals 1 and 5 are depauperate and imperforate. 14. Pentamerous ventrally, four ambulacra and interambulacra dorsally, four oculars and genitals. — A Strongylocentrotus drobachiensis from Dumpling Islands, Maine, (R. T. J. C-oll., 824), 18 mm. in diameter, is completely pentamerous ventrally. A slight distance above the ventral border ambulacrum V drops out and also the two associated half-interambulacra. Ocular V has quite disappeared. There are four oculars and genitals in place. This type differs from the last considered only in that there are four instead of five genital plates. 15. Pentamerous ventrally , four ambulacra and jive interambulacra dorsally. Jive oculars and genitals. — This is the commonest type of departure from the complete pentamerous system found, 17 cases having been seen. A small specimen of Strongylocentrotus drobachiensis (Plate 7, fig. 4) shows interesting features. The specimen is 17 mm. in diameter, 9 mm. high, 44 ROBERT TRACY JACKSON ON ECHINI. and misshapen on the abnormal side. \'entrally there are five ambulacra, interambulacra, and teeth, but ambulacrum II is developed for a short distance only. Dorsally there are five genitals and oculars, but ocular II is imperforate. The dorsal border of the ambulacrum is widely separated from its ocular. Ocular II lies dorsally and produces its share of interam- bulacral plates, although ambulacral plates ceased to be built. Columns 1 and 2 of interam- bulacrum 1 extend dorsally and abut against oculars I and II as usual. In interambulacrum 2, columns 1 and 2 extend dorsally, but only 2 reaches the apical disc, column 1 dropping out, so that in the last three rows or zones of growth there is only a single column of plates in this area, a condition somewhat comparable to that seen in area 4 of Arbacia (Plate 4, fig. 11) where also a single column of plates exists dorsally. Just above ambulacrum II in this speci- men (Plate 7, fig. 4) there are extra interambulacral plates that cannot be definitely referred to either areas 1 or 2. They are virtually space fillers, which, in my opinion, is what the interambulacral plates are. This specimen is structurally almost identical with the interesting Echinus esculentus described by Ritchie and Mcintosh (1908). In both, the ocular remained in place dorsally but imperforate, the ambulacrum existed ventrally only, and the adjacent interambulacra developed in a comparable manner, though with certain differences. In the Echinus only the four columns of the two interambulacra exist above the imperfect ambulacrum, and extend dorsally to their respective three ocular plates. A Strongylocentrotus drobachiensis (Plate 6, fig. 6) has five oculars and genitals as usual; there are five interambulacra continuous to the apical disc, but one of the five ambulacra, IV, dies out dorsally, so that interambulacra 3 and 4 come in contact and alone touch ocular IV. The distance of separation of the ambulacrum from its ocular is only slight, but it repre- sents a zone in which there are only four ambulacra. In this specimen ocular III is split, a rare variation, and genital 5 is divided. A few pores in the dorsal part of interambulacnun 3 are apparently supernumerary genital pores that lie below genital 3. An Arbacia punclulata, from Woods Hole, Massachusetts, in the American Museum of Natural History, is peculiar in that ambulacrum III fails to reach the apical disc so that there are only four ambulacra in that region. The structure is almost the same as in Plate 6, fig. 6, except that a different ambulacrum is involved. Six specimens of Strongylocentrotus col- lected at Dumpling Islands, Maine, (R. T. J. Coll., 825-830), one from Calderwood Island, Maine (R. T. J. Coll., 845), one from Frenchman's Bay, Maine (R. T. J. Coll., 846), and two from Friday Harbor, Puget Sound (R. T. J. Coll., 903, 904), a Strongylocentrotus eunjthrogravimus from New South Wales (R. T. J. Coll., 905), also a Tripneustes esculentus from Hayti (R. T. J. Coll., 831), and two specimens of Toxopneustes atlanticus from Bermuda (R. T. J. Coll., 894, 897), all show a similar structure to Plate 7, fig. 4, with five genitals and oculars in place and an ambulacrum dropping out at a greater or less distance from the apical disc. In all but one of these cases the ocular lying above the imperfect ambulacrum is imperforate. In the THE PENTAMEROUS SYSTEM AND VARIATION. 45 several cases, each of the five ambulacra is involved, but IV appears to be more frequently imperfect than any other area. 16. Pentamerous ventrally, four ambulacra and five interanibulacra dorsally, four oculars, five genitals. — This type is similar to that of 13 in that the ocular associated with an area has disappeared, but differs in that there are five interambulacra. A Strongylocentrotus from Calderwood Island, Maine (R. T. J. Coll., 832), 46 mm. in diameter, is completely pentamerous ventrally, but ocular II is wanting, and ambulacrum II falls short of the apical disc by 10 mm. The two bordering interambulacra, 1 and 2, are in contact dorsally. The dorsal plates of column 1, in interambulacrum 2 and of column 2, in interambulacrum 1 are large, not small as is usual in dorsal plates, indicating that no new ones have been recently added, which is in accord with the absence of ocular II, from which the plates of these two columns take origin. Three other specimens of the same species from Calderwood Island (R. T. J. Coll., 833-835) and a Colobocentroius atratus from the Hawaiian Islands, in the State Museum at Albany, New York, have a similar structure. In all, an ocular is absent, and the corresponding ambulacrum falls short of the apical disc, but the associated interambulacra extend to the apical disc simi- larly to the specimen just described. In the Colobocentrotus noted, ocular I is absent, and ambulacrum I falls short of the apical disc. 17. Pentamerous ventrally and throughout the corona, five oculars, but four genitals. — This line of departure from the pentamerous system is where only one genital is absent. It is a typical condition in spatangoids where genital 5 is always absent, but occurs only as an aber- rant variation in regular Echini. In Eucidaris tribuloides (text-fig. 185, p. 167) there are only four genital plates, 1 being absent. Oculars I and II are in contact, and as a result interambulacrum 1 comes in contact with two oculars as it does in Bothriocidaris archaica and in the posterior interambulacrum in spatangoids. This interambulacrum is also much narrowed dorsally and at one point a single plate fills the area laterally. A more striking case is the Arbacia punctulata shown in Plate 4, fig. 11. In this remarkable specimen there are five oculars but only four genitals. Genital 4 is absent so that oculars IV and V are in contact. The interambulacrum 4 dorsally drops out to a single column of plates; the single column and the ocular contact of the dorsal plate of the same as a regressive variation is an exact repetition of the character seen in Bothriocidaris archaica (Plate 1, fig. 2). A quite comparable case is seen in Strongylocentrotus drobachiensis (Plate 6, figs. 7, 8). In this specimen there are five oculars, of which I and II reach the periproct and V, IV, III are exsert. There are only four genitals, number 4 being absent, so that oculars IV and V are in contact. These plates are peculiar in that they are produced ventrally in an elongate fashion. Interambulacrum 4 ventrally consists of two columns of plates as usual; above the mid-zone it narrows, and the two columns give place to a single column of plates for three rows, that is, 46 ROBERT TRACY JACKSON OX ECHINI. three plates (Plate 6, fig. 8). Above this point two columns are built again for a short distance, there being at this point three plates in column 1 , and two in column 2. Proceeding dorsally, a change again occurs, and two single plates are built; the last is very high and narrow, unlike anything seen in any other echinoid. This structure is very difficult to see on the exterior of the test, but on the interior (Plate 6, fig. 8) it is perfectly plain. The dropping out to a single column is comparable to the typical character of Bothriocidaris and to the peculiar Arbacia (Plate 4, fig. 11). Dorsally, interambulacrum 4 does not reach the ocular plates, but is separated for a short distance in which ambulacra IV and V come in contact and alone reach oculars IV and V. Another case was found in an Ai-bacia punctulala from Florida (R. T. J. Coll., 889, Plate 4, fig. 12). The specimen is 33 mm. in diameter. It is pentamerous throughout except for the absence of genital 4, which is wanting, as in Plate 4, fig. 11. It differs from that specimen, however, in that interambulacrum 4 is completely developed, its two columns extending, as usual, to the apical disc, but abutting against oculars IV and V without any contact with a genital as that plate is absent. Again, a similar structure is shown in a specimen of Toxo- pneustes atlanticus from Bermuda (R. T. J. Coll., 898). This specimen measures 53 mm. in diameter. It is quite pentamerous throughout except that genital 4 is wanting. Oculars IV and V are therefore in contact and cover completely ambulacra IV and V and interambula- crum 4 as in the Arbacia, Plate 4, fig. 12. In this Toxopneustes there are two columns of plates in contact with the two oculars, but the plates are small. These several cases demon- strate that the absence of a genital does not cause a loss of the corresponding interambulacral area. Hexamerous. 18. Six ambulacra, interambulacra, oculars, genitals, and teeth. — In a fine large Tripneustes esculentus from Pernambuco, Brazil (Plate 6, figs. 2, 3) there are six areas throughout, a condition hitherto definitely known only in Strongylocentrotus Uvidus. The specimen measures 110 mm. in all tliameters as it is perfectly circular in outline. It is 58 mm. in height. In this species, when two oculars are insert, they are the bivium (see tabulation of 703 speci- mens, p. 161), so that the specimen is oriented on the basis of the insert bivium and position of the madreporite. Obviously the additional ambulacrum, ocular, and interambulacrum lie between interambulacrum 3 and ambulacrum IV. There are four normal genitals, but two, numbers 3 and 6, are fused into a single plate, a somewhat rare condition, but similar cases are seen in Strongylocentrotus and Tripneustes (text-figs. 195, 196, p. 169). There are six oculars, the sixth lying between the tips of genitals 3, 6. The ambulacra are all normal in appear- ance and at the mid-zone measure 28 mm. in width, except V, which is 29 mm. The width, 29 mm., is the same that is found in the ambulacra of a normal five-rayed specimen measuring * THE PENTAMEROUS SYSTEM AND VARIATION. 47 the same diameter of 110 mm. The interambulacra are normal in appearance superficially, but at the mid-zone they all measure 27 mm. in width, that is, they are narrower than the ambulacra. In a normal specimen of the same size the five interambulacra measure each 38 mm. in width when the ambulacra measure 29 mm. In the six-rayed specimen, therefore, evi- dently the space gained to add the extra ambulacrum and interambulacrum is attained by building ambulacra of practically the usual width, but narrowing all the interambulacra equally to much less than the usual width. This emphasizes the conclusion gathered from normal Echini that the interambulacrum is essentially a space filler and adapts itself to fill what space is available between the ambulacra which are the more essential structures. There are twelve cuts in the test ventrally for the peristomal gills, and the lantern has six teeth and all its other parts in sixes or multiples of the same. In the narrowed interambulacral areas the tubercles (Plate 6, fig. 3) occupy the same distance apart as in ordinary pentamerous speci- mens, but owing to the narrowness of the areas there are fewer tubercles on each plate. This case with the Arbacia lixula previously considered (p. 40) demonstrates that the distance apart and size of tubercles is the species feature and not the actual number of tubercles on a given plate. The same hexamerous structure as just described occurs in a Strongyloccntrotus drobachien- sis from Dumpling Islands, North Haven, Maine (R. T. J. Coll., 838, Plate 8, fig. 3). This choice specimen measures 53 mm. in diameter, and 25 mm. in height. There are six ambulacra and interambulacra complete, six oculars and six genitals, but genitals 3, 6 are fused, with ocular VI between them exactly as in Plate 6, fig. 2. The ambulacra at the mid-zone measure 11 mm. in width and the interambulacra 12 mm. In a normal pentamerous specimen of the same size the ambulacra measure 12 mm. and interambulacra 19 mm. in width. It is clear, therefore, that as in the Tripneustes, the space for the sixth areas is attained mainly bj' narrowing the interambulacra and retaining ambulacra of practically the usual width in all areas. Ventrally there are twelve cuts for the peristomal gills. Unfortunately the jaws are absent, but they were doubtless hexamerous as all other parts of the sea-urchin have this arrangement. I was startled by finding this same structure for a third time in a small Strongijlocentrotus drobachiensis, also collected at Dumpling Islands (Plate 7, figs. 7, 8). The specimen is 14 mm. in diameter and 6 mm. in height; it is perfectly shaped without any dis- tortion. There are six ambulacra and interambulacra throughout the corona, six oculars and genitals, but as in the two other cases, genitals 3, 6 are fused, with ocular VI between them, as seen in Plate 7, fig. 7. It is certainly most extraordinary that this parallel structure should exist in three specimens, and indicates what I have elsewhere pointed out, how very definite even extremely rare variation may be. Ventrally there are six teeth, twelve pri- mordial ambulacral plates around the mouth and twelve cuts for the peristomal gills (Plate 7, fig. 8). The three similar specimens of hexamerous Tripneustes and Strongylocentrotus just 48 ROBERT TRACY JACKSON ON ECHINI. described were evidently cases of congenital variation, whereas the next type considered is a variation to six parts taken on at a comparatively late stage in development. The specimen of Strongylocentrotus lividus described by Ribaucourt (1908) would belong in this series, as he says that it is completely hexamerous, having six teeth, six ambulacral and interambulacral areas, six oculars and genitals. It would be interesting to know in which area the modification takes place and if there is a fusion of genitals as in the specimens above described. 19. Pentamerous ventrally, six ambulacra and interamhulacra dorsally, six oculars, five genitals. — A most interesting condition occurs in a specimen of Tripneustes esculentus found in material kindly collected for me in Jamaica by Dr. Thomas Barbour. Tliis choice speci- men (Plate 6, fig. 4) measures 76 mm. through III, 5 and is 45 mm. in height; it presents a slight hump in the aberrant area. There are six oculars, but only five genitals. The addi- tional sixth ocular lies between ocular II and the madreporite. Ventrally the specimen is pentamerous and quite normal, but above the mid-zone a sixth ambulacrum appears interca- lated between ambulacrum II and interambulacrum 2. It extends dorsally, the two columns of plates reaching ocular VI in the usual fashion. About halfway, up the sixth ambulacrum a sixth interambulacrum appears. It consists of a single column of narrow plates which extend to oculars II and VI. The single column and ocular contact are comparable to the character seen in Bothriocidaris archaica (Plate 1, fig. 2). I believe it and the next described case are the only known instances since Ordovician times of an interambulacrum consisting of a single column of plates throughout its length. At two points the single column of interambulacral plates is separated dorso-ventrally, and at these breaks ambulacra II and VI are in contact for a brief space. Genital 3 is split horizontally, as in text-fig. 190. A like partially hexamerous condition exists in a Strongylocentrotus drobachiensis from Truro, Massachusetts (R. T. J. Coll., 836), 35 mm. in diameter. It is quite normal ventrally, but dorsally there are, as in the above case, five genitals and six oculars. Ocular VI lies between ocular V and genital 5 and is fused with the latter. A sixth ambulacrum for a length of 10 mm. lies between ambulacrum V and interambulacrum 5, the condition being as in Plate 6, fig. 4. A sixth interambulacrum, consisting of two plates in a linear series, lies between ambulacra V and VI as in the same figure. A small and distorted specimen of Arbacia punctulata (R. T. J. Coll., 882) kindly sent me from Johns Hopkins University through Dr. G. T. Hargitt, again shows a similar structure. The specimen measures 3G mm. in diameter through 4, I and 19 mm. in height. Pentamerous ventrally, dorsally a sixth ambulacrum exists for a distance of 6 mm. The additional area lies between interambulacrum 1 and ambulacrum II (compare Plate 6, fig. 4). A sixth inter- ambulacrum consisting of two plates in a vertical series lies between ambulacrum VI and ambulacrum II, as in Plate 6, fig. 4. There are six oculars and five genitals, but the latter are THE PENTAMEIIOUS SYSTEM AND VARIATION. 49 confused by secondary sutures. There are six periproctal plates, probably a coincidence in number only as six plates in that area may occur in a perfectly pentamerous specimen (text-fig. 204, p. 175). An Echinarachnius parma, from Chelsea Beach, Massachusetts, in the collection of the Boston Society of Natural History, no. 348, is another case of a similar structure. This speci- men (Plate 8, fig. 4; Plate 7, fig. 9) is 59 mm. in diameter. It is not unusual in appearance except, for the added areas. There are five ambulacra and interambulacra ventrally and nearlj' to the ambitus where the extra ambulacrum originates. The specimen is readily oriented by the position of the periproct in 5 (Plate 8, fig. 4) and also by the proportionate size of the primordial ambulacral plates (Plate 7, fig. 9), the la, Ila, III6, IVa, V6, being the larger plates as usual, according to Loven's law. The sixth areas as seen are late in development and occur between interambulacrum 1 and ambulacrum II. In the fused oculo-genital mass there are six ocular pores, one to each area as usual. There are, however, only four genital pores. One is wanting in 5 as usual, and also in 1, which is not usual. There is, however, a pore in the aberrant interradial area 6. Ambulacrum VI originates just below the ambitus, has two columns of plates and a petal as usual, and differs only in that plates below the petal are smaller, and the petal is slightly narrower than in the other areas. Interambulacrum 6 originates slightly later than ambulacrum VI and differs from the other interambulacra only in that the plates are smaller. In this specimen the hexamerous variation was evidently not congenital, for as shown, a pentamerous condition was maintained for a time, expressed in terms of growth by the introduction of six horizontal rows or zones of ambulacral plates, as seen in area II (Plate 7, fig. 9). Above this zone the added sixth ambulacrum is introduced, and it, with the extra interambulacrum, is continued throughout succeeding growth to the apical disc. 20. Pentamerous ventrally, six ambulacra, five interambulacra dorsally, six oculars, five genitals. — This structure is shown in a specimen of Strongylocentrotus drobachiensis from York Harbor, Maine (R. T. J. Coll., 847). The specimen is somewhat flattened, 27 mm. in diameter and 13 mm. in height. There are five ambulacra and interambulacra ventrally, but within 5 mm. of the peristomal border a sixth ambulacrum appears and is extended to the apical disc. The sixth ocular and ambulacriun lie between ocular and ambulacrum I and genital and inter- ambulacrum 1. The relation is comparable to that shown in Plate 6, fig. 4, except that the two ambulacral areas are in contact, as in Plate 7, fig. 5, since there is no intervening inter- ambulacrum. The genital plates in this specimen are considerably split by secondary sutures and somewhat distorted. De Loriol (1883, Plate 4, fig. 3a) describes a partially hexamerous Stomopneustes variolaris that is referable to this type of variation. I studied the specimen in Geneva. Ventrally there are five teeth, ten primordial ambulacral plates, five ambulacra, and five interambulacra. Dorsally there are six oculars and a sixth ambulacrum, but there are only five interambulacra and five genitals. 50 ROBERT TRACY JACKSON ON ECHINI. Bateson (1894) reports two cases of "total" hexamerous Echini. One is a Galerites, in which there are six ambulacra and interambulacra as seen from below; the dorsal portion and jaws are not shown. I have-not seen Meyer's original paper where this was described. The other is a six-rayed Amblypneustes, but there is no description of the specimen. Bateson also records three specimens in which there is an accessory ambulacrum. These, with de Loriol's and Ribaucourt's cases above mentioned and the eight more or less completelj' hexamerous specimens here described, include all cases I know of in which there is an increase in parts over the typical pentamerous system in Echini. It is evidently a very rare variation. Other variations from the normal have a certain interest. Four plates in the periproct is typical of Arbacia pundulata, but as later shown, variations of fewer or more than this number are common (text-figs. 200-205, p. 175). Variations in the number of genital pores are common, also as regards the distribution of madreporic pores. The variants of genital plates, or genitals and oculars, are all discussed later. The above variations from the pentamerous symmetry can all be considered as monstrosi- ties. Variation of another kind is more common, and interesting from another point of view; that is, variation in which the whole animal, or a part, is more or less fully developed in the direct line of the differential development or evolution of the species. This in Palaeozoic Echini is marked as radial variation, which was discussed in my earlier paper (1896, p. 151), and is shown in many cases here. An example is seen in Lovenechinus missouriensis, which has five columns of interambulacral plates with often a sixth column represented by one or two plates near the mid-zone. In Plate 41, fig. 1, the sixth column is represented in areas E, G, I, but in areas A, C, there are only five columns. In the young specimen (Plate 40, fig. 1) there are five columns in four areas, but in area E the fifth column is represented by only a single plate, a great departure from the typical and a close approach to the character seen in lower species of the genus, as L. lacazei (Plate 36, figs. 1-6), which has only four columns of interambulacral plates in an area as a specific character. As shown in my earlier paper in detailed studies of Melonechinus muUiporus, a given column of interambulacral plates, especially the columns from 5 upward, may come in at vary- ing zones in different areas of the same specimen, showing considerable variation in the rate of radial development. This is shown well in the zone of introduction of the seventh column in Lovenechinus septies (Plate 45, fig. 1). The actual number of columns, or differential char- acter, may vary in ilifferent areas of the same specimen, as in Hyattechinus rarispinus (Plate 23, fig. 3). In Melonechinus muUiporus (Plate 54, fig. 5) there are nine columns in area .\, but only eight in the four other areas; in Plate 60 the specimen figure 2 has nine columns in all areas, but figure 1 has eight in all areas; Plate 55, fig. 3, has nine columns in areas C, E, and G, but the other areas are not known definitely; the specimen, Plate 55, figs. 1, 2, and Plate 57, has only eight columns in all five areas. A specimen may have but seven columns of inter- STRUCTURE OF THE SKELETON AND GROWTH. 51 ambulacral plates in an area, a rather rare variation in this species. This same variation of greater or less degree of radial differential development is shown in many species in this paper. The radial . variation in a specimen may be as great as the variation of different specimens within the limits of a single species. Variation is marked as regards the number of ocular plates that meet the periproct in Recent and some Palaeozoic Echini, as fully discussed later. Variation occurs all through Echini in all parts of the organism, and numerous cases are considered in the following pages. All the evidence goes to show that aberrant variation is relatively rare, and in almost all cases variation in Echini is either arrested, progressive, or regressive on the direct line of differential development of the given character in the ontogeny of the individual and the adults of asso- ciated species and genera. A variant in a species can, therefore, usually be compared either as a direct connection, or, if not that, as a parallelism with the typical condition in some other species of the group. It seems that Echini are a particularly good group in which to study questions of variation because here variations can usually be expressed in very definite terms of numerical or other equally positive characters. Structure of the Skeleton and Growth. The skeleton of a sea-urchin is made up of plates, spines, jaws, and other parts which are situated more or less deeply, and the skeleton is covered by living ciliated epithelial tissue. The plates and spines, while apparently external, are in reality included within living tissue. An exception to this covering of living tissue occurs in the primary spines of Cidaris, where, as Lang shows (1896, p. 389), a cortical layer is formed, and the integument dies away from the area covered by that tissue and persists only around the base of the spine. This may he com- pared with the velvet on deer's antlers, which, after the completion of growth, dies away, leav- ing the antler bare. The secondary spines of Cidaris, however, have no cortical layer, and are always covered by a ciliated integument, like a permanent velvet in antlers, and which is seen in the horns of the giraffe and okapi. This condition of the skeleton of Echini, of being covered by living tissue, is of fundamental importance to a proper understanding of the structure. As the parts are internal, they are capable of being added to or resorbed at any part through- out the life of the individual. This is true of all parts except the distal points of the teeth, which are truly external. The individual plate of the test grows by a constant addition to the exterior and resorption of the interior, which latter is composed of open lattice-like or trabecu- lar tissue. The growth of a sea-urchin plate may be aptly compared to the growth of the head of a femur or similar bone with its hard dense exterior and open trabecular internal structure. Sectioning a sea-urchin plate, we find no trace of its earlier shape or character within, any more than we find the traces of a young femur within in sectioning the femur of an adult dog. 52 ROBERT TRACY JACKSON ON ECHINI. The mineralogical structure of the sea-urchin plate, as seen in a section of a plate of Recent Strongylocentroius drobachiensis, is crystalline calcite, with tlie axis of the crystal perpendicular to the surface of the plate, as shown by extinguishment under the polariscope. The plate is made up of a crystal which is optically continuous. Whether one, or more than one crystal enters into the composition of a single plate is doubtful, l)ut it may jierhaps be compared to ice on a pond, in which also the crystalline structure is optically continuous and the crystalline axis is perpendicular to the surface. It may be noted that the same crystalline structure, with axis perpendicular to the surface, was observed in the plates of Synapta, Asterias, and the calyx plates of a young Antedon rosaccus. In the stem of Antedon, however, the crystalline structure is parallel to the longer axis of the stem, not perpendicular to the surface. While in the sea-urchin the corona grows by increase in size of the plates, often to a relatively great size, as in Cidaris, or the two plates in the second row of the posterior interambulacrum of Micraster, it also grows by the addition of new plates. In the corona, the new plates, both ^n the ambulacral and interambulacral areas, are always added dorsally, in immediate contact with an ocular plate. This holds good for Palaeozoic as well as later types; the only excep- tion appears to be the aberrant Pourtalesia jeffreysi Wyville Thomson, in which, according to Loven (1883), in adults, at least, oculars are apparently wanting. In Bothriocidaris archaica (Plate 1, fig. 2) it is seen that the ocular plates cover both the ambulacrum and interambu- lacrum entirely, and in all other Echini, as already stated, the oculars overhang the ambulacrum entirely and the interambulacra in part on either side, as in Melonechinus (Plate 50, fig. 6), and Lovenechinus (Plate 41, fig. 3). It is true that in many, j^erhaps most Echini, the interam- bulacral plates originate in contact with both the ocular and the genital plates in the angle between them, but they apparently never fail to come in contact with the oculars and they may not touch the genitals, as seen in Bothriocidaris archaica (Plate 1, fig. 2). In the posterior interambulacrum of Ananchytes (text-fig. 175) and in adult Phormosoma placenta (text-fig. 170) interambulacral plates originate against the oculars without touching a genital, and the same may often be seen in abnormal specimens as in area 4 of Arbacia (Plate 4, figs. 11, 12), and in the extraordinarily modified specimen of Strongylocentroius lividus (Plate G, fig. 5). It is strik- ing in the Palaeozoic genera with many columns of plates that the new interambulacral plates originate against the oculars, Lovenechinus, Plate 41, figs. 2, 3, as tliscussed more fully later. What part, if any, the ocular plays in the origination of new ])lates is miknown, but at this area they originate. Mr. A. Agassiz (1904, p. 80) notes that in sjiecimens of Lovenechinus (Oligoporus) rnissouriensis and Lepidechinus iinhricatus the young interambulacral plates originate against the ocular. The specimen of Lovenechinus to which he refers is that figured in my Plate 42, fig. G, and the Lepidechinus in my Plate 03, fig. 7, as L. tessellatus sp. nov. The periproct grows by increase in the size of plates and the addition of new plates quite independently of the corona. The peristome grows in area by increase in the size of basicoronal THE AMBULACRUM OF THE CORONA. 53 plates of the corona and in cases by resorption of the base of the corona. The plates of the peristome are various in origin. The primordial ambulacral plates of regular Echini are appar- ently in this area from their inception (Plate 3, fig. 7). Additional ambulacral plates are derived by flowing down from the corona (text-figs. 41-48, p. 80). All other peristomal plates, it is believed, are formed on that area and are not derived from the corona (text-fig. 57, p. 84). These matters will be considered in detail in their appropriate places. The Ambulacrum of the Corona. The ambulacrum is the most essential feature of a sea-urchin, and has a first importance in classification and morphology, on account of the varied structure that it presents. We can conceive of a sea-urchin without an interambulacnini, although such is not known, or wanting in almost any other skeletal parts, but it seems that an ambulacrum is an essential character of the class. The detailed structure of the ambulacrum in post-Palaeozoic types is for the most part fairly well known, but the Palaeozoic types have not been closely studied, and throw a great deal of light on the structure of this area in the group as a whole. The ambulacrum is essentially an area for bearing tube-feet, which are important as a means of locomotion in most Echini, or are modified more or less fully as organs of respiration in other Echini (clypeastroids and others). The ambulacrum is never composed of less than two vertical columns of plates, and when more than two, it always has an equal number on each side; therefore there is a distinct median suture with an equal area on each side of it. From this it follows that half an ambulacrum, barring exceptional variation, is as good as a whole one for studj^, which is important in fossils where one half is often all that is available. A partial exception to the rule of never less than two columns of ambulacral plates occurs in Palaeolropus josephinae Loven, as figured by Loven (1874, Plates 13, 32). In this exception, dorsally in all five areas the two columns drop out to a single column for a distance of three to seven plates from the oculars. This indicates that a sea-urchin might have a single column of ambulacral plates throughout at least the greater part of an area, but such is unknown. The width of the ambulacrum varies greatly. It is narrow usually, in types with low imiserial plates, Eucidaris (text-fig. 4), Palaeechinus (text-fig. 9) ; but it may be wide, as in the petaloid area of Clypeaster. The ambulacnmi is wider usually when the plates are compound, Strongy- locentrotus (text-fig. 5si), or when four colunms, Lovenechinus (Plate 45), or more columns, Melonechinus (Plate 57), are developed. It may be much wider than the interambulacrum, Lepidesthes colletti (Plate 70, fig. 1), Meekechinus (Plate 76, fig. 1), and about twice the width of the interambulacrum in Bothriocidaris (Plate 1, fig. 1). In the primitive type, Bothriocidaris archaica (Plate 1, fig. 1), the ambulacral plates are high, hexagonal, and there are therefore few tube-feet, about sixteen, to each area excluding 54 ROBERT TRACY JACKSON ON ECHINI. the two peristomal rows. In young Goniocidaris (Plate 2, figs. 1-3) the ambulacral plates are also high, hexagonal, and there are few tube-feet, about ten to each area. From this condi- tion, which I would call primitive, increase of tube-feet or increase of locomotive power, is attained on three independent lines of development. P'irst and simplest, by producing low plates with pore-pairs uniserial, so that many plates are contained in the vertical extent of the area, as in Archaeocidaris (Plate 9, fig. 6), Eucidaris (text-fig. 4), and Palaeechinus (Plate 30, fig. 3). A second method of getting tube-feet is by increasing the number of columns of ambu- lacral plates from two to a greater number, but always by even numbers and always by simple, never compound plates (text-figs. 10-21). The increase is from two to four, six, eight, ten, twelve, sixteen, or twenty columns of plates in an area, which is the most at present known. This method is known in the Palaeozoic Echini only, and is especially developed in the Palae- echinidae and Lepidesthidae. It is, however, taken up as a method in other types, as Echinocy- stites. The third method of attaining increase of tube-feet is by building compound plates, formed by the coalescence of originally simple plates, Centrechinus (text-figs. 92, 94). The Text-figs. 2-14. — Character of the ambulacrum in representative Echini; left half represented. The horizontal dotted hne is on the plane of the mid-zone. 2. Bolhriocidaris archaica sp. nov. Ordovician. From Plate 1, fig. 1. 3. Goniocidaris canaliculala A. Agassiz. Young. From Plate 2, fig. 2. 4. Eucidaris irihidoides (Lamarck). Bahamas. 5. Sirongyloccntrolus drobachiensis (O. F. Miiller). Young. From Plate 3, fig. 11. 5a. The same. Adult. York Harbor, Maine. 6. Micraster cor-anguineum (Lamarck). Cretaceous, England. 7. Echinaraclmius parma (Lamarck). East port, Maine. 8. Melalia pedoralis (Lamarck). Bahamas. Showing plates of two are;is. 9. Palaeechinus elegans M' Coy. Lower Carboniferous. From Plate 31, fig. 1. 10. Maccoya hurlingiomnsis (Meek and Worthen). Lower Carboniferous. P'rom Plate 33, fig. 2. 11. Lovenechinus missouricnsis (Jackson). Lower Carboniferous. From Plate 43, fig. 3. 12. Oligoporus danae Meek and Worthen. Lower Carboniferous. From Plate 50, fig. 7. 13. Mclonechinus Tmdtiponis (Norwood and Owen). Lower Carboniferous. From Plate 56, fig. 4. 14. Lepidesthvs colklli White, Lower Carboniferous. From Plate 70, fig. 3. THE AMBULACRUM OF THE CORONA. 55 component elements of a compound plate are frequently low, but the plate as a whole is usually high. This method occurs in the Centrechinoida and is seen in some of the Holectypina, but is unknown in the Palaeozoic. A good example is Strongylocentrohis drobachiensis (text-fig. 5a), which at the mid-zone is characterized by compound plates composed of five or six component parts, each of which bears a pair of pores. As a very rare aberrant variant, pores may be wanting in such plates. In three specimens of Strongylocentrotus drobachiensis, collected in Maine, the plates are compound, as usual, but in one area of each specimen the plates dorsally are somewhat distorted and have no ambulacral pores. It must be strongly urged that the full character of the ambulacrum in a given type is to be based on the plates at the mid-zone. Below this point, or ventrally, they may not have attained the full differential characters, and dorsal to the mid-zone, one gets into the area of locaUzed stages in development, where the young plates again have not taken on the full char- acter; or again, one may also dorsally reach an area of senescent stages where the full characters are dying out. To illustrate this important principle, in Maccoya burUngloncnsis (Plate 33, figs. 1, 2) the plates at the ventral border are all primaries and pore-pairs uniserial; at the mid- zone the plates are alternately primaries and occluded, and the pore-pairs biserial; dorsally, the young plates are again all primaries and pore-pairs uniserial. In Lovenechinus missourien- sis (Plate 42, figs. 1-4) the plates ventrally are all primaries, at the mid-zone demi- and occluded, and dorsally again primaries. In Melonechinus multiporus (Plate 56, figs. 2-7; Plate 57; text- fig. 245) ventrally, plates are demi- and occluded; at the mid-zone demi-, occluded, and many isolated; close to the ocular, primaries only. In all of these series, and many others as well, the same system exists of simpler conditions ventrally in the plates built when the animal was young, and also in the nascent plates dorsally ; and from there passing to the plates of the mid- zone, it is easily seen how the complex conditions there existent are built up. From these plates also one can arrive at genealogical relations, it is believed, of a very definite character (text-fig. 237, p. 231), as discussed later. Of the Centrechinoida, in the young Strongylocentrotus, Loven (1892) showed that the plates are simple with a single pore-pair (Plate 3, fig. 11). These plates in the adult are re- moved by resorption, but still the plates at the ventral border are unlike those of the mid-zone. At the dorsal border in Strongylocentrotus, as Loven (1874) showed, and as Duncan (1885) showed in a number of types, the plates are simple, not compound. Duncan says (1885, p. 421), "It is an interesting and highly suggestive truth that all the regular Echinoidea should have their most radially situated plates in the form of the simple primaries of the Cidaridae." Loven showed in Strongylocentrotus drdbachiensis that these simple plates are shoved down- ward and become fused to form the compound plates, characteristic of the species. At this region of young plates we find, therefore, as a localized stage, a simplicity like that of less spe- cialized forms of Echini, as (Uidaris. This character of dorsal simple plates is shown very well 56 ROBERT TRACY JACKSON ON ECHINI. in Centrechinus (text-fig. 94, p. 107), Astropyga (text-fig. 99), Echinus (text-fig. 115), and Phor- mosoma (text-fig. 170). Alexander Agassiz (1874, p. 642) said that the compound ainbulacral plates are formed by the splitting of the original plates, but he was clearly mistaken. Passing dorsally, we may pass from compound plates into an area of simple plates which have dropped all attempt at fusion, as seen in Hemi('idaris of the Jurassic (p. 17). Or, as A. Agassiz (1904, Plate 20, figs. 1-4) shows in Salenocidaris miliaris A. Ag., there is a single primary plate ven- trally succeeded by one compound plate, which is again succeeded by simple plates throughout the area, a case of extreme reversion, in which simplicity is the dominant character, and com- poundness or specialization is shown b.y only a single plate. This may be aptly compared to Baculites amongst the ammonoid cephalopods, in which its coiled (ammonite) character is reduced to a minute, almost microscopical basal portion, which is rarely preserved. What- ever the complexity of the ambulacrum in Palaeozoic or later types, it is striking that a simple condition obtains in the nascent plates of the placogenous zone. This all shows the fact of stages in development as evinced by the ambulacrum passing dorsally or ventrally toward the mid-zone, and the importance of differentiating zonal areas in the description of the ambulacrum. While the regular Echini maintain a pair of pores in each ambulacral plate,. or plate ele- menrt in the case of compound plates, the case is different in the irregular post-Palaeozoic Echini. In the clypeastroids below the petaloid area, the ambulacral pores soon drop out, as in Echi- narachnius parma (Plate 8, fig. 4), and the ambulacral plates for the most part at least are non- poriferous. It is true that there are ^'ery fine pores on the ventral side, as described by A. Agassiz (1874, p. 703), but these are so peculiar that they can fairly be distinguished from the usual pores of the ambulacral system. In spatangoids ventrally the pores may be want- ing, or reduced to a single pore, instead of a pair of pores to a plate, in numerous cases. As stated, ambulacral plates typically bear a pair of pores representing a single tube- foot, or the pair may be reduced to a single pore, as in some spatangoids. In spatangoids, however, we find a remarkable exception to the rule of a single tube-foot to a plate. In Colly- rites (Plate 3, fig. 15) and other types, as shown by Loven (1874), A. Agassiz (1904), and others, in the basicoronal row, the la, Ila, Illb, lYa, Yb have two pairs of pores, and this is the only case of two pore-pairs, indicating two feet to a simple plate, known in Echini. Loven thought it indicated a compound plate because of the two pairs of pores, or in cases two separate single pores, but always representing two tube-feet. The fact that no suture has been seen in these plates in any of this group of Echini militates against this view, and it seems best to consider this as a peculiar case of two feet to a. simple plate rather than a compound plate, the suture of which has not been observed. As discussed under consideration of these plates (pp. 69, 71), the ambulacral plates in the basicoronal row of spatangoids and clypeastroids may reason- ably be homologized with the first row of ambulacral plates found around the mouth in regular Echini and transferred to the basal row of the corona in the process of evolution of these spe- THK AMHI'LACRUM OF THE CORONA. 57 cialized forms. In spatangoids, tube-feet as locomotive organs are largely abandoned, and they have only or mainly a respiratory funrtion; here no compound plates are known. With a loss of the locomotive importance the ])lates below the petaloid areas, as in Micraster (text- fig. 6), are relatively high, even hexagonal, as in the embryo and the ancient Bothriocidaris. This primitive character of plates may exist in only one area. In Metalia pedoralis (text-fig. 8), for example, the plates at the mid-zone of the bivium and posterior pair of the trivium are low and wide, with pore-pairs horizontal. In the odd anterior ambulacrum, however, the plates are as high as wide, pore-pairs are superposed and in the middle of the plate, as in Bothrio- cidaris. In such plates we have often in the shape and height, also the superposed position of the pores in each pair, a condition which is a closer approach to the embryonic character and also to the character of Bothriocidaris than is seen at the mid-zone in the adults of any regular Echini excepting the ancient Bothriocidaris. The position and mutual relation of the ambulacral pores is of considerable interest. It appears to be typical of the primitive and great majority of adult Echini to have two pores for each tube-foot, and the pair of pores to be surrounded by a more or less strongly marked peri- podium. In many spatangoids the peripodium is absent. In all Palaeozoic genera there are two pores to every ambulacral plate and a peripodium is present apparently without exception (Plate 23, fig. 1). Of course it is such a slight eminence that it is very commonly worn off, but it has been found so widely that its frequent absence is attributed to conditions of erosion. In all Palaeozoic types and most modern Echini, the pores lie either in the middle of the ambu- lacral plate, which is rare, or nearer the next adjacent interambulacrum than the middle of the plate, which is the usual position. In no Palaeozoic type do they lie nearer the median suture of the ambulacrum than the middle of the ambulacral plate. In the demi-plates of Echinosoma hispidum (A. Agassiz, 1904, Plate 48) the pores lie nearer to the mid-suture than to the middle of the plates in which they occur. The same is true of occasional spatangoids, but is exceptional. In Bothriocidaris (Plate 1, fig. 1) the ambulacral pores of each pair lie in the middle of the plate, superposed doi-so-ventrally, and surrounded by an oval perijjodium, the axis of which coincides with the vertical axis of the area. In young Goniocidaris (Plate 2, fig. 2) the pores of each pair also lie superposed as in Bothriocidaris. From the superposed position, which may be called jirimitive, the position of the pores in all Palaeozoic types except Bothriocidaris and in most other regular Echini takes on a change in which there is a revolu- tion through more or less of an angle of 90 degrees from the perpendicular, the revolution being in such a plane that the upper pore of the two is revolved toward the next adjacent interam- bulacrum. The revolution is therefore to the left of the perpendicular in the left half ambu- lacrum and to the right of the perpendicular in the right half-ambulacrum. Only rarely in regular Echini does the revolution exceed 90 degrees from the vertical, and in no case that I have found does the revolution take place toward the middle of the ambulacrum, but in the 58 ROBERT TRACY JACKSON ON ECHINI. opposite direction. From thi.s it occurs that the pores of each pair occupy usually an inclined position, and the outermost pore of a pair is a little higher than the inner pore, Hyattechinus (Plate 23, fig. 1). It may be on a line with the inner pore, as in Melonechinus (Plate 61, fig. 8), but it is rarely lower than the inner pore in regular Echini. At the ventral border of the test the pores of each pair lie much more nearly vertical than at the mid-zone in Strongylocentrotus and Arbacia. In the latter the pores of the two pore-pairs adjacent to the median spheridium are actually superposed, as in Bothriocidaris. It is to be noted also that the pores on the peristomal ambulacral plates, as in Eucidaris (Plate 2, fig. 6), Phyllacanthus (Plate 2, fig. 18), Phormosoma (text-fig. 43, p. 80), and Strongylocentrotus (text-fig. 50), are nearly or quite vertically superposed as in the young and in the primitive Bothriocidaris. Also it may be shown that in very young, newly formed plates next the ocular, of Goniocidaris (Plate 2, fig. 4), the plates are high, not low, and the pores are nearly or quite superposed. This is in a measure like the young of Goniocidaris (Plate 2, fig. 2) or adult of Bothriocidaris. I have seen the same character in very young plates of Eucidaris tribuloides close to the ocular. In many clypeastroids, as Echinarachnius parma (Plate 8, fig. 4), the pores revolve through an angle of more than 90 degrees from the vertical, so that the outer pore of each pair lies lower than the inner pore. In many spatangoids the pores ventrally, and in most of the other ambula- cral plates, are superposed as in the embryo and Bothriocidaris, but in the petaloid areas they are nearly or quite horizontal. In Mctalia pedoralis (text-fig. 8) the plates in ambulacrum III are high, without petaloid expansion, and the pores are superposed in the middle of the plate; in the four other areas the plates are relatively lower, petaloid dorsally, and in these areas at least the pores are horizontal, not superposed, and not in the middle of the plates. From this it seems that the relative position of the pores represents a greater or less departure from the primitive, and that a more primitive or more specialized relative position of pores occurs at definite parts of the test in the same specimen. In adult Bothriocidaris, Palaeechinus (text-fig. 15), and in the young, as seen in Gonio- cidaris, Strongylocentrotus (Loven, 1892); also in the young, as shown at the ventral border of the test, in Maccoya (text-fig. 16; Plate 33, fig. 1) and Lovenechinus (Plate 42, fig. 1), we find that the pore-pairs in succeeding plates lie over one another in a continuous or uniserial system. This arrangement is certainly primitive. It is to be observed that, in this condition all tube-feet extend to the ground from one continuous line, and when the plates are low, that they are proximally closely crowded as in Cidaris. From this condition of uniserial pores and tube-feet, we pass in many independent series to types in which the pore-pairs are biserial, triserial, or polyserial in each half-ambulacrum. This is accomplished by alternate plates failing to reach the interambulacrum in whole or in part, thus producing a biserial arrangement, Maccoya (text-fig. 16; Plate 33, fig. 1) and Lovenechinus (text-fig. 18; Plate 42, fig. 2). In addition isolated plates may be introduced to make a triserial arrangement of pores, Melon- THE AMBULACRUM OF THE CORONA. 59 echinus springeri (text-fig. 19; Plate 52, fig. 2), Perischocidaris (Plate 67, fig. 1); or still more additional isolated plates may be introduced to make many series of pores in each half-ambula- crum, Melonechinus muUiporus (text-fig. 20; Plate 56, fig. 4), Lepidesthes colletti (text-fig. 21; Plate 70, fig. 1), and Meekechinus (Plate 76, fig. 1). In the Centrechinoida the pores of the 3[o 2{^" 1 (CD 6KZ 7 \ 4(^ 5 > 2^ 3 > 1(0 ) Text-figs. 15-21. — Ambulacra of Palaeozoic Echini showing development of pore series by a drawing-out process of plate movement. 15. Palaeechinus elegans M'Coy. Lower Carboniferous. Adapted from Plate 31, fig. 1. All plates reach the interambulacrum . 16. Maccoya huiiiiiglotiensis (Meek andWoTthen). Lower Carboniferous. Adapted from Plate 33, fig. 1. Venlrally all plates reach the interambulacrum; further up, only alternate plates. 17. Lovenechinus sepHcs sp. nov. Lower Carboniferous. Adapted from Plate 45, fig. 2. Ventrally all plates reach the interambulacrum; further up, every alternate plate. At the mid-zone the plates are as in te.xt-fig. 18. 18. Loi'etiechinus missouriensis (Jackson). Lower Carboniferous. Adapted from Plate 42, fig. 2. Every second plate reaches the interambulacrum. 19. Melonechinus springeri sp. nov. Lower Carboniferous. Adapted from Plate 52, fig. 2. Every third plate reaches the interambulacrum. 20. Melonecldnus mulliporus (Norwood and Owen). Lower Carboniferous. Adapted from Plate 56, fig. 4. Every fifth plate reaches the interambulacrum. 21. Lejddesthes collclti Wliite. Lower Carboniferous. Adapted from Plate 70, fig. 3. Every eighth plate reaches the interambulacrum. 60 ROBERT TRACY JACKSON ON ECHINI. compound plates tend strongly to fall into arcs in such fashion that succeeding pore-pairs fall in diiTerent vertical series, as seen well in Strongylocentroius franciscanus. In Diplocidaris and Tetracidaris the pore-pairs, though in primary plates, are alternately pulled out of line, so that succeeding tube-feet would not lie quite over one another. This passage from the primitive condition of monoserial pore-pairs to the Ijiserial or poly- serial, which is attained by the three methods of more than two columns of simple plates (Palaeozoic), compound plates with pores tending to arcs (Centrechinoida), or primaries with pore-pairs biserial (certain Cidaridae), is all in accordance with the mechanical principle of each tube-foot attaining fullest play by being out of line with its fellows in so far as is possible. The arraiigement reminds one of the phyllotactic arrangement of leaves in plants, where e?tch leaf is not succeeded by another in a vertical line imtil one or more ha\'e been added which are off that vertical line, but in a succession of lines of their own series. In types, as spatangoids, in which below the petals the plates are relatively high and tube- feet therefore not crowded, the pore-pairs fall nearly in vertical uniserial lines, as in the J'oung and primitive forms. This is aided probably by the fact that the ambulacral feet are for the most part modified as respiratory rather than locomotive organs. Ambulacral plates on their proximal side differ from the same plates on their distal side, and this difference, as far as observed, is usually in the line of greater simplicity and relative primitiveness of structure. In Melonechinus mulliporus (Plate 56, figs. 4, 5) and M. gignn- teus (Plate 61, figs. 5-9), the ambulacral pore-pairs distally are eccentric and lie nearest to the interambulacral suture, but proximally, as shown, the pore-pairs of isolated plates lie in the middle of each plate, a primitive character. In general the pores proximally are nearer the middle of the plate than on the distal side. In Maccoya intennedia (Plate 34, fig. 2) the plates alternately reach and are cut off from the interambulacral suture and pore-pairs are biserial. All this is as seen from the outer or distal side. When seen from the inner or proximal side (Plate 34, fig. 3), all the plates at \\w mid-zone ci'oss the half-area instead of alternate plates being cut off from the interambulacral suture. iVlso the pore-pairs are uni- serial instead of biserial, as they are on the outer side of the ^•erv same plates, so that in all these characters they are primitive like the young and the next lower genus Palaeechinus (Plate 31, fig. 1). A similar condition is seen in distal and proximal sides of the plates in Maccoya burlinglonensis (Plate 33, figs. 4, 5). In Lovenechinus missouricnsis the outer demi-plates, as seen distally (Plate 43, fig. 3) are verj^ narrow, the inner occluded plates are relatively wide and the pore-pairs of both lie very near the interambulacral suture. In a proximal view of the same plates (Plate 43, fig. 4) the demi- and occluded plates are of about the same width, and the pore-pairs of both lie near the middle line of the half-ambulacrum. Also proximally the plate that lies opposite the horizontal suture lines of the adradial plates is spread out in a fan-shaped manner, whereas on the distal side the fan-shape is nearly or quite wanting. THE AMBULACRUM OF THE CORONA. 61 V A similar condition of fan-shaped plates proximally, which are not so shaped distally, is seen in Melonechinus multiporus (Plate 5G, figs. 4, 5) and M. giganteus (Plate 61, figs. 5-9). Comparable differences between the interior and exterior sides of the same plates are seen in interambulacral, ocular, and genital plates, as described in the consideration of these parts (pp. 75, 96, 172). All this shows that in the study of specimens and the description of species, one must bear in mind whether the specimen represents the internal or external characters of the test. Another feature noticed on the inside of ambulacral plates is elevated nodose or spinose projections that occur in some Echini. These were first seen in the fossil Hyattechinus (Plate 24, fig. 6), where low, knob-like or spinose elevations exist between th(> inner pore and the middle of the area. Alexander Agassiz (1904, p. 31) has described spines extending into the body from the inner face of the peristomal ambulacral jilates of Porocidaris cohosi, but I believe they have not been noticed before in coronal ambulacral ])lates. In Phyllacanthus (Plate 3, fig. 12) near to the ventral border of the test, there are small spines between the inner pores and the middle of the area; these seem to be the equivalent of those occurring in the Palaeozoic Hyattechinus. Close to the peristomal border these spines increase in height, arch over and then fuse in a continuous ridge (text-fig. 224, p. 193). Passing dorsally, in Phyllacanthus a second series of spines occurs, one over each inner pore, and above the ventral area these alone exist. In Eucidaris tribuloides I find a similar condition to that of Phyllacanthus, except that there are commonly three spinules over the inner pore (Plate 3, fig. 13). tSuch spinose growths are most delicate and are destroyed by a touch so that they can only be seen in a test cleaned without any brushing. They have not been noticed in any of the Centrechinoida. Closely associated with the ambulacrum are the spheridia described by Loven (1874), which, as he showed, are characteristic of all modern Echini except the Cidaridae. If they had existed in pits, as is so often the case, we might expect to find them in the Palaeozoic. The fact of their absence in the primitive Cidaridae is an argument for the assumption that they were also probably absent in Palaeozoic genera. Certainly they are not known. Pedicellariae have recently come into prominence through the critical studies of Mortensen and others. It is of interest to note that tridentate pedicellariae occurred in the Palaeozoic, as shown in Meekechinus elegans gen. et sp. nov. (Plate 76, figs. 8, 9). Except as shown in the Jurassic Pelanechinus by Clroom (1887) I believe these are the first found fossil, and they will probably always remain rarities. Special respiratory organs are also closely associated with the ambulacra. Charles Stewart (1879) first described internal branchiae in Eucidaris iribuloides which lie dorsal to the lantern and extend over the ambulacral areas radially from beneath the compasses and between these and the braces. The same structures were described by Ludwig (1880) and by Prouho (1887) in Cidaris papiUata. Prouho gave them the name of Stewart's organs, a convenient distinction from the quite dilTerent external gills. I have seen them in dissections of Cidaris affinis, 62 ROBERT TRACY JACKSON ON ECHINI. Eucidaris tribuloides and thouarsii. Stewart's organs are described in Asthenosoma by P. and F. Sarasin (1888), and in several echinothuriids by A. Agassiz and Clark (1909). External gills are developed as five pairs of branched appendages which are outgrowths of the oral integu- ment (text-fig. 55, p. 83). They are characteristic of the Centrechinoida and by inference the fos- sil Holectypina. Their presence is marked by the indenting of the interambulacral basicoronal plates so that they are recognizable in fossils. These peristomal gills, as I would call them for distinctiveness, are interambulacral in position in contradistinction to Stewart's organs, which are ambulacral or radial in position. In clypeastroids and spatangoids, as well as par- tially in some of the Centrechinoida (Arbacia), the function of respiration is maintained by the dorsal ambulacral tentacles, which have lost their function as locomotive organs. These I would call in distinction ambulacral gills. We do not and probably cannot know definitely what respiratory organs existed in Palaeozoic Echini. Branchial slits for peristomal gills I have found no evidence of, though carefully looked for. There is no evidence of specialization of tube-feet as ambulacral gills, though they may have performed that function in part. As the primitive Cidaridae have the Stewart's organs so well developed, it is not unlikely that these organs were the respiratory organs of all Echini in Palaeozoic times. The Interambulacrtjm. The interambulacrum in Echini functions chiefly as a space filler and a bearer of spines and pedicellariae. The spines serve for protection and more or less in locomotion, and pedi- cellariae as grasping, cleansing, and protective organs. In spite of this secondary physiological importance, the interambulacrum forms a large part of the test of the sea-urchin in most types, and is of very great interest, especially in Palaeozoic genera. The interambulacral plates originate in direct contact with the ocular plates and quite independently of the genitals. The young last added plates may always be found in direct contact with the oculars in Palaeozoic and later Echini, and their independence of the genitals is proved in cases where no genital reaches the interambulacrum, as in Bolhriocidaris archaica (Plate 1, fig. 2), the posterior area in spatangoids (text-figs. 174, 175, p. 149), the aberrant Arbacias (Plate 4, figs. 11, 12), and Tripneustes (Plate 6, fig. 4). As any given interambulacrum comes in contact with two oculars, one on either side, and as at the base of these two oculars new interambulacral plates originate (Melonechinus, Plate 56, fig. 6, and numerous other figures), it seems that an interambulacrum may theoretically be considered as composed of two halves, one half-interambulacrum being associated with the ocular and ambulacrum on one side, and the other half being associated with the ocular and ambulacrum on the other side. In other words, the corona may be conceived as made up of five areas, each surmounted by an ocular plate, an ambulacrum and adjacent half-interambulacrum on either side making up each THE INTERAMBULACRUM. 63 of these five parts. Such an ideal area composed of plates 'associated with a single ocular is shown in Palaeechinus and Eucidaris (text-figs. 217, 218, p. 191). Support for this view is found in the fact that the perignathic muscles of the lantern, which are nearly or quite ambulacral in position, are inserted wholly on the base of the associated two half-interambulacra, as in Eucidaris (text-fig. 218); or if not, they are inserted on two half-interambulacra and auricles of the ambulacrum itself, as in Centrechinus (text-fig. 219, p. 191). In the Echinothuriidae where oculars and genitals are separated by an interspace, Mr. Agassiz (1883, p. 32; 1904, p. 96), says that the interambulacral plates are derived from the periproct, as the two areas are there in contact. Dr. Mortensen (1903, p. 175) opposes this view. Evidently this contact is a mere coincidence of structure and has no bearing on the origin of plates. In the echinothuriids which I have seen, the young interambulacral plates are in contact with the oculars as usual (text-fig. 170, p. 149). Wliile in regular Echini outside the Echinothuriidae the oculars and genitals typically form a closed ring, yet occasionally in variants the ring may be open so that the periproct reaches the interambulacrum, as in Strongylocentrotus (Plate 5, fig. 15; Plate 6, fig. 5). The full differential characters of the interambulacrum as of the ambulacrum are expressed at the mid-zone of the adult. Here are usually found the full number of columns of plates characteristic of the species, also the typical tubercles, spines, imbrication, or other characters which go to make up the specific description. The ventral border in the basicoronal zone represents the earliest formed plates and the youth of the individual, as far as it can be gathered from the study of an adult specimen, though the actually first formed plates may have been resorbed in development. Passing dorsally, with later added plates, new characters may come in until we get the full differential features developed at or about the mid-zone. Dorsal to the mid-zone we pass into the area of young last formed plates which have not yet acquired the full characters. Or again dorsally, we may find senescent features in the loss of columns of plates. Passing from the basicoronal row dorsally, we find in most Palaeozoic types, and many post-Palaeozoic as well, stages in development strongly marked, which stages can be correlated with the adult condition of simpler genera or simpler species within the genus. The interambulacrum in Echini has from one to fourteen vertical columns of plates in each of the five areas, which represents the least and greatest number known at present. There are intermediate grades representing every step between this least and greatest specialization of the area, and it is a matter of great interest to follow the progressive series as represented by stages in development, and by adult types, to see how the progressive differential structure is built up. As the plates of the ventral border are the oldest or first formed of any plates seen in an individual specimen, and as the later added plates succeed one another as we pass dorsally, it might be thought that we could read stages in development as expressed by rows and columns of plates with ease and certainty, and such can be done in many types, as in 64 ROBERT TRACY JACKSON ON ECHINI. Hyatlechinus beecheri (Plate 26) aiid Perischodomus biserialis (Plate 64, fig. 2). Complications may come in, however, especially resorption of the base of the corona by encroachment of the peristome cutting off part of the ventral plates, and also rarely resorption within the corona, as exceptionally in Arachnoides, or differential growth of associated plates, which may separate plates originally in contact (Echinarachnius). There may be few plates in a given interambulacrum, as in Bothriocidaris (Plate 1, fig. 1) or Cidaris, or a very large number, as in Hyattechinus beecheri (Plate 26), and many other species. There is every evidence that these plates all originated as separate plates and never by division of previously formed plates. Mr. Agassiz (1881, p. 95; 1904, p. 104) saj's that in Phormosoma, interambulacral plates are formed by the splitting of earlier continuous plates. Mortensen (1904, p. 25) suggests that these planes of division may be due to fracture in the handling of the specimens which have excessively thin plates, and my limited observations of Echinothuriidae accord with this view. If these are natural suture lines in Phormosoma, the numerous plates thus formed cannot bo homologized with the numerous columns of plates in Palaeozoic types to which Mr. Agassiz compared them, for the latter originate independently, not by the splitting of previously continuous plates. While fusion into compound plates is seen in the ambulacrum of the Centrechinoida, such fusion is unknown in the interambulacrum in all Echini. Loven (1874, p. 51) assumed, it is true, fusion of certain plates around the base of the corona in spatangoids. No sutures exist, and it seems that single plates at this area in these and other types are not cases of fusion, but of failure to develop more than one plate in the row to which they belong. In the important ancient type Bothriocidaris, as best shown in B. archaica (Plate 1, fig. 1), there is a single column of interambulacral plates in each area extending from just above the second row of ambulacral plates of the peristome to the apical disc, where in this species they are completely covered by the large ocular plates and do not reach the genitals. The full details of this and other cases given are considered under the description of the species. This is the simplest and, I think, the most primitive condition known in any adult echinoid. In my paper on Echini (1896, p. 233) I considered Bothriocidaris as representing the archaic radical as regards interambulacral structure from which the interambulacra in all other Echini could be derived. Further evidence and stutly confirm this view. Mr. A. Agassiz (1904, p. SO) says, "The existence of an interambulacral zone composed of a single row of plates [in Bothriocidaris] does not give us any clue to the mode of formation of the Palaechinid type of interambulacra with its manifold rows of plates." I cannot agree with this view, as it seems that the single primordial plate found so universally in j'^oung Echini, or in adults when resorption has not removed it, may fairly be considered as representing in development a stage consisting of a single column which in all but Bothriocidaris is succeeded by additional columns during development, as shown in my first paper and in numerous cases THE INTERAMBULACRUM. 65 in this paper. Dr. Mortensen (1903) describe.s a very young Hypsiechinus (my Plate 3, fig. 6), in which the interambulacrum consists of but a single plate in each of the five areas. In young Echinus (Plate 3, fig. 5) Bury (1895) figures a similar single plate, and Loven (1874, Plate 17, fig. 49) figures a young echinoid in which there is only a single interambulacral plate in each area. This single plate, it seems, represents a single-column stage consisting of this one plate, and may fairly be homologized with the plate at the ventral border of the interambulacrum of Bothriocidaris. Loven (1892) showed that in young Goniocidaris (my Plate 2, fig. 1), also in Strongylo- centrotus (Plate 3, fig. 11), and Echinus, at the ventral border of the interambulacrum there is a single plate which lies dorsal to the continuous row of primordial ambulacral plates. This plate is evidently the single first formed plate, and is homologized with the ventral plate of the interambulacrum in Bothriocidaris. In the young of the Recent Echini mentioned, the first plate is succeeded in the second row by two plates introducing the character of a second column, which is the next stage in differential development of the interambulacrum. A similar single first plate succeeded by two plates in the second row is shown by A. Agassiz (1904) in young Salenia (his Plate 21, fig. 1), young Phormosoma (my Plate 3, fig. 10), and in young Arbacia (his Plate 54, fig. 2). In later life, in regular Echini, as shown by Loven, this single initial plate is usually resorbed by the encroachment of the actinostome, so that in the adult there are two plates on the peristomal border, as in Cidaris and most regular Recent Echini. Arbacia (text-fig. 227, p. 193) and Phormosoma (text-fig. 43, p. 80) are exceptions, for, as Mr. Agassiz showed (1904, p. 54, Plate 41), the single plate is retained at the ventral border of the adult in these genera. As Loven (1874) showed in the clypeastroids and spatangoids, the plate is retained at the ventral border of the corona in most cases, as the corona is not resorbed by the encroachment of the peristome. There are certain exceptions to this, as later discussed. Turning to Palaeozoic types, we find in Perischodomus (text-fig. 30, p. 70; Plate 64, fig. 2) that there is a single plate ventrally succeeded by two plates in the second row, as in young Cidaris, etc., but in the third row there are three plates, which introduces another stage in differential development, structurally an advance on that seen in Cidaris and all modern Echini. In the next row of Perischodomus there are four plates, and later a fifth column is introduced. In this type we see that a form with five columns of plates in the adult is built up by a series of stages starting with a single column represented by a single plate ventrally, and the succeeding stages are represented by additional rows or zones of growth progressively introducing new columns or new characters in differential development. In Hyattechinus beecheri (Plate 26) we find ventrally a single plate representing a single ' In his text Dr. Mortensen (1903, p. 89) calls this plate a genital, but he wrote me that this was an error, and in the second jjart of his Ingolf Eehinoida (1907, p. 172), corrects the error, saying that these plates are the first interambulaerals and the genitals are discernible on the abactinal side. 66 ROBERT TRACY JACKSON ON ECHINI. column of plates in each of the five areas. In the second row there are two plates, in the third three, in the fourth four, in the fifth five, and so on. There is not always a new column added in each new row or zone of growth in this species, but this is very nearly the case up to the maximum number of columns added, which is eleven. Dorsally the adradial columns 1 and 2 drop out, so that columns 3 and 4 come against the ambulacrals in further dorsal growth. The same series of stages of progressive development marked by adding columns, and regressive senescent stages marked by the dropping out of columns of interambulacral plates are shown in Hyattechinus rarispinus (Plate 23, figs. 1, 3) and H. peniagonus (Plate 25, figs. 1, 3). In this last species in senescence, several columns are dropped, so that close to the apical region there are ten columns instead of the fourteen characteristic of its full development near the mid-zone. H. pentagonus is also especially interesting, as it attains fourteen columns of plates in the adult in each area, the maximum specialization in this line of differential development of all known Echini. Yet as resorption has not removed any plates, we can trace the development from the simple ventral or young condition, represented by a single plate on the peristomal border, to the complex adult and senescent old age without a single gap in the series of stages in develop- ment. In other Palaeozoic genera a similar progressive individual development may be traced in which the single ventral plate is retained in the adult, as in Pholidechinus (Plate 28, fig. 1), Pholidocidaris (Plate 73, fig. 6), Lepidocentrus (Plate 20, fig. 7), and Lepidesthes (Plate 68, fig. 3). The above represent Palaeozoic genera in which there is no ventral resorption of the corona, and therefore all the interambulacral plates, built during the life of the individual, are retained and can be seen in a well preserved adult individual. Such successive stages in the development of an interambulacrum can be fairly compared to studying the development of a cephalopod or other mollusc, as has been done so successfully by Hyatt and others, by tracing the characters from the umbo or original apex of the shell by a series of steps or stages to the completed adult or old-age individual, all worked out in a single specimen. There is the great difference, however, that in the mollusc the shell is external, in the sea-urchin internal, so that in the latter the plates have grown since first formed and may have suffered changes, but from their presence stages can be read. Resorption, however, comes in in Palaeozoic as well as in post-Palaeozoic types. Studying genera representing the Palaeechinidae, we find that there are two plates at the ventral border of the interambulacra in all cases where the ventral part of the corona is complete, as seen in Palaeechinus (Plate 30, fig. 3), Maccoya (Plate 33, fig. 1), Lovenechinus (Plate 43, fig. 1), Melonechinus (text-fig. 25; Plate 50, fig. 2; Plate 57, fig. 1), and others. This character is explained by the fact that the single initial ventral plate, found so universally in young modern Echini, and retained ventrally in numerous fossil and living genera, has been resorbed in the advancement of the peristome, as in Cidaris and most modern regular Echini. Miss Klem THE INTERAMBULACRUM. 67 (1904, p. 3) criticizes this view and claims that there are more than two plates when tlie ventral area is complete. I can only say that Miss Klem is mistaken. I have never seen an exception. Two of the specimens here figured, showing this character of two plates ventrally, are from the Hambach Collection which Miss Klem studied and herself figured, but it is fair to state that they have been freed somewhat from matrix since she studied them. The specimens referred to are Lovenechinus septies (Plate 45, fig. 1 = Miss Klem's Plate 2, figs. 5a-5d) and Melonechinus muUiporus (Plate 57, fig. 1 = Miss Klem's Plate 3, figs. 6a-6d). In Palaeechinus quadriserialis (Plate 30, fig. 3) there are two plates in the first row or zone, three in the second, four in the third (excepting in one area, C), and above the introduction of the fourth column no more columns are added, but all are continued to the apical disc. Loven (1874, p. 39) made the acute observation that in Palaeozoic genera with multiple columns of plates, the adambulacral plates alone are continued from the peristome to the apical disc. This observation holds true of most of the family of the Palaeechinidae, as in Palaeechinus (Plate 30, fig. 3) and Lovenechinus (Plate 41, fig. 1). The statement, however, needs modification. In those types where there is a single plate at the ventral border, Perischodomus (Plate 64, fig. 2), obviously two columns do not reach the peristome. Also in senescence (Melonechinus indianensis, Plate 53, fig. 1), usually the two adambulacral columns drop out before reaching the apical disc; or in special- ized types they may drop out very early, as in Hyattechinus beecheri (Plate 26) or H. pen- iagonus (Plate 25, fig. 3). In the family of the Palaeechinidae the species have as a range from three to eleven columns of interambulacral plates. In the several species the early development is as in Palaeechinus quadriserialis, but in later growth additional differential characters are usually taken on. In a young Lovenechinus missouriensis (Plate 40) a fifth column comes in soon after the fourth, and in this specimen, which is very young and the only really young Palaeozoic echinoid yet known, the fifth column originates near the mid-zone. In an older individual (Plate 41, fig. 1) where many more rows of plates have been added dorsally, we find that the introduction of the fifth column has been relatively shoved ventrally by the later added plates. In addition, a sixth column is represented by one or two plates in three areas, E, G, I, but in two areas, A, C, no sixth occurs. This occurrence of a column represented by only one or two plates is very exceptional in Echini and suggests a disappearing part. In many specimens of the species it is entirely absent. In Lovenechinus septies the method of introduction of columns is well shown (Plate 45, fig. 1), as it is one of the best preserved specimens I have seen. In three areas the two plates are present in the basicoronal row, restored in the other two; from this point up or dorsally in the several areas the new columns come in quite regularly. In the very choice specimen of Melonechinus muUiporus (Plate 57, fig. 1; text-figs. 245, 246) there are two plates ventrally in areas G, I, one of the two in areas A and C, and they are restored in areas where wanting, as indicated by dotted lines. Above this zone the introduction of columns is shown without the absence of a single plate in all five interambulacral areas. 68 ROBERT TRACY JACKSON ON ECHINI. In Melonechinus muUiporus there are eight or nine, rarely seven, columns of plates in each area. In M. vanderbilti there are nine columns of plates, and in M. giganteus (Plate 59, fig. 14) there are eleven. The columns are all added in a perfectly definite method and are in this feature further developments of the system seen in Palaeechinus quadriserialis. The progressive addition of columns as we pass dorsally from the single column ventrally to the two to fourteen columns dorsally, I believe, represents distinctly stages in development through which the animal passes in its growth, the progressive stages being comparable to the conditions seen in the adult of simpler genera or simpler species allied to the type under con- sideration. The method of introduction of interambulacral columns was shown in detail by Jackson and Jaggar (1896) and Jackson (1896), but will be briefly stated here. In the Palaeechinidae and most Palaeozoic Echini, excepting those that have strongly imbricate plates, the plates of the interambulacrum are typically hexagonal excepting the adambulacral columns which are nearly pentagonal but crenulate or rounded on the adradial suture. WTiile the plates are typically hexagonal, this symmetry is modified where new columns are introduced. The initial plates of columns above the third are typically pentagonal with an apex of the pentagon pointing ventrally, and an adjacent ventral plate on the left or right is heptagonal, the added side com- pensating for the one side short in the initial pentagon. An exception to this rule of pentagonal initial plates of columns is seen in the initial plate of the third column, which is typically hex- agonal and is immediately succeeded by the initial plate of column 4, which is a pentagon, as seen in Melonechinus muUiporus (Plate 57) and many other species. If by rare exception the initial plate of column 3 is not immediately followed by the initial plate of column 4, as in area C of Lovenechinus septies (Plate 45, fig. 1), then the initial plate of column 3 is also pen- tagonal. When odd-numbered columns, as columns 3, 5, 7, 9, 11, etc., are introduced, they are typically introduced in the middle of the area with an equal number of columns on each side of them, and also typically the heptagonal associated plate lies on the right ventral border of the initial pentagon. When even-numbered columns are added, as columns 4, 6, 8, 10, etc., they are typically added to the right of the center with one more column on the left than on the right. While this is the rule in the large majority of cases, there are frequent exceptions in which even-numbered columns are added on the left instead of the right of the center. As a concurrent character, the associated heptagonal plate in even-numbered columns typically lies to the left of the initial pentagon. Why this systematic position of columns and associated heptagonal plates should come in with such regularity is not obvious, but it is a very constant character, as shown by Jackson and Jaggar {1896, p. 163) who worked out the percentages of normal cases and variations. In llie dorsal portion of the interambulacrum the plates are nearly or quite rhombic in outline and gradually take on the typical hexagonal outline, as they are pu.shed ventrally by later intercalated plates and come into mechanical contact with adjacent plates of their several associated columns (Plate 59, fig. 14). CHARACTERS OF BASICORONAL PLATES. 69 Dorsally each interambulacrum comes in contact with a genital and two adjacent ocular plates, except in Bolhriocidaris archaica (Plate 1, fig. 2) and in part in B. pahleni (Plate 1, fig. 6), and in the posterior interambulacrum of many spatangoids (Micraster), where the interambula- crum abuts against two ocular plates only (text-figs. 174, 175, p. 149). The interambulacrum also comes against oculars only in the remarkable variations in Arbacia shown in Plate 4, figs. 11, 12. The new interambulacral plates in Echini are formed against the ocular plates on either side of the area, never in the middle against the genital. The relation of the interambulacrum to the ambulacrum in development is of much interest, and it seems may be expressed in terms of acceleration of development. In Bothriocidaris (Plate 1, fig. 1) two continuous rows of ambulacral plates surround the mouth, forming the peristome of that type, before reaching the initial plate of the interambulacrum in the third aboral row or zone. This character in Bothriocidaris is discussed in detail under consideration of that type. In young Goniocidaris (Plate 2, fig. 1) and in young Strongylocentrotus (Plate 3, fig. 11), young Salenia (A. Agassiz, 1904, Plate 21, fig. 1), and young Phormosoraa (Plate 3, fig. 9), a single row of continuous ambulacral plates surrounds the mouth forming the peristome at that age, but the initial single plate of the interambulacrum appears in the second row which is the base of the corona, instead of the third row as in Bothriocidaris. In clypeastroids and spatangoids (Collyrites, Plate 3, fig. 15) the initial single plate of the interambulacrum appears intercalated between the plates of the primordial ambulacral row, which, therefore, are not continuous as in regular Echini, but discontinuous, and, moreover, do not form part of the peristome, but form with the primordial interambulacral plates the basicoronal row of plates of the corona proper. Characters of Basicoronal Plates. Basicoronal plates are the oldest or first formed plates existent in any given corona, and unless plates have been removed by resorption, or by flowing down on to the peristome, as in the ambulacrals of cidarids, they are the actual plates formed at this area in the young animal. Therefore, as elsewhere discussed, they often present simpler characters than plates dorsal to them, and thus show stages in development. These plates on the proximal side give rise to the attachment of part of the muscles of the Aristotle's lantern, either directly or through the elevated processes of the perignathic girdle, which is later discussed. There are certain types of structure in basicoronal plates which are comparatively constant in large groups, so that they are of more than specific or generic value. The characters of basicoronal interambulacral plates are the more striking and may be stated in brief. Where no plates have been removed by resorption, there is a single plate at the ventral border of the interambulacrum. The primitive type of this character is Bothrio- 70 ROBERT TRACY JACKSON ON ECHINI. cidaris (text-fig. 22), which continues to build a single column. This same character of a single plate ventrally, but succeeded by two plates in the second row, is characteristic of the young of all modern regular Echini, as shown by Loven (1892) (Goniocidaris, text-fig. 23). In the adult of most regular Echini the single plate and probably more have been resorbed by the advance of the peristome (Eucidaris, text-fig. 24). In the Palaeechinidae with many columns of plates, apparently only one plate has been resorbed, when we find two plates in the basicoronal 27 28 29 30 31 Text-figs. 22-31. — Characters of the base of the interambulacrum in representative Echini. 22. Bolhriocidaris archaica sp. nov. Ordovician. From Plate 1, fig. 1. 23. Goniocidaris canaliculala A. Agassiz. Young. From Plate 2, figs. 1, 2. , 24. Eucidaris tribuloidcs (Lamarck). Bahamas. 25. Melonechinus muUiporus (Norwood and Owen). Lower Carboniferous. From Plate 57. 26. Archaeocidaris wortheni Hall. Lower Carboniferous. From Plate 9, fig. 6. 27. Plexechinus cinclus A. Agassiz. Adapted from A. Agassiz, 1904, Plate 58, fig. 1. 28. Echinocyamus -pusiUxis (Miiller). Adapted from Lov6n, 1874, Plate 44. 29. Rotula dentata (Lamarck). Adapted from Lovfn, 1874, Plate 46. 30. Pcrischodomus hiscrialis M'Coy. Lower Carboniferous. From Plate 64, fig. 2. 31. Tiarcchinus princeps (Laube). Triassic. Adapted from Loven, 1883, Plate 13, figs. 152, 154. In figures 22, 23, 27-31 the primordial interambulacral plate is in the basicoronal row; in 24-26, it, with or without additional plates, has been resorbed. row (Melonechinus, text-fig. 25), or in the Archaeocidaridae, several rows of plates may have been resorbed, and we find four plates in the basicoronal row (Archaeocidaris, text-fig. 26). In modern Echini where there has been no resorption, the single primordial plate is retained in the adult. This character, as shown by Loven (1874) and others, occurs in most clypeas- CHARACTERS OF BASICORONAL PLATES. 71 troids and spatangoids (text-figs. 27-29). In Plexechinus (text-fig. 27) the .single plate is followed by another single plate, and these by a second column. This is a very rare exception, as the typical character is for the initial primordial plate to be immediately succeeded by two plates in all Echini except Bothriocidaris. In Rotula (text-fig. 29) the primordial interam- bulacral plate is separated from the next two of its area by the lateral introvenient ambulacral plates. As shown by Loven, plates originally in contact in the young may be thus separated during growth. A few modern regular Echini retain the primordial plate in the adult (Phor- mosoma, text-fig. 43, p. 80; Arbacia, text-fig. 227, p. 193). In Palaeozoic Echini with imbricate plates and many interambulacral columns, there is no resorption, and therefore the primordial interambulacral plate is retained in the adult, as in Perischodomus (text-fig. 30), Hyattechinus (Plate 25, fig. 1), Lepidocentrus, Lepidesthes, and Pholidocidaris. The primordial plate in all these is succeeded by two plates, then three, then four, etc. (text-fig. 30). In the remarkable Tiarechinus from the Trias, Loven (1883) showed that the primordial interambulacral plate is succeeded by three plates (text-fig. 31), and then no more plates are built, a unique condition. Considering the general characters of the basicoronal row, with special reference to the ambulacral plates, we find the following types of arrangement. In Bothriocidaris the basi- coronal row consists of two high hexagonal ambulacral plates with pores superposed in each ambulacral area and one interambulacral plate in each interambulacral area (Plate 1, fig. 1). This same character is seen in young cidarids (Plate 2, fig. 2) (Loven, 1892; A. Agassiz, 1904), young Strongylocentrotus (Plate 3, fig. 11), and Echinus (Loven, 1892), young Salenia, Arbacia, and Phormosoma (A. Agassiz, 1904). It is, I think, fair to call this a primitive character, and it represents what I (1896, p. 235) described as the protechinus stage. The protechinus stage is comparable in other groups of animals to the protoconch of cephalous Mollusca, what I (1890) described as the prodissoconch of Pelecypoda, and to Beecher's (1901) protegulum of Brachio- poda and protaspis of Trilobita. All are referable to what I termed (1890, p. 289) the phylenibryonic stage in development, a stage in which the differential characters of the class are established in ontogeny. A second type of basicoronal plates, in which there are two ambulacral plates alternating with one interambulacral plate in each area, is seen in types where the ambulacral plates have in part flowed down on to the peristome, so that the plates originally there are gone, and yet the primordial interambulacral plate has retained its original place. Such is seen in the Palaeo- zoic Hyattechinus (Plate 23, fig. 1), Pholidechinus (Plate 28, fig. 1), Lepidesthes (Plate 68, fig. 3), and Recent Phormosoma (text-fig. 43, p. 80). This last type differs markedly from those just considered in that the ambulacral basicoronal plates in the adult are compound instead of simple. No compound ambulacral plates on the peristome are known in any type. A third type of basicoronal plates is seen in clypeastroids (Echinarachnius, text-fig. 52, 72 ROBERT TRACY JACKSON ON ECHINI. p. 80) and spatangoids (Micraster and CoUyrites, Plate 3, fig. 15). In both of these the primordial ambulacral plates typically alternate with the primordial interambulacral plates to form the basicoronal row. In all other echinoids the primordial ambulacral plates are on the actinostome and form buccal plates both in the young and in the adult. Some very peculiar special modifications of this plan occur in aberrant cases in clypeastroids and spatan- goids, as follows. In Arachnoides placenta (Linne), Loven (1874, Plate 51) showed that in the young, ambulacral and interambulacral plates alternate as usual. In the adult, the posterior odd interambulacral plate has been pushed dorsally out of the basicoronal ring, and the other primordial interambulacral plates in areas 1, 2, 3, 4, have disappeared by intracoronal resorption, so that as a result, the ten primordial ambulacral plates have come in contact and alone make up the basicoronal ring. They are in contact as they are typically in the buccal membrane around the mouth in regular Echini (Bothriocidaris), but the contact in Arachnoides placenta is a secondary contact not homologous with, though analogous to the original primitive con- tact. In specimens of Arachnoides that I have examined, both smaller and larger than those figured by Loven, the primordial interambulacral plates are in place in the basicoronal row. It is probable, therefore, that Loven's larger specimen as described was an exceptional indi- vidual variant, and not typical of the species. In Lovenia forbesi Woods and Duncan, from the Miocene of Australia, the primordial interambulacral plates of areas 2, 3, 5, are in place in the basicoronal row, but the plates of 1 and 4 have been pushed dorsally so that the primordial ambulacrals 16, Ila, and lYb, \a have come in contact instead of being separated by an interambulacral plate as usual. In Pourtalesia jeffreysi Wyville Thomson, Loven (1883, p. 13) shows the extraordinary condition that the primordial interambulacral plates 2, 3 join posteriorly the base of primordial interambulacral 5 and shut out the primordial ambulacrals I, V and II, IV from the peristomal row. In this highly aberrant type, therefore, the ventral margin of the corona is made up of the two primordial ambulacrals Ilia, b, and the sum of primordial interambulacrals 2, 3, and 5, as far as known a unique condition. Dr. Mortensen (1907, p. 73, Plate 17, fig. 21) shows in Echinosigra (Pourtalesia) paradoxa Mortensen, a most excessive lengthening and also narrowing of the basicoronal plates in areas I, 5, and V, making them, perhaps, the most peculiar plates of any echinoid, in a type that takes delight in breaking all rules of echinoid structure. All the above are types of basicoronal plates in which there has been no encroachment by resorption of the ventral border of the corona. Wliere resorption has taken place to a greater or less extent, a different character of basicoronals prevails. In the Palaeechinidae resorption has apparently cut away only the single primordial interambulacral plate and at the same time, as gathered from Melonechinus (Plate 56, fig. 8), some ambulacral plates have been transferred to the peristome. In these types we find two IMBRICATION OF CORONAL PLATES. 73 plates at the ventral border of the interambulacrum and two in the ambulacrum (Maccoya, Plate 33, fig. 1; Lovenechinus, Plate 43, fig. 1; Oligoporus, Plate 50, fig. 8), or two in the interambulacrum and four in the ambulacrum (Melonechinus, Plate 56, fig. 2). Resorption may have cut away more than one row, probably three or more rows, when we have the condi- tion in Archaeocidaris (Plate 9, figs. 7, 8; Plate 10, fig. 10), where there are four plates in the interambulacrum and two in the ambulacrum in the basicoronal row. Resorption may cut away an indeterminate number of plates, when we find the condition seen in most modern regular Echini, as Cidaris and Strongylocentrotus, with two interambula- cral and two ambulacral plates in each area in the basicoronal row. This includes, I believe, the essential structure of the basicoronal row of all Echini. The proximal modifications of these plates by the development of the perignathic girdle are considered under that head (p. 190). The enlargement of the peristomal aperture in Echini is attained l^y two methods: by the growth of the individual plates, which will enlarge the opening to any extent ; or by cutting away the plates by resorption, which is a more obvious method of enlargement. The growth of the plates is, however, the more important factor. Even when resorption has not cut away any interambulacral plates, as in Echinarachnius, Spatangus, Arbacia (text-fig. 227, p. 193), and especially Phormosoma (text-fig. 43, p. 80), the size of the peristomal opening is many times larger than it was in the young individual. Imbrication of Coronal Plates. In Echini, in many types, there is no imbrication in the coronal plates, the suture lines of the plates being so nearly perpendicular to the surface that the sides of a plate are as nearly parallel as possible in a curved test. The sides of a plate may be far from -parallel where the plates are very thick, as in Lovenechinus nobilis (Plate 38, fig. 9), but still they are perpendicular to the surface and are therefore not imbricate. Certain plates in Echini may present a strong angle on the suture line in a given direction, which Bather suggestively calls beveling. Beveling is applied as a term where the inclination is in a limited number of plates only. An excellent example is in the adradial suture of the Palaeechinidae. Here in all species of the family, as shown in Lovenechinus septies (Plate 45, fig. 5), the ambulacral plates are inclined over the adjacent interambulacral plates, so that a strong bevel results in the plates concerned. A similar bevel is seen in the petaloid area in some clypeastroids (Clypeaster), where the ambulacra are strongly inclined over the interam- bulacra on the adradial suture. In Clypeaster rosaceiis a curious combination of direction of beveling occurs. As stated, in the petaloid areas, the ambulacra bevel over the interambulacra as they do in the Palae- 74 ROBERT TRACY JACKSON ON ECHINI. echinidae. On the other hand, ventrally in Clypeaster, the interambulacra bevel over the ambulacra (text-fig. 234, p. 197) as in Pholidechinus or other types that have imbricate plates. A succession of bevels, in a meridional or transverse series, as Bather (1909a, p. 64) says, produces imbrication properly so called. Imbrication is a character of many quite independent series in Echini, and while of genetic value in a limited series, cannot be used as a basis of close systematic connection in Echini as a whole, as I have shown (1896, p. 237). As imbrication is confusing, especially in fossils where specimens are often viewed from the internal side; diagrammatic figures are given (text-figs. 32-39), illustrating the chief char- acters. Imbrication in Echini is curiously constant in its direction. Interambulacral plates in the corona always imbricate aborally in the vertical plane, so that the dorsal border of one plate is inclined over the ventral border of the next adjacent dorsal plate of its series (text-fig. 32). Interambulacral plates also always imbricate outwardly or adradially from the center, so that each plate from the center of the area laterally imbricates over its neighbor toward the ambulacral area and on the adradial suture imbricates over the ambulacral plates, as seen in Lepidesthes wortheni (Plate 67, fig. 8). In the ambulacrum, on the contrary, the imbrication is always adoral, so that the ventral border of one plate inchnes over the dorsal border of the next adjacent ventral plate, in the opposite direction from that of the interambulacral plates (text-fig. 38). Laterally there is little or no imbrication in ambulacral plates (Lepidesthes) except on the adradial suture, where the ambulacral plates dip under the adjacent interambulacrals instead of over them, as they do in the Palaeechinidae and in some clypeastroids, as above noted. The imbrication of ambulacral and interambulacral plates, as described, occurs weakly in Archaeocidaris, Lepidocidaris, Lepidechinus, strongly in Lepidocentrus (Plate 20, fig. 8), Hyattechinus (Plate 22), Pholidechinus (Plate 28, fig. 2), Perischodomus (Plate 64, figs. 2, 3), Lepidesthes (Plates 66, 70), Pholidocidaris (Plate 73, fig. 4), and Meekechinus (Plate 76, fig. 1). Imbrication also occurs in the modern thin-plated Echinothuriidae. This character has been considered a basis of connecting the Echinothuriidae with Palaeozoic types having imbri- cate plates, but so many characters are opposed to it, that this must be considered a case of parallelism. In modern forms, imbrication is not limited to the Echinothuriidae, but occurs also in the Centrechinidae. This character is well developed in Aslropyga pulvinata (text-fig. 100, p. 109), where interambulacral plates iml^ricate dorsally and laterally, and the ambulacral ventrally. The plates are so thin that it is difficult to see the sutures of imbrication in cross section, but the imbrication is readily seen on the internal view of the test. The interambula- cral plates imbricate over the ambulacrals, as they do also in echinothuriids, in Centrechinus, and in the Palaeozoic types with imbricate plates (as noted ii»the Palaeechinidae, the ambula- cral plates are beveled over the ijiterambulacrals, but imbrication proper does not occur in this family). Mr. Agassiz (1881, p. 71) notes the occurrence of imbrication in Astropyga, but IMBRICATION OF CORONAL PLATES. 75 Text-figs. 32-39. — Method of imbrication of plates in Echini; the figures 32-38 are sj'nthetic, based on studies of Lepideslhes colktii (Plates 69-71). 32. External view, interambulacral plates are rounded in outline. Column 3 imbricates dorsally and laterally in two directions. Columns 1, 2, 4 imbricate dorsally and laterally in one direction and over ambulacrals. Ambulacral plates imbricate ventrally, bevel under adjacent interambulacrals, but do not otherwise have lateral beveling. 33. Cross section of the same. 34. Internal view of the same. Interambulacral plates are hexagonal in outline, angles of imbrication reversed from external view, ambulacral plates with pores in the middle of plates, instead of near the intcrambulacrum as in external view. 35. Cross section of same. 36. Optical section of plates seen from exterior. 37. Optical section of plates seen from interior. 38. Showing in vertical section direction of imbrication of plates in the interambulacrum and ambulacrum. 39. Eucidaris tribuloides {La,ma,Tck). Bahamas. R. T. J. Coll., 693. X 4. Base of corona and peristome. On the peristome both ambulacral and non-ambulacral plates imbricate adorally, a condition not known in the corona of any echinoid. 76 ROBERT TRACY JACKSON ON ECHINI. says the plates of both areas imbricate in the same direction dorsally. I have studied imbri- cation in only one specimen, an Astropyga pulvinata; but in that one the ambulacra imbricate ventrally, interambulacra dorsally as described. I believe the fact has been overlooked, but a weak imbrication occurs in Centrechinus setosus. It is not obvious in external view, but it is readily seen on viewing the interior of a test. The interambulacral plates imbricate dorsally as usual, and the ambulacral slightly ventrally; the interambulacra are inclined over the ambulacra on the adradial suture as usual in imbricate Echini, living or fossil. If a specimen of Centrechinus from alcohol is sectioned horizontally by cutting through the ambitus, it is found that there is much flexibility of the test. The same is seen in a dried specimen if it is first soaked in water. In Chaetodiadema the test is thin and highly flexible. It is very interesting that flexibility and imbrication are so charac- teristic of the Recent Centrechinidae, for many characters connect this family with the Echinothuriidae, as clearly shown by Messrs. A. Agassiz and Clark (1909). The rule of ventral imbrication for ambulacral and dorsal for interambulacral plates in the corona, is apparently without exception in the corona for all Echini where imbrication occurs. Certain exceptions according to the literature are said to exist. Worthen and Miller (1883) supposed that in Lepidesthcs ( = Hybochinus) spectabilis the interambulacral plates imbricate ventrally and the ambulacral dorsally. Their specimen was doubtless incorrectly oriented on account of the position of the teeth as discussed under the systematic description of that species, so that if I am correct it is no exception to the rule. In my earlier paper (Jackson, 1896, pp. 223, 224) a serioys error was made in regard to Lepidocentrus mulleri where the interambulacra were described as imbricating ventrally; this was wrong, since the specimen was inverted, as discussed under that species in this paper. Again, an error was made in describing Pholidocidaris meeki (1896, p. 211, Plate 9, fig. 54). The interambulacra and ambulacra were described as both imbricating adorally. The speci- men (Plate 73, figs. 6, 7) is imperfectly preserved, and I can only plead guilty of an error of observation. I was also partly misled by the fact that Meek and Worthen's (1873, Plate 15, fig. 9a) figure of Pholidocidaris irregularis, the only good figure published, was inverted, a fact that was since ascertained. There is no proof at present that imbrication of coronal plates is ever other than dorsal in the interambulacra and ventral in the ambulacra in types of Echini with imbricate plates. On the peristome, imbrication is adoral when it exists in both ambulacral antl interambula- cral plates, as in Eucidaris (text-fig. 39, also text-figs. 43, 46, 47, p. 80). When imbricate specimens of fossil Echini are viewed from within (text-fig. 34), the struc- ture appears very different from that as seen from without (text-fig. 32). This is shown well in Perischodomus biserialis (Plate 62, figs. 6, 7; Plate 64, figs. 2, 3). From the interior, the ambulacra appear to imbricate dorsally and laterally overlap the adjacent interambulacra; SPINES. 77 also the ambulacra are wide internally instead of narrow, on account of their lateral beveling. The interambulacra appear to imbricate ventrally, as seen from the interior, and dip under the adjacent ambulacra. AH this is reversed when the specimen is viewed from the exterior. This complication must be constantly borne in mind in a study of internal and external molds of fossils or confusion will ensue. Spines. Having discussed the corona, a brief consideration may be given to spines as an obvious and salient feature of Echini. The name Echinodermata implies the character of spines. It is a misnomer, if the group as a whole is considered, for in the Pelmatozoa, spines are rarelj^ developed, and in the holothurians are absent. Spines are, however, a feature of the Echini, and there is no known species which is without them. In the very young plates at the dorsal area of the test, spines are absent, not yet having developed. The nearest approach to this condition as a species character is seen in Bothriocidaris pahleni (Plate 1, fig. 3), which in the interambulacrum apparently has no spines in the adult. It is true that minute granules exist, which possibly bore spines, but such are not known. This species, however, has spines on the ambulacra and all other Echini have them on the interambulacra as well. Spines are classed as primary, secondary, and miliary, and are attached to tubercles of corresponding size. In the Palaeozoic, primary spines are especially well developed in Archaeo- cidaris, where in each interambulacral plate there is a single large central perforate tubercle with an associated spine of variable but always relatively large size. In the youngest plates at the dorsal area, as shown in Archaeocidaris rossica (Plate 11, fig. 2), neither tubercles nor primary spines are developed, and when the tubercles develop, they are at first imperforate as seen in the younger dorsal plates in the figure cited. In Eucidaris (Plate 3, figs. 1, 2) a similar condition exists: the young tubercle is imperforate and the young spine is a thin hollowed plate, like an inverted saucer, its spine-like character developing in later growth. A young spine of a cidarid is short, broad, and distally rounded, and reminds one of the character of the spines of Colobocentrotus, except for the absence of marginal angles commonly developed in the spines of that type by mutual pressure. Central primary spines are well developed in Lepidocidaris (Plate 16, figs. 1-3). Spines of larger size than associated spines and which may be fairly called primaries, are found irregu- larly distributed in Lepidocentrus (Plate 19, figs. 3-5), in HyaUechinus rarispinus (Plate 23, fig. 6), and on the adambulacral columns in Pholidocidaris (Plate 73, fig. 3), and, judging from the tubercles, in part in Perischocidaris (Plate 67, fig. 2). In the several types considered, there are also small secondary spines clustered around the primaries and alone existent on the ambulacral plates (Archaeocidaris). Or secondary spines and tubercles may occur with pri- 78 ROBERT TRACY JACKSOX ON ECHINI. maries on adambulacral columns and exist alone on other interambulacral colunuis and on the ambulacrum (Pholidocidaris, Plate 73, fig. 3; Perischocidaris, Baih', 1874, Plate 4). Secondary spines may exist without any primaries. Such is the condition in all the Palae- echinidae (Plate 52, figs. 10, 11), in Lepidesthes (Plate 70, fig. 4), and, judging from the small and uniform size of the tubercles, in Lepidechinus (Plate 63, fig. 3) such spines and tubercles may be fairly compared in size to those characteristic of Clypeaster and the usual type seen in clypeastroids. Such small spines as occur in Bothriocidaris (Plate 1, fig. 3) may probably be considered as primaries on account of the perforate tubercles and the small .size of the whole animal. As far as known, spines are very uniform in character within the species in the Palaeozoic, cases of marked deviation, such as occur in some Cidaridae, being almost unknown in these older types. From this uniformity in the Palaeozoic, spines present very good species characters, which is fortunate, as they so often occur in a dissociated condition among fossils. As discussed (p. 51), spines are typically internal, and the evidence indicates that they were internal, that is, covered by living epithelial tissue, in all Palaeozoic types excepting Miocidaris and possibly Archaeocidaris. Stout, thick, and club-shaped spines are largely characteristic of the Cidaridae and a few other types. A specimen of Strongylocentrotus drobachiensis in the Peabody Museum at Salem is interesting as showing thick club-shaped spines (Plate 6, figs. 11, 12), which are quite cidarid in appearance. These spines are numerous but rather local in distribution, not covering the whole specimen. The specimen is of normal size and appearance otherwise, and it appears to be merely a rare variation. This was the only such case seen in 33,000 specimens of this species examined, nor was such a structure seen in any other species of the family. Tubercles as well as the spines with which they are associated may be divided into primary, secondary, and miliary. Primary tubercles may be perforate or imperforate. They are perforate in all Palaeozoic species where they occur; they are also perforate in the Cidaridae, Centrechinidae, and some of the Saleniidae, also in some spatangoids. Primary tubercles are imperforate in the remaining Centrechinoida, the Holectypina, and many of the Spatangina. Secondary tubercles are imperforate in the Palaeozoic, whether they occur with primaries or alone, as in the Palaeechinidae. They are also imperforate in the Cidaroida and in Acrosalenia, but they are perforate in the Centrechinidae and certain clypeastroids and spatangoids (Clypeaster, Metalia, Meoma). Perforation of tubercles is evidentlj^ a character of primitive types and of some specialized types of clypeastroids and spatangoids. Primary tubercles in a species occur with a definite distribution which is characteristic, as seen especially well in the Centrechinoida, a given number typically existing on a plate. It seems that the distance between tubercles which is maintained in a species is the real feature of their distribution. If, by chance, a plate is narrower than usual, as in Tripneustes (Plate 6, fig. 3), the tubercles are their usual distance apart, but there are fewer to a given plate; THE PERISTOME. 79 or, if, by chance, a plate is wider tlian usual, as in the tetramerous Arbacia lixula figured by Verrill (1909), then again the tubercles are their usual distance apart, but there are more tubercles to a plate than is typical of the species (pp. 40, 47). The Peristome. The peristome is closely associated with the corona, and is next considered. The peris- tome, which is the tissue lying between the mouth opening and the basicoronal plates, presents various characters in different groups of Echini, and these characters fall into six quite distinct types as described below. The peristome in almost all Echini is more or less extensively covered with plates. There are in regular Echini ambulacral plates bearing tube-feet, either in one or in many rows. With these there may be in addition what I would call non-ambulacral plates, which do not bear tube-feet. Such non-ambulacral plates, where they occur in the Perischoechinoida and Cida- roida, occupy an interradial position ; or, where they occur in the Centrechinoida, they occupy also a radial position, being a more or less generally distributed series of plates (Toxopneustes, text-fig. 57). In spatangoids there are no ambulacral plates on the peristome, but non-ambula- crals only. These are generally distributed over the area. I have adopted the term non- ambulacral for such plates because, though they may be interradial only (Cidaridae), they may also have a general distribution, and they have no relation to the interambulacra of the corona. The most ancient type of peristome known is that represented by Bothriocidaris (text- fig. 40) in which there are two continuous rows of ambulacral plates, a structurally primitive character. The first row around the mouth can fairly be considered the primordial ambulacral plates, and, as shown in B. archaica (Plate 1, fig. 1), this row follows Loven's law of la, Ila, III6, IVa, V6 large and 16, lib, Ilia, IV6, Va small. Above the second row the corona proper begins with a row of five interambulacral and ten ambulacral plates. The line of separation between the peristome and the corona is not so strongly marked as in most types, but it is comparable to that of young Phormosoma (text-fig. 41), as pointed out by Mr. Agassiz (1904, p. 79). It is quite probable that the second row of peristomal plates in Bothriocidaris was derived by flowing down from the corona, but on the other hand there may have been two rows of peristomal plates formed before the development of the corona. In Phormosoma at a very young stage (Plate 3, fig. 9), as shown by Mortensen (1904, p. 54), there is only one row of iK'ri- stomal plates, whereas a little later (text-fig. 41; Plate 3, fig. 10), as shown by A. Agassiz, there are two rows of peristomal plates almost exactly as in Bothriocidaris, as Mr. Agassiz (1904, p. 79) pointed out. The peristome of Bothriocidaris makes also very close approach to the character of the young in cidarids (Plate 2, figs. 1, G) and young Strongylocentrotus (text-fig. 49), also other young Centrechinoida. It differs from these early stages principally in having 80 ROBERT TRACY JACKSON OX ECHINI. ^ nL THE PERISTOME. 81 Text-figs. 40-54. — Characters of peristome and base of the corona in repre.sentativc Ecliini. 40. Jiolliiiocklaris tirchiiica sp. nov. Ord()\ician, Russia. From Plate 1, fig. 1. On the peristome two rows of ambu- lacral plates. 41. t'lioniKixdiiKi iiliiccnlaWyvWU' TliomMm. Young. Diani. 9 nun. Adapted from A. Aga.ssiz, 1004, Plate 43, fig. 3. On the ])eri.slomc' two rows of ambulaeral plates. 42. The same. Diam. 7 mm. .\dapted from A. Agassiz, 1904, Plato 43, fig. 1. Peristomal jilates with pores in a central iierijiodium. 43. The same, adult . Off Cape May to Cape Sable, 956 fat h. Diani. 50 mm. R. T. J. Coll., 707. X 3. On the peristome many row's of ambidaeral plates. 44. The same. Peristomal gills enlarged. 45. (•oniocidaris canalicitlala A. Agassiz. Young. Diam. 1.45 mm. From Plate 2, fig. 1. On the peristome one row of ambulaeral plates. 40. Eiicidari.-i Irihidoiihs (hamiirck). Bahamas. Diam. 45 mm. R. T. J. Coll., 708. X 3. On tlie jteristome many rows of ambuhirnd ami in addition intcrradial non-a,inl)ulacral plates (compare with young, Plate 2, hg. 0). 47. Airhiicucidaris wnrlkini Hall. Lower Carboniferous. From Plate 9, fig. 0. Partially' restore > m C S 1 o QJ t—i )-H HH t~^ > »— f l-H i" > "3 u O 1— t 1— I l-H > g > 1— T 1 Q ■4-3 c l-H t-H .9 > l-H > 1— T 2 O l-H l-H HH OJ .9 <— ( ►— ( > hH"" £ o U QJ HH HH HH HH HH >' > hh" 2 o O C 2 3 O < a c3 •i £ 3 S "o a CD a tJ) c rt t.. (-1 < a> a> CD 0} g 1— 1 hH ,_( HH. HH h-i t—i 1— 1 kt t-H t-H HH ^ HH & - H- T »— T •—T ^'' -9 t— < 1—1 H- 1 HH Ch ci 03 m m 0 > > (—4 > 1— 1 o gj «; t-^ 3 .a 'G > -t-3 1 > 1— 1 > QJ o OJ 9i ii QJ .s •— 1 •— I M o P4 Si > > > 0) o M t— I > 1— r 1— r h-l a S3 "3 £ s £ QQ "B 0) *n o -2 _C3 _rt u a 1 o o C3 "5 3 "3 3 =) Fh o o o o O t-t !? > o c )— ( t— 1 > Li > 1— 1 fc-i a; C > > 1 "3 o +^ ^ ^ 1 "3 o o < 1— 1 "3 o O > O 3 o O 2 "3 o 1— 1 o O o a 1 50 Gullmar Fjord on the Swedish coast of Skagerrak SO ss 12 15 29 6 1,700 Pulpit Harbor, North Haven, Me., 30-.58 mm. diam. 10 O.Off 88 0.5 ; 173 1 1,503 9 14 194 St Pierre Newfoundland 6 9;g 0.5 2 12 178 1 3 Dumpling Islands, North Haven, Maine. (This series, given above, belongs here.) 6,0.50 developed g.5 0.03 95 /.; 2 specimens, 30-75 mm. diam. 7,600 Calderwood Island, Fox Island Thoroughfare, Maine 3.2 0.07 H 1.6 0.03 1 242 5 7,157 118 2 76 2,700 York, Maine (text-fig. 176, p. 153) 0.9 0.04 S7 ^ 0.04 1 25 1 2,610 38 1 25 1,200 Friday Harbor, Puget Sound, Washington, 40-85 0.9 96 g.e 0.1 / mm. diam. 11 1,149 31 1 8 588 Massachusetts Bay, mostly from Nahant, Mass. 14 95 2.2 0.^ 2 8 557 13 1 9 4,000 Frenchman's Bay, Maine 0.03* 1.9 94 ;J.3 0.05 / 1* 75 3,745 130 2 47 300 South Harpswell, Maine 2.7 0.3 92 3.7 ; 8 1 •271 11 3 Truro, Massachusetts, series 20-30 mm. diam. (1,100 specimens) 0.3 96 .3.5 1 2,800 3 1,0.52 32 13 Same locality, series 30-60 mm. diam. (1,700 speci- O.G H 4.3 O.Off / y mens) 11 1,595 73 1 20 200 Labrador 2.5 0.5 S7.5 7.5 2 T* 1 175 15 4 35 Iceland and Faroe Islands 11 eo ao .9 4 21 y 3 33,000 20 1,758 11 30,231 569 9 0 402 ' Italic numerals represent percentages, Arabic numerals the number of specimens observed. 144 ROBERT TRACY JACKSON ON ECHINI. pores and ocular I only is insert; an older, but still very young specimen, 22 mm. in diameter, has the bivium insert, the species character. Of adults, 179 were examined, including a superb series of 61 specimens from Friday Harbor, Puget Sound, Washington, kindly sent me by Mr. A. P. Romine. Of this species, in 87 %, oculars I, V are insert (text-fig. 152). In 1 % ocular I alone is insert as an arrested variant. Both of the two specimens showing this character are adults, one being very large, 145 mm. in diameter (text-fig. 151). Of progressive variants Text-figs. 150-153. — Ocular plate arrangement in Slrongylocentrotus franciscanus A. Agassiz. 150. Monterey Bay, California. Young. Diam. 5.5 mm. R.T. J. Coll., 552. X 6.5. Ocular I alone insert as a developing character, no genital pores. 151. Friday Harbor, Puget Sound, Washington. Diam. 145 mm. R. T. J. Coll., 906. X 1.6. Ocular I alone insert, an arrested variant. 152. Cahfornia. Diam. 154 mm. R. T. J. Coll., 724. X 1.6. Oculars I, V insert, the tjpical character. An exceptionally large specimen (text-figs. 166, p. 149; 199, p. 171). 153. Vancouver, British Columbia. Diam. 126 mm. R. T. J. Coll., 725. X 1.6. Oculars I, V, IV in.sert, a progres- sive variant. 11 % have oculars I, V, IV insert. The specimen from which text-fig. 152 was drawn is ex- ceptionally large, 154 mm. in diameter, the largest seen, yet it has only the bivium insert; the progressive variants were all considerably smaller. One specimen only is aberrant with oculars V, II insert, ocular I being excluded from the periproct by the fusion of genitals 5, 1 as in text-fig. 146. Of the three adults here figured it is noteworthy that the largest specimen (text-fig. 152) has the smallest apical disc and the next smaller specimen, which is an arrested variant (text-fig. 151), has the largest apical disc (p. 87). The most progressive species of the genus is Slrongylocentrotus purpuralus from California. Through the kindness of Dr. W. K. Fisher, I received a fine series of specimens; with others, 120 have been studied. In this species 77 % have oculars I, V insert, 1 % is arrested with I only insert, and 22 % are progressive variants with I, V, IV insert. The species eury- throgrammus, franciscanus, and purpuratus, although the\' have oculars I, V insert as a typical OCULAR AND GENITAL PLATES. 145 character, have a larger number of progressive variants as regards ocular plates than any other species of the genus (table, p. 162). Strongylocentrotus gibbosus is a most unusual sea-urchin in that it has deeply bedded in the periproct a commensal crab, Fabia chilensis Dana. This crab is a very constant associate; but one specimen in 24 studied has no trace of a crab (text-fig. 155). The anomalous associa- tion of the crab very deeply modifies the relative position of the ocular plates. The crab in the cases seen occupies a position close to ocular IV and genital 3, as shown in the figures, and no variation was noted in this position. This species and Gymnechinus are the only Echini studied in which an aberrant arrangement of ocular plates is the dominant one. In 24 specimens one has I, V., IV insert (text-fig. 154), and four have all the oculars insert 756 Text-figs. 154—157. — Oeiilar plate arrangement in Sirongylocenlrotiis giJihosus (Agassiz). Payta, Peni. 'I'lie tyjiical commensal crab, Fabia chilensis, in all but text-fig. 155. All figures X4. 154. Diam. 31 mm. R. T. J. Coll., 704. Oculars I, V, IV insert. 155. Diam. 28 mm. R. T. J. Coll., 702. Oculars V, IV insert. Crab wanting. 156. Diam. 42 mm. R. T. J. Coll., 703. Oculars V, IV, III in.sert, the typical character in tliis aberrant species; the periproct meets the interambulacra at the points marked X. The commensal crab is very clear and in its (ypical posi- tion (pp. 94, 128, 177). 157. Diam. 40 mm. R. T. J. Coll., 705. Ocular plates all insert. (text-fig. 157). These are the only cases of all oculars insert seen in the genus. It is also interesting that in text-fig. 157 there is an interspace between ocular III and genital 3, where the periproct reaches the interambulacrum, and the same feature is shown in three areas in text- fig. 156. This is comparable as a parallelism to a similar condition of separation of ocular and genital plates in Phormosoma (p. 110). All the other specimens, 79 %, are aberrant in ocular arrangement. In five, oculars V, IV are insert; one of these has no associated crab (text-fig. 146 ROBERT TRACY JACKSON ON ECHINI. 155), but yet retains the specific irregularity. Eight have V, IV, III insert (text-fig. 156), a very unusual arrangement, seen in only three other cases in the order Centrechinoida, yet it is the dominant character, 33 %, in S. gibbosus. This arrangement of oculars with V, IV, III insert was found as an aberrant variation in six cases in the Cidaroida, as described in Cidaris affinis and Eucidaris tribuloides (pp. 96, 97, 99). Six specimens, 25%, have oculars I, V, IV, III insert, again a rare arrangement in the order, but frequent in the Cidaroida. In young specimens of gibbosus a few mm. in diameter all oculars are exsert. In the Strongy- locentrotidae, of the species studied, two are characterized by having oculars all exsert; all others have typically the bivium insert iexcept the aberrant S. gibbosus (pp. 162, 164). The Echinometridae as here restricted includes Echinometra and allies, which are elliptical in outline through a sidewise axis, a character peculiar to this group of Echini only. In this family, as in the Arbaciidae, when ocular plates enter the periproct, it is in the sequence V, I, not I, V, as ill the Echinidae and Strongylocentrotidae. On this basis, when only one ocular is insert, it should be V, and this is true as a dominant rule; occasionally, however, ocular I alone is insert. Such cases may be considered variants toward the character of the associated families in which, if only one plate is insert, that plate is typically ocular I. The rule is not quite so constant as in the Arbaciidae, in which, if one plate is insert, it is ocular V with very rare exceptions. The most primitive species of Echinometra on the basis of ocular plate arrangement is E. oblonga. Of this species (44 specimens) in 98 % all the oculars are exsert, and in 2 % ocular V is insert as a progressive variant. In Echinometra viridis (25 specimens), 96% have all oculars exsert, and 4% ocular V insert. Echinometra mathaei {15Q specimens) has typically, 79 %, all oculars exsert; of progressive variants 19 % have V, and 1 % have V, 1 insert; 1 % is aberrant with oculars V, II insert. Of Echinometra lucunter a large series was studied, and it shows different facies in two widely separated localities. In the West Indian area (578 specimens) typically, 57 %, ocular V alone is insert (text-fig. 159). As arrested variants 17 % have all oculars exsert (text-fig. 158), the typical character of E. oblonga. As progressive variants 25 % have oculars V, I insert (text-fig. 160), like the typical character of E. van brunti, and 0.3 % have V, I, IV insert (text-fig. 161). In Echinometra lucunter from Bermuda considerable difference exists. In 176 specimens from that locality ocular V is typically insert in 46 %. The arrested variants, however, are less frequent, only 9 % having all oculars exsert. Also the progressive variants are much more frequent than in the West Indian form, there being 43 % with V, I, and 0.6 % with V, I, IV insert. The Bermudan form, while having typically ocular V insert as in the West Indian, is therefore distinctly more progressive as shown by its variants, and makes a closer approach to the character seen in the next higher species, E. van brunti. The Bermudan form of E. lucunter is notable for its large size and robust character. One specimen in my collection from that locality measures 90 mm. through the long axis. Wliile in Echinometra OCULAR AND GENITAL PLATES. 147 lucunter 413 specimens have ocular V alone insert, only four have ocular I alone insert, showing the strong dominance of this character. In the whole number (754) examined, there are but two specimens, 0.3 %, with an aberrant arrangement of oculars. In one, oculars V, IV are insert, and in the other V, II alone reach the periproct (tables, pp. 163, 164). Text-figs. 158-161. — Ocular plate arrangement in Eddnometra lucunlcr (Linn6). Hayti, West Indies. All figures X4. (Compare text-figs. 111-114, p. 115.) 1.58. Length 41 mm., through 3, I. R. T. J. Coll., 768. Oculars all exsert, an arrested variant. 159. Length 55 mm., tlirough 3, I. R. T. J. Coll., 769. Ocular V insert, the typical character. 160. Length 52 mm., through 3, I. R. T. J. Coll., 770. Oculars V, I insert, a progressive variant. 161. Length 41 mm., through 3, 1. R. T. J. Coll., 771. Oculars V, I, IV insert, an extreme progressive variant. Echinometra van brunti is an interesting form as it is apparently the highest species of the genus, judging from the ocular plates. In 76 specimens the typical character is to have oculars V, I insert, which occurs in 72 %. As arrested variants only 11 % have oculars all exsert, which is the character of E. mathaei; in 12 % V only is insert, which is the character of E. lucunter; and in 4 % ocular I only is insert. As a progressive variant, 1 % have V, I, IV insert. It would be most interesting to get a large series of this species for comparative study, but it is felt that the number seen is enough to give a fair basis for comparison. In Heterocentrotus mammillatus (35 specimens) 80 % have oculars all exsert, 11 % ^^re pro- gressive variants with ocular V insert; in one specimen ocular I, and in one oculars V, I are insert. One specimen is aberrant with oculars V, II insert. In the nearly allied Heterocentrotus trigonarius (47 specimens) typically, 60 %, all oculars are exsert; in 28 % ocular V is insert, in 4 % ocular I and in 9 % oculars V, I are insert. In Colobocentrotus atratus (82 specimens) 99 % have all oculars exsert, and in 1 % ocular V is insert, as a progressive variant. The Echinometridae as a family is characterized by very close adherence to the rules of ocular arrangement. In the whole 1,222 specimens examined, only four aberrants were found; of these, three have oculars V, II and one has V, IV insert (tables, pp. 163, 164). Having considered the character of ocular plates in their relation to the genitals and to 148 ROBERT TRACY JACKSON ON ECHINI. the periproct as typical developing and adult features, and as variants in many species of fossil and living Echini, the characters may be summarized as follows. In the Ordovician Bothrio- cidaris archaica the oculars form a continuous ring with genitals on their dorsal border (text- fig. 162). In the Devonian to Carboniferous usualty, as in Melonechinus (text-fig. 163), oculars and genitals all reach the periproct and the corona. In the Mesozoic usually, as in Cidaris coronata (text-fig. 164), the genitals alone reach the periproct and oculars are all exsert. In the Recent, oculars are all exsert in the young individual. In the adult typically all oculars may still be exsert, or one, or more to all plates may travel in so as to reach the periproct, and the incoming, barring relatively rare exceptions, is in a perfectly definite order. Different species or genera attain as a character definite points along this line of dilTerential development, and selected cases of coming in are shown in text-figs. 165 to 169. All plates may be typicallj' exsert, as in Echinus escidentus (text-fig. 116, p. 117). One plate may be typically insert, when it is either ocular I, as in Echinus niagellanicus (text-fig. 165), or ocular V, as in Echino- Text-figs. 162-175. — Ocular plate arrangement in typical Echini. Each figure represents the typical character of the several species represented. 162. Bothriocidaris archaica sp. nov. Ordovician, Russia. From Plate 1, fig. 2. X 7. Oculars very large, meeting in a ring, genitals small, dorsal to the oculars (p. 87). 163. Melonechinus mulliporus (Norwood and Owen). Lower Carboniferous, St. Louis, Missouri. From Plate 50, fig. 6. X 3. Oculars and genitals all meet the periproct and corona. Compare text-fig. 169. Genitals have many pores. 164. Cidaris coronata Goldfuss. White Jura, Sonntheim. Stuttgart Museum 9,782. Enlarged. All oculars exsert. Plates of periproct in place, very rare for fossils of the genus (pp. 96, 174). 165. Echinus magellanicus ThiWppi. Patagonia. Diam. 26 mm. R. T. J. Coll., 773. X 2.7. Ocular I insert (p. 119). 166. Slrongylocentrotuit franciscanu-s A. Agassiz. California. Diam. 112 mm. R. T. J. Coll., 699. X 1.7. Oculars I, V insert (p. 144). 167. Arbacia nigra (Molina). Chili. Diam. 92 mm. R. T. J. Coll., 796. X 2.6. Oculars I, V, IV insert (p. 116). 168. Acrosalenia pseudodecorala Cotteau. Bathonien, France. (After Cotteau, 1S7.5-'S0, Plate 246, fig. 6.) Oculars I, V, IV, II insert. Suranal plate is dorsal to genital 3 (p. 112). 169. Phyllacanlhusbacidosa (Lamarck). Mauritius. Diam. 41 mm. R. T. J. Coll., 695. X 2.7. All oculars insert. (Compare text-fig. 163, p. 102; Plate 3, figs. .3, 4.) 170. Phortnosoma placenta Wyville Thomson. Off Cape Sable to Cape May, 956 fath. Diam. 56 mm. R. T. .1. Coll., 707. X 3.5. All oculars insert and interspaces exist between oculars and genitals so that the jieriproct reaches the interambulacra. Genitals are split ventrally. Ambidacral plates are simple primaries in the placogenous zone. Inter- ambulacral plates originate again.st oculars as iisual (pp. 63, 110, 177; Plate 3, fig. 8). 171. IIolectypHS hemisphericus Desor. Inferior Oolite, Leckhampton Hill, England. Diam. 28 mm. R. T. J. Coll., 738. X 9. Oculars all excluded from the center by genitals (p. 170). 172. Cassidulus subquadratus Conrad. Upper Cretaceous, Holly Springs, Mississippi, .\fter W. B. Clark, 1893, Plate 31, fig. Ih. Enlarged. Oculars I, V reach the center, other oculars are excluded from the center by the genitals. 173. Toxobrissus pacificus A. Agassiz. Adapted from A. Agassiz, 1904, text-fig. 279, p. 193. Enlarged. Oculars I, V are virtually insert, although actually ocular I is shut out by the posterior extension of genital 2. 174. Micrnslercorangiiineum {Lamarck) . Cretaceous, England. Length 55 mm. R. T. J. Coll., 521. X 6.7. Oc\i- lars I, V, IV reach the center, others excluded from the center by the genitals. Compare text-fig. 167 (p. 92). 175. Ananchyles ovalus (Leske). Cretaceous, Sussex, England. Length 67 mm. Student Laboratory Coll., 172, Harvard University. X 2.4. Oculars I, V, IV, II reach the center, III only being excluded from the center by the genitals. Compare text-figs. 126 and 168 (pp. 92, 167). OCULAR AND GENITAL PLATES. 149 150 ROBERT TRACY JACKSON ON ECHINI. ynetra lucunter (text-fig. 159, p. 147). Two plates may be insert, when it is typically the bivium, as in Strongylocentrotus franciscanus (text-fig. 166). Three plates may be insert, when it is the bivium and left posterior plate of the trivium, as in Arbacia nigra (text-fig. 167). When four plates are insert, it is typically the bivium and posterior pair of the trivium, as in Acrosalenia pseudodecorata (text-fig. 168). I have shown this character in many cases as a variant but have not seen a species in which it is the character, so have copied the figure from Cotteau (1875-'80), as he says that four plates are typically insert in that species. All ocular plates may be insert, as in Phyllacanthus baculosa (text-fig. 169), when we have a character which is a close approach to that dominant in the Upper Palaeozoic (text-fig. 163). The next differential character to appear is for interspaces to develop between the ocular and genital plates, so that the periproct comes in contact with the interambulacra, as in Phormo- soma placenta (text-fig. 170). Turning to the irregular Echini, we find in part a parallel series to that seen in the regular Echini. As the periproct is eccentric, of course oculars cannot reach that structure, but if oculars reach the center line, they may be considered insert, or if excluded from the center line by junction of the genital plates, they may be regarded as exsert. Oculars may be all oxsert, as in Holectypus (text-fig. 171; compare text-fig. 164). The bivium alone may be insert, as in Cassidulus (text-fig. 172; compare text-fig. 166). In some species, as Toxobrissus (text-fig. 173), the plates of the bivium may be separated by the posterior extension of the madreporite, but as this plate radially speaking is out of place, the oculars I, V may still be considered as morphologically insert. Or it could be looked at from another point of view and ocular V alone regarded as insert (as is common in the Arbaciidae), the other four oculars being shut out from the center by the contact of the genitals. Three ocular plates may be insert, when it is the bivium and left posterior plate of the trivium, as in Micraster (text-fig. 174; compare text- fig. 167). Or, finally, four ocular plates may be insert, when it is the bivium and posterior pair of the trivium, as in Ananchytes (text-fig. 175; compare text-fig. 168). The typical character of arrangement of ocular plates and the range of variation are ex- pressed diagrammatically by block lines in text-fig. 176. Each block represents the frequency of the character in the species, as given in the tables (pp. 100, 101, 143, 154-163). An excep- tion to this statement is in the cases of Strongylocentrotus lividus (p. 126) and Phyllacanthus annulifera (p. 102) in which additional specimens, tabulated after this diagram was drawn, somewhat altered the facts and percentages, but it was not deemed of sufficient importance to omit the observations, and there was no opportunity to alter the diagram. The characters are all given correctly to scale excepting that when one exists in less than 1 % of the specimens, it is represented by a line the thickness of which has no relation to the frequency of the same. On the left of the diagram is given the character a, all oculars exsert. This is an invariable feature in the specimens of Echinus affinis observed (p. 118). It is also the character of the OCULAR AND GENITAL PLATES. 151 adults of most Mesozoic and the young of all modern species of regular Echini. Passing to Echinus esculentus, all exsert is still strongly the character, but one plate may be insert as an infrequent progressive variant (p. 118). In Arbacia pundulata, a similar condition prevails, but one plate insert is a more frequent variant and rarely two or three plates are insert. Strongylocentrotus lividus has typically all oculars exsert, but one insert is a frequent and two insert a rarer progressive variant (p. 126).^ Echinometra mathaei has a lower percentage of all exsert and a consequent gain of progressive variants (p. 146). In Echinus angulosus the progressive variants become more strongly marked, especially as compared with the northern species of the genus shown (p. 119). In Salenocidaris varispina, instead of all exsert, one ocular insert is the species character, and the feature of oculars all exsert passes into the phase of an arrested variant (p. 112). Echinus magellanicus has typically one ocular insert; all oculars are exsert as an occasional arrested variant, or I, V insert as an occasional progressive variant. In no case seen were oculars I, V, IV insert, the space for which is indicated in the diagram at X, but in one speci- men oculars I, V, IV, II are insert, as an extreme progressive variant (p. 119). Echinometra lucunter from the West Indian fauna (p. 146) has one ocular insert as the species character, but all exsert is a frequent arrested variant and two insert a more frequent progressive variant, with rarely three plates insert. In the same species from Bermuda (p. 146) a different condition prevails, as arrested variants are rarer and progressive variants are much commoner than in the southern form. With Strongylocentrotus fragilis two oculars insert becomes the dominant character, with all exsert or one insert as frequent arrested variants (p. 128). In Echinometra van brunti two plates insert is strongly the dominant character with all oculars exsert or one insert as rela- tively infrequent arrested variants; and three oculars insert as a somewhat rare progressive variant (p. 147). Strongylocentrotus drobachiensis from York, Maine, has very strongly two oculars insert, with but few arrested and progressive variants (p. 138). From here on, the species shown have less of the bivium insert, few arrested variants with all exsert or one insert, and progressively more with three or four plates insert. Toxopneustes variegatus shows a distinct gain of progressive variants over the last species considered, and a consequent dropping of the bivium insert character (p. 121). In Strongylocentrotus purpuratus there is a still further reduction of the I, V insert and gain of the I, V, IV insert. A striking case is the Toxo- pneustes atlanticus in which progressive variants are much in excess of what is found in the closely allied T. variegatus (p. 122). With Tripneustes esculentus from the West Indian fauna (p. 124), three oculars insert becomes the dominant character. In this species from this area two oculars insert is a very ' In the diagram (text-fig. 176) the first 170 specimens only of Siroiigylocentrotus lividus are included. Recently, and too late to alter the diagram, this number was increa.sed to 1,163 specimens. In this larger series the percentages (pp. 126, 162) are altered somewhat from that shown in the diagram and one specimen, 0.1% has oculars I, V, IV insert, a feature not given in this diagram. 152 ROBERT TRACY JACKSON ON ECHINI. common arrested variant in frequency closely approaching the dominant character, and four oculars insert is a common progressive variant; rarely all oculars are insert. Arbacia nigra has strongly three oculars insert with no progressive variants, but occasional arrested variants of all exsert, and frequent arrested variants with one or two oculars insert (p. 116). This species covers the same range of characters as shown in Arbacia punctulata, but in very different pro- portions. Centrechimis setosus has strongly three oculars insert as the species feature and is almost the only species in the order in which the variation covers six characters. All exsert or one or two oculars insert occur as rare arrested variants, and four oculars or all insert are frequent progressive variants (p. 108). Of Centrostephanus rodgersi there are few observations, but as far as they may be trusted, it is the strongest three oculars insert of any species known, with frequent progressive variants having four or all oculars insert (p. 110). Eiicidaris tribu- loides has strongly three oculars insert as a dominant character (p. 97). Its range of varia- tion covers practically all the characters known in regular Echini from the Lower Carboniferous up. The only exceptions to this statement are cases of Echini that typically have an aberrant ocular arrangement, as Gymnechinus pulchellus (p. 120) and Strongylocentrotus gibbosus (p. 145). The arrested variants of Eucidaris tribuloides are few, with all oculars exsert, 'or one or two insert. Progressive variants are common with four or much oftener five oculars insert. In no sea-urchin studied is four oculars insert a typical species character, though this is said (Cotteau, 1875-'80) to be the case in Acrosalenia pseudodecorata Cotteau. With Phyllacanthus annulifera all oculars insert is the character with three or four oculars insert as frequent arrested variants (p. 102). In Phyllacanthus baculosa all oculars are insert as a strong species feature (p. 102). The only possible variants on this line of development are arrested, and such occur with three or four plates insert as rare arrested variants. In Aspido- diadema nicobaricum all oculars are strongly insert and no variation is known (p. 104). Those cases given in the diagram are selected, but almost any of the species shown in the tables of ocular plate arrangement could be intercalated in the diagram and there find an harmonious place in the scheme of differential structure. The accompanying tables show the characters of ocular plates in the species studied. The table for the Cidaroida is on pp. 100, 101 and for Strongylocentrotus drobachiensis on pp. 142, 143. In tabulating characters the only specimens omitted were those that had no locality, where this was an important item; also 71 that had three, four, or six areas, and some eight speci- mens that were so distorted that they could not properly be included. The general relation of percentages is considered on p. 93. All aberrant variations in the Centrechinoida are given in the last table (p. 164). As the relation of ocular plates to the genitals and periproct plaj's such a distinct and definite part in geological sequence, in ontogeny, in variation, and in geographical distribution, it seems that they may be accepted as a feature of importance in the study of Echini. It seems also that they make a reasonable basis where differences occur on which to arrange the systematic sequence of species within the genus. OCULAR AND GENITAL PLATES. 153 V/////^////Y////Y/^///^^////>y////X/^Azy/////>//////^//////. 605 Echinus aiRnis 20(1 Et'hinus csculcntus 2,100 Arbacia punctulata, Woods Hole 170 Strongylocentrotus lividus 159 Echinomctra inathaci 100 Kchimis angulosus 2(3 Salcnocidaris varispina 1(J4 Echinus niagcUanicus .■(78 Ecliinomctra lueuntcr, West Indies and Florida 17.5 Ecliinomctra lucunter, Bermuda 5.5 Strongylocentrotus fragilis 76 Echinomctra van brunti 2,700 Strongylocentrotus drobach- iensis, York, Maine 1,013 Toxopueustes variegatus 120 Strongylocentrotus purpuratus 1,463 Toxopneustes atlanlicus, Bermuda (45-60 mm. diam.). 455 Tripneustes esculentus, West Indies and Florida 246 Arbacia nigra 1,168 Ccntrechinus setosus 12 Ccntrostcphanus rodgersi 849 Eucidaris tribuloides 12 Phj'llacanthus annulifera 106 Phyllacanthus baculosa 47 Aspidodiadema nicobaricum 154 ROBERT TRACY JACKSON ON ECHINI. Table of Typical Ocular Plate Arrangement and Variation in the Centrechinoida. 00 a a 1 a 3 Order CENTRECHINOIDA. Suborder A UWDONTA . £ 3 O < i a f— ( ►— 1 1— t l-H > > o GO .s i 3 o tf > a > 2 3 o > > _C3 3 o O Xt >< q; t-> 1— < > ►— 1 CO 3 o 1 X if .5 > > I— T 2 _d "s o o t4 c 2 "3 o o 5 a -i 2 a 3 u o ««-■ o a S O i Hemicidaridae. luu* 15* 100 6 S9 151 81 47 13 2 // 2 90 9 100 16 100 43 100 75 6 Jurassic, Europe. Heinicidaris hoffmanni Agassiz. Jurassic, Europe. Hemicidaris intermedia (Fleming). Jurassic, Europe. (Text-fig. 75.) Hemicidaris crenularis (Lamarck). Jurassic, Europe. Hemicidaris ludensis d'Orbigny. Jurassic, Europe. (Text-figs. 76, 77.) Hemicidaris langrunensis Cotteau. Jurassic, Europe. Hemidiadcma stramonium Agassiz. Jurassic, Europe. Asterocidaris minor Cotteau. Jurassic, Sollies, France. (Text-fig. 78.) Goniopygus peltaiits Agassiz. Jurassic, Europe. Glyplicus hierogbjphicus Agassiz. Jurassic, Europe. (Text-fig. 79.) 169 8 13 16 9 31 5 T 10 1 3 5 1 5e 9 67 12 58 ;2 16 1 18 10 16 43 1 75 426 ■ 366 32 0 27 0 0 0 1 Aspidodiadematidae. 44 AnTtitlndiadp.ma mriierei. Dodorloin. lOU 44 47 /OO 10 m 22 47 Hawaiian Islands, 241-294 fathoms. Aspidodiadema nicobaricum Doderlein. Hawaiian Islands, 165-500 fathoms. (Text-fig. 176.) Dermalodiadema antillarum A. Agassiz. West Indies, 687-955 fathoms. (Text-fig. 80.) Dermalodiadema horridum A. Agassiz. Galapagos Islands, 812 fathoms. 10 22 123 0 0 0 0 0 0 123 0 * Italic numerals represent percentages, Arabic numerals the number of specimens observed. OCULAR AND GENITAL PLATES. 155 Table of Typical Ocular Plate Arrangement and Variation in the Centrechinoida (continued). ■e 1 ■g -M (H ■4^ m OJ OJ txi ^ CO W X >1 y. — ' 1 I— 1 t« t—i t—i 1— 1 ! o hH h^ (— < »-H 1— ( ^ ^ t—i •-^ t—t HH *~* (—1 1 £2 • > > > tn CO M cj HH HH »-H Oj ^ 5 M o a > 1—1 Eh .S 1— 1 O *s Lh V t-l S > ►> ;-• "o 5S i S c 1— 1 to -t-3 en V CO .s .s > > >■ C OJ "o 1 t— ( > 1— r 1— T 1 H-T a c3 m m CO CD CO U] r^ Lh t4 (h t-< 3 •S Cj _S -2 ci ^ c3 o Q a a ^2; o "3 "3 "a "3 •3. o c! Suborder A ULU DON T A (contiiuRd). < o O o O O O < ^ Centrechinidae. 18 Pseudodimkma pseudodiadema (Lamarck). Jurassic, Trouvillc; Yonnc. (Text-fig.s. 81, 82.) 89* 11 16* 2 1 Magnosia punciala Quenstedt. Jurassic, Nattheim. (Text-fig. 83.) loo' 1 3 Cotlaldia granulosa (Munster). Cretaceous, Chamboy. 100 3 4 Phymechimis mirabilis (Agassiz). Jurassic, Europe. (Text-fig. 84.) 75 ;g5 S 1 7 Pediiia rolala Wright. Oolite, England. 100 7 4 Stomechinus granularis Agassiz. Jurassic, Europe. 100 4 28 Slo7>iechinus perlalus Desnioulins. Jurassic, Europe. 100 28 50 Stomechinus bigrauularis (Laniarcli). 88 2 2 8 Jurassic, Europe. (Text-figs. 85, 86.) 44 1 1 4 14 Polycyphus tiormannus Desor. Oolite, Europe. 93 7 13 1 37 Orthopsis niiliaris (d'Archiac). Cretaceous, Algiers; Europe. (Text-fig. 87.) 100 37 ' Cenlrechinus sctosus (Leske). Atlantic and Pacific Oceans, several localities. De- 23 5 9 55 5 3 ;2 veloping series, 8-40 mm. diam. (110 specimens). 25 5 10 60 5 3 2 1,278 (Text-figs. 88-92.) The same. Atlantic and Pacific Oceans, many localities. Devel- 0.2 0.2 0.; 1 57 /7 ^;2 S oped series, 40-108 mm. diam. (1,168 specimens). 2 2 1 ii 671 194 254 33 ■ (Text-figs. 93-95, 176.) 11 Echinoihrix calamaris (Leske). 5 91 Mauritius; Zanzibar; and loc. (?) 1 10 18 Echinoihrix diadema (Linnd). Mauritius; Hawaii; Bonin; Samoa. ;oo 18 ' Italic numerals represent percentages, Arabic numerals the number of specimens observed. 156 ROBERT TRACY JACKSON ON ECHINI. Table of Typical Ocular Plate Arrangement and Variation in the Centrechinoida (continued). i g "o u Xi a 3 Suborder A I/LODOATTA (continued). X 2 "3 o -H H-l h-T 1— 1 > > .s 1— ( "3 O O! I-H (— ( h- T 1— 1 > 1— T if .S > 2 o o ID hH hH ►-H h- T > 1— 1 tf '/.■ .S > o a> s§ 1— 1 l-H HH 1— 1 +3 > § o 1 1— 1 l-H 1— t ■*^ .s h— 1 HH k> »— ( > W "3 O o .s 'i O < O -i O S a > 3 o 5 1 69 11 100 2 70 7 73 19 81^ 13 > 65 40 2 92 93 se 50 6J 10 5S 7 162 19 3 * Italic numerals represent percentages, Arabic numerals the number of specimens observed. 158 ROBERT TRACY JACKSON ON ECHINI. Table of Typical Ocular Plate Arrangement and Variation in the Centrechinoida (continued). 0 E m 1 Suborder Sr/iJOZ)OA^rA (continued). £ O < 1-^ I— ( > > 1 a CO — o > 1— I 1— T .s > "3 O >> )— * % a > 1— 1 O ti O) tc tf 'fl a > > 1— T £ O 1 hH HH 1— I 1 I— I I— I > l-H > £ 3 o O 1 3 o 3 o o "o § bo c 10 Phyiuosomatidae . Phyinusonui dilii/imrei (Desor). Cretaceous, Algiers. (Text-fig. 106.) 100* 10* 10 Stomopneustidae . Stoinopneustes varluluris (Lamarck). Ceylon; Mauritius; Zanzibar. (Text-figs. 107-110.) 0 0 0 0 0 0 10 0 04 2 1 70 45 22 14 6 4 64 Arbaciidae. Arbacia sldlata Gray. Gulf of California; Panama. 0 1 0 45 14 0 0 4 78 95 74 H 132 87 1,832 76 174 87 58 9 7 1 3 100 2 0.1 2 3 8 7i 238 19 43 -4 3 30 23 2;? 55 1 141 ; lediterranean; Azores; Teneriffe. Arbacia punclulata (Lamarck). Woods Hole, Massachusetts (2,100 specimens). (Text-figs. 111-114, 200-205, 176.) The same. . Florida (229 specimens) . rhnn'yt. iliifrpfmi. Rlninvillp O.S 7 4 / 1 11 12 30 OJt 9 2 1 0.6 2,329 12 3 67 6 7 76 Cliili; Patagonia; Hawaiian Islands (?) Arbacia spatxdigera (Agassiz). Peru. Arbacia nigra (Molina). ChiU. (Text-figs. 167, 176.) Coelopleurus floridanus A. Agassiz. Cape Hatteras to Charleston, 63 fathoms. 53 25 57 139 5 13 246 10 2 19 2,939 2,282 2 373 53 176 0 0 53 * Italic uuiuerals represent percentages, Arabic numerals the number of specimens observed. OCULAR AND GENITAL PLATES. 159 Table of Typical Ocular Plate Arrangement and Variation in the Centrechinoida (continued). QQ g 1 & "o 1 Suborder CAMARODONTA. "3 O 1— 1 H-1 > > f ! .£ ! »— t ■3 0 Oculars V insert; I, IV, II, III exsert. t t— 1 > 1— 1 c « 0 1^ 1 % ! dJ 1 1— 1 1— 1 C > 1— T ■3 0 •2 i t— 1 1—1 > 1— T 2 1 0 a; 'n a 'Tl "3 0 -Q 0 a a; i c 7 Temnopleuridae. Glyphucyphus radialus Desor. Cretaceous, Europe. Trigonocidaris alhida A. Agassiz. West Indies. Temnopleunis hardwickii (Gray). Japan; Ceylon. Temnopleurus toreumaticus (Leske). China Seas; Japan; India. Temnopleurus reevesii Gray. Japan; Siam. Salmacis alexandri Bell. Australia. Salmaris bicolor Agassiz. Japan; Mauritius; Red Sea. Salmacis sphaeroides (LinnS). Australia; Philippines; and loc. (?) Mespilia globula (Linn6). Samoa; Japan; and loc. (?) Microcyphiis maculalus Agassiz. Mauritius; Japan, etc. Amblypneustes formosus Valentin. Australia; Tasmania. Amblypneustes griseus (Blainville). Australia. Amblypneustes pallidus (Lamarck). Australia. Amblypneustes ovum (Lamarck). Australia. Holopneusles purpurescens A. Agassiz. Australia. 7* 11 100 11 100 7 100 34 63 10 100 22 100 14 100 12 97 33 9S 136 100 100 12 100 10 97 37 97 32 7 34 19 47 9 22 14 12 34 3 1 139 1 1 g 2 12 10 38 3 1 33 3 1 400 378 11 1 1 0 0 7 2 ' Italic numerals represent percentages, Arabic numerals the number of specimens observed. 160 ROBERT TRACY JACKSON OX ECHINI. Table of Typical Ocular Plate Arrangement and Variation in the Centrechinoida (continued). B Suborder CAM A RODONTA (continued). o > > > O .s > i 605 10 200 129 299 10 127 100 200 16 14 Echinidae. Echinus affinis Mortensen. Off east coast of United States. 1,022-1,106 fathoms. (Text-figs. 115, 176.) Echinus elegans Diiben and Koien. Off Cape Remain and loc. (?) Echinus gracilis A. Agassiz. Off Carolina and Martha's Vineyard. Echinus esculentus Linne. Plymouth, England; Isle of Man; Mediterranean. (Text-figs. 116, 117, 176.) Echinus miliaris Milller. Norway; England; Isle of Man. Echinus microluberculalus Blainville. Mediterranean. Echinus melo Lamarck. Mediterranean; England. Echinus acutus Lamarck. Plymouth, England; Norway; Bergen; Mediterranean. Echinus angulosus Leske. Algoa Bay; Cape Colony; South Africa. (Text-fig. 176.) Echinus magellanicus Philippi. Patagonia; Straits of Magellan. Developing series, 2.5-5 mm. diam. (36 specimens). (Plate 3, fig. 14.) The same. Falkland Islands; Straits of Magellan. Developed series (164 specimens). (Text-figs. 165, 176.) Echinus margaritaceus Lamarck. Patagonia; Kerguelen Islands; Antarctic. Gymnechinus robillardi (Loriol). Mauritius. (Text-fig. 179.) Gymnechinus pulcheUus Mortensen. Singapore; Gulf of Siam. (Text-figs. 177, 178.) 100* 605* 1_(J0 8 100 10 97 195 9S 126 96 287 90 9 83 104 54 54 97 35 5 9 6 1 2 5 W 1 IS 23 SO 30 145 6 1 n 1 3 2 0.3 1 4 7 56 OM 1 e_ 1 1 2 6 1 S9 8 100 14 * Italic numerals represent percentages, Arabic numerals the number of specimens observed. OCULAR AND GENITAL PLATES. 161 Table of Typical Ocular Plate Arrangement and Variation in the Centrechinoida (continued). Eh 1 1 X CD 1 1 4) -*-3 l-H ^ l~J »— I c .a K o 1 Suborder CAMARODONTA (continue!). CO a o < > > CO _c l-H o t— t > 1— I a* > O o > -S 1 O Vj > > "a O CD en ^B HH > "3 o o (-1 GJ i 1 < •i s OS g o "3 a 60 o 2 < Echinidae (continued). 5 Toxop/ieimles maculalus (Lani.arek). Fiji Island.s; Mauritius. 20* 1* 50 4 38 Tnxopneusles pileolus (Lamarck). 8 a^' Acapulco; Panama; Mauritius. 3 35 25 Toxopneusk'S semUuherculalus Valentin. Galapagos Islands. 1 5e 24 1,043 Toxopneustes variegatus (Lamarck). 1 0.^ 90 5 0.^ 0.8 West Indies and Florida. (Text-figs. 170, 184.) 11 1 937 84 2 8 cToxopneusles allanlicus (A. Agassiz). 0.9 0.2 5; i6 o.s 2 Bermuda. Developing series, 35-45 mm. diam. 5 1 475 92 2 12 (587 specimens). The same. 0.1 0.5 0.; 67- 28 .f.g 2.5 2,643 ■ Developed series, 45-60 mm. diam. (1,403 speci- mens). (Text-figs. 118-122, 176.) 1 8 2 983 415 18 36 The same. 0.7 0..3 68 ;g9 0.7 1.8 . Developed series, 60-77 mm. diam. (593 specimens). 4 2 401 171 4 11 16 Tripneusles depressus A. Agassiz. 6 7.5 19 La Paz, California; and loc. (?) 1 12 3 72 Tripneusles variegatus (Leske). 7 ; 6S 5 11 4 Mauritius; Jeddah, Red Sea; Durb.an; Natal; Samoa; 5 1 49 6 8 3 Indo-Pacific; Lord Howe Island; H.awaiian Islands. Tripneusles esculenlus (Leske). 2 (J7 35 2 0.5 1 Bermuda. Developed series, 50-145 mm. di.am. (193 3 117 67 3 1 o specimens). (Text-fig.s. 123, 196.) The same. West Indies and Florida. Developing series, 19-50 .9 5 76 42 13 7 ^> 703 1 mm. diam. (55 specimens). The sa}ne. West Indies and Florida. Developed series, 50-127 2 3G 165 172 ^5 84 5 ff 27 mm. diam. (455 specimens). (Text-figs. 124-127, I 176.) 20 Ei'echinus chlorolicus (Valentin). New Zealand. 90 18 5 1 5 1 6,292 1,444 259 19 3,290 1,021 123 7 129 * Italic numerals repi-esent percentages, Arabic numerals the number of specimens observed. 162 ROBERT TRACY JACKSON ON ECHINI. Table of Typical Ocular Plate Arrangement and Variation in the Centrechinoida (continued). p. a 3 10 48 291 1,163 60 55 Suborder CAMARODONTA (continued). Slrongylocenlrolus alhu Patagonia. 39 12 56 179 120 24 2,063 Strongylocentrotidae. Pseudoboletia indiana (Michelin). Mauritius; Isle de la Reunion. Sphaerechinus ptdcherrimus A. Agassiz. Japan. Sphaerechimis granulans (Lamarck). Mediterranean; Azores. Stro7igylocentrotiis liiridus (Lamarck). Mediterranean; Fayal; England. (Text-figs. 128-130, 176.) (Molina). Strongylocentrotus fragilis sp. nov. Catalina Islands, California; off California and Ore- gon coasts. (Text-fig. 176.) Slrongylocenlrolus mexicanus A. Agassiz. Cape St. Lucas. Slrongylocenlrolus tuberculalus (Lamarck). Japan; Sidney, Australia. Slrongylocenlrolus depressus A. Agassiz. Japan and loc. (?) Slrongylocenlrolus drbhachiensis (O. F. Miillcr). See tabulation of 27, 417 specimens exclusi\'e of devel- oping series, p. 143. The percentages only are given here. (Text-figs. 135-149.) Slrongylocenlrolus eurylhrogrammus (Valentin). Australia. Slrongylocenlrolus franciscanus A. Agassiz. California; Washington. (Text-figs. 151-153, 106.) Slrongylocenlrolus purpuralus (Stimpson). California; Puget Sound. (Text-fig. 176.) Slrongylocenlrolus gibbosus (Valentin). Payta, Peru. (Text-figs. 154-157.) 876 53 32 13 0.004 03 "3 O 1 2 IL 195 2 1 29 16 915 220 26 S 3 17 1 > C3 "3 O 0.04 I 100* 10* 98 47 59 258 50 15 56 31 S3 5 92 36 100 12 94 93 52 S7 156 77 93 31 765 c > 3 o > > o o 3 1 3 n 20 22 26 i 1 56 0.03 17 4 C3 9^ 25 15 IS 1.2 79 19 69 * Italic numerals represent percentages, Arabic numerals the number of specimens observed. OCULAR AND GENITAL PLATES. 163 Table of Typical Ocular Plate Arrangement and Variation in the Centrechinoida (continued). 03 § - S P. s 3 2; Suborder CAMARODONTA (continued). 03 u o 1 HH > > a> CO -S t— 1 t-, 3 o H- 1 t—t 1— 1 f > o O t-i 1— 1 1— 1 > C ;-< C3 o 1 h- ( i > o o 1 cJ h- 1 t— 1 H-t .^ .9 1— < > > "3 o o CO G 1 o o < a o3 b« -i Mi t~i "3 o o "o c a> S 0) bD G t-i 44 Echinometridae . Echinontdra ohlonga Blainville. Samoa; Hawaiian Islands; South Africa. Echinometra viridis A. Agassi z. West Indies. Echinometra malhaei Blainville. Red Sea; Durban; Natal; Mauritius; Bonin; Hawai- ian Islands. (Text-fig. 176.) Echinometra lucunter (Linn6). West Indies and Florida (578 specimens). (Text- ■ figs. 158-161, 176.) The same. Bermuda (176 specimens). (Text-fig. 176.) Echinometra van brunti A. Agassiz. Lower California; La Union; South America. (Text- fig. 176.) Heterocentrotus mammillatus (Leske). Mauritius; Indian Ocean; Bonin Island; and loc. (?) Heterocentrotus trigonarius (Lamarck). Hawaiian Islands; Mauritius; Tahiti; Palmyra Is- land. Colobocentrotus atralus (Linne). Hawaiian Islands; Mauritius; Zanzibar; and loc. (?) 98* 43* 96 24 79 126 IT 97 9 15 // 8 80 28 GO 28 99 81 o.s 2 / 2 4 3 S 1 4 2 1 / 1 19 31 57 332 46 81 /^ 9 // 4 ;25 13 1 25 159 1 25 144 76 72 55 S 1 5 4 1 754 o.s 2 0.ff 1 ; 1 1 1 0.6 76 1 35 3 47 1 82 1,222 450 10 473 281 4 0 0 4 ' Italic inunerals represent percentages, Arabic munerals the number of specimens observed. 164 ROBERT TRACY JACKSON ON ECHINI. Table of Aberrant Ocular Plate Arrangement in 4S,541 Specimens of the Centrechinoida} DO 1 1 "S B 3 en g 1 a m V HH ^H > 1— ( 1 > u ■i b. ! S! £ > > .£ t— < ^—^ m s u O % X > fi a 3 O o % X > bl en .£ > o 0) 1—4 > > b< _c )— 1 2 "3 O 1—1 I— ( > »— » > i" .s 1— t 1—1 2 ta "3 £ 1— ( t— ( »— 1 )— c > > CO (-< 1 o 4^ (-> X > bi 0) en -S 1— 1 t>" t s "3 o O t-i > 1— < 1—1 > tn 1 <5 en > tn C > 1— 1 tn t> _S 3 o 1 ID 1— 1 > )— 1 (-> _c > m t^ _2 "3 o O b> X > > JS "B tj O •4-J bi X t- OJ .s > > CO "3 o OJ 0^ 1— 1 > > cfi *3 O o t-, OJ 4; tn C > O 58 Hemicidaris crenularis (Lamarck). Centrechiniis selosus (Leske). Acrosalenia heniicidaroides Wright. Acrosalenia pustulata Forbe.s. Shmopneustes variolaris (Lamarck). Arbacia lixula (Linne). Arbacia punclulata (Lamarck). Arbacia dufresni Blainville. Arbacia spatuligera Agassiz. Arbacia nigra (Molina). Microcyphus maculalus Agassiz. Echinus microluberculatus Blainville. Echinus angulosus Leske. Echitius magellanicus Philippi. Echinus margarilaceus Lamarck. Gymnechinus robillardi (Loriol). Gymnechinus ptdchellus Mortensen. Toiopneusles variegatus (Lamarck). Toxopneusles atlanticus (A. Agassiz). Tripneusles variegatus (Leske). Tripneusles esculentus (Leske). Evechinus chloroticus (Valentin). Sphaerechinus granulans (Lamarck). Strongytocentrotus Hindus (Lamarck). Strongylocentrotus albus (Molina). Slrougyloceidrolus luberculalus (Lamarck). Strong yloceidrotus drobachiensis (0. F. Miiller) Strongylocentrotus franeiscaniis A. Agassiz. Strongylocenlrolus gibbosus (Valentin). Echinometra mathaei Blainville. Echinometra lucunler (Linn6). HeleroccHlrolus mammillalus (Leske). 2 3 7 6 6 1 0.6 7 13 8 1 1 1 1 6 89 100 0.8 2 4 4 5 9 1 13 3 1.2 1 79 1 0.3 3 1 1 278 1 1 1 32 58 1 1 2 16 64 1 1 2 2 3 141 . . 2 329 1 1 14 3 10 13 67 76 246 1 1 4 139 1 299 2 1 100 200 1 16 1 9 8 14 14 1,043 2 643 2 1 1 18 1 1 5 25 13 1 72 2 703 1 1 20 2 1 20 291 2 23 1,163 5 4 2 3 60 4 2 1 39 1 81 33,000 179 3 2 29 33 34 1 215 1 2 •> 24 5 8 0 159 1 1 1 63 754 1 35 3 8 4 300 1 11 17 Total number of aberrants 704 = 10 97 1 56 82 4 47 'This table includes all aberrant variants of the Centrechinoida listed in the tables, pp. 142, 143, 154 to 163. See text- figs. 110, p. 114; 140-149, p. 134; 155, 156, p. 145; 177-179, p. 165; 182, p. 165; 196, p. 169. SPECIAL CHARACTERS OF GENITAL PLATES. 165 Special Characters of Genital Plates. Certain characters of genital plates are here taken up aside from theii- relations to the oculars, that have just been considered. The characters in genitals are subject to a great deal of variation. While the bearing of these variations is often obscure, for the most part they follow very definite lines. In my studies of post-Palaeozoic fossil and Recent Echini, over Text-figs. 177-184. — Showing genital plates excluded from the periproct. 177. Gymrwchinus pulchellus Mortensen. Gulf of Siam. Diam. 15 mm. R. T. J. Coll., 863. X 8.2. Genital 3 exsert by the contact of genitals 2 and 4. Oculars I, II insert, the species character (pp. 93, 120). 178. The same. Diam. 10 mm. R. T. J. Coll., 864. X 8.7. Genital 4 ex.sert by the contact of genitals 3 and 5. 179. Gymnechinus robillardi {Lonoi). Mauritius? Diam. 26 mm. R. T. J. Coll., 811. X 4.5. Genital 3 exsert. Oculars I, II insert, the species character (p. 120). 180. Strongylocenlrotus drobachierisis {O.F.MulleT). YorkHaiboT, Maine. Diam. 44 mm. R. T. J. Coll., 782. X 2.81. Genital 3 exsert by the contact of genitals 2 and 4, as in text-figs. 177, 179, 184. 181. The same. Chelsea Beach, Massachusetts. Diam. 3 mm. R. T. J. Coll., 880. X 28. Genital 4 exsert by the contact of genitals 3 and 5 as in text-fig. 178. No genital pores (compare text-fig. 133, p. 129). 182. The same. Dumpling Island, North Haven, Maine. Diam. 50 mm. R. T. J. Coll., 866. X 2.7. Genital 4 exsert by the contact of genital 5 and ocular IV (compare text-fig. 144, p. 134). 183. The same. Calderwood Island, Maine. Diam. 32 mm. R. T. J. Coll., 865. X 2. 8. Genital 4 exsert by the contact of genital 3 and ocular V. 184. Toxopneastes varicgatus (Lamarck). Boca Ceiga Bay, Florida. Diam. 58 mm, R. T, J, Coll., 872. X 3.7, Genital 3 exsert (p. 121). 166 ROBERT TRACY JACKSON ON ECHINI. 250,000 genital plates have been examined, and the principal striking variations noted. In the 33,000 specimens of Strongylocentrotus drobachiensis studied, detailed lists of the variations of genital plates were kept as a basis for determining the numerical frequency of occurrence. Under ocular plates it is shown that there is great diversity as to whether these meet the periproct or are shut out from it. On the other hand, genital plates show great uniformity in that all the plates meet the periproct. No exception to this is known in fossil regular Echini; in Recent Echini, however, some exceptions have been found as individual variations. Dr. Mortensen (1904, p. 114) says of Gymnechinus pulchellus that sometimes one of the genitals, mostly 3, is excluded from the periproct. He writes me that in about thirty percent of the specimens genital 3 and more rarely 4 is exsert. In ten selected specimens of G. pulchellus that Dr. Mortensen generously sent me, four have genital 3 exsert (text-fig. 177), two have genital 4 exsert (text-fig. 178), and four have all genital plates reaching the periproct. In a specimen of Gymnechinus robillardi (text-fig. 179) genital 3 is exsert. De Loriol (1883) figures the same species with genital 4 exsert. In his collection I saw two specimens with genital 4 exsert, as in text-fig. 178. It is striking that this very exceptional character is common in two related species. Specimens with exsert genitals have been found in a few other species. One specimen in 139 Microcyphus maculaius (this is in de Loriol's collection in Geneva), one in 1,043 Toxopneustes varicgatus (text-fig. 184), and two in 2,643 Toxopneusies atlanticus have genital 3 exsert. In 33,000 Strong ylocentrotus drobachiensis 37 specimens have genital 3 exsert (text- fig. 180). In all the cases seen of genital 3 exsert in the several species, this plate was similarly excluded by the contact of genitals 2 and 4 as figured. In S. drobachiensis eight specimens have genital 4 exsert. This, however, is not excluded by the same plates in all cases. In a very young individual (text-fig. 181) genital 4 is excluded by the contact of genitals 3 and 5, as in Gymnechinus pulchellus (text-fig. 178). Another method of exclusion of genital 4 is by the contact of genital 5 and ocular IV (text-fig. 182) ; and in a third method, which may be considered more normal for the species, genital 4 is excluded by the contact of ocular V and genital 3 (text-fig. 183). In the Museum of Comparative Zoology, a Microcyphus maculatus from Mauritius has genital 4 excluded by the contact of ocular V and genital 3, as in text-fig. 183; also a Heterocentrotus trigonarius from the same locality has genital 3 excluded by the contact of genitals 2 and 4, as in text-fig. 184; and a Microcyphus annulatus Mortensen, from Bass Straits, Australia, has genital 1 excluded from the periproct by the contact of genitals 5 and 2. The oculars of this specimen are all exsert, as usual in the genus. In all, 61 cases have been seen in 50,000 specimens with a genital excluded from the peri- proct, 47 of the cases being of genital 3, thirteen of genital 4, and one case of genital 1. Why genital 3 or 4 should be excluded and almost no others is not obvious, Init it shows how definite even a very rare variation can be. While genitals are rarely excluded from the periproct as variants, they are still more con- SPECIAL CHARACTERS OF GENITAL PLATES. 167 stantly in contact with the interambulacrum ventrally. In Bothriocidaris archaica (Plate 1, fig. 2) the genitals Ue wholly dorsal to the oculars, and in B. pahleni (Plate I, fig. 6) they do so in part. In one specimen of Strongylocentrotus drdbachiensis (Plate 5, fig. 16) genital 1 lies dorsal to the oculars, as in Bothriocidaris archaica; but in this peculiar specimen interambula- crum 1 is wanting (p. 42), which accounts for this exceptional genital exclusion. Otherwise, as far as seen, a genital, when existent, always extends to the interambulacrum of its area. Genital plates are of secondary importance. One may be absent without visibly affecting the corona (spatangoids, text-figs. 174, 175, p. 149). Rarely a genital may be wanting in regular Echini (p. 45), when the other portions of the test have the usual pentamerous system. Such a case is seen in Eucidaris (text-fig. 185). In this specimen there are five oculars, I and II being in contact from the absence of genital 1. In interambulacrum 1 there are two young plates lying against oculars I and II ; the next older plate belonging to column 2 is large and extends across the area, whereas the third older plate is small. Otherwise the corona is quite as usual. In two specimens of Ai'bacia (Plate 4, figs. 11, 12) and in a Strongylocentrotus (Plate 6, fig. 7), genital 4 is want- ing and interambulacrum 4 is narrowed, consisting in part of only a single column of plates. In Tripneustes csculcntus (Plate 6, fig. 4) there are six oculars, but no additional genital, so that the extra genital may theoretically be considered as wanting, and there is only a single column of interambulacral plates through- out the added area as far as developed. In all of these specimens in the area where the genital is wanting, the interambulacrum dorsally abuts against the two over- lying ocular plates in a similar manner to that of Bothriocidaris and the posterior area in spatangoids (text-figs. 162, 174, 175 p. 149). Genital and ocular plates are fused in a mass in clypeastroids, and two genitals may be fused in spatangoids (A. Agassiz, 1904, p. 161), but in regular Echini these plates are typically separate. Occasionally, however, in regular Echini two genitals or a genital and an ocular are fused. In a specimen of Centrechinus setosus (text-fig. 186) genitals 2, 3 and ocular III are fused into a perfectly symmetrical mass, no trace of sutures being visible. A similar fusion of two genitals, or a genital and an ocular, has been seen in many cases in the Echinidac and Strongylocentrotidae. In Strongylocentrotus drdbachiensis genitals 2, 3 were fused in 36 cases; 3, 4 in 32 cases (text-fig. 195) ; 4, 5 in 68 cases (text-fig. 147, p. 134) ; and 5, 1 were fused in 19 cases (text-fig. 148, p. 134). Genitals 1, 2 were fused in only five cases. The fusion of genitals 185 Text-fig. 185. — Euciilaris Iribidoides (La- marck). Jamaica. Diam. 34 mm. R. T. J. Coll., 802. Genital 1 wanting. Young interam- bulacral plates originate against ocular.s I and II, as usual (p. 45). 168 ROBERT TRACY JACKSON ON ECHINI. was seen in other species also, as Tripneustes esculentus (text-fig. 196). In 103 cases of S. drobachiensis an ocular was fused with a genital (text-fig. 143, p. 134), any ocular of the five fusing with one or rarely two of the associated genitals. As far as the fusion of genitals, or genitals with an ocular goes, it may be considered a parallel variation to the typical condition in certain irregular Echini, where such fusion normally occurs. While fusion of genitals is rare, splitting on lines of solution to make divided plates is more common. This splitting of genitals by secondary sutures is a typical character in the Echino- thuriidae only (Asthenosoma, text-fig. 101, p. 109, and Phormosoma, text-fig. 170, p. 149). Such splitting is shown fully by Mr. Agassiz (1904) in many of this family. The splitting is t86 Text-fk:s. 1S6-1S9. — Fusion, or siilitting of genital i)Iatcs in Coilrechinus seiosus (Lcsko). 186. .Jamaica. Diam. 49 mm. R. T. J. Coll., 803. X 2.8. Genitals 2, 3 and ocular III arc fused in a mass; oculars I, V, IV, II insert, bilaterally symmetrical through III, 5. 187. Bahamas. Diam. 85 mm. R. T. J. Coll., 714. X 2.8. Genitals .spht by secondary sutures. 188. Bahamas. Diam. 85 mm. R. T. J. Coll., 689. X 2.8. Madreporic pores in genitals 2, 3, .and ocular III. 189. Jamaica. Diam. 71 mm. R. T. J. Coll., 772. X 2.8. Madreporite split by secondary sutures. especially marked on the ventral border of genital plates or may pass directly through the plate. I have seen no splitting of genitals in fossil Echini, but it is rather common as a variant in Recent species. Such secondary division of genitals is seen in Centreehinus (text-figs. 187, 189). In Sirongylocentrotus drobachiensis many cases of split genitals were found. In 275 cases genital 3 is spht by a transverse suture parallel to the base, as in text-fig. 190. This seems to be a family peculiarity, as it was observed in several species of the Echinidae as in Tripneustes esculentus (Plate 6, fig. 4), and in the Strongylocentrotidae. Instead of splitting by one trans- verse suture, as in text-fig. 190, genital 3 may be divided by one or more sutures in other planes, as in text-fig. 190a and 19t)b; such were noted in 90 cases. Genital 3 is split much more SPECIAL CHARACTERS OF GENITAL PLATES. 1G9 Text-figs. 190-196. — Splitting, fu.sion, etc., of genital and ocular plates, in iStrongyloccntrotu.s and Trii)iu'ustos (p. 28) . 190. Slrongyloccntrotus drdbachiensis (O. F. Mtillei^. York Harbor, Maine. Diam. 54 mm. R. T. J. Coll., 773. X 3. Genital 3 split by one horizontal suture (compare Plate 6, fig. 4). 190a. The same. Diam. 38 mm. R. T. J. Coll., 774. X 3. Genital 3 split by one vertical suture. 190b. The same. Diam. 49 mm. R. T. J. Coll., 775. X 3. Genital 3 split by three sutures. 191. The same. Calderwood Island, Maine. Diam. 46 mm. R. T. J. Coll., 879. X 3. Genital 2 spht. 192 sutures. 193 The same. Harpswell, Maine. Diam. 43 mm. R. T. J. Coll., 776. X 3. Several genitals split by secondary The same. York Harbor, Maine. Diam. 46 mm. R. T. J. Coll., 711. X 4. Genitals 2, 3, and oculars HI, V are split by secondary sutures; the splitting of oculars is a very rare variation (Plate 6, fig. 6). 194. The same. Diam. 42 mm. R. T. J. Coll., 700. X 4. Genitals 1 and 2 much split up; ocular V split; extra genital pores, but there is no genital pore in the madreporite (compare text-figs. 197- 199, p. 171). 195. Thesame. Diam. 41 mm. R.T.J. Coll., 777. X 3. Genitals3, 4fused (comparePlate6, fig. 2; Plate7, fig. 7). 196. Tripneustes esculenlus (Leske). Bermuda. Diam. 138 mm. R. T. J. Coll., 787. X 2. Genitals 5, 1 fused; oculars V, IV, II insert (pp. 93, 124). 170 ROBERT TRACY JACKSON ON ECHINI. frequently than an}^ other plate in the families of the Echinidae and Strongylocentrotidae. Genital 2 is also frequently divided in .S'. drobachiensis by secondary sutures (text-figs. 191- 194.) Divisions of this plate were found in 120 cases in this species. Genitals 1, 4, and 5 are occasionally divided by secondary sutures, as seen in 134 cases, but 2 and 3 take the lead by a strong majority. Very rarely oculars are split (text-figs. 193, 194), but only ten cases were noted; otherwise, genitals divided by secondary sutures are the only cases I have seen in Echini of plates derived by the fission of preexisting plates. In the young of echinothuriids, genitals are not split, and in Strongylocentrotus the splitting was very rare in young specimens, wliile rather frequent in adults. It is evident, as in echinothuriids, that this is a character that comes in with increasing age. Occasionally a line of secondary suture only goes partially through a plate, demonstrating that these are true cases of splitting and not cases of accessory plates. In very young Echini genital pores do not exist, as shown fully by Loven (1892) and A. Agassiz (1904). As seen in Strongylocentrotus drobachiensis (p. 131, text-figs. 131-134), no genital pores were found in specimens up to 5 mm. in diameter, after which age they come in rapidly, and only 128 in 800 were without such pores in the series 5 to 10 mm. diameter; only six in 2,000 were without genital pores in the series 10 to 15 mm. diameter, and no larger specimen was wholly imperforate. Occasionally, one, more rarely two, and in three cases three genitals were imperforate in adults. In a fresh Toxopneusies atlaniicus examined, in which a genital was imperforate, the corresponding genital gland was absent. The absence of a genital pore as a variant may be compared with Holectypus (text-fig. 171, p. 149), and some spatangoids (A. Agassiz, 1904) in which one or more genitals are typically imperforate. Typically, in post-Palaeozoic regular Echini there is a single genital pore within the con- fines of each genital plate. In Goniocidaris nutrix and canaliculata (text-figs. 72-74, p. 99) the ventral border of the genitals is open, doubtless resorbed so that the genital pores impinge on the interambulacra. This character has been seen as a variant in Centrechinus setosus, Arbacia nigra, and Strongylocentrotus drobachiensis, but is of uncommon occurrence. Mr. Agassiz (1873) shows that in the clypeastroid Peronella peronii (Agassiz) the genital pores lie in the interambulacra widely removed from their usual position. In an Arbacia punctulata in one area the genital pore is not in its usual plate, but passes through the third interambula- cral plate from the apical disc. In Strongylocentrotus the genital pores very rarely perforate an interambulacral plate, and in four cases found perforated an ocular plate. While genital pores are usually found in genital plates, it seems that there is no necessary correlation, and the pore when developed may perforate any portion of the test. Genital pores are usually perfectly visible in dorsal view, are even salient features, but in Salenia pattersoni they are invisible externally though plainly seen in an internal view of the same specimen (Plate 4, figs. 1,2). Wliile a single genital pore to a plate is the character in Recent regular Echini, supplementary pores are not rare. They are especially common in Arbacia SPECIAL CHARACTERS OF GENITi^L PLATES. 171 punclulnla (text-figs. 197, 198), in which I previously described them (Jackson, 1899, p. 130). Simikir extra genital pores have been seen in Eucidaris tribuloides, Echinus affinis (text-fig. 115), Strongylocentrotus franciscanus (text-fig. 199), ;S'. drohachiensis (text-fig. 193), and many other species. In S. drobachiensis extra pores were seen in 186 specimens. Dr. Mortensen writes me that such extra pores are connected with the genital glands. They are always sporadic Text-figs. 197-199.— Extra pores in genital plates. 197. Arbacia pimclulala (Lamarck). Woods Hole, Massachusetts. Diani. '6-1 mm. oculars exsert. 198. The .same. Diam. 35 mm. R. T. J. Coll., 655. X 3. Ocular V insert (p. 115). 199. SlrongyloceiUrolus franciscanus A. Agassiz. California. Diam. 123 mm. 11. T. J. Coll., 72(j. X 2. V insert (compare text-fig. 115, p. 117). R. T. J. Coll., 656. X3. All Oculars I, and a]:)parently may be considered a parallel variation to, rather than a genetic connection with, the Palaeozoic types where extra pores typically occur. In the Ordovician Bothriocidaris (Plate 1, fig. 2) genital pores are unknown; it may be that they were wanting, as such pores are wanting in young Echini. More likely they existed but do not show in external view, as noted in Salcnia pattersoni (p. 112). In an ancient fossil it would be easy for small genital and madreporic pores to be so filled up as not to be recognizable, for frequently in fossils pores cannot be seen when we know from other specimens that such existed. In Lepidechinus (Plate 63, figs. 7, 8) there is one pore in each genital plate in the only species of the genus in which the apical disc is known. In other Palaeozoic Echini genital plates typically have more than one pore to a plate. There may be two or three, as in Lepidesthes (Plate 68, fig. 5), or there may be three to five in a plate, as in Palaeechinus (Plate 29, fig. 6; Plate 30, fig. 4), Lovenechinus (Plate 41, fig. 3), and Melonechinus (Plate 56, fig. 6). Instead of a few pores there may be numerous genital pores to a plate, even as many as ten or eleven, as shown in Lepidocentrus (Plate 21, fig. 5), Pholidechinus (Plate 28, fig. 10), and Perischocidaris (Plate 67, fig. 3). It is po.ssible that in types where fine madreporic pores are unknown, some of the larger pores served as madreporic 172 ROBERT TRACY .TAf'KSOX OX ErUIXl. openings. Otherwise all the pores in genital plates doubtless connected with genital glands, as in Recent Echini with accessory pores. In very young Recent Echini, as shown by Loven (1892), there is a single madreporic opening in genital 2 (Plate 2, fig. 3; Plate 3, fig. 14; text-fig. 131, p. 129). Fine madreporic pores soon appear, and in the adult are usually numerous (Strongylocentrotus, text-figs. 132- 139, p. 132). In some types there are few madreporic pores in the adult, a primitive character (Salenia, Plate 4, fig. 1). Usually the madreporite is perforated by many pores, as seen from the interior as well as from the exterior. In the Cidaridae, however (text-figs. 59, 60; 70, 71, pp. 95, 98), while there are many on the exterior of the plate, there is only a single large madreporic pore on the interior, so that at this part of the plate the youthful character is retained in the adult. The same condition exists in Salenia -patter soni (Plate 4, figs. 1,2). In Echinocyamus (Loven, 1874) there is a single external madreporic pore. In Hahrocidaris scutata (text-fig. 206), as shown by Messrs. A. Agassiz and Clark (1908), no fine madreporic pores exist in genital 2, but, instead, there is a single pore in the middle of the plate, which appears to be a retention in the adult of the single madreporic opening characteristic of the young. This is the only case known in an adult regular echinoid. In Palaeozoic Echini a typical madreporite with numerous fine pores is known in a few species. It exists in Lovenechinus lacazei, as observed by Bather (text-fig. 243) ; also, it has been found in Lepidesthes formosa (Plate 68, fig. 5), L. colletti (Plate 71, fig. 1), Meekechinus elegans (Plate 76, fig. 6), and Echinocystites pomum (Plate 18, fig. 6). In a number of Palaeo- zoic genera the apical disc is not known well enough to enable one to state anything with assur- ance. In the Palaeechinidae the apical disc is known in many species and yet the madreporite has been seen in Lovenechinus only, as above noted, and reported in Melonechinus multiporus by Keyes (1894). I have not seen a specimen of Melonechinus with madreporic pores. In post-Palaeozoic regular Echini the madreporic pores exist in genital 2, and are typically limited to that plate. In examining large series of specimens, it is frequentlj' found that the madreporic pores extend beyond genital 2. When this occurs, in the great majority of such variants, they extend to genital 3 and frequently ocular III as well (Centrechinus, text-fig. 188) ; or they may extend to ocular III without invading genital 3 (text-fig. 146, p. 134). This extension of madreporic pores has been seen in many species, but in Strongylocentrotus dro- bachiensis, on account of the number examined, a greater range is known than in others studied. Of this species in 33,000 specimens, 928 cases were noted in which madreporic pores extended from genital 2 to 3 (Plate 5, fig. 13). In 128 cases madreporic pores occur in genital 2 and ocular III (text-fig. 146, p. 134), and in 64 cases in genitals 2, 3 and ocular III, as in Centre- chinus (text-fig. 188). In 80 cases, madreporic pores are in genitals 1, 2, 3, and in 60 cases in genitals 2 and 1 (text-fig. 142, p. 134). Other variations in the extension of madreporic pores beyond genital 2 are only rarely found. In 15 cases, they exist in genital 2 and ocular II; in THE PERIPROCT. • 173 six cases in 2, III, II; in three cases in 2, 3, III, and II; in four cases in 2, 3, II (text-fig. 148, p. 134). In seven cases madreporic pores occur in genitals 1, 2, 3, and ocular III; in four in 1, 2, 3, and II; and in eleven in 1, 2, 3, and II, III (Plate 5, fig. 14). The most extreme range observed was one specimen in which the madreporic pores extended from genital 2 to genitals 1, 3, 4, and oculars III, IV, V. A similar range of variation in the distributipn of madreporic pores and in about the same proportions has been found in other species, especially in the families Echinidae and Strongylocentrotidae. In the young, madreporic pores very rarely extend beyond genital 2, whereas they frequently do in the adult. This change evidently is taken on as the animal increases in size to the adult condition. Occasionally madreporic pores invade interambulacrum 2, and in one somewhat distorted Strongylocentrotus drobachiensis this invasion was extensive. All this goes to show that while typically madreporic pores are limited to genital 2, they may extend from this plate to adjacent genitals, oculars, or interambulacrum, but apparently never to plates of the periproct. They seem never to be absent from genital 2 in Recent regular Echini. When madreporic pores extend beyond genital 2, in the great proportion of cases they move to the left of the antero- posterior axis, emphasizing by this a bilateral axis or plane of symmetry through III, 5. As seen from these studies, the genital plates have nothing to do with the interambulacrum, which develops on either side of the oculars (p. 62). The genitals typically possess genital pores, and one of them possesses madreporic pores, but both of these structures may pierce other parts of the test. Genital plates may, therefore, be considered as structures of secondary importance, of much less morphological value than are the oculars. The Periproct. The periproct is an area of considerable interest. Unfortunately in fossils it is rarely preserved, as the plates easily drop out and are frequently wanting, even in museum specimens of Recent Echini. It is worth noting that in Recent Echini periprocts and peristomes may be rendered strong and safely fastened in place by dipping the specimens in dilute shellac dissolved in alcohol, or gelatine dissolved in water. The same treatment is very eiTective in strength- ening the thin tests of delicate Echini ami in keeping the spines firmly in place. In Palaeozoic Echini the plates of the periproct are small, angular, and completely fill the area, as in cidarids. In Bothriocidaris archaica there are nine small plates that fill the area dorsal to the genitals (Plate 1, fig. 2). In Hyatlechinus beecheri (Plate 25, fig. 5) a few peri- proctal plates are preserved (others restored in the figure), indicating a periproct essentially as in Cidaris. In Palaeechinus (Plate 31, fig. 4), Maccoya (Plate 34, fig. 6), Lovenechinus (Plate 42, fig. 6), and Melonechinus (Plate 56, fig. 1) the periprocts as far as preserved are made up of many polygonal, rather thick plates filling the area, that are essentially like those of 1 74 ROBERT TRACY JACKSON ON ECHINI. Eucidaris tribuloides in structure (text-figs. G2-69, p. 98). In the Lepidesthidae, the peri- proctal plates in the known types are essentially the same, like cidarids, but are rather thinner and more scale-like; such are shown in Lepidesthes formosa (Plate 68, fig. 5), L. coUetti (Plate 71, fig. 1), and Meekechinus elegans (Plate 76, fig. 6). Palaeozoic types as far as known do not throw any light on the suranal plate, which is such a prominent feature in the young of some Recent Echini and the adults of certain genera. There is no evidence that a suranal plate was differentiated in the Palaeozoic, and, as it is apparently wanting in the Cidaroida and Aulodonta, both young and adult, it is also doubtless wanting in the Palaeozoic. In the Cidaroida, periproctal plates are thick and angular, filling the area, with the anus central. They are essentially like those known in the Palaeozoic and in so far may be con- sidered primitive. These plates are shown in representative cidarids in text-figs. 59-74, 164 (pp. 94-99, 149). Loven (1892) figures a young Goniocidaris with a single plate filling the periproct (Plate 2, fig. 3); but Mortensen (1911) thinks this a mistake, as in a specimen of a similar stage he found three plates. Mr. Agassiz's (1904) figures of young cidarids, and those young specimens which I have studied, show no indication of a dominance of a first, over later added plates. It seems, then, that we have here no suranal such as exists in the Saleniidae and in young Echinidae and Strongylocentrotidae, in which groups alone has it been demon- strated. The existence of a suranal plate is apparently a secondary and specialized rather than a primitive character. Dr. Mortensen wrote me in effect that he concurs in this view, and he expresses the same in a recently published paper (1911). In the Centrechinoida the character of periproctal plates presents wide differences in various genera. Of the Aulodonta there are large plates on the periphery of the periproct with small isolated plates within, as seen in Chaetodiadema, Centrostephanus, Astropyga (text-figs, 96-99, pp. 108, 109), or young Centrechinus (text-fig. 88); or there may be small, isolated plates only, with tissue largely leathery, as in adult Centrechinus (text-figs. 93-95, p. 107). Of the Stirodonta in Salenia and Salenocidaris (Plate 4, figs. 1, 6; text-fig. 102, p. Ill) we find a prominent suranal plate lying dorsal to genital 3 and with small anal plates in addition. In the fossil Peltastes (Plate 4, fig. 7) the suranal occupies a prominent place but lies dorsal to ocular III, thus differing from Salenia and young Strongylocentrotus and Echinus, in which the suranal lies dorsal to genital 3. In a series of 74 specimens of Peltastes wrighti one speci- men has two large plates, as in Acrosalenia hemicidaroides (text-fig. 104, p. Ill), and as in that figure the larger plate, which may be considered the suranal, lies against genital 3, as in Salenia. This is a striking difference from the other specimens, in which there is only a single plate lying dorsal to ocular III, as shown in Plate 4, fig. 7, which is the generic character. The fact that the suranal may occupy one of two positions has a certain bearing on other genera. In Acrosalenia spinosa (text-fig. 103, p. Ill) the suranal lies dorsal to ocular III, as in Peltastes, but in A. hemicidiiroides (text-fig. 104, p. Ill) there are two large plates; the larger THE PERIPROCT. 175 of these two, which appears to be the suranal, hes dorsal to genital 3, as in Salenia. In Acro- salenia wiltoni (text-fig. 105, p. Ill) there are four large periproctal plates besides a vacant space, doubtless for small anal plates as in Salenia. The largest of these four is probably the suranal and lies dorsal to ocular III, as in Acrosalcnin spinosa and Peltastes. In Arbacia there are typically four plates in the periproct, l)ut as variants there may be fewer or more than this number. Bell (1879) notes that there may be as few as three or as many as ten, and Mr. Agassiz (1881, p. 57) notes that there may be more than four up to thir- teen periproctal plates. I noted (1899, p. 131) that in Arbacia punctulata there may be as few as three or as many as nine plates as variants from the typical four. In Arbacia the typical four periproctal plates lie in such planes that two are in the antero-posterior axis III, 5, and two are on each side of this axis (text-figs. 111-114, p. 115; text-figs. 107, p. 149; 202). The Y 204 205 205a 206 Text-pigs. 200-20G. — Variation of periproctal plates in Arbacia with the tj'pical character in Habrocidaris. 200. Arbacia punclulala (Lamarck). Woods Hole, Massachusetts. Diam. 25 mm. R. T. J. Coll., 727 Only two periproctal plates; suranal dorsal to genital 3. 201. The same. Diam. 45 mm. R. T. J. Coll., 728. Diam. 47 mm. R. T. J. Coll., 729. Diam. 31 mm. R. T. J. Coll., 730. Diam. 36 mm. R. T. J. Coll., 731. Diam. 43 mm. R. T. J. Coll., 732. X3. 202. The same. X 3. Only three periproctal plates. X 3. Four periproctal plates, the typical character (p. 1 1."").) X 3. Five periproctal plates; compare text-fig. 206. X 3. Six periproctal plates. X 3. Many periproctal plates. Florida. Diam. 32 mm. R. T. J. Coll., 890. X 4. One large and about fifty small periproctal plates. The specimen is so distorted its ocular arrangement is omitted in tabulation of the species, pp. 115, 153. 206. Habrocidaris scutata (A. Agassiz). Santa Cruz, West Indies. Diam. 17.5 mm. Adapted from A. Agassiz and Clark, 1908, Plate 54, fig. 5. X 3. Five periproctal plates; compare text-fig. 203. There is only a single madreporic pore in genital 2 (compare Plate 3, fig. 14). The genital pores lie very far tlorsally (p. 172). 203. 204. 205. 205a. The same. The same. The same. The same. 170 ROBERT TRACY JACKSON ON ECHINI. plate lying dorsal to ocular III is apparently the equivalent of the suranal, as in Acrosalenia wiltoni (text-fig. 105), which also has four large plates. In examining some 2,300 specimens of Arbacia punctulata, considerable variation was found -in the periproctal plates, and they varied in adults from two to many (text-figs. 200-205). Two specimens were found with only two plates and many with three. In both of these forms what I take to be the suranal overlaid genital 3, as in Salenia, not ocular III as usual. A number of specimens were found with five plates and several with more than five, one being shown with six and one with thirteen (text- figs. 204, 205) . In one abnormal specimen from Florida (text-fig. 205a) there is one large peri- proctal plate facing ocular III, as usual, and, in addition, some fifty small rounded periproctal plates of a character similar to those usual in the Echinidae. When there are five periproctal plates in Arbacia, as in text-fig. 203, the condition is exactly like that of Habrocidaris (text- fig. 206). In Parasalenia gratiosa A. Agassiz, as shown by Mr. Agassiz (1873, p. 435), there are four periproctal plates, as in Arbacia, but they lie in a different plane so that two are on each side of the antero-posterior axis through III, 5. Of five specimens of Parasalenia gratiosa seen in de Loriol's collection, in the Geneva Museum, four have the typical four plates in the periproct, but one has only three. Variations of periproctal plates in Arbacia dufresni were also noted as described, p. 115. In the young of the Echinidae and Strongylocentrotidae a suranal plate fills the periproct, as shown by Mr. Agassiz (1874, p. 732). This is typically developed in young Echinus (Plate 3, fig. 14) and Strongylocentrotus (text-figs. 131-134, p. 129). As the animal grows, the suranal gradually loses its prominence, new plates forming along its right posterior border and the suranal retaining while recognizable a position dorsal to genital 3. In the adult of Strongylo- centrolus drobachiensis a plate is often found dorsal to genital 3 somewhat larger than other periproctal plates (text-figs. 135-139, p. 132). It seems that this may be recognized as the suranal still holding a slight supremacy in size. A similar plate similarly located is frequently recognizable in other genera, as Toxopneustes (text-fig. 122, p. 122) and Echinus (text-fig. 115, p. 117), and may fairly be considered as the suranal. There is considerable individual variation, some specimens having no such larger plate at this area and others possessing one. Dr. Mortensen (1911) expresses the view that the suranal is not a primitive structure form- ing an essential part of echinoid morphology, but is a specialized structure found only in certain groups of Echini. I heartily agree with this view. It seems that the suranal is simply one of the plates of the periproct which has attained exceptional prominence and which in youth entirely fills this area in those families in which it is a marked feature (Saleni- idae, Echinidae, Strongylocentrotidae). In the adults of the Temnopleuridae, Echinidae, Strongylocentrotidae, and Echinometridae (text-figs. 115-161) the periproctal plates are usually relatively large and solid, more or less numerous, but not definitely limited to a small number (except in Genocidaris and Parasalenia), ARISTOTLE'S LANTERN AND PERIGNATHIC GIRDLE. 177 and fill the periproctal area. In irregular Echini periproctal plates are relatively numerous and always fill the area completely. As clearly shown by Mortensen (1907, Plate 13) in Brisaster (Schizaster) fragilis, the periproct in the very young individuals is within the apical disc, as in regular Echini. It early travels out and soon assumes its adult position in the posterior inter- ambulacrum. In regular Echini the periproct is typically enclosed by a ring of genital, or genital and ocular plates. In the Echinothuriidae, on account of the separation of genitals and oculars, the periproct maj^ come in contact with the interambulacra (text-fig. 170, p. 149). As a rela- tively rare variation, a similar condition may exist in other regular Echini, as Sirongylocentrotus gibbosus (text-fig. 156, p. 145), S. drobachiensis (Plate 5, fig. 15), and S. lividus (Plate 6, fig. 5). In such types the young interambulacral plates develop in contact with the oculars as usual, and the periproct has no relation with the interambulacra excepting that of mechanical contact (pp. G3, 110). In the Exocycloida the periproct lies in interambulacrum 5; it is therefore separated from the ocular and genital plates of the apical disc and completely surrounded by interambulacral plates. The Aristotle's Lantern and Perignathic Girdle. Comparatively little has been pubhshed in regard to the Aristotle's lantern in Palaeozoic Echini. Trautschold (1868) figured complete lanterns in Archaeocidaris rossica, SoUas (1899a) figured a lantern in Palaeodiscus, and Meek and Worthen (1873) in Lepidocidaris in part. The lantern and its muscles in living Echini have been worked out quite fully by a number of authors, the most important being Valentin (1841), A. Agassiz (1872-'74, 1904, 1908, 1909), T. H. Stewart (1861), and Lov4n (1892). The jaws and muscles are often very inadequately or incorrectly given in text-books. For comparison with fossils, a lantern with soft parts was worked out iij Strongylocentrotus (Plate 5) as a basis for comparison. The lantern of this genus and a number of others not previously described were studied, and are given more or less fully in the following pages. The perignathic girdle might properly be described with the lantern in the several types, but it seemed best to treat it separately (pp. 189-198). It is believed that the structure of the lantern is of great value in systematic classification, and that the structure of its several parts presents characters that are of ordinal or subordinal value. As Dr. Mortensen pointed out (1904, p. 54), the structure of the teeth, keeled or un- keeled, is "a very important character, though it has hitherto received very little attention." Besides the teeth there are other features of value. Briefly stated, the essential points are: teeth grooved or keeled ; epiphyses narrow, or wide and united by suture ; the top of the pyramids, as seen when the epiphyses are removed, a smooth floor, or pitted ; foramen magnum deep, or shallow; angle of outline of the lantern depressed or erect; compasses present or absent. 178 ROBERT TRACY JACKSON ON ECHINI. The lantern of Strongylocentrotus drobachiensis is taken up first as a type of structure, because opportunity offered to study living material and abundant fresh specimens for dis- section. The structure has not been published in this type excepting in so far as Lov6n (1892) described the jaws and muscles in the young; the study has, therefore, interest from making known the structure in a common type and adds some features not previously recorded. In Strongylocentrotus there are five keeled teeth lying free about the mouth. The tooth is held in place by the two half-pyramids which ventrally enclose it closely by an inward extension of the base of the lateral wings (Plate 5, fig. 5). The tooth rests against the dental slide (s. d.) on the inner face of the pyramids. On each side the dental slide is produced dorsally as a guid- ing or styloid process, which in face view is seen projecting above the base of the foramen mag- num (Plate 5, figs. 4, 7). After traversing freely the space of the foramen magnum, in this type the tooth rests against two guiding processes, the crests of the epiphyses (Plate 5, figs. 2-5, 7, 9). The tooth then curves inward, takes a second sharp curve on itself and in the same plane, and terminates in a free point in the dental capsule (Plate 5, fig. G). From the mouth to a point just above the crests the tooth is calcified; beyond this point to the proximal tip the tooth is soft and pulpy and easily destroyed. In area 1 of Plate 5, fig. 9, the soft part of the tooth has been removed, whereas it is in place in areas 2 and 3. The calcified part is that usually shown in figures of lanterns, and is the only portion preserved in fossils, as in Archaeocidaris (Plate 12, figs. 4-8) and Pholidechinus (Plate 27, figs. 4-6). An entire tooth spread out fiat is shown in Plate 5, fig. 8; it is seen that at the dorsal, young or growing portion of the tooth it is grooved, but, passing ventrally, the character of the keel is soon taken on. That is, the young, last added portion of the tooth as a localized stage in development is grooved, as is the whole tooth in the Aulodonta, Cidaroida, and Palaeozoic forms; whereas the older or earlier formed portion of the tooth has taken on the full specific and subordinal character of a keel. The pyramids are each composed of two pieces joined by a median or pyramidal suture (Plate 5, figs. 4, 7, p. s.), and each piece is spoken of as a half-pyramid. Dorsally the pyramid in- cludes a wide angle, the foramen magnum, which in this and allied species is rather deep. On the inner side at the base (Plate 5, fig. 5) the wings of the pyramid curve around so as to embrace the tooth tightly at its point of exit. Each half-pyramid consists of a peripheral portion, the outer face of which gives rise to two muscles, and the inner bears one of the two dental slides which support each tooth, and a lateral wing which is strongly ridged horizontally for attach- ment of the interpyramidal muscles. The dorsal face of the half-pyramid, seen only when the epiphysis is removed (Plate 5, figs. 9, 10), presents a series of pits of considerable size and depth. These pits have apparently been overlooked by previous observers. They are important, as they are characteristic of the order of the Centrechinoida, but are not found in other orders of Echini. In 100 specimens of Strongylocentrotus drobachiensis from Frenchman's Bay, Maine, examined for this character, pits exist in all the pyramids, but there is considerable variation as regards the pattern and depth of the pits; occasionally they are very shallow. ARISTOTLE'S LANTERN AND PERIGNATHIC GIRDLE. 179 An epiphysis surmounts each half-pyramid to which it is joined by close suture (Plate 5, figs. 2-7, 9). The epiphysis extends proximally, capping the half-pyramid, and laterally ex- tends over the foramen magnum and meets its fellow of the opposite side in a median suture. Each epiphysis presents a glenoid cavity and an internal and an external tubercle (Plate 5, figs. 3, 5, 9), which serve in articulation with the brace. It also bears an elevated process or crest which is apposed to the tooth and dorsally supports that organ. The extension of the epiphyses over the foramen magnum so as to unite in suture and the crests developed on the same, ai-e important characters seen in tlie four families of the Temnopleuridae, Echinidae, Strongylocentrotidae, and Echinometridae, comprising the new suborder Camarodonta, but are absent in other Echini (p. 183). The brace is a block-shaped plate, often called rotula, which rests on and interlocks with the two opposed epiphyses (Plate 5, figs. 9, 11, 12). On its outer lateral borders it presents two condyles which fit into the glenoid cavities of the epiphyses. The compass (Plate 5, figs. 2, 9) rests on top of the brace, extends over its whole length, is attached to the brace on its inner proximal end by a tiny ligament, and is bifurcated on its outer end. Each compass consists of two parts, a suture just within the circular compass muscles separating the compass into an inner and an outer piece. The structure of the compass and brace is quite uniform in Echini, excepting the clypeastroids. There are thus forty pieces in the lantern of Strongylo- centrotus, and in all other Echini that possess a lantern, excepting the clypeastroids. These parts are in brief : five teeth ; ten half-pyramids ; ten epiphyses ; five braces ; ten pieces making up the five compasses. Next taking up the soft parts associated with the lantern of Strongylocentrotus drdbachiensis, we find that each tooth at its basal poi'tion is enclosed in a voluminous, very delicate trans- parent sac that may be called the dental capsule. A dental capsule is figured by the Sarasins (1888) in Asthenosoma,' and is mentioned in Cidaris by T. H. Stewart (1861) and in Echinus by Chadwick (1900), but I think it has not been adequately shown before. In order to see the sac properly, a specimen should be opened alive, right out of the sea. It is so delicate and sensi- tive, that if opened a few hours later, though the animal be still active, the sac has shrunk to small proportions. In alcoholic material as far as observed it is always collapsed. When opened thus alive, the capsules are so inflated that they look like five bladders radially arranged, and so large that they actually touch one another at the area nearest the oesophagus (Plate 5, fig. 1). The sac envelops the base of the tooth completely, lies free, but on its outer border passes over the epiphyses and some distance down the face of the area of the foramen magnum (Plate 5, fig. 6). Similar capsules were seen in fresh specimens of Arbacia ipunctulata, Sphaer- echinus granularis, and Strongylocentrotus lividus, and very small ones in Echinarachnius parma. It would be interesting to study the capsules in perfectly fresh material of other Echini. ' Unfortunately in Lang's (1896) copy of this figure, tlie dental capsule is labeled Polian vesicle. 180 ROBERT TRACY JACKSON ON ECHINI. The muscles of the lantern are very complex. They have not been described in adult Strongylocentrotus, though Loven (1892) showed them in a very young one and in exquisite detail in the adult of several types. There are sixty muscles in Strongylocentrotus and other regular Echini. There are ten protractors (Plate 5, figs. 1, 4, pr.; text-fig. 229), which take origin on the upper outer face of the half-pyramids and epiphyses and are inserted at the base of the two half-interambulacra which are associated with the intervening ambulacrum (compare text-figs. 218, 219, p. 191). The protractors serve to extend the mouthparts, which the animal actively does when in good condition. The ten retractors (Plate 5, figs. 1, 4, re.), which serve to open the jaws, take origin on the lower outer face of the half-pyramids and are inserted on the auricles. The retractors of the two half-pyramids that are united by the interpyramidal muscle pass to the two auricles of the same ambulacrum (compare text-fig. 219, p. 191). Ten radial compass muscles take origin from the bifurcated termini of the compasses and are in- serted at the base of the two next adjacent half-interambulacral areas. From studying their motions in life, and from their position and wide divergence, it seems that the function of the radial compass muscles is to maintain and restore the vertical position of the lantern. When alive, the mouth with the peristome is freely and actively extended and retracted, and not in a vertical plane only, but with much sidewise or inclined motion, so that the radial compass muscles are in just the position to effect the motion and restore the perpendicular. It is note- worthy that in clypeastroids, where the lantern is in almost immediate contact with the base of the test and little motion is possible, the protractor muscles are very small and the compasses with their muscles are absent. The distribution of protractor, retractor, and radial compass muscles is immediately con- nected with the view expressed (pp. G2, 190), that the corona may be considered as made up of five rather than ten areas. Five large but short interpyramidal muscles (Plate 5, fig. 2, ip.), extend from the corrugated lateral wing of each half-pyramid to the similar wing of the next adjacent half-pyramid. These muscles serve to close the jaws and from their size must be very powerful. Ten tiny internal brace muscles (Plate 5, fig. 12, i. b.) extend from the brace to the two associated epiphyses, and ten similar external brace muscles, (e. b.), extend also from the brace to the epiphj^ses, binding these parts together and doubtless giving some mobility. Five circular compass muscles (Plate 5, fig. 1, i.) extend between the compasses, being inserted at the proximal end of the outer of the two pieces of which each compass is composed. Their function is not evident. In brief the sixty lantern muscles of Strongylocentrotus and other regular Echini are as follows: ten protractors; ten retractors; five interpyramidal muscles; ten internal brace muscles; ten external brace muscles; five circular compass muscles; ten radial compass muscles. Such being the character of the lantern of the adult, it is important to see what is the ARISTOTLE'S LANTERN AND PERIGNATHIC GIRDLE. 181 character in the young. Loven (1892, p. 13, Plate 4) described the lantern of a young Strongy- locentrotus drobachiensis only 1.2 mm. in diameter. At this stage the pyramids are wide-angled, so that the whole lantern is broadly inclined instead of being nearly perpendicular, as in the adult; also the sides of the half-pyramids bearing the interpyramidal muscles are curved instead of being in a nearly vertical plane as in the adult, and as a result the interpyramidal muscles are relatively longer than in the adult. In these several characters the lantern of young Strongylocentrotus point for point agrees with the typical Palaeozoic lantern as seen in Archaeo- cidaris (Plate 12, figs. 4-6) and Pholidechinus (Plate 27, figs. 4-6). The same agreement with the Palaeozoic characters is seen in the lantern of young Goniocidaris (Plate 2, fig. 17). Having considered the lantern and muscles of Strongylocentrotus, representative lanterns of Echini will be taken up in the sequence of their structural differentiation. Of Bothriocidaris nothing is known but the tips of the teeth, or pyramids. They lie opposite the ambulacra (Plate 1, fig. 1). That they occupy this anomalous position is without explanation unless by an accidental twist they have come to assume it. It is the most ambiguous feature of this important type. I have been so fortunate as to secure for study some excellent lanterns of Palaeozoic genera. These are described under their several species, but are briefly considered here. The essential character of the lantern in the Echinocystoida and Perischoechinoida is represented by Archaeocidaris (Plate 12) and Pholidechinus (Plate 27) ; other genera as far as known present only slight differences. Of other genera it is known more or less completely in Palaeodiscus, Oligoporus (Plate 50, figs. 11, 12), Melonechinus (Plate 56, figs. 9, 10), Lepidesthes (Plate 68, figs. 9-14), Pholidocidaris (Plate 74, figs. 2, 6, 7), and Meekechinus (Plate 76, fig. 7). In the Palaeozoic genera the teeth are grooved, and when in place, extend slightly above the base of the foramen magnum (Pholidechinus, text-fig. 207, Plate 27, figs. 5, 6). While the teeth are usually obtusely acuminate orally, as in Pholidechinus and Lepidesthes (Plate 68, fig. 9) , in one genus, Meekechinus (Plate 76, fig. 7), the teeth are deeply serrate distallj^ a unique character. The pyramids are wide-angled in outline and as a result the lantern is inclined at an angle of about 45 degrees instead of being nearly perpendicular as in the adults of most living regular Echini; also the lateral wings of the pyramids are curved, indicating quite long interpyra- midal muscles (Plate 27, figs. 4, 6). In these characters the Palaeozoic lantern approaches closely that in the young of modern types (Goniocidaris, Plate 2, fig. 17), and also the adult of Recent echinothuriids (Plate 2, figs. 19-21). The foramen magnum is moderately deep (Archaeocidaris, text-fig. 208, Plate 12, fig. 4). It appears very shallow in Pholidechinus (Plate 27, fig. 6), but this is in part due to the position in which the specimen was drawn. The lateral wings of the pyramids present a corrugated surface for attachment of interpyra- midal muscles (Plate 12, fig. 8; Plate 13, fig. 13; Plate 68, fig. 12). The dorsal surface of the half-pyramids, seen when the epiphyses are removed, is smooth, as in the Cidaroida 182 ROBERT TRACY JACKSON ON ECHINI. (Plate 2, fig. 9), not pitted as in tiic Centrechinoida. The half-p3'ramid seen from within shows a dental slide for the support of the tooth (Plate 13, fig. 14; Plate 50, fig. 12), as in Recent Echini. The styloid processes do not extend above the base of the foramen magnum, in this feature being like the Cidaroida, but unlike the Centrechinoida, where the styloid processes are visible in face view (text-fig. 211). The epiphyses cap the half-pyra- mids and are narrow, projecting only slightly beyond the limits of the outline of the half- pyramids (text-figs. 207, 208; Plate 12, figs. 1, 7; Plate 27, figs. 4-6). Each epiphysis pos- sesses a glenoid cavity for the interlocking of the brace (Plate 12, fig. 8; Plate 68, fig. 12). The brace is a block-shaped plate, practically identical with that of living regular Echini. It rests on the epiphyses, and its condyles fit into their glenoid cavities (Plate 12, figs. 3, 6, 7; Plate 27, figs. 4-6). A compass is such a delicate structure that it is rarely preserved, but one was found in Archaeocidaris (Plate 12, figs. 1, 2) resting on the brace and with the typical bifid outer end. A compass is also shown in Pholidechinus (Plate 27, fig. 5), but the bifid end is gone, or possibly it did not have such a bifurcation, as it is not existent in all Recent compasses. These are the first cases of compasses described from Palaeozoic species. It is seen that in the Palaeozoic Echinocystoida and Perischoechinoida the lantern is made up of forty pieces, as in Recent regular Echini. In structure it approaches closest to that of Cidaris. There were doubtless the same sixty muscles as described in Strongj'locen- trotus. It is interesting to see how near the characters of these ancient genera are to those of living forms, and also that their closest comparison is with the young of the latter. In the Cidaroida the lantern is the most primitive of any Recent Echini. Loven (1892) showed the character in the young of Goniocidaris ; here at a very early stage the pyramids are very wide-angled, with long interpyramidal muscles and a deep foramen magnum (text- fig. 209; Plate 2, fig. 17). In these characters it differs from the adult, but is practically identi- cal with the Perischoechinoida. According to Loven, at this early stage the protractor and retractor muscles are inserted directly on the primordial interambulacral plates, which are still in place. This condition again by inference is close to that in the Palaeozoic genera where (excepting Miocidaris) apophyses and auricles are apparently always absent, and the pro- tractor and retractor muscles were without doubt attached directly to the basicoronal inter- ambulacral plates (text-fig. 221, p. 193). Loven (1892) described the lantern and muscles of "Cidaris papillata" {Cidaris cidaris (Linne)) with his usual care. To show another type and to bring out certain points, the lan- tern of Eucidaris tribuloides is here shown in text-fig. 210 and Plate 2, figs. 7-16. In this species the teeth are grooved and extend far beyond the pyramids ventrally (Plate 2, figs. 7, 8). The tooth rests against dental slides on the inner face of the pyramids, but the styloid processes are slight and do not extend above the foramen magnum so as to be seen in face view (Plate 2, fig. 10), as they do in the Centrechinoida. The pyramids are comparatively narrow-angled, stand- ARISTOTLE'S LANTERN AND PERIGNATHIC GIRDLE. 183 ing erect, and the foramen magnum is extremely shallow, the pyramidal suture extending nearly the length of the outer face (Plate 2, figs. 8, 10). The pyramid has the usual corrugations on the lateral wings (Plate 2, fig. 7). When the epiphyses are removed, the dorsal faces of the pyramids present a smooth surface (Plate 2, fig. 9, area II). This is an important character in which it resembles Palaeozoic genera, but differs from the Centrechinoida in which pits typically occur in this area. The epiphyses are narrow, extending only slightly beyond the upper face of the half-pyramids. Each presents a glenoid cavity and internal and external tubercles for articulation with the brace (Plate 2, figs. 7, 9, 14, 15). The brace is of the usual shape with rather strongly marked condyles and external and internal foramina for articulation with the glenoid cavities and tubercles of the epiphyses. Each ossicle of the compass is composed of two pieces and is strongly arched, with a bifid outer end (Plate 2, fig. 12). A lantern is shown on Plate 2, fig. 9, drawn to give the full characters from the dorsal view. In area I all the parts are in place ; in V the compass is removed to show the brace ; in IV the compass and brace are removed to show the epiphyses; and in area II the epiphyses also are removed, so as to show the dorsal face of the half-pyramids. This figure may be compared with similar drawings of Phormosoma (Plate 2, fig. 20), Salenia (Plate 4, fig. 4), and Strongylocentrotus (Plate 5, fig. 9). These figures bring out essential differences in their several separate groups. The lantern of Eucidaris as shown has the character of its order and is essentially similar to that of the Perischoechinoida. The chief differences are the angles of the pyramid and the shallow foramen magnum. These differences, however, are bridged by the young. In the Centrechinoida the structure of the lantern is extremely interesting and affords data for grouping the families in three distinct suborders on the basis of the characters of the teeth and the epiphyses. The order as a whole differs from other orders in that the dorsal face of the half-pyramids (seen when the epiphyses are removed) shows a series of pits instead of plane surfaces. The order is characterized by having a deep foramen magnum instead of a very shallow one, as in the Cidaroida, or a moderately deep one, as in the Perischoechinoida. The first suborder, which I would call the Aulodonta, (avXds, a groove, and oSovs, tooth) is characterized by grooved teeth and narrow epiphyses, not meeting in suture over the foramen magnum (Centrechinus, text-fig. 211; Phormosoma, Plate 2, figs. 19-21). The second sub- order or Stirodonta (a-relpa, a keel, and oSous, tooth) is characterized by keeled teeth and narrow epiphyses, not meeting in suture over the foramen magnum (Salenia, Plate 4, figs. 3, 4; Glyptocidaris; Stomopneustes, Plate 4, figs. 8-10; Arbacia, text-fig. 212). The third suborder or Camarodonta (/ca/iapa, anything with an arched covering, and oSovs, tooth) is characterized by keeled teeth and wide epiphyses, meeting in suture over the foramen magnum (Tripneustes, text-fig. 213; Strongylocentrotus, Plate 5, figs. 1-12). A more detailed consider- ation of these suborders and their characters follows. Aulodonta. — In Centrechinus setosus (text-fig. 211, text-figs. 219, 220, p. 191) the teeth 184 ROBERT TRACY JACKSON ON ECHINI. 212 Text-figs. 207-216. — Essential features of the lantern in representative Ecluni. In each species figured are shown a pyramid with associated parts in place seen in face view, with a cross section of the tooth; also an epiphysis (A) seen from above, and a top view of a half-jjyramid (B) from which the ejtiphysis has been removed. 207. Pholidcchinus brauni sp. nov. Lower Carboniferous, Crawfordsville, Indiana. Yale University Museum Coll., 321. X 3.4. (Compare Plate 27.) Tij) of compass restored as indicated by dotted lines. Tooth grooved and epi- physes narrow, foramen magnum moderately deep. 208. Archaeocidaris rossica (Buch). Lower Carboniferous, Miatschkowa, Province of Moscow, Russia. Mus. Comp. Zool. Coll., 3,181. X 3.2. (Compare Plate 12.) Top of pyramid smooth, tooth grooved, epiphyses narrow, foramen mag- num moderately deep. Tip of the tooth restored a.s indicated by dotted lines. 209. Goniocidaris canaliculala A. Agassiz. (Adapted from Plate 2, fig. 17.) Young lantern, epiphyses narrow, fora- men magnum moderately deep. 210. Etiddaris Iribuloides (Lamarck). Bahamas. R. T. J. Coll., 694. X 3.2. Top of pyramid smooth, tooth grooved, epiphyses narrow, foramen magnum very shallow, (Compare text-fig. 218, p. 191; Plate 2, figs 7-16.) ARISTOTLE'S LANTERN AND PERIGNATHIC GIRDLE. 185 2n. Cenlrechinus selosus (Leske). Bermuda. R. T. J. Coll., 788. X 1.6. Top of pyrainid pitted, tooth grooved, epiphyses narrow, foramen magnum deep (compare text-figs. 219, 220, p. 191). 212. Arhacia punclulata (Lamarck). Woods Hole, Massachusetts. R. T. J. Coll., S83. X 1. Top of pyramid pitted, tooth keeled, epiphyses narrow, foramen magnum deep. 213. Tripneusles esculenlus (Leske). Florida. R. T. J. Coll., 884. X 2.4. Top of pyramid i)illcd, tooth keeled, epiphyses wide, meeting in suture with crests supporting tooth, foramen magnum deep (compare Plate 5, figs. 1-12) . 214. Clypeasler rosaceus {lAnwi). Bahamas. R. T.J. Coll., 885. X 1.6. Top of pyramid smooth, pyramids later- ally flaring, epiphyses narrow, compass wanting, foramen magnum very shallow. 215. The same. Tooth keeled, in side and face views and in section. 216. The same. Revolved outward to show ventral tip of tooth. Text-figs. 207-209, pyramid wide-angled, lantern inclined; 210-213, pyramid narrow-angled, lantern erect; 214, pyramid flaring, lantern procumbent. Lettering: 6, brace; c,compass; cr., crest; cp., epiph3'sis, top view and in place; /, foramen magnum; p, top of pyramid; St., styloid process; (, tooth. are grooved and continue calcified high above the base of the foramen magnum. They are supported by the dental slides, as usual, and the long styloid processes, typical of the order, are a prominent feature in face view. The pyramids are narrow-angled, with a deep foramen magnum, and the upper borders of the pyramids curve inward over the space of the foramen magnum and a slight distance beyond the epiphyses. The top of the pyramids has deep pits seen when the epiphyses are removed (text-fig. 211JB). The epiphyses are narrow, not quite covering the dorsal face of the half-pyramids. The brace and compass are as usual except that the compass may be outwardly rounded instead of bifid as in most Echini. In text-fig. 219 (p. 191), the protractor and radial compass muscles are shown passing from the lantern to the two half-interambulacra associated with ambulacrum III and the retractor muscles passing to the auricles that originate on ambulacrum III. The genus Phormosoma is most interesting as representing the remarkable Echinothuriidae. Mr. Agassiz (1904, Plate 47) figured the structure of the lantern in Echinosoma hispidum (A. Agassiz), and I show it in another species, Phormosoma placenta. The teeth are grooved, narrowly acuminate distally (Plate 2, figs. 19-21). They are supported by the dental slides and styloid processes as usual. The pyramids are very wide-angled with long interpyramidal muscles. As the angles are wide, the lantern is inclined instead of nearly perpendicular, the usual character of the suborder, and in this respect it is closely like that in the adult of Palaeo- zoic and the young of Recent species. This is best seen by comparing the dorsal view of Phormo- soma (Plate 2, fig. 20) with that of Pholidechinus (Plate 27, fig. 5). The foramen magnum is deep and widely curving, a very distinctive character. The dorsal face of the pyramids has the pitting seen in all the order when the epiphyses are removed, as shown in Plate 2, fig. 20, area II. In side view (Plate 2, fig. 21) the lateral wing of the pyramid is corrugated as usual, and is strongly bowed on its outer line. The epiphysis is very narrow with the usual glenoid cavity (Plate 2, figs. 19, 21, and 20, area IV). Duncan (1889a, p. 40) is mistaken in saying of the Echinothuriidae that epiphyses are wanting; they doubtless exist in all regular Echini 186 ROBERT TRACY JACKSON ON ECHINI. possessing jaws. The brace is of the usual shape, and the compasses are strongly arched, curving over the outer border of the braces (Plate 2, fig. 19). The muscles of Phormosoma are of the usual number in lanterns of regular Echini, but in addition extra muscles are given off from the auricles radially. These muscles (text-fig. 226, p. 193) are very delicate and may fairly be called radial peristomal and radial somatic muscles, as they extend like tent-ropes from the auricles radially to these two areas. They have been described in Asthenosoma as longitudinal muscles (Lang, 1896, p. 471), but this name seems inappropriate. These muscles apparently cause movements of the highly flexible test of the corona and peristome (p. 195). The Aulodonta have the primitive structure of grooved teeth and narrow epiphyses seen in the Echinocystoida, Perischoechinoida and Cidaroida. With this they have the deep foramen magnum, prominent styloid processes, and especially pits in the top of the pyramids character- istic of the Centrechinoida. Stirodonta. — Salenia is a type of much interest. Duncan (1889a) gives the characters of the lantern, but I believe they have not been shown in detail or figured. The teeth are keeled and supported by the dental slides and styloid processes as usual (Plate 4, figs. 3, 4). The pyramids are rather wide-angled, have a deep foramen magnum (Plate 4, fig. 3) , and when the epiphyses are removed, show pits in their dorsal faces (Plate 4, fig. 4, area I). The epi- physes are very narrow, only slightly more than covering the dorsal faces of the half-pyramids (Plate 4, fig. 4, area I). The foramen magnum is therefore open dorsally. The braces and compasses have the usual shape. The protractor and radial compass muscles pass from their origin in the half-pyramids and compasses to the basicoronal plates of the half-interambulacra associated with the area in which they originate (Plate 4, figs. 4, 5). The retractor muscles pass from the half-pyramids to auricles developed from the ambulacra. In Salenia the keeled teeth, deep foramen magnum, pits in the dorsal face of the pyramids, and the auricles are all characters which unquestionably locate this type as one of the Centrechinoida and equally separate it from the Cidaroida with which group Salenia was formerly associated (p. 195). The lantern of Stomopneustcs variolaris, I believe, has not been described. It is erect, with teeth keeled, epiphyses narrow, not meeting in suture over the foramen magnum (Plate 4, figs. 8-10). The dorsal ends of the half-pyramids extend inward over the foramen and at their free ends are produced ventrally as spurs which meet and give support to the teeth. The function of these pyramidal spurs is similar to that of the crests developed on the epiphyses of the Camarodonta (text-fig. 213), but they cannot be homologized with them as they arise from a different structural part. I believe this is a feature which has not hitherto been noted in the lantern of Echini. Stomopneustcs has been placed in the Echinometridae, but the structure of the lantern would locate it in this suborder, and it seems that it may be safely followed as a basis of classification. In Glyptocidaris (Phymosoma) crenulare, the only living representative of its family, the Phymosomatidae, the structure of the lantern is as in Stom- ARISTOTLE'S LANTERN AND PERIGNATHIC GIRDLE. 187 opneustes, teeth keeled, foramen magnum open, epiphyses narrow, and spurs from the produced dorsal tips of the pyramids supporting the teeth. These characters link these two families together and equally separate them from other families of regular Echini. In Arbacia, as in Salenia, the teeth are keeled (text-fig. 212). The pyramids have a rather deep foramen magnum, and the upper faces of the half-pyramids, seen when the epiphyses are removed, show a pitted surface. The epiphyses are narrow, little more than capping the half- pyramids, so that the foramen magnum is open dorsally. The braces of Arbacia are of the usual type, but the compasses are distally rounded instead of bifid as in most Echini. This feature was found in Arbacia lixula, A. pundulata, and A. nigra, so that it is probably generic. The Stirodonta as regards the lantern present features intermediate in character, having narrow epiphyses as in the Aulodonta and still more primitive types, but keeled teeth like the Camarodonta. Keeled teeth are evidently mechanically more powerful than grooved teeth (text-figs. 207-212, p. 184). Therefore this structure can be considered a mechanical as well as a structural advance over what obtains in the lower groups of Echini. Camarodonta. — Tripneustes represents the most complex structure known in the Centre- chinoida. The teeth are keeled (text-fig. 213), and are supported by the dental slides. The epiphyses are wide and meet in a median suture over the foramen magnum. In addition, the epiphyses bear crests, which are strongly developed in Tripneustes, and these processes bear against and give support to the teeth at their upper portion. The pyramids are nearly per- pendicular and have a deep foramen magnum, which is roofed over by the joined epiphyses. The dorsal face of the half-pyramids, seen when the epiphyses are removed, shows the pits characteristic of the order. The styloid processes are strongly developed, as seen in face view. The epiphyses have the usual glenoid cavity and tubercles for articulation with the brace. The brace is of the usual character, and the ossicles of the compass are divided into two parts and are distally cleft as usual. Strongylocentrotus as previously described (Plate 5) is essen- tially like Tripneustes. A peculiar variation was found in a specimen of Tripneustes esculentus from Bermuda. In this there are no jiits in the top of the pyramids, and as such it may be considered a regressive variant. It is tlie only case seen in the Centrechinoida in which these characteristic pits are wanting. The character of the Camarodonta as described is to have keeled teeth and wide epiphyses meeting in suture over the foramen magnum ; also the epiphyses bear crests which give support to the teeth. This is the feature of the Temnopleuridae as ascertained in Temnopleurus, Microcyphus, and Amblypneustes. It is the character of the Echinidae as ascertained in Echinus, Toxopneustes, and Tripneustes, and of the Strongylocentrotidae, as seen in Sphaere- chinus and Strongylocentrotus. Finally, it is the character of the Echinometridae, as ascertained in Echinometra and shown by Mr. Agassiz (1908) in Colobocentrotus. The lantern of the Camarodonta is mechanically as well as structurally the most highly evolved known in Echini. 188 ROBERT TRACY JACKSON ON ECHINI. The keeled teeth as in the Stirodonta are a mechanical advance over grooved teeth (text-figs. 207-213, p. 184). In addition the confluence of the epiphyses (text-fig. 213, p. 184; Plate 5, figs. 2, 5, 9) is a mechanical advance, as, besides supporting the teeth, it makes with the braces a continuous rigid base as fulcra for the action of the interpyramidal muscles, instead of a ^^^//t/^CV*^-^vA/%<^C'^"0'fc^tinct continuous base as occurs in all lower Echini (text-figs. 207-212, p. 184; Plate 2, figs. 19, 20; Plate 27, figs. 5, 6). Turning to the irregular Echini, Loven (1892) and Hawkins (1909) showed that in Discoidea, representing the Holectypina, the pyramids are wide-angled. This is like the character in the young of Recent regular Echini and in the adult of Palaeozoic species, so that in Dis- coidea it may be considered a regressive type of lantern (p. 218). Loven (1892) worked out the lantern and muscles of several of the Clypeastrina with minute care. The lantern in this suborder is highly flattened, broadened, and specialized in character, being very different from the same structure in any regular Echini. A representative portion of a lantern of Clypeaster is shown in text-figs. 214-216 for comparison. The tooth is short and narrow, and the keel, which is relatively very wide (text-fig. 215), makes up a large part of the mass of the tooth. A little depression in the tip of the tooth, represented in section, is probably due to mechanical wear. The keeled teeth of the Clypeastrina indicate a relation of this group to regular Echini with keeled teeth. When the lantern is held in its natural plane (text-fig. 214), only the dorsal tip of the tooth is visible, but if rotated upward (text-fig. 216), the ventral tip is seen as well. The pyramids are broadly triangular and procumbent with a lateral expansion into great wings which are deeply plicate on their outer faces. The median suture exists as usual ; the foramen magnum is extremely shallow, almost a straight line. On the lateral wings of the half-pyramids there are no corrugations, as in regular Echini, for the attachment of interpyramidal muscles, although these are well developed. On the dorsal face of the half-pyramid seen when the epiphysis is removed (text-fig. 214B), there are no pits as in the Centrechinoida, but the sur- face is smooth. The epiphyses are very small and the brace is a small, narrow, block-shaped plate quite different from that of the regular Echini. The compasses are wanting, apparently an ordinal character. The Aristotle's lantern is an internal structure and as such is not so likely to be affected by external conditions as the more superficial parts of an urchin. It seems, therefore, that its structure as a complicated mechanism should yield data of classificatory value, binding together the larger groups, and I believe that the facts fully support this view. Summing up the characters briefly: in the Palaeozoic Echinocystoida and Perischoechi- noida (text-figs. 207, 208), the lantern is inclined at an angle of about 45 degrees, not erect. The teeth are grooved, pyramids wide-angled, foramen magnum moderately deep, epiphyses narrow, and there are no pits in the top of the pj'ramids. In the ancient Cidaroida (text-fig. 210) the structure is similar, but the lantern is erect, the foramen magnum very shallow, and there are no pits in the top of the pyramids. ARISTOTLE'S LANTERN AND PERIGNATHIC GIRDLE. 189 In the Centrechinoida, as an order, there are pits in the top of the pyramids beneath the epiphyses, the foramen magnum is always deep, styloid processes are visible in face view, and the lantern is erect excepting in Astropyga and the Echinothuriidae, where it is inclined as in the young, and the adult of Palaeozoic types. Other characters of the lantern in this order are sufficiently distinctive to justify the formation of three suborders, as shown (p. 183). In all the above the lantern is made up of forty pieces, braces and compasses are nearly alike, and corrugations exist on the pyramidal wings for attachment of interpyramidal muscles. In the Holectypina the structure of the lantern in Discoidea was shown in part by Loven (1892), and has recently been shown very completely by Hawkins (1909) in Discoidea cylin- drica. As Hawkins shows, the teeth are narrow, elongate, and keeled; pyramids are wide- angled with a rather deep foramen magnum and laterally with corrugations for the attachment of interpyramidal muscles ; the epiphyses are relatively large but narrow and closely resemble those of the Stirodonta. In the whole structure the lantern approaches much nearer to that of the Stirodonta than of any other group of Echini. Of course it may differ in other genera of the Holectypina where it is at present unknown (see p. 218). In the Clypeastrina the lantern is procumbent, but is more erect in the young, as I have seen in Echinarachnius parma. The teeth are keeled, the foramen magnum is extremely shallow (text-figs. 214-216), pyramids are highly modified with wing-like extensions without ridges for the attachment of interpyramidal muscles, or pits in the top of the pyramids; brace and epiphyses are very small, and compasses are absent. The whole lantern is much specialized, the keeled teeth presenting the best connecting character (see p. 218). The Spatangina as a suborder are characterized by the absence of the lantern, and it is of highest interest that a lantern exists in young Echinoneus, as shown by Mr. Agassiz (1909). In a very early stage this type has a lantern with keeled teeth, deep, open foramen magnum and bifid compasses, the retractor muscles are attached to the ambulacral auricles, and the protractor and radial compass muscles are attached to the base of the single primordial inter- ambulacral plates. It is interesting and suggestive that this combination of characters shown by Mr. Agassiz in young Echinoneus finds an exact parallel in adult Arbacia (text-fig. 212, p. 184; text-fig. 227, p. 193), and further, the Stirodonta, as here defined, is the only group of regular Echini that does possess such a combination of characters (p. 217). Before considering the perignathic girdle and the relation of the lantern muscles to the same, I would call attention to the relation of the lantern O.V 7'. 1. Ambulacral plates compound, or largely simple. Primordial interambulacral plates resorbed. Base of corona resorbed. Oculars exsert or becoming insert in the .seciuence I, V, IV, II, III. Periproct with a permanent large suranal, or more large plates, with small anal plates. Primary tubercles imperforate, or perforate. No spurs from pyramids supporting teeth. Family Saleniidae. Ambulacral plates compound. Primordial interambulacral plates resorbed. Base of corona resorbed. Oculars becoming insert in the sequence I, V, IV, II, III. Periproct with numerous small plates only. Primary tubercles imperforate. Spurs from pyramids support the teeth dorsally. . Family Phymosomatidae. Ambulacral plates compound, composed of three elements each, at the mid-zone every four or five ambulacral plates are bound together and grown over by one primary tubercle. Primordial interambulacral plates resorbed. Base of corona resorbed. Oculars becoming insert in the sequence I, V, IV. Periproct with many small plates only. Primary tubercles imperforate. Spurs from pyramids support the teeth dorsally Family Stomopneustidae. Ambulacral plates compound. Primordial interambulacral plates in the basi- coronal row. Base of corona not resorbed. Oculars all exsert or becoming insert in the sequence V, I, IV. Periproct with four or fi\e large plates only. Primary tubercles imperforate. No spurs from pyramids supporting teeth. Family Arbaciidae. . EE. Epiphyses wide, meeting in suture over the foramen magnum. Ambulacral plates compound. Coronal plates not imbricate. Ambitus circular, or elliptical through a sidewise axis. Primordial interambulacral plates resorbed. Base of corona resorbed. Oculars all exsert or becoming insert in the sequence V, I or I, V, IV, II, III. Periproct usually plated witli many small plates (in one genus, Parasalenia, with four large plates). Peristome with ten (in one species five) primordial ambulacral plates and more or fewer non-ambulacral plates; rarely the latter are absent. Lantern erect. Primary tubercles imperforate. Suborder CAMARODONTA. Ambitus circular. Pits, or sculpturing in coronal plates dorsally. Ambulacral plates at mid-zone composed of three elements each. Oculars usually all exsert. Family Temnopleuridae. Ambitus circular. No pits or sculpturing in coronal plates dorsally. Ambula- cral plates at mid-zone composed of tliree elements each, rarely dorsally of two elements. Oculars all exsert, or becoming insert in the sequence I, V, IV, II, III Family Echinidae. Ambitus circular. No pits or sculpturing in the coronal plates. Ambulacral 204 ROBERT TRACY JACKSON ON ECHINI. plates at mid-zone composed of from four to ten elements each, rarely (some Echinostreplius) of three elements each. Oculars all ex.sert or becoming insert in the sequence I, V, n', II. . . . Family Strongylocentrotidae. Ambitus elliptical in a sidewise axis. No pits or sculpturing in coronal plates dorsally. Ambulacral plates at mid-zone composed of four or more elements each, rarely (Parasalenia) of three elements each. Oculars all exsert, or be- coming insert in the sequence V, I, IV. . . . Family Echinometridae. BB. Periproct outside of oculo-genital ring, in interambulacrum 5. Regular in form, or more fre- quently more or less markedly bilaterally symmetrical through the axis III, 5. Slight or no resorption of base of corona by the advance of the peristome. Lantern present or absent. Peri- stomal gills, or ambulacral gills only. Sphaeridia present. . . Order EXOCYCLOIDA. Ambulacral plates compound, or largel\' simple, areas not petaloid dorsally. Primordial ambulacral plates imknown. Primordial interambulacral plates in basicoronal row, or in part resorbed. Base of corona slightl.y resorberl. Oculars and genitals all present and dis- tinct, or fused, or genital 5 absent; when present, it is imperforate. Periproct unknown. Peristome central, structure unknown. Lantern inclined so far as known. Teeth keeled, epiphyses narrow. Foramen magnum moderately deep. Pyramids with ridges on lateral wings. Perignathic girdle consisting of low apophyses and auricles. Peristomal gills present Suborder HOLECTYPINA. Ambulacral plates simple, areas petaloid dorsally. \'entrally ambulacral pores are minute and specialized. Primordial ambulacral plates in basicoronal row. Primordial inter- ambulacral plates in basicoronal row, or exceptionally (Arachnoides, p. 72) pushed dorsally and in part resorbed by intracoronal resorption. Base of corona not resorbed. Ocular anrl genital plates fused in a mass, usualh- no genital pore in area 5. Genital pores witliin the fused mass or outside in interambulacra 1, 2, 3, 4. Periproct plated. Peristome central, leathery. Lantern procinnbent, highly modified, teeth keeled, foramen magnum very shallow, small epiphyses and liraces, but no compasses. Pyramids usually without ridges on lateral wings. Perignathic girdle consisting of auricles only, on ambulacral, or on inter- ambulacral plates. No peristomal, but ambulacral gills only. Suborder CLYPEASTRFX.l. Ambulacral plates simple, areas commonly petaloid dorsally; in some types pores are absent in part of the plates. Ambulacrum III often differs markedly in character from other areas. Ambulacral plates often highly specialized in form and size. Primordial ambulacral plates in the basicoronal row, or (Pourtalesia) in part pushed dorsally. The basicoronal plates la, Iln, lllb, TVa, Yb are larger and with two pairs of pores or two separate single pores, whereas the 16, lift, II I«, IV6, Va are smaller with one pore-pair or one single pore. Primordial interambulacral plates in basicoronal row or (Lovenia, Pourtalesia) pushed dorsally. Base of corona not resorbed. Oculars and genitals separate, or genitals partially fused. Oculars apparently absent in some Pourtalesias. Genital 5 absent, and some additional genitals rarely absent. Periproct plated. Peristome eccentric, plated with non-ambulacral plates onl\'. Lantern and perignathic girdle absent (present in the young of Echinoneus, pp. 189, 217). No peristomal, but ambulacral gills only. Suborder SPAT AN GIN A. SYSTEMATIC CLASSIFICATION OF ECHINI. 205 AA. Two or more columns of simple plates in each ambulacral area, and three or more columns of plates in each interambulacral area (for A, see p. 201). B. Genitals largely covering the dorsal surface. Two columns of low plates in each ambulacral area and three columns of plates in each interambulacral area. Plates not imbricate. Prim- ordial interaml)idacral plates in basicoronal row. Base of corona not resorbed. Oculars small, strongly exsert by the contact of large genitals. Periproct central, structure unknown. Peri- stome central, structure unknown. Lantern and perignathic girdle unknown. Order PLESIOCIDAROIDA. BB. Genitals small. Two to twenty columns of plates in each ambulacral area and three to four- teen columns of plates in each interambulacral area. Plates not imbricating, or imbricating; when the latter, ambulacral plates imbricate ventrally and interambulacral plates dorsally and from the center laterally and over the ambulacra on the adradial sutures. Primordial ambulacral plates on the peristome. Base of corona resorbed or not. Peristome central with many rows of ambulacral plates only, or the same with non-ambulacral plates. Lantern inclined, composed of 40 pieces, teeth grooved. Foramen magnum moderately deep, epiphyses narrow. No pits in top of pyramids. No perignathic girdle. No peristomal gills. Primary spines and perforate tubercles with secondary spines and imjierforate tubercles, or secondaries only. (". Irregular, periproct apparently in an interambulacrum. Two to four columns of plates in an ambulacral area, anil eight to nine columns of plates in an interambulacral area. Plates thin, imbricating. Oculars and genitals doulttful. . . . Order ECHINOCYSTOIDA. Two columns of plates in an ambulacral area and eight to nine colunms of plates in an interambulacral area. Primordial interambulacral plates in basicoronal row. Base of corona not resorbed. Peristome with ambulacral plates only. Lantern typically echinoid. Numerous fine spines. (One genus and species.) Family Palaeodiscidae. Four columns of plates in an ambulacral area, and eight columns of plates in an interam- bulacral area. Small primary spines and tubercles. (One genus and species.) Family Echinocystidae. CC. Regular, periproct within oculo-genital ring. Two to twenty columns of plates in each ambulacral area, and three to fourteen columns of plates in each interambulacral area. Plates imbricate or not. Primordial interambulacral plates in basicoronal row, or resorbed. Base of corona not resorbed or resorbed. Oculars usually all insert. Genitals typically with more than one pore each; rarely (Lepidechinus) with one pore each. Madreporite usually not recognizable. Periproct covered with many thick plates. Peristome with many rows of ambulacral plates only, or in addition with interradial non-ambulacral plates. Spines primary and secondary, or the latter only. Primary tubercles perforate, secondary tubercles imperforate Order PERISCHOECHINOIDA. D. Two columns of plates in each ambulacral area, four to fourteen columns of plates in each interambulacral area. Plates thin, imbricating. E. Four to eight columns of plates in an interambulacral area. Primordial and additional interambulacral plates resorbed in the advance of peristome. Ocu- lars, genitals, and periproct imperfectly known. Peristome with many rows of 206 ROBERT TRACY JACKSON ON ECHINI. ambulacral and interradial non-ambulacral plates. Spines large primaries with perforate tubercles in the center of each interambulacral plate, also secondary spines and imperforate tubercles Family Archaeocidaridae. Fragmentarily known from dissociated interambulacral plates and spines. Plates similar to those of Archaeocidaris but primary tubercles without a basal terrace Eocidaris (one species). Ambulacral plates all alike, of equal height. Four columns of plates in an interambulacral area. Primary spines large, tapering, or inflated, smooth, or with lateral spinules. Primary tubercles with basal terrace and scrobicular ring Archaeocidaris (many species). Ambulacral plates low, w'ith also higher, wider and wedge-shaped plates. Six to eight columns of plates in an interamljulacral area. Primary spines cylin- drical. Primary tubercles with no basal terrace, but with scrobicular ring. Lepidocidaris (one species). EE. Five to fourteen columns of plates in an interambulacral area. Primordial interambulacral plates in basicoronal row. Base of corona not resorbed. Ocu- lars insert. Genitals with many pores each. Peristome with many rows of ambulacral plates only. Spines small eccentrically placed primaries with secon- daries, or the latter only Family Lepidocentridae. Test high, probably spheroidal; ambulacral plates high, two or three equaling the height of an adambulacral plate, pore-pairs uniserial. Seven to eight columns of nearly rhombic plates in an interambulacral area. Koninckoddaris (two species). Test high, spheroidal, ambulacral areas narrow throughout; ambulacral plates low, about eight equaling the height of an adambulacral plate, pore-pairs uniserial. Five to eleven columns of plates in an interambulacral area. Small primary spines and tubercles with secondaries on interambulacral plates. Lepidocentrus (five species). Test depressed to flattened; tlirough the ambitus circular, pentagonal, or cly- peastriform. Ambulacral areas broad, petaloid ventrally, narrow dorsally, pore-pairs uniserial. Eleven to fourteen columns of plates in an interambula- cral area. Small primary- and secondary tubercles on interambulacral plates. Hyattechinua (three species). Test high, spheroidal, ambulacral areas narrow throughout, pore-pairs mod- erately biserial. Nine to ten columns of plates in an interambulacral area. Secondary spines and tubercles only. . . . Pholidechinus (one species). DD. Two to twenty (usually more than two) columns of plates in each ambulacral area, three to eleven columns of plates in each interambulacral area. Plates not imbricate or imbricate. E. Test elliptical, obovate, spherical or subspheroidal. Two to twelve col- umns of plates in each ambulacral area, three to eleven columns of plates in each SYSTEMATIC CLASSIFICATION OF ECHINI. 207 interambulacral area. Plates not imbricate, but ambulacral plates bevel over the interambulacral on adradial sutures. Primordial interambulacral plates resorbed. One row only of interambulacral plates resorbed in advance of the peristome. Oculars usually all insert, genitals usually with three to five pores each. Peri- stome with many rows of ambulacral ami some interradial non-ambulacral plates (Plate 5(5, figs. 7, 8). Secondary spines and imperforate tubercles only. Family Pal,\eechinidae. Two columns of plates in each ambulacral area, consisting of plates which are all primaries; pore-pairs uniserial. Four to six columns of plates in each interambulacral area Palaeechinus (four species). Two columns of plates in each ambulacral area, consisting of plates which are alternately primaries and partially or completely occluded; pore-pairs biserial. Four to eight, or nine columns of plates in each interambulacral area. Maccoya (six species). Four columns of plates in each ambulacral area, consisting of demi- and occluded plates; pore-pairs biserial. Four to seven columns of plates in each inter- ambulacral area. Lovenechimis (six species). F o ■-;:: ij <\i ' cs 1 Ph a < CO O CO ft > -2 — S Ti != =3 ? C5 S o O O (jj g a U B (1 1 S < n g Id < Ui K Q « « S 2 1- 0 3 u >i 3 >. < III < 13 bl a < Q Z 0 ^ n ^ ^ - -1 K h z < < « a. (0 (209) 210 ROBERT TRACY JACKSON ON ECHINI. (Plate 1, fig. 1), in the corona each ambulacrum consists of two cokmins of high hexagonal plates, each with two pores, superposed, surrounded by a peripodium, and situated in the middle of the plate. Such a structure is not known in any other adult regular echinoid, but is almost identical with that seen in young Goniocidaris (Plate 2, figs. 1-3), Strongylocentrotus (Loven, 1892), and Phormosoma (Mortensen, 1904, p. 54). Each interambulacrum of Bothriocidaris consists of a single column of plates, which is represented by a single plate at the ventral border of the interambulacra in the young of all other Echini (Goniocidaris, Plate 2, fig. 1 ; Strongylo- centrotus, Plate 3, fig. 11; Phormosoma, Plate 3, fig. 10). This single plate is also seen in adults where resorption has not removed the base of the corona, as in the Palaeozoic Hyattechinus rarispinus (Plate 23, fig. 1), H. pentagonus (Plate 25, fig. I), H. beecheri (Plate 26), Perischo- domus (Plate 64, fig. 2). Again, the single ventral plate is seen in adult Recent Phormosoma (text-fig. 43, p. 80), Arbacia (text-fig. 227, p. 193), in the Triassic Tiarechinus (text-fig. 31, p. 70), and in nearly all clypeastroids and spatangoids (text-figs. 27-29, p. 70, and 52, 54, p. 80; Plate 3, fig. 15). It seems that there is ample proof that the interambulacrum begins with a single plate, as shown by Loven (1874), and Mortensen (1903) in Hypsiechinus (Plate 3, fig. 6). This structure with less evidence I correlated (Jackson, 1896, p. 233) as a stage in development with the single column of plates in Bothriocidaris, naming it the protechinus stage. As Palaeozoic types with many columns of interambulacral plates begin at the ventral border, the young, with a single plate representing a single column, and later add their several columns during development (Plate 26), it seems that Bothriocidaris throws great light on the numerous columns there existent (p. 64). In Bothriocidaris a row of primordial ambulacral plates forms a continuous ring around the mouth, as in all succeeding regular Echini, and they are relatively large, as in the young of modern Echini, Goniocidaris (Plate 2, fig. 1). In Bothriocidaris a second row of plates sur- rounds the first, completing the peristome. This second row was probably derived by flowing down from the corona, and is closely comparable to the second stage in development of Phor- mosoma (Plate 3, fig. 10), as pointed out by Mr. Agassiz (1904, p. 79). A striking feature of Bothriocidaris is the great size of the oculars and the very small genitals (Plate 1, fig. 2). This may fairly be compared with the similar condition in very young Echinus microtubercu- latus (Plate 3, fig. 5), as shown by Bury (1895), though the adult of the same species has large genitals which exclude the oculars from the periproct. The most difficult thing to account for in Bothriocidaris is the fact that the teeth seem to be radial in position, as discussed (p. 242). Of the three species of Bothriocidaris, the salient features are, briefly, as follows. In B. archaica sp. nov. the oculars meet in a continuous ring with genitals dorsal to the same (Plate 1, figs. 1, 2), interambulacral plates bear spines. In B. pahleni Schmidt (Plate 1, figs. 3-6), part of the genitals lie dorsal to the oculars, and part separate the oculars, reaching the inter- ambulacra; interambulacral plates are without spines. In B. globulus Eichwald (Plate 1, figs. SYSTEMATIC CLASSIFICATION OF ECHINI. ■ 211 7-9) all of the genitals separate the oculars and meet the interambulacra; interambulacral plates bear spines. These several species occur in the Ordovician of Russia (p. 238). The feature of Palaeozoic Echini is that they have more than two columns of plates in each interambulacral area. This is true of all known forms excepting Bothriocidaris and Miocidaris as far as the latter occurs in the upper Palaeozoic. Gregory (1897), SoUas (1899), and others have assumed that the most primitive form of Echini had many columns of interambulacral plates in an area, and several authors have considered Palaeodiscus as the most primitive known type. On this basis evolution would entail a loss of such parts, as our modern tj^pes all have two columns of interambulacral plates in an area. The evidence of development and adult structure is opposed to this view. At the ventral border of the young of all known modern types, and at the ventral border of the adult where not removed by resorption, we find a single primordial plate in each interambulacral area succeeded in the second row by two plates. There is no evidence in development of a larger number of columns dropping out to two in any known living form, or indeed, in any fossil form excepting as seen in senescence (Perischocidaris, Plate 65, figs. 1, 2; Plate 67, figs. 2, 3), and in the little known Tetracidaris of the Cretaceous. I, therefore, consider the Echini usually classed as the Euechinoida, with a geological range from the Lower Carboniferous to Recent inclusive, and comprising the orders Cidaroida, Cen- trechinoida, and Exocycloida as next related to Bothriocidaris. This view is based on structure and development. I am well aware of the intervening geological gap, but can only appeal to the rarity of all forms in the Silurian and Devonian to account for the absence of intermediate types. Wliile the Palaeozoic orders Echinocystoida and Perischoechinoida are not considered as in the direct line of ancestry of modern forms, they yet show many primitive characters, especially as regards the peristome and lantern, which can well be compared with the condition in the young of later types. The order Cidaroida is placed as directly derived from the Bothriocidaroida without known intermediate forms. The Cidaridae as regards the structure of the young and adult are the least removed from Bothriocidaris of any known echinoid, living or fossil. The young (Plate 2, figs. 1-3, 6) have high hexagonal ambulacral plates with the pores of the pore-pairs superposed. Each interambulacrum has a single plate ventrally, succeeded by two plates in the next row. The peristome has a single row of primordial ambulacral plates which are like those of Bothriocidaris excepting that in that type there are two peristomal rows. The base of the corona has not yet been resorbed, exactly like adult Bothriocidaris. In j'oung cidarids the genitals are large and oculars small and exsert, unlike Bothriocidaris. The young lantern is inclined (Plate 2, fig. 17), with deep foramen magnum, long interpyramidal muscles (Loven, 1892) as in the Perischoechinoida, and the lantern muscles are attached directly to the basi- coronal interambulacral plates without apophyses (Loven, 1892), as is the apparent condition in the Perischoechinoida (text-fig. 221, p. 193). 212 ROBERT TRACY JACKSON ON ECHINI. Adult cidarids differ from the j'oung principally in having low instead of high ambulacral plates, the ventral border of the corona resorbed, many rows of ambulacral, also (with rare exception) interradial non-ambulacral plates in the peristome. Many rows of ambulacral peri- stomal plates is distinctly a Palaeozoic character (Plate 23, fig. 1), and some genera as Archae- ocidaris have non-ambulacral plates as well. The oculars of adult cidarids may be all exsert as in the young, or one or more to all insert, as shown here. The periproct is plated with numer- ous heavy plates as in the Perischoechinoida. The lantern of the adult is nearl,y vertical (Plate 2, figs. 7-10), instead of inclined, with a very shallow foramen, and short interpj'ramidal muscles. The perignathic lantern muscles are all inserted on the elevated apophyses, a feature peculiar to this order only (text-fig. 222, p. 193). The absence of pits in the top of the pyra- mids is a feature like the Perischoechinoida but unlike the Centrechinoida. Teeth are grooved as in the Echinocystoida, Perischoechinoida, and Aulodonta. Primary spines are strikingly developed, naked of epidermis, with a cortical layer and a collar about the base, thus differing from the next order. The presence of frondescent Stewart's organs and the absence of peri- stomal gills and sphaeridia are distinctive characters of the order. The Cidaroida present distinctly a combination of Palaeozoic with modern characters. Apparently only two species in the Palaeozoic are definitely referable to the Cidaridae, the Lower Carboniferous Miocidaris cannoni sp. nov. from America (p. 247), and the Permian Miocidaris keyserlingi (Geinitz) from Europe (p. 245). The Centrechinoida as an order differs from the Cidaroida in that the ambulacral plates are usually compound; the peristome (excepting the Echinothuriidae) has only one row of primordial ambulacral plates, with non-ambulacral plates. The lantern has a deep foramen magnum, the tops of the pyramids are pitted, and auricles are developed, on which are inserted the retractor muscles of the lantern. Peristomal gills and sphaeridia are developed. The young of the Centrechinoida (Plate 3, figs. 9-11) make a close approach to cidarids in having simple ambulacral plates, and large central spines and tubercles in the middle of the inter- ambulacral plates. Further, the young have a single primordial interambulacral plate in each area, peristome covered with ambulacral plates only, inclined lanterns with long interpyramidal muscles recalling young cidarids, and the characters seen in the adults of certain Palaeozoic types, as previously discussed. Of the three suborders of the Centrechinoida, the first has the most characters like the Cidaroida, and the last has the fewest (p. 183). The suborder Aulodonta, while having the characters of its order, makes an approach to the Cidaroida in that the teeth are grooved, epiphyses narrow (text-fig. 211, p. 184), and primary tubercles perforate. Some members also approach the Cidaroida in possessing simple ambulacral plates, or Stewart's organs, or many rows of ambulacial plates in the peristome. The Hemicidaridae being wholly a fossil group, some characters are wanting. The peri- stome is unknown. They are primitive in that oculars are mostly all exsert, regressive in that SYSTEMATIC CLASSIFICATION OF ECHINI. 213 Hemicidaris in the upper half of its test has returned to simple ambulacral plates. In my tables of ocular plates (pp. 154-159) Duncan is followed in the grouping of fossil genera in families, and it is possible that some genera placed in this family may have other affinities. The plates are thick and most genera have perforate tubercles. The Aspidodiadematidae are essentially characterized by simple ambulacral plates, large primordial ambulacral plates which nearly cover the peristome (text-fig. 51, p. 80), and a very large apical disc, all of which are primitive characters. There are very wide insert oculars (text-fig. 80, p. 104). The test is thin and fragile. The Centrechinidae have compound ambulacral plates of usually three elements each. The Mesozoic species have relatively thick plates, not imbricate, oculars usually all exsert. The living species have thin tests more or less imbricate, three or more oculars usually insert (text-figs. 81-99, pp. 105-109); while the sequence of incoming of oculars is typically I, V, IV, II, it is not rarely {Centrechinus setosus) I, V, IV, III, a feature rare in the order as a whole, but a typical variation of many cidarids (p. 96). There are only ten primordial ambulacral plates of moderate size on the peristome with non-ambulacral plates (Loven, 1892, Plate 12, figs. 153, 154), a strong distinction from the Echinothuriidae. While the lantern is typically erect, it is inclined in Astropyga, as in the next family. In the Echinothuriidae the test is remarkably thin and strongly imbricating, and ambu- lacral plates are compound, composed of three elements each. The primordial interambulacral plates are retained in the basicoronal row (text-fig. 43, p. 80). It is remarkable that a peri- stome of the size attained in this group comes about by the growth of the plates and not by resorption of the base of the corona. Perhaps the most distinctive character of the family is the fact that the peristome is plated with many rows of ambulacral plates only, a feature rarelj' seen in the Cidaroida and in no other post-Palaeozoic group ; but a character of many Palaeo- zoic genera, as Palaeodiscus (Plate 18, fig. 2), Hyattechinus (Plate 23, fig. 1), and Lepidesthes (Plate 68, fig. 3). Mortensen (1904, p. 53) on the strength of my observations classed the Echinothuriidae with the Lepidocentridae because of the similarity of the peristome. I cannot agree with this view; the ambulacra and interambulacra differ radically from those of the Lepidocentridae. In addition, the auricles, peristomal gills, and pits in the top of the pyramids class this family with the Centrechinoida and with close affinities to the Centrechinidae ex- cepting for the peristome. This is the relationship recently adopted by Messrs. Agassiz and Clark (1909). The character of the peristome with many rows of ambulacral plates only may perhaps be best considered a localized reversion, or parallelism to some ancient type. Such is seen, in fact, in the young of all regular Echini where the peristome for a time is covered with ambulacral plates only. The oculars of the echinothuriids are exsert in the young (Plate 3, fig. 8), but in the adult are all insert and often are separated from the genitals (text-fig. 170, p. 149), a 214 R()BP:RT TRACY JACKSON ON ECHINI. feature characteristic of this family only. The lantern is inclined, with long interpyramidal muscles (Plate 2, figs. 19-21), a feature like the Palaeozoic, and also the young of types that have erect lanterns in the adult. It may be considered a retention in the adult of a youthful character, seen also in Astropyga. Stewart's organs, more or less developed, are a feature of this family, as of the Cidaroida, although they differ markedly, being vermiform or sausage- shaped rather than frondescent as in the Cidaroida. Radial peristomal and somatic muscles (text-fig. 226, p. 193) are peculiar features of the Echinothuriidae and known in no other Echini. The suborder Stirodonta differs from the Aulodonta in that the teeth are keeled, but agrees with it in that the epiphyses are narrow (text-fig. 212, p. 184). In some the primary tubercles are perforate as in the Aulodonta, but in most genera tubercles are imperforate as in all the Camarodonta. This group, as most others, occupies an intermediate position, possess- ing in some genera characters like the preceding, and in others characters like the succeeding group. Such is found to be true of any classification if the groups are at all nearly related. The Saleniidae is one of the most interesting groups of Echini. The permanence in the adult of the suranal is one of the most striking features (text-figs. 102-105, p. 111). It may lie against genital 3 as in Salenia or facing ocular III (Peltastes), or may be one of several large plates as in some Acrosalenias. This is apparently the first appearance of the suranal plate in Echini, as discussed (p. 174). The ambulacral plates may be compound throughout, or only ventrally and simple above, indicating extreme reversion. The large primary spines and prominent central tubercle Hke Cidaris may fairly be considered a retention in the adult of characters common to the young of the whole order, indicating with the simple ambulacral plates, when they occur, a reversionary rather than a progressive type. The retention of a single madreporic opening on the proximal side of genital 2 (Plate 4, figs. 1, 2) is a fact in accordance with the same line of reasoning. Ocular plates are exsert or reach the periproct in the sequence I, V, IV, II, III, a contrast to the Arbaciidae where the sequence is V, I, IV. The Phymosomatidae Mortensen (1904, p. 55) classes with the Saleniidae and Arbaciidae, expressing closer relations with the latter. Duncan (1889a) also places this familj^ next to the Arbaciidae. Ambulacral plates are compound, of five elements each in the Recent Glypto- cidaris crenulare, or with more or fewer elements in some fossils. One or more to all oculars may be insert (text-fig. 106, p. 113). Mr. Agassiz's photographic figure (1873, Plate 7a, fig. 8) of Recent Glyptocidaris crenulare shows ocular I only insert. This character, with the somewhat prominent suranal plate opposite genital 3 in the figure cited, inclines one to affiliate the family rather more closely with the Saleniidae than the Arbaciidae. The teeth are keeled, epiphyses rkarrow, and spurs from the dorsal tips of the pyramids support the teeth as in Stomopneustes (Plate 4, figs. 8-10). This structure is peculiar to' these two families only, as far as known. The Stomopneustidae contains only the Stomopneustes variolaris. This genus has been classed with the Echinometridae, but Mortensen (1903, p. 133) separated it as a distinct family SYSTEMATIC CLASSIFICATION OF ECHINI. 215 on account of differences in spicules of the tube-feet which are very large and peculiar, and on differences of the pedicellariae. It differs from the Echinometridae also in that the oculars enter the periproct in the sequence I, V, IV (p. 113), not V, I, IV, as in that family. The most important character, however, is that the teeth are keeled and epiphyses narrow, not meeting in suture over the foramen magnum (Plate 4, figs. 8-10). Other important peculiarities exist as described (p. 186). The character of the lantern places this genus and family as a member of the Stirodonta. Its affinities are nearest to the Phymosomatidae as indicated by spurs from the dorsal tips of the pyramids, which support the teeth (Plate 4, figs. 8-10). The ambu- lacral plates are composed of three elements each, and where fully developed, as at the mid- zone, four or five plates are combined and grown over by a large primary tubercle, a very strik- ing character. The periproct has numerous small plates, as in Phymosoma. The ocular plates of the bivium are typically insert in Stomopneustes variolaris, but I only or I, V, IV may be insert as described (p. 113). Also, as an aberrant variation, three specimens in a total of 64 have oculars I, V, II insert. This is an essential character of the Echinidae and Strongylo- centrotidae, and suggests a connection between the families. The small suranal plate of Glypto- cidaris crenulare, as figured by Mr. Agassiz (1873, Plate 7a, fig. 8), suggests the same affinity, and it seems that in one of these two families may lie the ancestral stock of the Echinidae and allies. Of the Arbaciidae the most striking feature is that there are only four, or five, nearly equal plates in the periproct, a feature otherwise known only in Parasalenia. Another important feature not so readily ascertained is the retention of the primordial interambulacral plates in the basicoronal row, as pointed out by Mr. Agassiz (1904, p. 54). A third unusual character is that ocular plates, when they enter the periproct, do so in the sequence V, I, IV (p. 158). This is known elsewhere only in the Echinometridae, as here shown. The auricles exist as widely separate styloid processes (text-fig. 227, p. 193) or slightly arched and joined by suture (text-fig. 228). Tube-feet are dorsally modified as ambulacral gills in Arbacia (A. Agassiz, 1872, p. 264). In the genera Habrocidaris and Podocidaris, as shown by A. Agassiz and Clark (1908, p. 77), and Pygmaeocidaris, as shown by Doderlein (1906), the primordial interambulacral plate is retained as in Arbacia. The suborder Camarodonta may be considered the most specialized of modern regular Echini on the basis of the lantern, and also in various genera by the sculptured test, the degree of specialization of the ambulacrum, peristome, perignathic girdle, or the elliptical form through a sidewise axis. The essential feature is the fact, that while the teeth are keeled as in the last ' Doderlein figures (1906, p. 184, fig. 36e) a third median plate in tiie third coronal row of Pygmaeocidaris. As this genus so closely resembles Habrocidaris, in which there is a median tubercle but no median third plate, it is possible that Doderlein was misled in introducing the sutures of this plate, a point difficult to ascertain in such a small specimen (4.8 mm.) as he had. If Doderlein is correct, it is a unique case in the Centrechinoida. 216 ROBERT TRACY JACKSON ON ECHINI. suborder, the epiphyses are wide and meet in a medium suture over the foramen magnum (text- fig. 213, p. 184; Plate 5, fig. 2). The epiphyses further develop crests that serve as supports for the upper part of the tooth. These features are known in no other Echini and express the fullest differentiation attained in the lantern of regular Echini (p. 187). In the Temnopleuridae, the circular form of the test, the ambulacral plates of three ele- ments each, with the curious pitting or sculpturing developed on at least the upper part of the test are the most distinctive features. The peristome is mostly naked except for the primordial ambulacral plates. Of Pleurechinus doederleini, Mortensen (1904, p. 78) says that there are only five, instead of ten, buccal plates, a unique character. This is a very remarkable departure, as the ambulacrum, both in the peristome and in the corona of Echini, so constantly consists of a right and a left half for each area. The ocular plates are mostly exsert in modern types, a primitive character, but one or more are said to be insert (Duncan, 1889) in some fossil types. In the Cretaceous Glyphocyphus radiatus all oculars are insert, p. 117. This family on account of its peculiarities represents probably a side branch rather than the basal series of its suborder. The Echinidae is a large family including many genera and species; the test is circular or slightly pentagonal; ambulacral plates have three elements each. Plates with two elements each occur in Echinus affinis, but this is exceptional, and they exist dorsally only (p. 118). The peristome has ten primordial ambulacral plates of moderate size (Loven, 1892, Plate 12, fig. 156) with more or fewer large or small to granular non-ambulacral plates. Exceptionally there is a dense covering of non-ambulacral plates (Toxopneustes, text-fig. 57, p. 84). The oculars may be all exsert, or ocular I, or very frequently I, V, or rarely I, V, IV may be insert as a species character; as progressive variants, oculars I, V, IV, II, or all plates may be insert. The range of variation is wide in many species, but with rare exceptions oculars reach the periproct in the sequence I, V, IV, II, III (text-figs. 115-127; pp. 160, 161). The commonest aberrant variant for oculars is for I, V, II to be insert, a combination seen only in this family, the Strongylocentrotidae, Stomopneustidae, and Saleniidae (p. 164). In Gymne- chinus (p. 120) typically an abnormal arrangement of oculars occurs as described. The periproct is plated with many larger or smaller plates, and in adults frequently a somewhat larger plate, which is apparently the suranal, lies against or near genital 3 (text-fig. 122, p. 122). The size which renders this plate distinguishable is subject to much individual variation. It is markedly developed in some, as Toxopneustes variegatus. Of the perignathic girdle the auricles are moderately developed and commonly meet in an arch over the ambula- rum; apophyses are usually only slightly developed. The Strongylocentrotidae (Gregory, 1900, p. 313) includes the large genus Strongylocen- trotus and a few others. The test is circular as in the Echinidae, but, with the exception of some species of Echinostrephus, there are from four to ten elements to an ambulacral plate. As in the Echinidae, oculars may be all exsert or otherwise reach the periproct in the sequence SYSTEMATIC CLASSIFICATION OF ECHINI. 217 I, V, IV, II (text-figs. 128-153). Many species typically have the bivium insert, and one, Strong ylocentrotus (jibbosus (p. 145), typically has an aberrant arrangement of oculars. Strongy- locentrotus albus is peculiar in having frequent and also very rare forms of aberrant arrange- ment of ocular plates (pp. 127, 162, 164). The Echinometridae, as here restricted, includes genera which are elliptical through a sidewise axis, not through III, 5. As a secondary character, the ocular plates, when reaching the periproct, do so in the sequence V, I, IV, not I, V, IV, as in the Echinidae and Strongy- locentrotidae. This sequence is strongly held as shown in the table (p. 163), but occasionally ocular I comes in first as a variant. These characters eliminate vStrongylocentrotus and its allies, which have usually been included here, but which I include in Gregory's (1900) family Strongylocentrotidae placed next to and associated with the Echinidae. The Echinometridae besides the above have in some genera other striking and very specialized characters. In Echinometra lucunter the auricles are extraordinarily developed as high spoon-shaped arches (text-fig. 230, p. 193). In Heterocentrotus the primary spines attain excessive size and bulk, and in another direction the spines of Colobocentrotus are most abbreviated, truncate; and in C. atratus the spines are polygonal from mutual pressure, so as to resemble angular plates. The Exocycloida present a remarkable series of forms of Echini, all characterized by having the periproct eccentric in interambulacrum 5. Assuming a monophyletic origin for the group, the three sul)orders present a striking series of structural departures from the regular Echini from which they doubtless originated. Considering the characters of the group as a whole in brief, the compound ambulacral plates and peristomal gills of the Holectypina and the auricles of that group and the Clypeastrina, the existence of keeled teeth, where teeth are known, and the presence of sphaeridia, are all characters which unquestionably associate the Exocycloida with the Centrechinoida and not with the Cidaroida, where these structures are non-existent. Mr. Agassiz (1909) has shown that in the young of the spatangoid Echino- neus a well developed lantern exists. This discovery is of the greatest interest and importance, as previously teeth were unknown in this group. On examining his figures and description, it is seen that the lantern is erect, teeth are keeled and epiphyses narrow, not extending over the foramen magnum. Further, the interpyramidal muscles are short, the protractor and compass muscles are inserted on the primordial interambulacral plates, and the retractors are inserted on the auricles which are situated on the ambulacra. These details are shown more fully by Westergren (1911), in a monograph on the genus Echinoneus, which death pre- vented Mr. Agassiz from writing. Looking back to the Centrechinoida, we find that this type of lantern exists only in the suborder Stirodonta. Further, the attachment of muscles, as stated, occurs only in Arbacia and probably other members of its family. Looking at the matter from the view-point of Arbacia, in that genus the teeth are keeled, epiphyses narrow (text-fig. 212, p. 184); the primordial interambulacral plates are retained in the basicoronal 218 ROBERT TRACY JACKSON ON ECHINI. row; auricles exist as usually separate styles (text-fig. 227, p. 193): ambulacral tube-feet dorsally are modified as ambulacral gills, and locomotion is effected in considerable part (A. Agassiz, 1872, p. 264) by the spines. Any one of these characters might well be a case of parallelism, but the sum of them gives strong evidence of kinship with the Exocycloida. I therefore consider the Exocycloida as connected with the Arbaciidae, probably through some early common ancestral stock. The suborder Holectypina is nearest akin to the Centrechinoida as it possesses compound ambulacral plates and peristomal gills. The ambulacra are not petaloid dorsally as they usually are in clypeastroids and spatangoids. The lantern is inclined with moderately deep foramen magnum, and the wings of the pyramids are horizontally striated for attachment of inter- pyramidal muscles as in regular Echini (Loven, 1892). The teeth are narrow, elongate, and keeled, and the epiphyses are large, but narrow (Hawkins, 1909). These characters make a near approach to those of the Stirodonta, but in part are like those of the clypeastroids. In addition, in the Holectypina auricles exist on the ambulacra and there are low interambulacral apophyses, both of which occur in the Centrechinoida, but auricles only exist in the clypeas- troids. In the Holectypina the base of the corona is only partially resorbed so that one or more primordial interambulacral plates are retained in the basicoronal row of the adult (Loven, 1874). In this suborder there are usually five genital plates with the madreporic pores in genital 2, but in genital 5 there is no pore (text-fig. 171, p. 149), or genital 5 may be absent. The test is usually circular and not specially modified as in the two following sub- orders. In the suborder Clypeastrina the test is more or less flattened to discoid, circular, penta- gonal, or other shape, and elongate and bilaterally symmetrical through III, 5. Internal pillars and stalactitic growths are peculiar to the group, also lunules, or reentrant emargina- tions may develop. Ambulacral plates are simple with the typical ambulacral pores limited to a petaloid area dorsally ; ventrally there are numerous very minute pores, described by Mr. Agassiz (1874, p. 695). They are so small as to be recognizable onlj- with difficulty. Both the primordial ambulacral and interambulacral plates are in the basicoronal row (text-fig. 52, p. 80) a character not known in regular Echini at any stage in life. Rarely (Arachnoides, p. 72, according to Loven, 1874), one interambulacral plate is pushed dorsally and the other four are resorbed by intracoronal resorption so that the basicoronal row may be composed of ten ambulacral plates only, a unique condition for Echini. The peristome is leathery without plates, as far as I have been able to ascertain, a condition unknown in other Echini. Oculars and genitals are fused into a mass; comparable to this is the partial fusion of genitals typical of some spatangoids, and the partial fusion of genitals or genitals and oculars occurring as variants in regular Echini (p. 167). There are four or five genital pores, but the posterior is usually wanting. In Laganum diploporum, described by Messrs. Agassiz and Clark (1907), curiously enough there are six genital pores, two being in area 5. The lantern is usually SYSTEMATIC CLASSIFICATION OF ECHINI. 219 procumbent (text-figs. 214-216, p. 184) though inclined in Echinocyamus (Loven, 1892). The teeth are keeled, foramen magnum shallow, epiphyses small and narrow, brace small, wanting in Laganum (Loven, 1892), compasses wanting. The pyramids are more or less highly modi- fied (least in Echinocyamus, Loven, 1892) with wing-like lateral expansions and without ridges for the attachment of interpyramidal muscles. The perignathic girdle consists of auricles only, which are situated on the ambulacral plates (text-fig. 231, p. 197) as in the Centrechinoida, or may be transferred to the primordial interambulacral plates (text-fig. 236, p. 197), as shown by Loven (1892). Each pair of auricles bears one or two pairs of retractor muscles, and in addition one pair of protractors. This last is a peculiar feature of the group, as in all other Echini with lanterns the protractors are inserted on the interambulacral plates, or on apophyses arising from the same, but not on auricles. Respiration is apparently by ambu- lacral gills only. Sphaeridia are situated in enclosed pits (Loven, 1874). The suborder Spatangina is characterized by tests which are conical, heart-shaped, or of various forms, often extraordinary (Pourtalesia), but bilaterally symmetrical through III, 5. The ambulacral plates are simple and the areas are usually petaloid dorsally. There may be two pores or one to a plate, or plates may be in part imperforate. The plates may be low, or high, and of very specialized form, especially in the bivium ventrally. Plates, especially in ambulacrum III, may be high, nearly or quite hexagonal in form, with pores superposed in the middle of the plate (text-fig. 8, p. 54), a feature not seen in the adults of other forms since the Ordovician Bothriocidaris, though common in the young of regular Echini. The primordial ambulacral and interambulacral plates are typically in the basicoronal row (Plate 3, fig. 15), but, in the shifting of parts, one or the other may in part be pushed dorsally. The base of the corona is apparently never resorbed by the advance of the peristome, which latter is usually densely covered with small non-ambulacral plates (text-figs. 27-28, p. 70, and 54, p. 80). Ocular plates may be all exsert, that is, do not reach the center line, or oculars I, V, or I, V, IV, or I, V, IV, II may be insert (text-figs. 172-175, p. 149). In cei-tain specialized types, as seen in some species of Pourtalesia (Loven, 1883; A. Agassiz, 1904), the oculars are apparently partially or wholly absent in adults. Four genitals are typically present, but 5 is absent and more genitals may be wanting. Genitals 2, 3 may be typically fused (Cystechinus, A. Agassiz, 1904), as seen in variants of regular Echini (p. 167). The madreporic pores are in genital 2 or extend to 3 where these two plates are fused. An extension to 3 is a coimnon variant in some regular Echini (p. 172). Genital 2 may be much enlarged and extended posteriorly so as to reach interambulacrum 5 (text-fig. 173, p. 149). The lantern is wanting in adults, but occurs in young Echinoneus (pp. 189, 217), as shown recently by Mr. Agassiz (1909). Spines are short and undifferentiated, or may be highly produced and with marked areal differentia- tion. Tubercles in some genera are subject to much areal differentiation, making patterns on the test. The primary tubercles in Metalia, Meoma, and Brissus are perforate and crenu- 220 ROBERT TRACY JACKSON ON ECHINI. late, and in Metalia even the secondary tubercles are perforate. This perforate character is a parallelism or a reversion to a feature typical of Palaeozoic species, the Cidaroida and Aulo- donta, but is not known in other regular Echini excepting a few of the Saleniidae. Respiration is apparently by ambulacral gills only. Sphaeridia are present. All the orders thus far considered have two columns of plates in each ambulacral area; and one column (Bothriocidaroida) or two columns (Cidaroida, Centrechinoida, Exocycloida) in each interambulacral area. In the latter the two columns succeed a single column repre- sented by a single plate in the basicoronal row, seen in the young and also in the adult if not resorbed. In succeeding orders we find two or more columns of ambulacral plates, and three or more columns of interambulacral plates in each area respectively, and on this basis they are considered a further remove from the primitive than any of the above types (p. 70). Of the Plesiocidaroida the type is Tiarechinus, which presents certain extraordinary characters as shown by Loven (1883). The ambulacra in each area are composed of two columns of low simple plates. The interambulacra in each area retain the primordial inter- ambulacral plates in the basicoronal row, but in the second row there are three plates (text-fig. 31, p. 70). For a row of three plates to follow immediately the primordial interambulacral plate of the first row is a character known in no other Echini. In Tiarechinus the three plates of the second row are elongate, reaching the apical disc, so that each area consists of but two rows of plates, a unique condition. The genitals are exceptionally large, nearly covering the dorsal field of the urchin, and oculars are small and strongly exsert. With present knowledge it is not closely affiliated with anj^ other group. The three columns of plates in an inter- ambulacral area, the retention of the primordial interanibulacral plates, and the imperforate and peculiarly distributed j^rimary tubercles are all objections to including Tiarechinus in the Cidaroida as done by Bather (1909a, p. 66). The Palaeozoic Echini included in the orders Echinocystoida and Perischoechinoida structurally are more removed from the primitive as regards the ambulacrum and interambula- crum than any Recent form. Further, there is no evidence in the development of modern types that they were derived from these more complex forms. Following the facts there is no alternative, as I see it, except to consider the Palaeozoic genera of the Echinocystoida and Perischoechinoida as structurally differentiated on independent lines from later known forms and on those lines further removed from the primitive than are the tj^pes now living. A some- what comparable case is seen in Cephalopoda in which the extinct Devonian to Cretaceous Ammonoidea are further removed structurally from the primitive Cephalopoda than are the species of living Nautilus. Certain broad characters of these Palaeozoic orders may be considered first as they are features of the group as a whole and can thus be disposed of and save repetition. Ambulacral plates are always simple, with two pores each. There may be two columns of such plates in SYSTEMATIC CLASSIFICATION OF ECHINI. 221 each area or more than two up to twenty, but with an equal number in each half-ambulacrum. The primordial interambulacral plates may be retained in the basicoronal row, or maj^ be re- sorbed in the advance of the peristome. There are three or more, usually more, columns of plates in each interambulacral area, and there may be as many as fourteen, the highest number at present known. The plates may be imbricate or not, but when imbricate, the ambulacrals imbricate adorally and the interambulacrals aborally and from the center laterally and over the ambulacrals (text-figs. 32-38, p. 75). The primordial ambulacral plates are situated on the peristome around the mouth as in Cidaris, as known in some types and inferred in others. Succeeding these on the peristome there are many rows of ambulacral plates only, extending to the corona, or in addition there may be non-ambulacral plates in certain genera. In the Echinocystoida the apical disc is doubtful, but in the Perischoechinoida it is small proportion- ately to the diameter of the test, a progressive character (Plate 55, figs. 2, 3). Oculars are typically all insert, but rarely one or more to all may be exsert. Ocular pores are rarely visible. Genital plates are of moderate size and have from two to ten or eleven pores each. In one species only (Lepidechinus tessellatus, Plate 63, figs. 7, 8) there is a single genital pore in a plate as far as preserved. A madreporite is only occasionally recognizable. The periproct where known is covered with many thick plates as in Cidaris, and no suranal is recognizable. The lantern is inclined (Plate 27, figs. 4-6) like that of the young of modern regular Echini, and as in that group, is composed of forty pieces. The teeth are grooved as in the Cidaroida and Aulodonta. The epiphyses are narrow; brace and compass as in modern forms. The foramen magnum is moderately deep and the lateral wings of the pyramids have ridges for the attachment of interpyramidal muscles which were relatively long. The dorsal face of the pyra- mids is smooth as in the Cidaroida, not pitted as in the Centrechinoida. No perignathic girdle is known and lantern muscles were apparently attached directly to the base of the inter- ambulacral plates (text-fig. 221, p. 193), as in young Cidaris. There may be primary spines with secondaries, and the tubercles of primary spines perforate; or there may be secondary spines only, the tubercles of which are imperforate. Respiration was quite likely effected by Stewart's organs, as in the Cidaroida. There certainly were no peristomal gills as in the Cen- trechinoida. Sphaeridia are unknown. The Echinocystoida as an order is characterized by the fact that the anus was apparently eccentric in an interambulacrum. The material is not well preserved and is imperfectly known. There are two or four columns of ambulacral and eight or nine columns of interambulacral plates in each area respectively. The plates are thin and imbricating. What the relations of this group are is uncertain, though it has certain approximations to the Lepidocentridae and may be an offshoot from a common early stock. As an irregular group in the Palaeozoic it is considered first in order to dispose of it (p. 250). The Palaeodiscidae as a family is the least specialized of its order as there are only two 222 ROBERT TRACY JACKSON ON ECHINI. columns of plates in an ambulacral area. These plates are wide and low with two pores each. The pores are not wanting ventrally, as stated by Gregory (1897). There are apparently eight or nine columns of irregular interambulacral plates in an area and the primordial inter- ambulacral plates are in the basicoronal row. The peristome is covered with ambulacral plates only, and the lantern, as shown by SoUas (1899; my Plate 18, fig. 5), has the usual features of Echini. The anus is apparently eccentric in an interambulacrum, as shown by Spencer (1904), but ocular and genital plates are unknown. An attempt has been made to correlate the only species in this family with Asteroidea and to regard it as a primitive echinoid, which I believe is unreasonable as considered under the description of the species. There is only one genus and species {Palaeodiscus ferox) from the Lower Ludlow, Silurian of England (Plate 18, figs. 1-5), (p. 250). In the Echinocystidae the ambulacra consist of four columns of plates in each area, being evidently an independent taking on of this character of many columns, which is a feature of the Palaeozoic. There are eight columns of plates in an interambulacral area. The anus apparently, as figured, is eccentric in an interambulacrum (Plate 20, fig. 1). There is only one genus and species which is imperfectly known: Echinocystites pomum Wyville Thomson from the Lower Ludlow, Silurian of England (Plate 16, fig. 4; Plate 18, figs. 6-8; Plate 20, figs. 1-4), (p. 252). The order Perischoechinoida includes all the remaining and the best known Palaeozoic genera. In this order the periproct is within the oculo-genital ring. While in many genera there are two columns of plates in each ambulacral area, there is in other genera an excess of this number up to twenty columns, and this is the one group of Echini characterized by extreme development in this direction. The interambulacra rarely have as few as three columns of interambulacral plates in an area, usually there are more, and as an extreme, as many as four- teen occur in each area. This character of the ambulacra and interambulacra is the most striking Palaeozoic feature. Coronal plates may be imbricate or not, varying with the families; also the primordial interambulacral plates may be in the basicoronal row or resorbed, vary- ing with families. Of course this is a rather nice point to determine as a specimen must be perfect ventrally in order to ascertain it, so that the evidence may be taken from only a few species, or even a single species in a family, and is open to correction with further knowledge. The peristome may have many rows of ambulacral plates only, the more primitive character, or may have the same with in addition some interradial non-ambulacral plates. Ocular plates are typically all insert, but in a few cases one or more to all are exsert, foreshadowing the typical Mesozoic and immature (also often adult) modern character. The genital plates are of moderate size and usually have from two to many genital pores. The madreporic pores have been rarely found, but usually are not distinguishable and perhaps were not existent in some species. The Palaeozoic periproct is well known in this order, and is practically uniform throughout. SYSTEMATIC CLASSIFICATION OF ECHINI. 223 composed of many rather thick plates close to the character of the periproct in the Cidaroida. The lantern is known well for the Palaeozoic onlj' in this order, and is strikingly uniform in character. Mr. Agassiz (1881, p. 80) suggested and Duncan (1889a, p. 11) stated that there are passages for peristomal gills in Archaeocidaris, but such have not been figured. As there was no indication of such slits in the best specimens which I studied (Plate 9, fig. 6 and Plate 11, fig. 1) and as such structures are otherwise known only in the Centrechinoida and Holec- typina, I think this is a mistake, and peristomal gills may be considered as wanting in Archaeocidaris and other Palaeozoic genera as well (p. 253). The Archaeocidaridae is a family with only three genera, Eocidaris, Archaeocidaris, and Lepidocidaris. Eliminating Eocidaris, which is most imperfectly known, there are two columns of ambulacral plates and four, or six to eight of interambulacral plates in each area respectively. The plates are rather strongly imbricate, and each interambulacral plate bears a central pri- mary perforate tubercle and large spine, also secondary spines and tubercles. The base of the corona is extensively resorbed, there being four plates in the basicoronal row in each interambu- lacral area. The peristome has many rows of ambulacral plates with non-ambulacral plates in Ai'chaeocidaris (Plate 9, figs. 6, 7) and probably was similar though unknown in Lepido- cidaris. The apical system is doubtfully known, which is rather remarkable with so many species. The lantern is well known in Archaeocidaris (Plate 12) and consists of forty pieces as in modern regular Echini (p. 254). The genus Eocidaris has large interambulacral plates which bear a central perforate tubercle, as in Archaeocidaris; there is a scrobicular area but no basal terrace. This genus is most imperfectly known, but Bather (1909), who has studied the question and material critically, feels that it should be retained. Only one species is recognized, the Devonian E. laevispina (Sandberger) , Plate 15, figs. 11, 12. The absence of a basal terrace, which is the essential difference from Archaeocidaris, could easily be accounted for by wear as it is not always ob- servable in true Archaeocidaris. The young dorsal plates of Archaeocidaris have no basal terrace (Plate 11, fig. 2), so that a genus founded on this character should be considered the more primitive of the two (p. 254). In Archaeocidaris the ambulacral plates are narrow and low, with pore-pairs uniserial (Plate 12, fig. 9). In the few species where the corona is fairly known, there are four columns of large plates in each interambulacral area (Plate 10, fig. 10). Each plate bears a large central perforate tubercle with a basal terrace and scrobicular ring (Plate 11, fig. 4). The basal terrace is so slight an eminence that it is often worn off, and this feature can only be ascertained in fairly well preserved specimens. From the center rises a primary spine of considerable size and various ornamentation; in addition, secondary and miliary spines and tubercles occur on the periphery of the plates. The young plates dorsally (Plate 11, fig. 2) are at first smooth, then an imperforate tubercle arises (as in developing plates of cidarids, Plate 3, figs. 1, 2) 224 ROBERT TRACY JACKSON ON ECHINI. but without a basal terrace, a feature which, with the perforation, developed later as the plate was pushed ventrally by younger newly added plates. The base of the corona is extensively resorbed so that several rows of plates have evidently been cut away in the advance of the peri- stome (Plate 9, figs. 7, 8). This is the most extensive resorption definitely known in any Echini, though it may have been as much, or more, in Recent regular Echini, where data are not avail- able to measure the amount of resorption (p. 256). The species of Ai'chaeocidaris are numerous but are largely described from isolated spines and plates. With available material perhaps the best way to group the species is on the basis of the character of the spines, as is done under the description of species. The smooth-spined forms are considered first, as smoothness is in general a primitive character; next are grouped the species with spines bearing short small spinules, then those with long thorn-like spinules, etc., associating the species on the similarity of characters in these respects (p. 258). Only six species are known with a more or less complete test, and of these the best material occurs in Archaeocidaris wortheni (Plate 8, figs. 5, 6; Plate 9, figs. 6-11) and A. rossica (Plate 10, fig. 10; Plates 11, 12). The species of Archaeocidaris occur in the Lower Carboniferous, Carbon- iferous or Coal Measures, and Permian. The genus Lepidocidaris is known only from L. squamosa Meek and Worthen (Plate 16, figs. 1-3; Plate 17). The ambulacral plates are not all alike, but every third plate is larger and more or less completely abuts against two smaller plates of the opposite half-area (Plate 17, figs. 1, 3, 8). There are six or eight columns of plates in an interambulacral area; these are high, rounded, with a scrobicular area, but no basal terrace. The margin is thickly studded with secondary tubercles. The primary spines are feebly striate longitudinally and very cylindrical (p. 282). The Lepidocentridae is a family of Silurian, Devonian, and Lower Carboniferous Echini that have strongly imbricate plates and includes most striking and interesting species. There are but two columns of plates in each ambulacral area, and from five to fourteen columns of plates in each interambulacral area; the latter is the extreme limit of known Echini. The primordial interambulacral plates are in place in the basicoronal row (Plate 23, fig. 1), as ascer- tained in five species. The peristome is covered with many rows of ambulacral plates only (Plate 23, fig. 1). Oculars where known are small, all insert, and the genitals where known, are wide with many (eight or nine) genital pores (Plate 28, fig. 10). The periproct has numerous plates (Plate 25, figs. 3, 5) and an exceptionally perfect lantern is known in one species (Plate 27), presenting the typical Palaeozoic character. The spines are small, eccentrically placed primaries, with secondaries, or the latter only (p. 284). The genus Koninckocidaris DoUo and Buisseret was based on K. cotieaui Dollo and Buis- seret, which has not been figured. The most important character is that the ambulacral plates are high, two or three equaling the height of an adjacent interambulacral plate; pore-pairs SYSTEMATIC CLASSIFICATION OF ECHINI. 225 are uniserial. The interambulacra have seven or eight columns of strongly imbricate polyg- onal plates in an area. As high ambulacral plates are a primitive character, Koninckocidaris is considered a lower genus than Lepidocentrus which has low ambulacral plates (p. 284). In Koninckocidaris cotteaui as described, two ambulacral plates equal the height of an interambulacral ; there are seven columns of interambulacral plates in an area, the median column being the narrowest; spines and tubercles are of two sizes. This, the type of the genus, is from the Lower Carboniferous of Belgium (p. 285). Koninckocidaris silurica sp. nov. (Plate 19, fig. 1; Plate 20, figs. 5, 6) has three ambulacral plates equaling the height of one adambulacral. There are eight columns of rhombic inter- ambulacral plates in an area, the plates of column 7 in each area being very narrow in the type. This species is from the Niagara Group, of Rochester, New York, and is of interest as by far the oldest echinoid yet found in America and also the oldest known in the family (p. 285). Lepidocentrus is characterized by relatively high spheroidal tests, ambulacral areas narrow throughout, ambulacral plates low, interambulacrals with small eccentric primary and second- ary tubercles. The species are from the Devonian or Lower Carboniferous, and the best known is L. mulleri (Plate 19, figs. 2-5; Plate 20, figs. 8-13), (p. 286). Hyattechinus gen. nov. — This genus includes some of the most striking of known Echini. The test is depressed or flattened. Horizontally the outline is circular, pentagonal, or clypeas- triform. The most distinctive character is that the ambulacra are broad, almost petaloid ventrally, narrow dorsally, ambulacral plates are low, pore-pairs uniserial. Interambulacra have from eleven to fourteen columns of plates in each area and a very accelerated development so that new columns are added early and rapidly during development. There are small primary and secondary tubercles on the interambulacral plates. As the species of Hyattechinus present very exceptional characters, these may be briefly stated. In Hyattechinus rarispinus (Hall) (Plate 21, fig. 6; Plates 22, 23) the test is nearly circular, and flattened probably as much as in Recent Phormosoma; ambulacra ventrally are twice as broad as dorsally, with ventral internal spurs as in beecheri. Interambulacra have eleven to thirteen columns of plates in each area, varying within these limits in different areas of the same specimen (Plate 23, fig. 3). This species was described as a Lepidechinus by Hall, but is very different from that genus. In Hxjattechinus fentagonus sp. nov. (Plate 24, figs. 1-4; Plate 25, figs. 1-4) the test is compressed, strongly pentagonal, the most so of any known regular echinoid. Ambulacra are ventrally twice as broad as dorsally. Interambulacra have fourteen columns of plates in each area, the highest number known in any echinoid. Dorsally, the adambulacral and some addi- tional columns drop out and therefore fail to reach the apical disc, an indication of senescence. In the remarkable Hyattechinus beecheri sp. nov. (Plate 24, figs. 5-8; Plate 25, fig. 5; Plate 26) the test is depressed with a flat base, moderately pentagonal and elongate, and bilaterally symmetrical through the axis of an ambulacrum and opposite interambulacrum which I take 226 ROBERT TRACY JACKSON ON ECHINI. to be the axis III, 5. This is the onlj' known case of a regular sea-urchin with centrally placed periproct that is bilaterally symmetrical through an antero-posterior axis. The ambulacra ventrally are twice as wide as dorsally and the ventral ambulacral plates develop a series of short spurs which extend into the body of the animal. Somewhat comparable proximal growths are known in Recent cidarids (p. 61; text-fig. 224; Plate 3, figs. 12, 13), but excepting Hyatt- echinus are unknown in other Echini. There are eleven columns of plates in each inter- ambulacrum in beecheri. The plates by their incoming show an accelerated development, and dorsally the adradial columns and some others drop out, showing senescence. The only known specimen is an internal mold so that the exterior of the plates is unknown, yet so perfect is the specimen that it is represented spread out by the Loven method (Plate 26), (p. 291). Pholidechinus gen. nov. is known only from a single species, P. hrauni sp. nov. (Plates 27, 28). The test is high and spheroidal, ambulacra narrow throughout; at the mid-zone alternate plates are • somewhat pinched off at the suture with the interambulacrum, by the broadening of the plates between. Occasional!}' a plate is quite cut off from the interambula- crum and thus becomes occluded, but this is exceptional. There are nine or ten columns of plates in each, interambulacral area. The plates are scale-like and imbricate strongly. Spines and tubercles are secondaries only, on both ambulacra and interambulacra. Genitals are very wide with many genital pores. A very perfect lantern is known, as figured (p. 299). The family of the Palaeechiuidae (commonly known as the Melonitidae) contains the most species, and as a whole is the best known of the Palaeozoic groups of Echini. The test is sphe- roidal or occasionally elliptical and in one species obovate. There are from two to twelve columns of plates in each ambulacral area according to the genus or species. It must be urged that this character should be ascertained at or near the mid-zone, as ventral or dorsal to this zone the number of plates may difTer (text-fig. 237, p. 231), and it is at this area that the full differential specific characters are evinced, as elsewhere discussed (p. 55). In the interambul- acra there are from three to eleven columns of plates in each area varying with the species, also varying somewhat in the species and even somewhat in different areas of the same indi- vidual. The plates of the corona are not imbricate, but in all species the ambulacral plates bevel over the interambulacral on the adradial sutures (Plate 38, fig. 9). A reverse condition exists in all Echini with imbricate plates, for in these the interambulacrals are beveled over the ambulacrals on the adradial suture (text-fig. 32, p. 75). Coronal pfcites are very thick, more so than in any other group of Echini, the thickness often exceeding the width or height of a plate, a distinctly specialized character. The primordial interambulacral plates are resorbed, and apparently these only, as there are two interambulacral plates in each interambulacral area in the basicoronal row (text-fig. 25, p. 70). The peristome is known onl}^ in Melon- echinus multi-porus (text-fig. 48, p. 80). On the basis of this species there are many rows of ambulacral with some interradial non-ambulacral plates on the peristome. Ocular ]ilates are SYSTEMATIC CLASSIFICATION OF ECHINI. 227 imperforate in the known species. They are typically all insert, or reaching the periproct, but exceptionally one or more to all may be exsert (Plate 33, figs. 11, 12). Genitals have from two to five pores each, and a madreporite is rarely recognizable (text-fig. 243, p. 331). The periproct is plated with numerous thick plates (Plate 34, fig. 6), as in the Cidaroida, but these plates are rarely preserved. The lantern is imperfectly known, but has the essential Palae- ozoic characters as far as preserved. The spines in the whole family are essentially alike, small secondaries on ambulacral and interambulacral areas; tubercles are imperforate and evenly distributed over the corona (Plate 61, fig. 8). The existence of secondary spines and imper- forate tubercles only may both be considered specialized characters. One species, Maccoya phillipsiae, is said to be Silurian, but all others in the family are Lower Carboniferous (p. 302). The family characters are of course understood in the brief description of generic characters given here. Palaeechinus M'Coy has only two columns of plates throughout each ambulacral area, all plates are primaries of equal height, pore-pairs uniserial. Interambulacra with four to six columns of plates in each area. The best known species are Palaeechinus quadriserialis (Plate 29, fig. 1; Plate 30, figs. 1-7) and Palaeechinus elegans (text-fig. 237; Plate 29, figs. 3, 4, 6; Plate 31, figs. 1-5), (p. 304). The genus Maccoya Pomel differs from Palaeechinus in that at the mid-zone pore-pairs are biserial. All ambulacral plates extend to the middle of the area, but marginally alternate plates meet the interambulacrum, or are nearly or quite cut off from it by the outer enlargement of their fellows {M. intermedia, Plate 34, fig. 2). This makes alternate plates primaries and occluded instead of all primaries as in Palaeechinus. In some species the alternate plate may weakly reach the interambulacrum instead of being cut off, M. sphaerica (Plate 34, fig. 5). Such make an approach to the character of Palaeechinus, but, as the pore-pairs are biserial, it seems best to consider them in this genus. On the interior of the test the plates all cross the half-areas and pore-pairs are uniserial, a radical difference from the exterior of the same structures (Plate 33, figs. 4, 5). The interambulacra have from four to nine columns of plates in each area in the several species. A very perfect periproct is known in M. sphaerica. The best known species are Maccoya burlingtonensis (Plate 32, figs. 1-3; Plate 33, figs. 1-5), M. intermedia (Plate 33, figs. 6-12; Plate 34, figs. 1-3), and M. sphaerica (Plate 32, figs. 4, 5; Plate 34, figs. 4-10), (p. 311). The genus Lovenechinus gen. nov. includes species in which at the mid-zone the ambula- crum consists of four columns of plates. In each half-area there is a vertical series of demi- plates which extend from the interambulacrum inward and a similar series of occluded plates that extend from the middle of the ambulacrum outward, but neither series crosses the half- area (Plate 42, fig. 2). Ventrally and dorsally the plates for a brief space are all primaries, and, between these and the character plates, are plates which are alternately primaries and occluded (Plate 42, figs. 1, 3 and 4; text-fig. 237). In this genus there are no isolated plates. 228 ROBERT TRACY JACKSON ON ECHINI. such as are present in Oligoporus (Plate 50, fig. 7), this being the distinction between the two genera. In the interambulacra of Lovenechinus in the known species there are from four to seven columns of plates in an area. Of the several species the most completely known are Lovenechinus missouriensis and L. septies (Plates 39-45 and 46, fig. 4), (p. 324). The genus Oligoporus Meek and Worthen is characterized by the fact that the ambulacrum at the mid-zone has four columns of plates and, in addition, in the middle line of each half-area isolated plates at intervals, but not in a continuous series so as to make an additional column (Plate 50, fig. 7). This structure is distinctly intermediate between that of Lovenechinus and the simplest species of Melonechinus. The youthful Oligoporus (Plate 50, fig. 8) has exactly the structure of adult Lovenechinus, and the youthful Melonechinus as a second stage in devel- opment (Plate 56, fig. 3) has exactly the structure of adult Oligoporus. In the interambula- crum of Oligoporus there are from four to nine columns of plates in an area according to the species. All known species are American, from the Lower Carboniferous, and the best known is Oligoporus danae (text-fig. 237; Plate 47, fig. 13; Plate 49, figs. 4, 5; Plate 50), (p. 350). The genus Melonechinus Meek and Worthen has many species with a very wide range of characters. The ambulacrum at the mid-zone has from six to twelve somewhat irregular col- umns of plates. The two median columns of occluded plates that extend from the middle of the ambulacrum outward are wider than other plates of the area, and are principally involved in the more or less high ridge that usually marks the central line of the ambulacrum. The demi-plates which extend from the interambulacrum inward do not differ externally from others of the area except the occluded plates, with the exception of the fact that they bevel over the interambulacra. On the outer face of the plates the pore-pairs are surrounded by a peripodium which occupies that part of the plate nearest the next adjacent interambula- crum. On the other hand, on the internal face of the plates the pore-pairs are very near the center of the isolated plates but in demi- and occluded plates, the pore-pairs lie nearer to the center of the half-area than to the middle of the plates (Plate 56, figs. 4, 5). The ambulacrum of Melonechinus on the ventral border has four columns of plates like the typical character at the mid-zone of Lovenechinus. Passing from this area dorsally, scattered isolated plates appear, the generic character of Oligoporus; further dorsally the isolated plates are more fre- quent and fall into columns which are more or less in number accoi'ding to the species in hand. This is shown in text-fig. 245, p. 382; Plate 56, fig. 3; Plate 57, fig. 1 ; and others. In one area of a specimen of M. muUiporus, as a regressive variant there are simple primary plates ventrally (Plate 57, fig. 3). This shows that while the genus typically has four columns ventrally, it may revert to two columns as a variation, repeating the character seen at this area in the lower genera of the family, and likewise the character seen at the mid-zone in Palaeechiuus, the lowest genus of the family. In the dorsal region of Melonechinus near the apical disc, in the region of young last added plates, the ambulacrum is also simpler. As few as two plates may systematic: classification of echini. 229 be found next the ocular (Plate 56, fig. 6), and passing ventrally, a progressively increasing number of plates occurs until the full species character is attained. This is shown in outline in text-fig. 237. The interambulacra at or near the mid-zone have from three to eleven columns of plates in each area varying with the species, and somewhat in the species, or indeed in differ- ent areas of the same specimen. As in other genera where there is a difference in this respect, the ambulacrum is considered the more essential part and of first consideration in classifica- tion ; the species are therefore erected first on the basis of specialization of the ambulacrum and secondarily on that of the interambulaci'um. Other characters are for the most part essentially similar in all species of the genus. The best known species is Melonechinus multi- porus (Plate 54, fig. 5; Plate 55; Plate 56, figs. 2-13; Plate 57; text-figs. 237, 245, 246, p. 382; Plate 60, figs. 1, 2), (p. 359). Having considered briefly the family of the Palaeechinidae, it is seen that the genera, and in atldition in Melonechinus the species, present a progressive series as regards differential evolution of ambulacral characters; also, where a complex ambulacrum is attained as a character gathered from the mid-zone, the ventral plates built in the youth of the individual are simpler. The young last added dorsal plates as a localized stage in development also present a simpler condition. This relation is expressed diagrammatically in text-fig. 237. This is not a schematic diagram, as each of the several figures is taken directly from the Plates as indicated. In the figure the species characters are given in the middle horizontal row representing the mid-zone. In the lowest row are given the characters taken at or close to the ventral border of the corona ; in the uppermost row are given the characters of plates at or close to the ocular plates. Between the lowest row or the uppermost row and the mid-zone are given the intermediate characters, both ventral and dorsal. In Palaeechinus elegans and other species of the genus the plates are simple primaries throughout the length of the ambulacrum, the simplest condition in the family. The plates are wanting at the ventral border in the known specimens of this species, therefore in the diagram a gap occurs at this point. In Maccoya biirlingtonensis the plates ventrally are simple pri- maries, at the mid-zone primaries and occluded plates alternating, and dorsally simple primaries. In Lovenechinus missouriensis ventrally the plates are simple primaries, higher up alternately primaries and occluded, and at the mid-zone demi- and occluded plates. Passing dorsally, we find primary and occluded plates alternating, and close to the dorsal border primaries only. In Oligoporus danae again, the plates ventrally are simple primaries, above that alternately primaries and occluded, above that demi- and occluded plates. At the mid-zone there are in addition scattered isolated plates which is the generic feature. Passing dorsally we find demi- and occluded plates only, which doubtless were succeeded near the oculars by primaries only, but such are not preserved in available specimens. In Melonechinus we have the most specialized genus with from six to twelve columns of 230 ROBERT TRACY JACKSON ON ECHINI. ambulacral plates at the mid-zone. In this genus ventrally there are four plates in each area instead of two as in the preceding genera, although exceptionally in one known case, as a re- gressive variant, in one area simple primaries occur (Plate 57, fig. 3). In Melonechinus springeri ventrally the plates are demi- and occluded, higher up isolated plates appear; at the mid-zone there are three definite columns in a half-area. Above the mid-zone we pass into an area of demi-, occluded, and scattered isolated plates, then demi- and occluded onh', and near the ocular primaries only. We see in this and other species of the genus that dorsally a simpler condition exists than that found at the ventral border of the corona, emphasizing the value of localized stages in studies of development. In Melonechinus indianensis ventrally the plates are demi- and occluded, higher up demi- and occluded, with scattered isolated plates; higher again three columns, and at the mid-zone four columns of plates in a half-area, the species character. Passing dorsally we find three col- umns, then two, and near the ocular primaries only. In Melonechinus multiporus the same condition occurs, but a little more complex. At the ventral border plates are demi- and occluded, higher up the same with scattered isolated plates, then three columns, then four, and at the mid-zone five columns in a half-area as the full specific differential. Passing dorsally, we fall into four columns, then three, two, and near the ocular primaries only may occur as in the same area in all lower species and associated genera. The extreme condition of the family occurs in Melonechinus giganteus. Here from conditions of preservation certain stages are wanting, but ventrally there are demi- and occluded plates, next as far as preserved an area with three columns, then one with five, and at the mid-zone there are six columns in the half-area. Melonechinus keepingi at the mid-zone has six columns in the half-area as in giganteus, and the dorsal area of this species was therefore used to complete the diagram, which I could not do in the case of giganteus. Passing dorsally from the mid-zone, we find a condition of five columns, then four, three, two, and finally primary plates only close to the dorsal border of the area. Text-fig. 237. — Illustrating the structure of the ambulacrum in representatives of the Palaeechinidae as seen at the mid-zone, with the phylogenesis of the same as shown by ventral youthful stages and dorsal localized stages of the species shown. Numerals indicate number of columns of ambulacral plates in a half-area. (Compare text-figs, on pp. 54, 59). Pulaeechinus elegans M'Coy (from Plate 31, fig. 1). Maccoya burlinglonensis (Meek and Worthen) (from Plate 33, figs. 1, 2). Lovenechinus missouriensis (Jackson) (from Plate 42, figs. 1-4). Oligoporus danae Meek and Worthen (from Plate 50, figs. 1, 7, 8). Melonechinus springeri sp. nov. (from Plate 52, figs. 2, 3). Melonechinus indianensis (Miller and Gurley) (from Plate 53, figs. 1-3). Melonechinus multiporus (Norwood and Owen) (from Plate 56, figs. 2, 3, 4, 6; Plate 57, fig. 1, and text-fig. 245, p. 382. Melonechinus giganteus (Jackson). Of this species the structure from the ventral border to the mid-zone is repre- sented below the black line (from Plate 59, figs. 12, 13, and Plate 61, fig. 8). Melonechinus keepingi sp. nov. at the mid-zono has the same character as giganteus, and its dorsal area is represented above the black line (from Plate 59, fig. 1). SYSTEMATIC ( I.ASSIFK'ATJON OF ECHINI 23] Si o 3 ° o < c tr c*- ■— 1 » T; S' oq ■i rt) m 3 n. o *- ni fr o Tl r^ N o c 3 g to (2- O B m ro o >-^ » r P N t-) n> n a. "-1 (fi fo (TO r- i/: 3 R ^ 11 <• ff O Q o N SI O 0 tD 3 o' •C! O tS) o 0 Palacechinus elegans. Maccoya burlingtonensis. Lovenechinus missouriensis. Oligoporus danae. Nl 0 Melonechinus springer! . JNIelonechinus indianensis. 0 Mcloncchinus multiporus. «^ CO', '.to 0 GO 0 Melonechinus giganteus below the heavy line, and Melon- echinus keepingi above the heavy line. 0-. -fc" CO ?^ 232 ROBERT TRACY JACKSON ON ECHINI. As shown in the diagram by connecting dotted Hnes, each type in the developnient of its ambulacrum presents a series of stages, either ventrally as youthful or dorsally as localized stages or both, which are directlj' comparable to the adult (mid-zone) character of all the simpler preceding types of the family. As the result, a complete genealogical history of the family can be read from the study of the ambulacrum. It is also seen how completely developing plates as localized stages repeat the characters seen as stages in tlie developing individual gathered from the plates from the ventral border up to the mid-zone. This is comparable to the developing characters occurring in successively added septa in ammonites, as shown by Hyatt and others, where also localized stages occur, as I showed (Jackson, 1899, p. 134). The family Lepidesthidae is enlarged over what it was when I first published it (1896) to include certain old genera and one new one. It presents a wide range of characters, yet it is difficult with present knowledge to make two distinct fainilies for the included genera. The sum of the characters affiliates the group as an offshoot from the Palaeechinidae, and the extreme characters seen in several species mark this as one of the most specialized of all groups of Echini. There are from two to twenty columns of plates in each ambulacral area and from three to twelve columns of plates in each interambulacral area. The plates are moderately or more usually strongly imbricate, and bear secondary spines with imperforate tubercles only, or in some genera part or all of the interambulacral plates bear also a primary spine with a perforate tubercle. Where the ventral border of the corona is known, the primordial interambulacral plates are in place, but this area is unknown in some of the genera. In Lepidechinus and Lepi- desthes the peristome is covered with ambulacral plates only, and the same condition probably exists in the other genera. Oculars are all insert or may be exsert. Genitals have from one to many pores each, and the periproct is heavily plated. The lantern has the usual Palaeozoic characters, but in one genus the teeth are distally denticulate. The simplest of the Lepides- thidae are so similar to the simplest members of the Palaeechinidae that the Lepidesthidae may well have been derived from that stock (p. 393). The genus Lepidechinus Hall makes a close approach to Palaeechinus. It differs horn it mainly in that the plates imbricate moderately, and that interambulacral plates bevel over the ambulacrals instead of the ambulacral plates beveling over the interambulacrals, as in Palaeechinus and all of its family. The ambulacral plates are all primaries, nearh' uniform in height, or with a slight tendency for alternate plates to become enlarged against the interam- bulacral suture, pore-pairs are nearly or quite uniserial. Interambulacral plates are in four to eight columns in each area. These plates are rounded hexagonal, with curved surfaces bearing secondary spines and tubercles only. Oculars are all exsert and genitals with a single pore each as far as preserved in L. tessellatus, the only species of the genus in which they are known (Plate 63, figs. 7, 8). The species are all known from fairly complete specimens (Plate 62, figs. 1-5; Plate 63; Plate 64, fig. 1), (p. 394). SYSTEMATIC CLASSIFICATION OF ECHINI. 233 The characters of the genus Perischodomus M'Coy (Plate 62, figs. 6, 7; Plate 64, figs. 2-8) are taken from P. biserialis M'Coy, the only satisfactorily known species. The ambulacra have two columns of plates in each area at the mid-zone, they are alternately wedge-shaped, or a plate may be quite shut out from interambulacral contact; pore-pairs are more or less biserial, but dorsally all plates are similar primaries with pore-pairs uniserial. Interambu- lacra have five columns of rounded strongly imbricate plates in each area; the plates of the adradial columns bear each an eccentric perforate pi'imary tubercle. The primordial inter- ambulacral plates are in the basicoronal row, there being no coronal resorption. Genital plates have numerous pores. This genus has several characters like the Lepidocentridae, in which family I previously placed it, but the sum of the evidence seems to place it in this family. One additional species, P. illinoisensis, is known incompletely. The genus Perischocidaris Neumayr (Plate 65, figs. 1, 2; Plate 67, figs. 1-3) is known only from one species, P. harteiana (Baily). It has a high rounded test with six columns of plates in depressed furrows in each ambulacral area. The interambulacra have five columns of rounded hexagonal plates which indicate moderate imbrication. The actual plates are not preserved as the only known specimens are two external sandstone molds. Part of the adradial inter- ambulacral plates bear an eccentric perforate primary tubercle and all interambulacral plates bear secondary tubercles. Dorsally three of the interambulacral columns drop out, so that in each area only two columns exist next the apical disc, a degree of dropping out seen in no other Palaeozoic species. The ocular plates are all insert and genitals have many pores. The ventral half of the test is unknown. Proterocidaris Koninck (Plate 65, fig. 3; Plate 67, figs. 4-7) contains only one species, P. giganteus Koninck. I have not seen specimens, but observations were made from Fraipont's excellent photographic figures, which he refers to Oligoporus soreili Fraipont. It is certainly not Oligoporus, and is referable to de Koninck's species. The test is very large, apparently nearly or quite spherical. There are four columns of plates in each ambulacral area and from eleven to thirteen columns of plates in each interambulacral area, varying in areas of the same specimen. This is the only species in the family with many interambulacral columns. The plates are strongly imbricate and bear rather small primary with numerous secondary spines. Lepidesthes Meek and Worthen, is a most interesting genus; the species are mostly small and elliptical or spherical. The ambulacra are wide and in each area consist of from eight to sixteen columns of very regular rhombic or more or less hexagonal plates, and the interambulacra, which are relatively narrow, of from three to seven columns of plates in each area. Plates are thin, strongly imbricating, bearing secondary spines and tubercles only. The primordial inter- ambulacral plates are in the basicoronal row, and the peristome is covered with ambulacral plates only. Ocular plates are insert, or in one species exsert, and genitals have few pores. The periproct is covered with many moderately thick plates. This is one of the most extreme 234 ROBERT TRACY JACKSON ON ECHINI. genera of Echini for ambulacral differentiation. The best known species are Lepidesthes for- rnosa (Plate 66, figs. 4-7; Plate 68, figs. 3-14) and L. colletti (text-figs. 14, p. 54; 32-38, p. 75; 251; Plates 69, 70; Plate 71, fig. 1). Pholidocidaris Meek and Worthen is one of the most specialized in certain respects of Palaeozoic genera. There are from four to six columns of plates in each ambulacral area, but the plates are very large ventrally and small dorsally. A peculiarity is that the pores are situated in the middle of the plates, a primitive character. The interambulacra as far as known have five or six columns of plates in an area and the plates are rounded polygonal. The adambulacral plates dorsally are very much larger than the intermediate plates, one equaling in height the distance of two or three of the adjacent interambulacra! plates. This is a character known in no other Echini; In addition the adambulacral plates bear each one eccentric primary spine and perforate tubercle, with secondary tubercles. The intermediate plates dorsally bear secondary tubercles only. The primordial interambulacral plates are in the basicoronal row, and the imbrication of coronal plates is extreme. The genital plates have many pores. No species is known perfectly, but the best known is the genotype, Pholidocidaris irregularis (Plate 73, figs. 3-7; Plate 74, figs. 1-7; Plate 75, figs. 1-5). Meekechinus gen. nov. (Plate 75, figs. 6-8; Plate 76) is in some respects the most specialized of known Echini, and is based on the single species M. elegans sp. nov. There are twenty columns of low rhombic plates at the mid-zone in each ambulacral area, the highest number known in any sea-urchin. With this great expanse of ambulacrum there are only three columns of small plates in each interambulacral area. All plates are strongly imbricate, all interam- bulacral plates bear a small primary spine and tubercle, with secondary tubercles, with the same on the ambulacral plates. Ocular plates are apparently all exsert, and genitals are large, the madreporite being exceptionally large and with distinct madreporic pores, the clearest seen in any Palaeozoic species. Pedicellariae were found in this remarkable species, and are the only ones yet known from the Palaeozoic. The lantern has the typical Palaeozoic characters except that the teeth are distally denticulate, a unique character for Echini. This species is the highest geologically and the niost specialized structurally of any known Palaeozoic echinoid. If this classification is adopted the names Palaeechinoida and Euechinoida will naturallj' lapse as the major divisions of the Echini are ordinal only as shown in the table, p. 209. PART III. PALAEOZOIC ECHINI. In considering the systematic part of this memoir, attention is called to the general state- ments of the history of the subject, sources of material, methods of work, etc., as given in the Preface and Introduction. The morphological part of this memoir is based in considerable part on the study of Palaeozoic species, so that considerations of these older forms and their relations are scattered all through the morphological sections. As references to the pages of these earlier considerations of Palaeozoic genera and species can be gathered from the index they are largely omitted in the descriptions. A key to the classification of Palaeozoic Echini is given in the last section (p. 201-208), and a systematic table (p. 209), showing the genealogical relations, as understood, of the Palaeo- zoic Echini to each other and to the larger groups of the post-Palaeozoic Echini. An asterisk before the name of a species indicates that specimens have been studied. Under each species the synonymy is given in full as far as ascertained, and references to the principal literature. Miss Klem (1904) has recently published so full a list of references in her Revision of Palaeozoic species, that it seems unnecessary to repeat minor references. All text-books are omitted unless there is special need of comment, also works in which species are simply listed without description or discussion, as Miller's Catalogue of North American Geology and Palaeontology, Etheridge's and Morris's Catalogues of British Fossils, d'Orbigny's Prodrome, Dujardin and Hupe's Echinodermes, etc. Where species are simply listed without description or discussion, as in parts of Loven's (1874) and Miss Klem's (1904J memoirs, no reference is deemed necessary. Messrs. Lambert and Thiery's Essai de Nomenclature (1910) has recently come to hand (March, 1911). Their distribution of Palaeozoic species in genera and their generic synonyms do not appear to me well considered. Wliile there are no descriptions of species in Lambert and Thiery's Essai, names from this work are included as a matter of synonymy. In my descrip- tions of species I have given what are considered the essential features with new matter. Where further information is desired, it may be obtained from the authors cited. Geological Distribution. Considering in brief and in a broadly inclusive way the geological distribution of Palaeozoic Echini, their occurrence in formations is as follows. In the Ordovician, Echini are known only from Russia where the primitive genus Bothriocidaris is represented by three species. (235) 236 ROBERT TRACY JACKSON ON ECHINI. In the Silurian, Echini are represented thus far only in America and England. In the Niagara Group of New York State is found the oldest known representative of the Lepidocen- tridae, Koninckoddaris silurica sp. nov. The position of the Niagara is about in the middle of the Silurian. In England, Maccoya phillipsiae (Forbes) is ascribed to the Llandovery which is at the base of the Silurian, but this horizon for the species may be doubted. In the British Museum there is an excellent echinoid lantern from the Wenlock Limestone of Dudley, the oldest assured horizon from which an echinoid is known in Great Britain. In the Lower Ludlow, which is the upper part of the Silurian, just below the Devonian, occur in England Palaeodiscus ferox Salter, and Echinocystites pomum Wyville Thomson, the sole representatives of the order Echinocystoida. In the Devonian, as in the Silurian, only a few Echini are known. In the Devonian of Germany Eocidaris laevispina (Sandberger) is fragmentarily known. This is the only species here ascribed to that genus and it is the earliest, also the only known representative of the Archaeocidaridae in the Devonian. In the Devonian of Germany also are found Lepidocentrus rhenanus (Beyrich), L. muUeri Schultze, and L. eifelianus Miiller. Lepidocentrus drydenensis is found in the Upper Devonian of America, in New York State. All species of Lepidocentrus are Devonian excepting one, which is found in the base of the American Lower Carboniferous. In England Lepidesthes devonicans Whidborne and Pholidocidaris acuaria (Whidborne) occur in the Devonian; they are the oldest representatives of their genera and of the family Lepi- desthidae. The imperfectly known Xenocidaris, with three species, is found in the Devonian of Germany. The term Subcarboniferous is commonly used in America for the lower part of the Carboni- ferous, below the Coal Measures. I use here the term Lower Carboniferous. This is a more nearly correct name for the horizon and has the advantage of coinciding with foreign usage. The Lower Carboniferous is the period of greatest development of Palaeozoic Echini and it is in America that the most species are found. Here occurs the Miocidaris cannoni sp. nov. which is important as the geologically oldest known member of the Cidaridae, which family is a salient feature of the Echinoid fauna from Mesozoic to Recent times. The genus Archaeocidaris also first appears in the Lower Carboniferous, and in this formation there are many species in both America and Europe." The genus Lepidocidaris, structurally the highest member of the .\i-chaeocidaridae, ' This lantern, which w;is recently discovered in Jthe matrix and developed by Dr. Bather, is well preserved. It h;is wide-angled pyramids, a moderately deep foramen magnum, and a brace in place. The sides of the pyramids have ridges for the attachment of interpyramidal muscles. It has the essential characters of lanterns, as seen in the Lower Carboni- ferous. As there is no portion of the test in association with the lantern, its generic position is unknown, but it is quite probable that it is a Lepidocentrus or a near ally of that genus. 2 Dr. Bather called my attention to the fact that Austin (1848) reported that a large species of Echinocrinus [Archaeo- cidaris] occurs in the Wenlock Limestone, Silurian, of Glidden Hill, which is near Coalbrookdale in Shropshire. There is no recognizable description, and the specimen is unknown. If this observation should be confirmed, it would greatly ex- tend the geological range of the genus. GEOLOGICAL DLSTRTBUTION. 237 is known from one species in the American Lower Carboniferous. The Lepidocentridae is represented in America by Lepidocentrus whitfieldi sp. nov., also H yattechinus rarispinus (Hall), H. pentagonus sp. nov., and H. beecheri sp. nov., all from the Waverly Group, the basal member of the Lower Carboniferous. The Lepidocentridae also is represented in America by Pholid- echinus brauni gen. et sp. nov., from the Keokuk zone, and in Europe by Koninckocidaris cotteaui Dollo and Buisseret, from Belgium, and Tornquistellus gracilis (Tornquist)" fi'om Ger- many. Of the Palaeechinidae, Maccoya phillipsiae is attributed to the Silurian of England, but with this exception the family of the Palaeechinidae is known only from the Lower Car- boniferous of America and Europe. Of this family the genera Palaeechinus, Maccoya and Lovenechinus are represented by both American and European species, but Oligoporus, is American only. Of the highest genus of the family, Melonechinus, eleven species are American, two are British, and one Russian. Of the Lepidesthidae, most of the species are from the Lower Carboniferous, but two are found in the Devonian as above noted, and two occur in later hori- zons. Lepidechinus, the lowest genus of the family, is known wholly from the Lower Car- boniferous, three species, L. iowensis sp. nov., L. tessellatus sp. nov., and L. imbricatus Hall, being found in America, and one species, L. irregularis (Keeping), in Ireland and France. Perischodomus is represented by one well known species, P. biserialis M'Coy, in the Lower Carboniferous of Ireland and England, and one imperfectly known species in America. Peris- chocidaris is known only from P. harteiana (Baily) in the Lower Carboniferous of Ireland. Proterocidaris is known from the striking species, P. giganteus Koninck, from the Lower Car- boniferous of Belgium. The relatively large genus Lepidesthes has one species in the British Devonian and one in the American Coal Measures, but all others are from the Lower Car- boniferous. Of these, six species are American, one British, and one Russian. As above stated, one species of Pholidocidaris is from the Devonian, but the others known are from the Lower Carboniferous. One species, P. irregularis Meek and Worthen, the best known is American. P. tenuis Tornquist is known in England and Germany and P. gaudryi Julien in France. In the Upper Carboniferous, or Coal Measures, the Palaeozoic Echini have dropped out with extreme suddenness and relatively few species are known. The genus Archaeocidaris is represented by 19 species recorded from the Coal Measures, all known fragmentarily only from spines and plates. Seventeen of these have been recorded from America and two from Europe.' Besides Archaeocidaris in the Coal Measures of America is found Lepidesthes extremis sp. nov., structurally and geologically the highest known species of this very interesting genus. ' The following species of Archaeocidaris have been found in the Coal Measures of North America as recorded by various authors: A. longUpina Newberry, A. agassizi Hall, A. coloradensis nom. nov., A. gracilis Newberry, A. aculeala Shumard and Swallow, A . edgarensis Worthen and Miller, A . trudifer White, A . dininnii White, A . oralis White, A . biangidala Shumard andSwallow, A. ourai/ensis Girty, A. Iriplex White, A. megastyla Shumard and Swallow, A. triserrala Meek, .4. halliana (Geinitz), A. spinoclavala Worthen and Miller, ,4. sp. b. Girty. From Europe there are only two species recorded : .4. paradoxa (Eichwald) in Russia, and specimens which are here referred to A. acanlhifera Trautschold in England. 238 ROBERT TRACY JACKSON ON ECHINI. In the Permian, very few forms of Echini are known. In England and Germany Mipcidaris keyserlingi (Geinitz) occurs, and is important as the geologically second species known in the Palaeozoic with two columns of plates in an interambulacral area. As far as at present defi- nitely known, it and the new Miocidaris cannoni from the American Lower Carboniferous are the geologically oldest members of the Cidaroida. In America, Archaeocidaris aculeata Shumard and Swallow is reported as occurring in the Permian, and in India Archaeocidaris is doubtfully represented by one species, A. forbesiana (Koninck), which is imperfectly known, but is provisionally referred to this genus. Archaeocidaris selwyni occurs in the Permian of Australia. In the American Permian is found Meekechinus elegans gen. et sp. nov., which is a very interesting type, as it has twenty columns of ambulacral plates in an area at the mid-zone, being in this respect the most specialized of known Echini. It is the highest of the Lepides- thidae and geologically the last of the series with more than two columns of ambulacral and interambulacral plates in an area which is the essential feature of most Palaeozoic Echini. Systematic Descriptions. Order BOTHRIOCIDAROIDA Jackson. Schmidt, 1S74, p. 37. Bothrioddaridae Zittel, 1879, p. 480 Bothriocidaroida Jackson, 1896, p. 238. Regular Echini, with periproct enclosed by the oculo-genital ring. Two columns of plates in each ambulacral area and one column of plates in each interambulacral area. Family BOTHRIOCIDARIDAE Klem. Bothriocidaridae (as an order) Zittel, 1879, p. 480; (as a family) Kleni, 1904, p. 14. With characters of the order. BoTHRiociDARis Eichwald. Bothriocidaris Eichwald, 1860, p. 654. Botriocidaris Neumayr, 1881, p. 4. Botryocidaris Loxen, 1883, p. 57. Small spherical or elliptical Echini. Ambulacral areas relatively wide, about twice as wide as the narrow interambulacra with their single columns of plates. Ambulacral plates high, hexagonal, the pores of each pore-pair superposed in a depression in the center of each plate. Interambulacral plates high, hexagonal, equaling the ambulacrals in height. Plates ROTHRIOCIDARIS. 239 of test not imbricate. Primordial interambulacral plates are in the basicoronal row. Basi- coronal ambulacra! plates are not sharply distinguished from the peristomal plates. Spines few on ambulacral and interambulacral, or on ambulacral plates only. Peristome with two rows of ambulacral plates only. Oculars very large, genitals small. Periproct with many small, angular plates, filling the area. Ordovician, Russia. This is the oldest known genus of Echini and also one of exceptional interest on account of its features which in many respects resemble the young of later Echini. The type species is Bothriocidaris globulus Eichwald, from the Ordo- vician of Russia. (See pp. 54, 64, 70, 80, 87, 149, 208.) ■ Kri/ in Ihr Species of Bothriocidaris. Oculars continuous, forming a closed ring; small genitals dorsal to tlie oculars; tuhercles on ambulacral and interambulacral plates, . . . B. archaica sp. nov., p. 2.39. Oculars in part continuous and in part separated by tbe small genitals; tubercles on ambulacral plates only, B. pahleni Schmidt, p. 242. Oculars all separated by the small genitals; tubercles on ambulacral and interambulacral plates,. B. globul-us Eichwald, p. 243. *Bothriocidaris archaica sp. nov. Text-figs. 2, p. 54; 22, p. 70; 40, p. 80; 1C2, p. 149; Plate 1, figs. 1, 2. Bothriocidaris globulus (pars) Jaekel, 1894, text-figs. 1, 2: (pars) A. Agassiz, 1904, p. 79; (pars) Klem, 1904, p. 15. Bothriocidaris [globulus pars] Bather, 1902, p. 620, text-fig. 5B. The precious and only known specimen of this species is in the Museum ftir Naturkunde in Berlin, and I am under greatest obligations to Dr. Otto Jaekel for the opportunity to study this which, I feel, is perhaps the most interesting of known Echini. The details of structure are remarkably clearly preserved, but are naturally difficult to study in such a smair specimen. It is probably the best preserved specimen known in the genus, and is marvellously perfect when its age is considered. Its perfection is a tribute to Dr. Jaekel's skill as a preparator. The test is slightly elliptical, 12 mm. in height, 11.2 mm. in diameter through the mid- zone. The circumference at the ambitus is 35.5 mm.; circumference in a vertical plane around the poles 36 mm., width of an ambulacrum at the mid-zone 4.2 mm., width of an interambula- crum at the same area 2.2 mm. Each ambulacrum consists of two columns of high hexagonal plates; at the mid-zone a plate measures 2.2 mm. in its greatest width. Ambulacral pores are superposed in a sunken area in the center of each plate. At the ambitus there are typically four, in some cases perhaps fewer, tubercles on each plate near the margin of the peripodium. Interambulacral plates are high, hexagonal, equaling the height of the ambulacrals, with from one to three tubercles on each plate at the mid-zone. The plates of the corona are not im- bricate. There are two rows of peristomal ambulacral plates. The diameter of the apical 240 ROBERT TRACY JACKSON ON ECHINI. disc through the plane III, 5 is 4 mm. This measurement includes from the adoral border of ocular III to the aboral tip of the youngest plate of interambulacrum 5. The apical disc measures proportionately 36% of the diameter of the test. This proportion is about the same as in the modern Cidaridae and is relatively larger than in other known Palaeozoic species (pp. 87, 104). Oculars are large, forming a closed ring, thus covering the ambul- acra and inter-ambulacra completely, a unique character. Genitals are small and shut out from contact with the interambulacra by the continuous character of the oculars (p. 87). The periproct has many small angular plates filling the area. This species differs from B. pahleni in that there are perforate tubercles on both ambulacral and interambulacral plates, whereas in pahleni they are absent in the interambulacrals. It also differs in that the genitals are wholly dorsal to the oculars instead of part of them separating the oculars as in pahleni. Boihriocidaris archaica differs from globulus in that the oculars form a closed ring, whereas in globulus the oculars are separated by the genitals which meet the interambulacra of their several areas. Lyckholm Schicht, Ordovician, Hohenholm, Island of Dago, Russia. The type and only known specimen is in the Berlin Museum as above stated. In the morphological part of this memoir much attention is given to Bothriocidaris as a primitive type, and the consideration is largely based on the study of this species. The ambula- crum is a feature of particular interest. The whole of ambulacrum IV measured on the curve from the oral termination to the ocular is 15 mm. in length. The two ventral rows of ambula- cral plates are considered as peristomal, from which the basicoronal plates of the corona are not sharply defined. In the corona proper, and excluding the peristome, there are eight ambu- lacral plates in each column of each area excepting Ilia, in which there are nine plates, as a very young plate is in contact with the ocular, but so low that it does not quite reach the inter- ambulacrum. Pore-pairs are superposed in the middle of each plate, a condition known in the adult of regular Echini only in this genus (text-fig. 22, p. 70). At the mid-zone there are four perforate tubercles to a plate, or in some plates fewer; when fewer at this area, the differ- ence may be due to loss from erosion. Ventrally and dorsallj' there are fewer tubercles as a character. In the sixth plate from the base in ambulacrum II o, the tubercles were not seen, but are filled in by comparison with adjacent plates, otherwise the tubercles of ambulacral plates are figured as observed. The hexagonal shape and superposed pores are like the young of later Echini (pp. 54, 57). Interambulacrum 3 measures on the curve 12 mm. in length. In each interambulacrimi there are nine plates, and the basicoronal plate of each area is a pentagon, the ventral apex of which impinges on the second row of peristomal ambulacral plates (compare Plate 2, fig. 1; Plate 3, figs. 9, 10). The tubercles of these plates are few in number, but striking. There is a single tubercle on the basicoronal plate in each area, also a single tubercle on the second BOTHRIOCIDARIS. 241 plate in each area excepting 2, where it is wanting, doubtless from erosion; in the third and succeeding plates to the mid-zone there may be one, two, or three tubercles varying with the areas, the highest number occurring in area 4. Dorsally, for one or more plates in an area, there is only one central tubercle to a plate like that in the basicoronal plates. In the youngest interambulacral plates in contact with the oculars, there are no tubercles, a common character of plates next the apical disc. The characters of the tubercles on the interambulacra express radial variation, ventral development, and dorsal localized stages. The distribution of tubercles on interambulacral plates is striking from the difference in different areas. Of course some tubercles may have been worn off, but those shown are probably essentially the number that existed when the animal was alive. Dorsally (Plate 1, fig. 2) the interambulacra abut against two oculars, a condition otherwise known only in spatangoids (text-fig. 175, p. 149), and in aberrant variants of regular Echini (Arbacia, Plate 4, figs. 11, 12; Eucidaris, text-fig. 185, p. 167). The single column of interambulacral plates is considered a primitive character, because in the young of all later Echini there is a single plate in the basicoronal row representing a single column as a stage in development (compare text-figs. 22-31, p. 70). There is no resorp- tion of the base of the corona, a character of the j^oung in all later Echini and of certain adult types of later Echini (p. 70). The peristome of this ancient type (Plate 1, fig. 1; text-fig. 40, p. 80) consists of two rows of ambulacral plates. The first row represents the primordial ambulacral plates, and examining them, we find that the la, Ila, III6, IVa, V6 are larger plates, and the 16, 116, Ilia, IV6, Va are smaller. This is in accordance with the law worked out by Loven in Recent Echini (Plate 2, fig. 1), and it is most striking that it was already at work in these ancient times. By means of it I oriented the specimen as shown in the figures. I could distinguish no tubercles on this first row of plates. The second row of the peristome surrounds the first and aborally is in contact with the basicoronal interambulacral and ambulacral plates. The plates of this second row are distinguished from the coronal plates only by their position and interambulacral relation, but the same thing is true of the young of some Recent types (Plate 2, fig. 6). The second row was probably derived by flowing down from the corona (pp. 79, 86). On some plates of this second row one or two tubercles were observed. The oculars are very large relatively, and are unique in that they meet in a continuous ring. Ocular III measures 1.4 mm. in height, but the others somewhat less. All have three tubercles excepting II, which has only two. Ocular III has in addition two tiny sinuous ele- vations which appear to be part of the plate. Jaekel thought these indicated a madreporite; this may be correct, but it is doubtful. The small plates in the dorsal interradial angles of the oculars I consider as genitals (as discussed, p. 88). Genitals 1, 2 and 4, 3 are in contact ' By an error in making my drawing in Berlin, from wliich original Mr. Blake drew Plate 1, fig. 1, the initial plate of area 1 rests against the tliird plate of 16, imstead of the second plate as it evidently should. 242 ROBERT TRACY JACKSON ON ECHINI. dorsally, as seen in Plate 1, fig. 2, but a periproctal plate separates genitals 2 and 3 and others separate genital 5 from 4 and 1 on either side. They thus present a certain bilateral symmetry, but this may be only a coincidence. All the genitals bear a tubercle excepting genital 2, where it is probabh' worn off, but if madreporic pores existed in 2, that might account for the absence of a tubercle. The small relative size of the genitals and their position dorsal to the oculars suggests a comparison with the somewhat similar condition in embryonic Echinus as seen in Plate 3, fig. 5. The periproct contains nine or ten minute angular plates, the sutures of which are difficult to make out, but apparently are as shown in the figure. They are similar to plates seen in other Palaeozoic periprocts and present no striking feature (p. 174). Teeth, or more probably, dental pyramids exist in areas I, II, IV, and V. Jaekel considered them parts of the dental system, and I agree with him, but Bather differs because of their radial position. No other known structure occurs at this area in Echini, and their position, it seems, may be accounted for by a possible accidental twist, as a considerable twist is occasionally found in dried Recent material. To accept a radial position as normal in this type is opposed to morphological principles, and in so far I should entirely support Bather's opinion. Bothriocidaris pahleni Schmidt. Plate 1, figs. 3-6; Plate 8, fig. 1. Bothriocidaris pahleni Schmidt, 1874, p. 38, Plate 4, figs, la-lg; Zittel, 1879, p. 481, text-figs. 339a-339c; A. Agassiz, 1892, p. 72, Plate 29, fig. 1; Jaekel, 1894, p. 246; Jackson, 1896, pp. 233, 238, text-fig. 4; Gregory, 1900, p. 300, text-figs. 1-3; Lambert and Thie'ry, 1910, p. 118. Botriocidaris pahleni Neiimayr, 1881, p. 152, Plate 1, fig. 6. Botryocidaris pahleni Loven, 1883, p. 57, 2 text-figs. Bothriocidaris palheni Pomel, 1883, p. 117. Bothriocidaris phalcni Klem, 1904, p. 15. The test is small and nearly spherical; according to Schmidt, the type is 15.5 mm. in height and 16 mm. in diameter. Ambulacral plates are high, hexagonal. Schmidt says there are ten plates in a column, and eliminating the two peristomal rows, we have, therefore, eight ambulacral plates in a column in the corona, as in B. archaica. The individual ambulacral plates at the ambitus are 3 mm. wide and 2 mm. high. The pits for pore-pairs are about 0.5 to 0.75 mm. broad, showing two pores which are superposed. There are two tubercles to a plate, and some of the spines are in place. They are longitudinally striate, up to 4 mm. long and about 0.5 mm. thick; these are the only spines known in the genus. The interambulacral plates are of the same height as the ambulacral, but narrower, 2.5 mm. wide at the ambitus. The primordial interambulacral plates are pentagonal, ventrally in contact with the dorsal border of the second row of peristomal ambulacral plates. The youngest plates dorsally against the apical disc are pentagonal, or so low that they are nearly triangular. There are eight or BOTHRIOCIDARIS. 243 nine plates in a column according to Schmidt, and nine as seen in Plate 8, fig. 1. The surface of these plates is finely granulated, but with no perforate tubercles, indicating that in this species there were no large spines, possibly no interambulacral spines at all. If the latter, it is a unique condition in Echini. There are two rows of peristomal ambulacral plates, the second being in contact with the ventral border of the primordial interambulacral plates. The oculars are large, of which the ocular considered III is the largest. Each plate bears two or three tubercles. Oculars IV, V and I, II are in contact (Plate 1, fig. 6), but IV, III and II, III are separated so that genitals 2, 3 reach the interambulacra. Thus it is structurally bi- laterally symmetrical through the axis III, 5, but this may be a mere coincidence. Four small genitals are in place, but genital 5 is wanting; 2 and 3 reach the interambulacra as in B. globulus, but the others are shut out by the contact of the oculars. Each genital bears a small per- forate tubercle. Considering these plates as genitals, there are three other plates within the apical disc that may be considered as plates of the periproct. This species differs from the other two in that there are no perforate tubercles on the interambulacral plates, and it is inter- mediate between B. archaica and B. globulus in the lateral contact of the ocular plates (p. 87). The type is from the Jeweschen Schicht, Ordovician, Nommis, Esthland; a second speci- men, the original of Plate 8, fig. 1, is from the Diluvial-Geschichte, Ordovician, Rostov, Russia. Loven (1883, p. 57) described and figured in this species, presumably the type specimen, long cylindrical bodies having one end in close relation to the ambulacral pores. He took these for tube-feet, and said that they even showed traces of having their tubes strengthened by arcuate spicules. It certainly is a remarkable condition of fossilization if these are tube-feet. In the figure which I published (1896, text-fig. 4, p. 234) of this species, the error was made of taking the perforation of tubercles for ambulacral pores, so that these were repre- sented incorrectly, in a horizontal plane instead of superposed. The same mistake was made by Gregorj^ (1900). The photographic figure (Plate 8, fig. 1) is taken from a cast that was made by Dr. Jaekel and which he kindly gave me. It shows enlarged the general appearance of the species, relative width of plates, and the whole length of one interambulacrum. The interambulacral areas are colored to differentiate them. Bothriocidaris globulus Eichwald. Plate 1, figs. 7-9. Aldrovandus, 1618, p. 136, fig. 5. Bothriocidaris globulus Eichwald, 1860, p. 55, Plate 32, figs. 22a, b, but not fig. 23; Schmidt, 1874, p. 40, Plate 4, figs. 2a-2c; (pars) Jaekel, 1894; (pars) Jackson, 1896, p. 238; (pars) Klem, 1904, p. 15; (pars) A. Agassiz, 1904, p. 79; Lambert and Thiery, 1910, p. 118. Botriocidaris globulus Neumayr, 1881, p. 152. Eichwald's original figures and description are not very detailed, but Schmidt gives an excellent description and figures of this species, from which my account is taken. Test globular. 244 ROBERT TRACY JACKSON ON ECHINI. in Schmidt's specimen 18 mm. high, 18.5 mm. in diameter. Eichwald's type was smaller, onlj' 12.5 mm. in diameter. There are two columns of high hexagonal plates in each ambulacral area, and the plates at the ambitus are 4 mm. wide and 3 mm. high. Interambulacral plates are somewhat narrower, 3 mm. wide. Schmidt saj's that there are ten ambulacral plates in each column, which, subtracting two for the peristomal rows, leaves eight in a column for the corona, as in the two other species. The pore-pairs are superposed in the middle of each plate, and at the ambitus there are four perforate tubercles around the peripodia. The interambula- cral plates at the ambitus have two or three perforate tubercles, but ventrally and dorsally only one. Schmidt says that the spines attributed to this species by Eichwald have nothing to do with it, but appear to be small Bryozoa. The peristome consists of two rows of ambula- cral plates as in the other species; the dorsal border of the second row is in contact with the ventral border of the primordial interambulacral plates. The oculars are large, the largest of which I take to be III, as in B. archaica. They bear one, two, or five tubercles each, but ocular IV has no tubercle, probably worn off. Four genital plates are in place, but genital 4 is absent or displaced (Plate 1, fig. 9). The genitals are larger than in the other species; each bears a tubercle and separates the oculars, therefore reaching the interambulacra as they do in all later Echini, but in this feature differing entirelj' from archaica and partially from pahleni (p. 240). There are only two periproctal plates in place b'ing dorsal to oculars II and III and separating the associated genitals. Schmidt figures five minute plates ventral to the primordial ambulacral plates, and which I take to be teeth or dental pyramids, as in the other species (discussion, pp. 181, 242). Schmidt's specimen is from the Lyckholm Schicht, Ordovician, Hohenholm, Island of Dago, Russia. The type is from the same locality. It is very interesting historically that Aldrovandus in 1618 gave a figure that is recognizable as belonging to Bothriocidaris and probably to this species. The figure is entitled "Echinus lapis spoliatus a spinis," but there is no description. Wliile somewhat rough, the figure shows two columns of high hexagonal ambulacral plates, with a cup-shaped depression in the center of each, and a single column of high hexagonal interambulacral plates which apparentlj- meets the genitals. There are from one to three tubercles on the interambulacral plates; these features apply to this species and no other. It is remarkable that this excessively rare echinoid should have been found so early and then lost sight of until recent years. Order CIDAROIDA Duncan. Cidaroida Duncan, 1889a. Regular Echini with periproct enclosed by oculo-genital ring. Two columns of low, narrow ambulacral plates and two columns of pentagonal interambulacral plates, the latter with a large, central primary tubercle and spine, also marginal secondary tubercles and spines. Other MIOCIDARIS. 245 characters are of importance if the whole order is to be considered (see pp. 201, 211) but for the Palaeozoic, this is a sufficient diagnosis. It is quite possible that the Devonian genus Xenocidaris, with three species, considered under Incertae Sedis, belongs to the Cidaroida. Family CIDARIDAE Gray. Cidaridae Gray, 1825, p. 4. With characters of the order. MiociDAKis Doderlein. Miocidarls Doderlein, 1887, p. 40; Bather, 1909, p. 61 ; 1909a, pp. S3, 251. Eotiaris Lambert, 1899, p. 82; Lambert and Thiery, 1910, p. 127. Permocidaris (pars) Lambert, 1899a, p. 39. Eocidaris (pars) Lambert and Thiery, 1910, p. 12(). Test of moderate size, adradial margin of the interambulacral plates beveled over the ambulacrals, interambulacral plates relatively f€w, or many in M. cannoni, often wide, with scrobicules which are circular or elliptical, distinct or confluent. This genus contains but two Palaeozoic representatives, the other species being Triassic or Jurassic. The type-species as selected by Bather (1909, p. 61) is the Triassic Miocidaris cassiana Bather (a new name for Cidaris (Miocidaris) klipsteini Desor, which was preoccupied). Dr. Bather (1909; 1909a, pp. 83, 251) has given a critical consideration to this genus and to the species attributed to it. The genus Miocidaris is of especial interest as it contains the geologically oldest known forms of the Cidaridae, and at the same time the oldest known Echini with two columns of plates in each interambulacrum, which is the feature of all modern Echini. Miocidaris keyserlingi under various generic names has long been known from the Permian of Europe. I have the satis- faction of describing a new species, M. cannoni, from the Lower Carboniferous of .■Vmerica, which greatly extends the geological and geographical range of the genus and family. Kci/ to the Palaeozoic Species of Miocidaris. Test small, about six plates in an interambulacral column .... .1/. ke]iserJhi(ji (Geinitz), p. 245. Test large, ai)oiit fourteen plates in an interambulacral column . . . .1/. eannoni sp. nov., p. 247. *Miocidaris keyserlingi (Geinitz). Text-figs. 238, 238 bis; Plate 9, figs. 1-5. Cidaris kcijserlimji Geinitz, 1848, p. 16, Plate 7, figs, la, lb, 2a, 2b. Cidaris verneuiliana King, 1848, pp. 6, 7. Archaeocidaris verneuiliana King, 1850, p. 53, Plate 6, figs. 22-24. Palaechinus verneuiliana King, 1850, description of Plate 6, figs. 22-24. Eocidaris kaiserlingii Desor, 18.58, p. 156, Plate 21, figs. 15, 16. 246 ROBERT TRACY JACKSON ON ECHINI. Eocidaris venieuiliana Desor, 1858, p. 156, Plate 21, figs. 13, 14; Loven, 1874, p. 43. Eocidaris keyserlingi Geinitz, 1861, p. 108, Plate 20, figs. 5-9; Loven, 1874, p. 42 ; Klem, 1904, p. 68. Eocidaris kaiscrlingi Kolesch, 1887, Plate 38, figs. 1-27; Doderlein, 1887, p. 39, Plate 11, fig. 8; Spandel, 1898, p. 33, Plate 13, figs. 1-6. Eotiaris keyserlingi Lambert, 1899, p. 82; Lambert and Thiery, 1910, p. 128. Permocidaris venicuili Lambert, 1899a, pp. 39, 47. Eocidaris vcrneuiliatius Klem, 1904, p. 70. Miocidaris keyserlingi Bather, 1909, p. .54, text-figs. 2, 3, Plate 1, figs. 6-17; 1909a, p. 86. Eocidaris rerneuilli Lambert and Thiery, 1910, p. 126. This important species has recently received critical consideration from Dr. Bather. The test is small, low, subspheroidal. Ambulacra unknown, but crenulations on the adradial suture face of the interambulacral plates indicate that about four ambulacral plates equal the height of an interambulacral (text-fig. 238 bis). Two columns of relatively high pentagonal plates in each interambulacral area, beveled over the ambulacrals on the adradial suture. The tubercles are prominent, crenulate, with large scrobicules and numerous secondary tubercles on the extra-scrobicular surface. Bather shows that there are low but clearly developed 238 238 bis Magnesian Limestone, Permian, Tunstall Hill, County Text-figs. 238, 238 his. — Miocidaris keyserlingi (Geinitz). Durham, England. 238. One of the most perfectly preserved specimens of an interambulacrum, showing the association and general shape of the plates. The scrobicules are confluent. X6. (After Bather, 1909, te.xt-fig. 2, p. .58.) 238 bis. Greater part of an interambulacrum, interior view, showing beveled sutures, denticulation of adambula- eral margin and apophyses of perignathic girdle. (After Bather, 1909, Plate 1, fig. 9.) apophyses on the basicoronal plates, as seen from the interior of the test (text-fig. 238 bis). These are the only such structures known in the Palaeozoic (p. 190). The fragment of a test in the Dresden Museum (Plate 9, fig. 2), which is one of Geinitz's original specimens, consists of six plates and measures 5.3 mm. in width. The primary spines are up to about 8 mm. in length, above the lower part with small MIOCIDARIS. 247 spinules directed distally. Bather gives much detail in regard to the structure of this species, and saj^s there is no good ground for separating verneuiliana and keyserlingi specifically. Lower Zechstein, Permian, Corbusen, Ronneburg, Germany, cotypes, Dresden Museum; near Posneck, Thuringia, Museum of Comparative Zoology 3,193; Permian, Zechstein, Freiburg i. B. Museum; Permian, Humbleton Hill and Tunstall Hill, British Museum. * Miocidaris cannoni sp. nov. Text-figs. 239a— 239e This species is represented by a single specimen kindly sent to me by Mr. George L. Cannon, instructor in geology and biology in the Denver High School, of Denver, Colorado. It was received too late (August, 1911) to be figured otherwise than by insert cuts, which with the description are inserted in proof-sheets. Text-figs. 239a-239e. — Miocidaris cannoni sp. nov. MilLsap Formation, Lower Carboniferous, near Denver, Colorado. Holotj-pe. Museum of Comparative Zoology Collection 3,201. a. View of the specimen. X 2. A few interambulacral plates are in place in interambulacra A, C, and E. The rest of the specimen consists of an internal siliceous mold. b. Outline of an internal impression of an interambulacral plate from area C. X 4. Showing elevated mamelon. c. View across the dorsal part of interambulacrum C. X 4. Showing fragments of three plates in place which meet the median suture; the outlines of the plates are restored, as indicated by dotted lines. d. Interambulacral plate from near the mid-zone in area E, with about four opposing ambulacral plates, which are indicated by siliceous casts of the pore-pairs. X 4. e. Section of an interambulacral plate in the dorsal part of area E, showing its beveled edge on the adradial suture. X 4. The outline of tlie ambulacrum is seen in section and siliceous molds of the pore-pairs exist as elevated plugs. The specimen is a siliceous internal mold and about half of it is embedded in a hard sili- ceous chert. A few fragmentary worn interambulacral plates are preserved. The specimen shows two ambulacral areas, one interambulacrum complete from the ventral border to the apex, and parts of two next adjacent interambulacra. The impressions of sutures on the mold are clear only in parts, but they are perfectly clear in the plates, as far as these are preserved. The test is depressed spheroidal, free from all distortion, and measures about 20 mm. in 248 ROBERT TRACY JACKSON ON ECHINI. height by 29 mm. in diameter. No ambulacral plates are preserved, so that the characters of the ambulacra are derived wholly from the mold. The ambulacra at the mid-zone measure about 7.5 mm. in width. As this is a measurement of the interior, on the exterior the width would be somewhat less, because the interambulacral plates bevel over the ambulacrals on the adradial sutures (text-fig. 239e). There are two columns of low plates in each ambulacrum. The position of the plates is indicated in part by rather obscure impressions of sutures, but mainly by the siliceous plugs which represent the casts of pores of the pore-pairs (text-fig. 239e) . The pores of each pore-pair at the mid-zone are 1 mm. apart, and the distance from the outer pore of one plate in a right half-area to the outer pore of the adjacent plate in a left half-area is 6 mm. Four ambulacral plates equal the height of an interambulacral plate at the mid-zone, as gathered from the number of pore-pairs in the mold opposed to an interambulacral plate which is in place (text-fig. 239d). This number is similar to the number of ambulacral plates opposed to an interambulacral in Miocidaris keyserlingi, as stated above, but it is much smaller than the usual number in the adults of Recent cidarids (compare text-fig. 218, p. 191). In the young of Recent cidarids, however, the ambulacral plates are higher and in very young speci- mens may be even as high as the interambulacrals (Plate 2, fig. 2). The only interambulacrum which is entirely exposed to view measures 12 mm. in width at the mid-zone. In the same plane the width from the right outer pore of ambulacrum D across interambulacrum C to the left outer pore of ambulacrum B is 13 mm., this being only slightly more than the width given for the interambulacrum itself. There are two columns of plates throughout the interambulacral area, and, as gathered from the impressions on the mold, there are fourteen plates in each interambulacral column. In each of the two lateral inter- ambulacral areas, A and E, which are partially visible, there are five plates in place, and these from their height also indicate that there would be about fourteen plates in one column. This is a very large number for the genus and also for the family. In Recent Eucidaris tribuloides I have found fourteen plates in a column, but such a number is rarely attained in the Cidaridae. In the dorsal portion of interambulacrum C there are two fragmentary plates (text-fig. 239c) which meet in a median angular suture, as usual in cidarids. As seen at one point dorsally where the plates are preserved, the interambulacral plates bevel strongly over the ambulacrals on the adradial suture (text-fig. 239e). It is from this structure that I refer the species to the genus Miocidaris. The interambulacral plates at the mid-zone measure 2.5 mm. in height by about 6 mm. in width, measuring to the middle of the interambulacral area, and about 0.75 mm. in thickness. The measurement of the height and width is taken from the internal mold, as no plate preserved is complete in width, but the height as given is confirmed by the plates which are in place. The most complete plate preserved at the mid-zone in area E meas- ures 4 mm. in width, but its admedian portion is hidden beneath the hard siliceous matrix. Unfortunately the worn plates that exist do not show a complete outline nor surface characters, MIOCIDARIS. 249 excepting that one plate in area E shows the remains of the perforation of a primary tubercle. In the mold the horizontal boundaries of the interambulacral plates are fairly well defined as depressed furrows extending from the adradial suture to near the middle of the interambula- crum, but the median sutures are mos'ly indistinct. On examining the interior of a half- grown Eucidaris tribuloides it is seen that the horizontal boundaries of the interambulacral plates stand out as prominent ridges, whereas the median boundaries do not, and this may well account for the differences seen in the mold. The mold of each plate presents a slightly convex form with a small raammillate elevation (text-figs. 239a, b) which corresponds with the concave form and deeper impression beneath the tubercle on the interior of the plate of a cida- rid, as seen well in an immature Eucidaris tribuloides. This species is of exceptional interest, as it is the first sea-urchin recorded from the Palaeo- zoic of America with two columns of plates in an interambulacrum ; also it extends the range of the genus Miocidaris and the order Cidaroida to the Lower Carboniferous, whereas previously they were not known below the Permian. The occurrence of this type with two columns of interambulacral plates in the Lower Carboniferous adds support to my view, as earlier pub- lished (1896, p. 237, table facing p. 242) and maintained in this memoir, that two columns of plates in an interambulacral area is a primitive structure and types having this structure are to be derived from an ancestor with one column of plates in an area, and not to be derived from an ancestor with many columns in an interambulacral area, as has been usually held (p. 211). Miocidaris cannoni, which I take pleasure in naming for Mr. Cannon, from whom I received the specimen, differs from other species of the genus in its large size and numerous interambula- cral plates in a column. The number of plates is perhaps sufficient to justify generic distinction, but as the surface characters are unknown, it seems best to place the species in the genus Mio- cidaris, to which at least it is very closely allied. While this species occurs in the Lower Carboni- ferous and is therefore very much older than Miocidaris keyserlingi of the Permian, it is placed higher than that species because, on account of having a large number of plates in an inter- ambulacral column, it is farther removed from the primitive than is keyserlingi with a smaller number of plates in a column. Mr. Cannon informs me that he recently obtained the specimen, and that it is from the Millsap Formation of the Rocky Mountain Lower Carboniferous, from near Denver, Colorado. The specimen was collected by Mr. George Day, and was found in one of the loose chert concre- tions in the Platte River drift. While not in place, material of this lithological character according to Mr. Cannon and other authorities is known in Colorado only from the Millsap Formation, which occurs as outcrops in the Front Range. Girty (1903, pp. 162, 167) ascribes the Millsap Formation to the Mississippian, or Lower Carboniferous. The specimen is now in the collection of the Museum of Comparative Zoology 3,201. 250 ROBERT TRACY JACKSON ON ECHINI. Order ECHINOCYSTOIDA nom. nov. Cystoddaridae Zittel, 1879, p. 480; A. Agassiz, 1881, p. 81. Cystoddaroida Duncan, 1889a, p. 20; .lackson, 1896, p. 242; Gregory, 1900, p. 300. Cysticaroidea Tornquist, 1897, p. 730. Professor Zittel based his name of the order on the name Cystocidaris which he gave to replace the genus Echinocystites of Wyville Thomson. As Cystocidaris is a synonym and cannot stand, neither, by the rules of nomenclature, can the name of the order based on it; I therefore substitute a new name for this order. Test spheroidal or flattened, apparently irregular, with the anus in an interambulacral area. Ambulacra narrow, with two or four columns of low plates in an area. Interambulacra wide, with numerous, eight or more, columns of very thin, rather irregular polygonal plates in an area. Lantern well developed, typically echinoid in character. The known specimens of the two families and genera of this order are imperfectly preserved, therefore are incompletely known; further knowledge is much to be desired. Family PALAEODISCIDAE Gregory. Palaeodisddae Gregory, 1897, p. 133; Lambert and Thierj-, 1910, p. 117. Test depressed, nearly or quite circular. Gregory says pentagonal, but I think this is hardly correct. Ambulacra narrow, compo.sed of two columns of low plates in each area, with pores throughout the areas, not imperforate ventrally, as Gregory says. Anus eccentric in an interambulacrum at a considerable distance from the apex and quite near the mid-zone, as figuredby Spencer (1904, Plate 1, fig. 1). Earlier Gregory (1897) said that the anus is central, but he did not figure it. I have not seen a specimen showing this structure. A typical echinoid lantern exists. There is only one genus and species. Palaeodiscxjs Salter. Pnlacodiscus Salter, 1857, p. 332; Zittel, 1879, p. 4.53; Duncan, 1889a, p. 6; Gregory, 1900, p. 301; Lam- bert and Thiery, 1910, p. 1 17. With characters of the family. The type and only recognized species is P. ferox Salter, of the Silurian of England. Palaeodiscus gothicus is considered under Incertae Sedis. *Palaeodiscus ferox Salter. Plate 18, figs. 1-5. Palaeodiscus ferox Salter, 1857, p. 332, Plate 9, fig. 6; Wyville Thomson, 1861, p. 13, Plate 4, figs. 6, 7 Neumayr, 1881, p. 155, Plate 1, fig. 8; Gregory, 1897, p. 129, text-figs. 4, 5; Plate 7, figs. 5a-5b Sollas, 1899, p. 701, text-figs. 6-11; 1899a, p. 277; Spencer, 1904, p. 31, text-figs. 1-8, Plate 1, fig. 1 Lambert and Thiery, 1910, p. 117. PALAEODISCUS. 251 The test is very much flattened in known specimens, as it probably was in Ufe, and is nearly or quite circular in outline. Ambulacra are narrow with two colurtins of low plates, of which about four equal the height of an adambulacral plate. The pore-pairs are about in the middle of the plates and are clearly developed ventrally, as shown in my figures. I cannot understand why Dr. Gregory should say (1897, p. 129) that there are no pores through the plates, but that the podia must have been extended between the sutures. The specimens Dr. Bather kindly loaned me, from the British Museum, have pores distinctly through the plates as in other tj^pical echinoids. Interambulacra with many, apparently about eight or nine columns of highly irregular plates, and, as pointed out by Spencer, the primordial plates are in the basi- coronal row. The plates were evidently extremely thin and bore numerous fine acicular spines. On the peristome there are ten columns of ambulacral plates only. The apical disc is unknown but I think Gregory is not justified in saying probably absent. It is not known to be absent in any echinoid. The lantern, as shown by Sollas, is of the typical character, inclined, with pyra- mids, epiphyses, braces, and even compasses, as figured by Sollas. Lower Ludlow, Silurian, near Ludlow, and Church Hill, Leintwardine, England; British Museum, a number of specimens, including those described by Gregory (1897) ; Oxford Museum. This species, representing its genus and family, has been much discussed, and on account of imperfections of material, a number of points are still uncertain. Salter referred the type to starfishes, and Zittel (1879, p. 453) followed him. Several authors have considered the type as intermediate between starfishes and echinoids, and as a primitive type. I think recently published material distinctly shows that Palaeodiscus is an echinoid, has no connection with starfishes, and is not a primitive type. As regards the ambulacrum, I find no evidence of pores between the plates, as claimed by Gregory, but rather they are through the plates as usual in echinoids. As regards the roofing system of plates described by Sollas and Spencer, they claim that above the typical ambula- cral plates (Sollas, 1899, p. 705, fig. 9) there is a set of narrower plates roofing over the outer . flooring plates. This narrower set is supposed to correspond with the ambulacral plates of a /^^U^^'^^^-4\^^a^i¥ehm, superposed on those of a stai'fiA, anotnus the connection of the two groups is supposed to be solved. It is a somewhat novel morphological reasoning from the facts. It would require that we consider the wider or flooring ambulacral plates of Palaeodiscus (which Sollas, 1899, pp. 705, 706, considers the equivalent of the ambulacral plates of later Echini) as equivalent to the adambulacral plates of a starfish grown across the area and meeting in the center line, but I see no basis for such an argument. I think the roofing plates of Sollas are the narrower ambulacral plates of the dorsal side brought in mechanical contact with the wider ventral plates by the flattening of the test, exactly as shown in area B of Hyattechinus rarispinus (Plate 23, fig. 1). The interambulacra are typically echinoid, and from the large number of columns of plates appear to me to indicate a specialized, not a primitive type. Ambulacral 252 ROBERT TRACY JACKSON ON ECHINI. plates on the peristome are a typical echinoid character. The inclined lantern is of the typical character seen in Palaeozoic species, is purely echinoid, and has no relation to the structure of any known starfish. There are grooved teeth, narrow epiphyses, braces, and even rather deeply forked compasses as shown bj^ Sollas (1899, text-figs. 7, 8) whose figure 7 is in part reproduced in my Plate 18, fig. 5. Some knowledge of ocular and genital plates is much to be desired, and the position of the anus should be more positively settled. If Spencer is correct in showing the anus in an interambulacral area, we must consider Palaeodiscus a peculiar and irregular echinoid. If the anus is in the center dorsally, as Gregory says (1897, p. 129), then Palaeodiscus is not irregular, is wrongly placed in this order, and could properly find a place in the Lepidocentridae, from which it differs essentially in this character only. Family ECHINOCYSTIDAE Gregory. Echiiiocystidac Gregory, 1897, p. 133; Lambert and Thiery, 1910, p. 118. Test spheroidal, ambulacra narrow with at the mid-zone four columns of plates which are demi- and occluded. Interambulacra broad with many columns of irregular plates. Plates thin with small primary and secondary tubercles and spines. Apical plates doubtful, but the anus is eccentric in an interambulacrum. A well developed typical echinoid lantern exists. One genus and species. EcHiNOCYSTiTES WyviUe Thomson. Echinocystiics WyviUe Thomson, 1861, p. 106; Duncan, 1889a, p. 20; non Hall, 1865; lS6Sa, p. 316. Cystocidaris Zittel, 1879, p. 480. Echinocystis Gregory, . 1897, p. 124; Lambert and Thiery, 1910, p. 118. With characters of the family. The type-species is E. pomum Wyville Thomson, from the Silurian of England. *Echinocystites pomum Wy\ine Thomson. Plate 16, fig. 4; Plate IS, figs. 6-8; Plate 20, figs. 1-4. Echinocystitcs pomum WyviUe Thomson, 1861, p. 109, Plate 3, figs. 1-3; Plate 4, figs. 1-5. Echinocystiics uva Wyville Thomson, 1861, p. 110, Plate 4, figs. 4,5. Cystocidaris pomum Zittel, 1879, p. 480. Cystocidaris uva Zittel, 1879, p. 480. Echinocystis pomum Gregory, 1897, p. 124, text-figs. 1-3, Plate 7, fig. 4; Sollas, 1899, p. 707, text-figs. 12-14; Klem, 1904, p. 13; Lambert and Thiery, 1910, p. 118. Echinocystis [pomum] Spencer, 1904, pp. 31, 34. Echinocystis um Lambert and Thiery, 1910, p. 118. The test is spheroidal, ambulacra narrow with four columns of plates in an area, demi- and occluded, pore-pairs are biserial and pass through about the center of each plate. Inter- PERISCHOECHINOIDA. 253 ambulacra wide with in each area about eight columns of very irregular thin polygonal plates which bear small primary and secondary tubercles and spines. The apical plates are doubtful ; but a madreporite is described by Wyville Thomson and Gregory in an interambulacrum. As this is a unique condition in Echini and morphologically difficult to understand, it may not be too much to suggest that in the squeezing of this rather badly preserved material, it may have got displaced. According to Wyville Thomson, the anus with apparently periproctal plates is in an interambulacrum, making it an irregular type. The lantern is well developed, and Gregory shows that it has typical echinoid pyramids. The material I have seen is so crushed and badly preserved that it is very difficult to make out much structure. There seems no reason for considering this type as presenting any possible connec- tion with starfishes, or as a primitive type. Its four columns of ambulacral plates, and especi- ally its eccentric anal area, would lead one to consider it a specialized and irregular rather than a primitive echinoid. Lower Ludlow, Silurian, Leintwardine, England ; a very fine specimen is in the Museum of Practical Geology, London 7,385, one of Sir Wyville Thomson's originals. This is shown in Plate 16, fig. 4, from a photograph kindly sent me by Dr. Kitchin. There are several specimens in the British Museum, part of which Dr. Bather kindly loaned me for study. Oxford Museum (Sollas). Order PERISCHOECHINOIDA M'Coy. Perischoechimda ]\I'Co.v, lS-t9, p. 251; 1S54, p. 114; Jackson, 1896, p. 239. Tesselati Desor, 1858, p. 152. Perischoechinidar Lnven, 1874, p. 39; A. Agassiz, 1874, p. 644; 1881, p. 3; Zittel, .1879, p. 4S1. This large order consists of regular Palaeozoic Echini in which the periproct is within the oculo-genital ring. The test is of various shapes and horizontal outline throughout the ambitus, but is usually spheroidal. Ambulacral areas are narrow, or wide, with in each area from two to twenty columns of plates, all simple and all bearing one pore-pair each. There are from three to fourteen columns of plates in each interambulacral area. Usually the plates of the adambulacral columns are pentagonal, and tho.se of the median columns hexagonal, except where new columns are introduced, and in young plates dorsally. Plates may be thick, or thin, and bear primary perforate tubercles and spines with secondaries, or the latter only. Plates of the corona may be imbricate or not; when imbricate, ambulacral plates imbricate adorally and bevel under the adambulacral plates laterally. Interambulacral plates imbricate aborally and from the center laterally and over the adjacent ambulacral plates. When plates are not imbricate, then the sides of the plates are as nearly parallel as is mechanically possible in a curved test, but they may be very far from parallel, as in the thick-plated Melonechinus. When there is no imbrication, the ambulacral plates may bevel over the adambulacrals, as is char- 254 ROBERT TRACY JACKSON ON ECHINI. acteristic of the Palaeechinidae. As far as the evidence goes, the primordial ambulacral plates are around the mouth on the peristome, and the primordial interambulacral plates are in the basicoronal row, or are resorbed in the advance of the peristome. The peristome is plated with many rows of ambulacral plates only, or may in addition have non-ambulacral plates situated interradially. Oculars are usually imperforate and all insert, but rarely are perforate, and exceptionally one or more to all may be exsert. Genitals usually have three or more to numer- ous genital pores; the madreporite is rarely distinguishable. The periproct is covered with many angular plates which fill the area. The lantern is inclined, subtending an angle of about 90 degrees, and as in modern regular Echini, is composed of forty pieces. The teeth are grooved, pyramids wide-angled, foramen magnum moderately deep, epiphyses narrow, .capping the half-pyramids; brace and compass as in modern Echini. This group includes all Palaeozoic Echini excepting those already considered, and comprises the best known and, excepting Both- riocidaris, the most interesting genera of these ancient formations (pp. 361-363). Family Archaeocidakidae M'Coy. Echinocrinoidea T. and T. Austin, 1S42, p. 111. Archaeocidaridae M'Coy, 1849, p. 253; Loven, 1874, p. 42. Lcpidocidaridac. Bather, 1907, p. 456. The test is spheroidal or depressed; ambulacra are narrow, with in each area two columns of plates imbricating moderately adorally and beveled undei; the adambulacrals. Pore-pairs are uniserial or slightly biserial. There are from four to eight columns of plates in each inter- ambulacral area, plates are large, thin, imbricating moderately aborally and laterally and over the ambulacrals. The interambulacral plates bear a large central perforate primary tubercle and large scrobicule, with a large primary spine, also secondary tubercles and spines. The base of the corona is resorbed in the advance of the peristome, strongly so in Archaeocidaris. The peristome has many rows of ambulacral and at least in Ai-chaeocidaris interradial non-ambula- cral plates as well. Oculars and genitals unknown or very doubtful. There are three genera in the family. The name Lepidocidaridae was proposed by Dr. Bather to take the place of Archae- ocidaridae on the basis of giving up Archaeocidaris for the earlier but misleading name of Echinocrinus. As I have decided to cling to the old established name Archaeocidaris, pending possible consideration of the matter, I therefore retain also the name Archaeocidaridae. EociDARis Desor. Eocidaris Desor, 1858, p. 155; Lov(5n, 1874, p. 42; Bather, 1909, p. 44; (pars) Lambert and Thiery, 1910, p. 126. This genus, most imperfectly known, is based on isolated interambulacral plates and primary spines. Interambulacral plates are probably hexagonal, or pentagonal in adradial EOCIDARIS. 255 columns, as in Archaeocidaris, but the shape is not definitely known in the only species attri- buted to this genus. The plates have a large central perforate tubercle and scrobicule and marginal secondary tubercles, like Archaeocidaris, but differ from that genus in that there is no basal terrace within the scrobicular circle. Primary spines are stout. This genus has recently received most careful consideration from Dr. Bather, and he concludes from all the evidence that of known species, only the Cidaris laevispina and scrobi- culata Sandberger are referable to it. Many species have been referred to the genus by various authors, but I agree with Bather that all excepting those noted, are clearly referable to other genera. This genus is closely related to Archaeocidaris, from which, so far as known, it differs only in the absence of the basal terrace. As the known species occur in the Devonian, it is older than Archaeocidaris, which first appears in the Lower Carboniferous. The youthful plates in the dorsal region in Archaeocidaris rossica have not acquired the basal terrace, and in this feature are therefore like the condition ascribed to the geologically older but closely related Eocidaris. The type species is E. laevispina (Sandberger) from the Devonian of Germany. *Eocidaris laevispina (Sandberger). Plate 15, figs, lla-d, 12. Cidaris laevispina Sandberger, 1850-'56, p. 382, Plate 35, figs. 2-2d. Cidaris scrobicuhda Sandberger, 1850-'56, p. 382, Plate 35, fig. 3. Eocidaris lacmspiHa Desor, 1858, p. 156, Plate 21, figs. 18-21; Loven, 1874, p. 43; Klem, 1904, p. 69; Bather, 1909, te.\t-fig. 1, p. 51, Plate 1, figs. 1-1; Lambert and Thiery, 1910, p. 126. Eocidaris scrobiculata Desor, 1858, p. 157, Plate 21, fig. 17; Loven, 1874, p. 43; Klem, 1904, p. 71; Bather, 1909, Plate 1, fig. 5; Lambert and Thiery, 1910, p. 126. This species, the only one which I recognize in the genus, is based on isolated interambula- cral plates and spines. Bather recently borrowed Sandberger's original specimens from the Wiesbaden Museum and figured them, and my figures are based on his. The interambulacral plates show beveled edges, indicating strong imbrication. As the outlines of the plates are not preserved, their shape, hexagonal or otherwise, cannot be definitely stated. The central perforate mamelon is elongate, scrobicular area large, with no trace of a basal terrace; second- ary tubercles are rather widely and irregularly spaced on the extra-scrobicular area excepting the beveled edge, which apparently in life was overlapped by the next adjacent plate. Primary spines are stout, cylindrical, smooth, with a weakly developed annulus, which does not show vertical fluting as in the typical milled ring of Archaeocidaris. The holotype of Cidaris scrobiculata Sandberger is a single interambulacral plate (Plate 15, fig. 12), and is so close to laevispina in character, that it is here included as a synonym. As Bather says (1909, p. 53), " The Sandbergers may have been right in regarding this specimen as 256 ROBERT TRACY JACKSON ON ECHINI. of a different species; but if one were to maintain that it came merely from a different part of the test of C. laevispina, I do not see how it could be disproved." Sandberger's material all came from the Middle Devonian, Stringocephalus Limestone of Villmar, and is in the Wiesbaden Museum as above noted. Two interambulacral plates from the Devonian of Villmar are in the Museum of Comparative Zoology 3,192, from the Schiiltze Collection. Archaeocidaris M'Coy. Echinocrinm L. Agassiz, 1841, p. 15; Bather, 1907; 1909; 1909a, p. 264. Archaeocidaris M'Coy Mss., 1844, p. 173; M'Coy, 1849, p. 252; Young, 1873; 1876; Loven, 1874, p. 43; Duncan, 1889a, p. 11; A. Agassiz, 1881, pp. 79, 80; Lambert and Thiery, 1910, p. 124. Palaeocidaris L. Agassiz and Desor, 1846-'47, p. 340. Cidarotropus Pomel, 18S3, p. 113; Lambert and Thiery, 1910, p. 125. Permocidaris (pars) Lambert, 1899a, pp. 39, 47; Lambert and Thiery, 1910, p. 127. The test is depressed spheroidal, ambulacra narrow, sinuous in outline, conforming to the outline of the massive adradial plates. Ambulacral plates are low, of uniform character, imbricating moderately adorally and beveled strongly under the adradials; the pore-pairs are uniserial. Interambulacra, in all species where a fairly complete test is known, with four columns of plates in each interambulacral area. The adradial plates are pentagonal, but rounded on the adradial suture line; plates of the intermediate columns are hexagonal. Each plate bears a prominent perforate median tubercle with a wide scrobicular area and a basal terrace, though this terrace, being a slight feature, may be worn off in eroded plates and is absent {A. rossica) in young plates dorsally. Beyond the border of the scrobicular circle are secondary tubercles which differ in their number and in the extent of surface which they cover both in different species and somewhat in plates of different parts of the test in the same species (A. rossica). Primary spines are relatively large, often very large, with a concave base, marked milled ring, and a shaft tapering or enlarged, smooth or ornamented with striations, spinules, or rarely flange-like vertical ridges. The primordial ambulacral plates are on the peristome around the mouth, and the primordial interambulacral plates with additional rows are resorbed in the advance of the peristome, as there are four plates in the basicoronal row (Plate 9, figs. 7, 8). The peristome is covered with radially situated ambulacral and interradially situated non-anibulacral plates, all small and imbricating adorally (text-fig. 47, p. SO). Ocular and genital plates are doubtful, periproct with many small angular plates. The lantern is well developed, of the typical Palaeozoic character (Plate 12, figs. 1-8). Of this genus a few species are known fairly completely,' but there are numerous species described from incomplete material, often only one or more dis- ' The best known species, especially as regards the test, are Archaeocidaris worlheni, legrandensis, rossica, agassizi, and urii. ARCIL\EOCIDARIS. 257 sociated plates and spines. Such are most unsatisfactory, often difficult to distinguish, and doubtless there are too many recognized species, of little interest except as expressing the geo- graphical distribution of the genus. With available material the best way, it seems, is to group the species on the basis of spine characters, associating those species that have smooth spines, those in which the spines bear low nodose spinules, and those with spinules more elevated and directed at right angles to the shaft of the spine, or pointing distally, or spines with lateral flanges, spines triangular in section, or spines inflated. This has at least some resemblance to a natural classification. The type species, as indicated by Bather (1907, p. 453), is A. urii (Fleming) from the Lower Carboniferous of Europe. The genus Archaeocidaris is structurally closely related to the Cidaridae, but is more complex in that there are four columns of plates in an interambulacral area instead of two columns. Archaeocidaris may be derived from some early cidarid, perhaps Devonian or Silurian. The fact that I describe a true cidarid (Miocidaris cannoni) from the Lower Carboniferous, which is geologically as early as any species of Archaeocidaris known lends weight to this argument. (See pp. 70, 77, 80, 184, 223, 361, 363.) Lambert and Thiery (1910, pp. 124, 125) under what I consider as Archaeocidaris recognize two genera, Archaeocidaris, which they say has interambulacra formed of from five to eight columns of plates, and Cidarotropus, in which they say that the interambulacra are formed of four columns of plates. I know no species of Archaeocidaris with more than four columns of plates in an interambulacral area, and many of the species which they list in one of their two definite genera are known only fragmentarily from isolated plates so that the number of columns in an area is quite unknown. The proper name to apply to this genus is open to question. As shown positively by my friend. Dr. Bather, on the basis of priority, Echinocrinus is the correct name. This name, however, is misleading, was based by Professor Agassiz on a misconception of affinities, and has been abandoned for some sixty years in favor of the entirely appropriate Ai-chaeocidaris of M 'Coy. It is not a case of Archaeocidaris being preoccupied but simply of priority. To revive the old name, Echinocrinus, in accordance with rules of nomenclature, will make confusion at present and in future, and no gain to any one. As Dr. Bather justly says (1909a, p. 264), "While there can be no doubt as to the conse- quences of the rules, this seems to me just one of those cases that should be settled by a properly constituted authority in defiance of the rules." Such being the case, pending the action of some formal authority, I feel it best to follow the conservative action of retaining the entirely satisfactory and thoroughly established name Archaeocidaris. The name Palaeocidaris Agassiz and Desor is a pure synonym, Cidarotropus Pomel with Archaeocidaris wortheni Hall as the type, and Permocidaris Lambert with Archaeocidaris forbesiana (Koninck) as the type, I do not recognize as distinct genera. 258 ROBERT TRACY JACKSON ON ECHINI. With present knowledge it is impracticable to make a key for distinguishing each of the numerous and imperfectly known species of Archaeocidaris. An attempt has been made, however, to give a key that will distinguish groups of species which have similar characters as regards the structure of the primary spines, and these will serve as an aid in identifying any case in hand in which the spines are in place. Key to the Groups of Species of Archaeocidaris.^ Spines circular in section, smooth, without spinules or nodose elevations: A. worthnii Hall (America), p. 259. A. hvujiKpina Newberry (America), p. 261. A. leqrandcnsis Miller and Gurley (America), A. (jJuhrispina (Phillips) (Europe), p. 2G1. p. 2G0. Spines circular or rarely elliptical in section, with numerous short spinules which are nearly uniform in size and are directed distally : A. nrrei (Miinster) (Europe), p. 262. A. aculeata Shumard and Swallow (America), A. rossica (Buch) (Europe), p. 263. p. 268. A. agassizi Hall (America), p. 266. A. shumardana Hall (America), p. 268. A. iUinoiscnsis Worthen and Miller (America), A. edgarcnsis Worthen and Miller (.America), p. 266. p. 269. A. coloradensis nom. nov. (America), p. 267. A. newberri/i Hambach (America), p. 269. A. kcokuk Hall (America), p. 267. A. frudifer White (America), p. 269. A. gracilis Newberry (America), p. 267. Spines circular in section, with large thorn-like spinules directed distally: A. norwoodi Hall (America), p. 270. A. cratis White (America), p. 272. A. parado.ra (Eichwald) (Europe), p. 270. A. acanthifcra Trautschold (Europe), p. 272. A. mucroruda Meek and Worthen (America), 'A. pizzulana Gortani (Europe), p. 272. p. 271. A. megastyla Shunmrd and Swallow (America), A. dininnii White (America), p. 271. p. 273. Spines with two lateral wing-like expansions: .1. Jiiangvlata Shumard and Swallow (America), p. 273. Spines densely covered with long spinules directed distally: A. ornata (Eichwald) (Europe), p. 274. A. ouraycnsis Girty (America), p. 274. Spines triangular in section, with spinules on the angles and directed distally: A. triplex White (America), p. 274. A. triscrrata Meek (America), p. 275. A. triserialis (M'Coy) (Europe), p. 275. ' A number of species of Archaeocidaris, or of species referable to that genus, which are not known well enough to intercalate in the systematic series are taken up later. These are: Archaeocidaris konincki, ladina, scolica, selwyni, sixi and trautscholdi, Cidarites tennesseae, Echinocrinus anceps, spinosus and siriatus, considered under Incertae Sedis. Also Archaeocidaris tirolensis, Echinocrinus cidnriforniis and pomum considered under Nomina Nuda. ARCHAEOCIDARIS. 259 Spines with four vertical rows of spinules: A. rankini Young (Europe), p. 276. Spines with six vertical rows of spinules: A. prisca (Miinster) (Europe), p. 276. Spines with many vertical ridges, which bear spinules directed distally: A. wcrwlcci Tornquist (Europe), p. 276. A. halliana (Geinitz) (Europe), p. 279. A. vrii (Fleming) (Europe), p. 276. A. muenstrriana (Koninck) (Europe), p. 280. Spines inflated, club-shaped: A. forhvsiana (Koninck) (India), p. 280. A. sp. h. Girty (America), p. 281. A. spinoclaoata Worthen and Miller (America), A. clacata (Eichwald) (Europe), p. 282. p. 281. *Archaeocidaris wortheni Hall. Text-figs. 26, p. 70; 47, p. 80; Plate 8, figs. 5, 6; Plate 9, figs. 6-11. Archaeocidaris wortheni Hall, 1858, p. 700, Plate 26, figs. 4a-4g; Loven, 1874, p. 4-3; Qiienstedt, 1875, p. 372, Plate 75, figs. 10-14; Zittel, 1879, p. 485, figs. 34:3a-343e (not 343d, which in this and other editions is evidently Archaeocidaris keokuk Hall); (?) Waleott, 1884, p. 212; Keyes, 1894, p. 128, Plate 17, figs. 3a, 3b; 1895, p. 187, Plate 20, figs. 3a, 3b; Jackson, 1896, p. 214, Plate 8, figs. 43-46; Tornquist, 1897, p. 770, Plate 21, fig. 5; Klem, 1904, p. 61. Cidarotropus wortheni Pomel, 1883, p. 113; Lambert and Thiery, 1910, p. 125. Test spheroidal, but flattened mechanically in the known specimens. The ambulacra are narrow, sinuous, conforming to the outline of the adambulacral plates. Ambulacral plates are low, imbricating moderately adorally, beveled under the adradials, with poi-e-pairs uni- serial. Interambulacra are broad, with four columns of plates in each area, imbricating mod- erately aborally and from center laterally and over the ambulacrals. There are four plates in the basicoronal row of each interambulacral area, indicating an extensive resorption of the corona in the advance of the peristome (p. 73). Interambulacral plates hexagonal, except the adradials, somewhat wider than high, with a wide scrobicular area as seen well in Plate 8, fig. 5, and Plate 9, fig. 9. Secondary tubercles are limited to a narrow border parallel to the edges of each plate. The primary spines are long, slender, slightly arcuate, from the milled ring tapering to the distal end. Small secondary spines are associated with the secondary tubercles. The peristome has ambulacral plates radially and numerous scale-like non-ambula- cral plates interradially situated, all imbricating adorally. The dorsal area is known only partially (Plate 8, fig. 6). The lantern is of the typical character of the genus (Plate 9, fig. 10). St. Louis Limestone, Lower Carboniferous, St. Louis, Missouri. The cotypes and several other specimens including one fine slab with ten tests are in the American Museum of Natural 260 ROBERT TRACY JACKSON ON ECHINI. History; specimens from the same locality in the Museum of Comparative Zoology ; F. Springer Collection; (?) Lower Carboniferous, Eureka District, Nevada (Walcott). The type specimen in the American Museum of Natural History is one of the most nearly complete specimens known in the genus, and measures 53 mm. in diameter through J, E (Plate 8, figs. 5, 6; Plate 9, figs. 6-8). It is not quite complete in the basicoronal row, but excepting area I is very nearly so. In areas A and C, in the basicoronal row, the plates of columns 1 and 4 are nearly whole, whereas the plates of columns 3 and 2 are small, representing only the dorsal part of each plate, of which the ventral portion has been resorbed. In areas E, G, and probably I, in the basicoronal row, the plates of columns 1 and 3 are small, of which the greater part has been resorbed, whereas the basicoronal plates of columns 4 and 2 are nearly complete plates with slight ventral resorption. A similar condition is shown in A . rossica (text-fig. 239 bis, p. 264) , and in part in A. urii (Plate 15, fig. 1). This alternation of large and small plates in the basi- coronal row is due to the cutting off of the ventral part of the plates by resorption in the advance of the peristome (text-fig. 26, p. 70). The relative position of the large and small plates differ- ing by the two arrangements I earlier ascribed (1896, p. 219) to the relative position of the in- troduction of column 4, as seen in the restoration (Plate 9, fig. 8). By restoring schematically the plates that have been resorbed, we see that if column 4 is introduced to the right of the center, then in the basicoronal row the plates of columns 1 and 4 are large; or if column 4 is in- troduced to the left of the center, then in the basicoronal row the plates of columns 1 and 3 are small. There are no gill-slits on the peristomal border (p. 223). The peristome in this fine specimen is the most nearly complete one known in the genus. There are two columns of ambulacral plates in an area, doubtless formed by flowing down from the corona as in Cidaris (text-figs. 46, 47, p. 80). There are also many irregular scale-like non-ambulacral plates imbricating strongly. These plates, it is believed, are formed in place, and are not derived from the corona, as discussed under consideration of the peristome (p. 86). It was in this specimen that secondary spines were first shown in the genus. A complete lantern is not known, but the essential parts are shown in Plate 8, fig. 6, and Plate 9, fig. 10. The pyramid is wide-angled with rather deep foramen, epiphyses narrow, capping the half- pyramids. *Archaeocidaris legrandensis Miller and Gurley. Plate 8, figs. 7, 8; Plate 9, figs. 12, 13; Plate 10, figs. 3, 4. Archacocidaris legranch'iisis Miller and Gurley, 1SS9, p. 373, Plate 10, fig. 1.5; Keyes, 189.5, p. 185; Klem, 1904, p. 50; Lambert and Thiery, 1910, p. 124. Eoeidaris blairi Miller, 1892, p. 683, Plate 22, fig. 1; Klem, 1904, p. 67. Test in side view (Plate 8, fig. 8) is only moderately compressed, and measures 26 mm. in diameter, and the ventral view (Plate 8, fig. 7) is of about the same size. The ambulacral plates are not preserved, but their position is located by the outlines of the adambulacrals. ARCHAEOCIDARIS. 261 There are four columns of plates in each interambulacrum ; these plates are low, wide hexagons, the basal terrace is clearly marked in the scrobicular area (Plate 8, fig. 7). Spines are smooth, tapering, slightly arcuate. Secondary spines are on the borders of plates associated with secondary tubercles (Plate 9, fig. 13). The type of A. legrandensis (Plate 10, fig. 4) is a very small and not well preserved speci- men, but it has low hexagonal plates and smooth, slightly arcuate spines. As I cannot dis- tinguish this species from hlairi, therefore legrandensis, which is the older name, has priority. Miller was mistaken in referring hlairi to the genus Eocidaris, for since the basal terrace is perfectly clearly marked, this featiire indicates that it is an Archaeocidaris. The type specimen of legrandensis is from the Kinderhook Group, Lower Carboniferous, Le Grand, Iowa, now in Chicago University Collection 6,198. A second specimen from the same locality and collection is no. 12,314. The specimens originally described as Eocidaris blairi, here considered a synonym, are from the Keokuk Group of Boonville, Missouri, now in Chicago University Collection 8,855. Keokuk Group, Canton, Indiana, a ventral view, with some spines in place. Museum of Comparative Zoology 3,198; Keokuk Group, Bono, Lawrence County, Indiana, Museum of Comparative Zoology, two good specimens, one (3,199) showing four columns of plates in each interambulacral area and the other (3,200) spines in place. In the Springer Collection a specimen 8,024, from the Kaskaskia Group, Pulaski County, Kentucky, and one 8,083, from the Keokuk Limestone of Bono, Indiana, are referred to this species. *Archaeocidaris longispina Newberry. Plate 10, fig. 1. Archaeocidaris longispina Newberry, 1861, p. 116, Plate 1, figs. 1, la; Keyes, 1895, p. 191; Klem, 1904, p. 51; Lambert and Thiery, 1910, p. 124. Known only from primary spines which are long, smooth, narrow above the milled ring, inflated in the lower half and thence tapering to a point. Newberry says 2.5 to 3.5 inches in length. The longest of the three type specimens measures 63 mm. in length and 6 mm. in diameter at the widest part. Crinoidal Limestone, upper part of Carboniferous (Coal Measures), on the banks of the Colorado River. The type consists of three spines on one slab. It is in Columbia University Collection 6,419 G, and bears the label "Coal measure limestone. North Arizona." *Archaeocidaris glabrispina (Phillips). Plate 10, figs. 2a-2b, 8a-8e, 9. Cidaris glabrispina Phillips, 1836, p. 208. (?) Eckinocrinus glabrispina M'Coy, 1844, p. 173. Archaeocidaris stellifera Baily, 1877, p. 18, text-figs. a-e. 262 ROBERT TRACY JACKSON ON ECHINI. Archaeocldaris lacvi^ Trautscholfl, 1879, p. 7, Plate 2, fig. 1 u (the number of the figure is wanting on Plate). Archacocidaris glabrispina Etheridge, 1SS8, p. 221 ; Klem, 1904, p. 62 ; Lambert and Thit'ry, 1910, pp. 124, 125 . Primary spines smooth, straight, swollen about the middle, above which tapering to a point. Interambulacral plates hexagonal, about 5.2 mm. wide, and wider than high. The species stellifera and laevis may differ from glabrispina if complete material is found; but with present evidence they cannot be distinguished. The plates figured by Baily (my Plate 10, figs. 8a-8c) are so schematic as probably to be unreliable. Lower Carboniferous, the original locality given by Phillips is Northumberland; Bundaran, Ireland, in the Griffith Collection, Science and Arts Museum, Dublin (this' is doubtless the material referred to by M'Coy, 1844, p. 173). The specimen figured (Plate 10, fig. 2) is from Hook Head, Ireland, and is in the Sedgwick Museum, Cambridge, England; Clitheroe, England, British Museum Collection E 10,691, this is a small slab with spines and interambulacral plates; Frome, England, a small slab with spines and interambulacral plates, British Museum Collection 38,550; Ireland, from one or more of several localities given by Baily for his A. stellifera, which is considered a synonym of glabrispina, as indeed Baily suggested as possible; Miatschkowa, Province of Moscow, Russia (Trautschold, his A. laevis). * Archaeocldaris nerei (Munster). Plate 10, figs. 5a-5d, 6, 7. Cidarites nerei Munster, 1839, p. 40, Plate 3, figs. 6a-6d; Koninck, 1842-'44, [description of] Plate E. Cidaritcs protci Munster, 1839, p. 40. Echinocrinns nerei L. Agassiz, 1841, p. l(j; d'Orbigny, 1850, p. 154. Echinocrinus proteih. Agassiz, 1841, p. 16; d'Orbigny, 1850, p. 154. Cidaris nerei Koninck, 1842-'44, p. 34, Plate E, figs, la-li. Pcdaeocidaris nerei L. Agassiz and Desor, 1846-'47, p. 340. Palaeocidari^ protei L. Agassiz and Desor, 1846-'47, p. 340. Archacocidaris nerei Miiller, 1857, p. 262, Plate 4, figs. 11, 12a, 12b; Desor, 1858, p. 154, Plate 1, fig. 6; Dujardin and Hupe, 1862, p. 465; Loven, 1874, p. 43; Barrois, 1882, p. 321; Julien, 1896, p. 126, Plate 6, figs. 6, 7; Lambert and Thicry, 1910, p. 124. Archaeocidaris nerii Klem, 1904, p. 52. Archaeocidaris protei Lambert and Thierj', 1910, p. 124. Known fragmentarily, interambulacral plates hexagonal, or pentagonal with rounded adradial border, primary tubercle pronounced, basal terrace marked; around the terrace on the outer portion of the scrobicular area, are numerous fine radial plications; minute secondary tubercles are outside of the scrobicular area. Primary spines cylindrical, vertically finely striate, and as Desor figures them, terminally with well developed spinules directed distally. Miiller figures an excellent pyramid (my Plate 10, figs. 6, 7) which is wide-angled, indicating an inclined lantern, with moderately deep foramen magnum, and laterally with ridges for the ARCHAEOCIDARIS. 263 attachment of interpyramidal muscles (p. 363). Miiller's figures of interambulacral plates are rather doubtfully referable to this species, but, as I show in A. rossica, plates differ enough in different parts of the test so that we must allow considerable latitude. Julien's figures are not recognizable, but his description appears to make his specimen referable to this species. Lower Carboniferous, Regnitzlosau ; Tournay, Belgium; Miinster's cotypes of both nerei and protei are in the Munich Museum; Tournay, Belgium, British Museum Collection 32,847; Museum of Comparative Zoology Collection; La Varville and Sigaret (Julien) ; Assise de Lena; Villaneueva, Spain (Barrois). The references given to American localities by Desor and other European authorities are probably incorrect. *Archaeocidari3 rossica (Buch). Text-figs. 208, p. 1S4; 239 bi^, p. 264; Plate 10, fig. 10; Plate 11, figs. 1-5; Plate 12, figs. l-13k. (?) Cidaris dcuailionis Eiehwald, 1S41, p. SS. [Description is unrecognizable so the name cannot hold.] Cidaris rossicus Buch, 1842, p. 323. Cidarites rossicus Murchison, Verneuil and Keyserling, 1845, p. 17, Phite 1, figs. 2a-2e. Palaeocidaris rossica L. Agassiz and Desor, 1846-'47, p. 367. Echinocrinus rossica d'Orbigny, 1850, p. 154. Palaeocidaris {Echinocrinus) rossica Vogt, 1854, p. 314. Eocidaris rossica Desor, 1858, p. 156, Plate 21, figs. 3-6. Echinocrinus dcucalionis Eiehwald, 1860, p. 652. Eocidaris rossicus Geinitz, 1866, p. 61. Archacocidaris rossicus Trautschold, 1868, Plate 9, figs. 1-lOb; 1879, p. 6, Plate 2, figs, la-lf, Ih, li, Ik, 11; Quenstedt, 1875, p. 373, Plate 75, fig. 12; Klem, 1904, p. 55. Archaeocidaris rossica Loven, 1874, p. 43; Tornquist, 1896, text-fig. p. 27, Plate 4, figs. 1-5, 7, 8. Archacocidaris rossica var. schcllwieni Tornquist, 1897, p. 781, Plate 22, fig. 12. Cidarolropus rossica Lambert and Thiery, 1910, p. 125. This species is known from the most nearly complete material of any species of the genus ; coming from the soft yellow calcareous clay beds of Moscow, the material is not distorted and is easily cleaned. Test compressed, spheroidal, about the form of a living cidarid. Ambulacral areas are narrow, sinuous, conforming to the outline of the adambulacral plates (Plate 10, fig. 10). Ambulacral plates are low, imbricating moderately ventrally and strongly beveled under the adambulacrals (Plate 12, fig. 9). Pore-pairs are uniserial. There are four columns of plates in each interambulacral area. The plates at the mid-zone are higher than wide ; ventrally they are not relatively so high, but dorsally are much higher than at the mid-zone; near the apical disc the young plates are very small, and smooth. The primary tubercles are strongly marked, scrobicular area large, basal terrace well developed at the mid-zone (Plate 11, fig. 4), and ventrally, but dorsally in the younger plates no terrace is developed. Passing dorsally to the still younger plates, it is seen (Plate 11, fig. 2) that the scrobicule is imperfectly developed, 264 ROBERT TRACY JACKSON ON ECHINI. then the tubercle is imperforate, and in the youngest plates, no primary tubercle exists. Sec- ondary and very small miliary tubercles are thickly distributed along a narrow border outside the scrobicular circle, and dorsally this border is much larger on the aboral aspect of each plate. Primary spines are stout, inflated in the lower third, tapering from there distally; the milled ring is strongly marked, above which the spine is vertically finely striate for a short distance, and above this part densely studded with short blunt knob-like spinules, set without any definite order. Secondary spines 3 to 4 mm. long are situated on secondary tubercles on the borders of the plates; thej^ are short, flattened, spathulate, vertically finely striate. Between the secondary spines are found some miliaries (Plate 11, fig. 4) which are minute and connected with miliary tubercles. These are the only miliary spines yet seen in the Palaeozoic. The peristome is plated with ambulacral, and many rounded strongly imbricate non-ambulacral plates. The lantern is typical of the Palaeozoic as described below. Lower Carboniferous, Miatschkowa, Province of Moscow, Museum of Comparative Zoology; British Museum; Berlin Museum flir Naturkunde, two specimens including the one described by Tornquist (1896); Strassburg Mu- seum; Munich Museum; Freiburg i. B. Museum. The var. schellwieni of Tornquist, Fusilinenkalk of the Carnic Alps; Miatschkowa and many other localities in Russia (Eichwald). A very choice and complete test of this species is in the Munich Museum (text-fig. 239 bis; Plate 10, fig. 10; Plate 11, figs. 1, 2). This is the best pre- served specimen known in the genus. On the peristome there are a few am- bulacral and non-ambulacral plates all small and imbricating adorally. In the basicoronal row of the corona, in the interambulacra of areas A, I, G, the plates of columns 1 and 4 are whole and those of columns 3 and 2 are half plates, in which the ventral half has been re- sorbed. On the other hand, in areas C and E, in the basicoronal row, the plates of columns 1 and 3 are half plates, of which the ventral half has been resorbed and the plates of colurans 4 and 2 are whole. This, it is believed, as in A. ivortheni (Plate 9, figs. 6-8) is associated with the introduction of columns in development, If, as in the figures cited, column 4 originates 239 b/s Text-fig. 239 bis. — Archacocidaris rossica (Buch) . X 4. Ven^ tral border of corona and peristome, same specimen as Plate 1 1 , fig. 1 ARCHAEOCIDARIS. 265 to the right of the center, then in the adjustment of parts we find the arrangement of areas as in A, I and G in text-fig. 239 bis. Or if column 4 originates to the left of the center, we then have the arrangement as in areas C and E of text-fig. 239 bis. There are no gill slits (p. 223). Passing dorsally up to the mid-zone, the hexagonal plates progressively increase in height relatively to their width and above the mid-zone this is still more marked. In the drawings of the Munich specimen the basal terrace is not brought out. My attention was not called to this feature when the drawings were made, but other specimens show it clearly on all plates up to the mid-zone, as in Plate 11, fig. 4. Above the mid-zone, however, in younger, though still large plates, the basal terrace is wanting. This is of much interest, as if such plates occurred alone as fossils, one would assume that the species had no basal terrace, and this probablj^ accounts for the absence of a terrace as far as known in some species. Also, it is a condition like that of Eocidaris which as a genus (most imperfectly known) has no basal terrace. Desor ■referred A. rossica to Eocidaris, and he was probably misled by seeing only dorsal immature plates in which the terrace had not yet developed. Still further dorsally, as we pass into the area of really young plates, the scrobicule is first small and then wanting, the tubercle is imper- fectly developed, then imperforate, and finally, in the younger plates, quite wanting, the plates close to the apical area being smooth without tubercles of any kind. This progressive develop- ment of characters of interambulacral plates as they grow older and are pushed ventrally, is exactly the same as may be traced in the development of fossil or Recent cidarids (Plate 3, figs. 1, 2) in some species of which it is especially marked (p. 106). In the A. rossica it is dis- appointing in such a perfect specimen not to have the oculars and genitals defined. There are many small plates in the center which are evidently periproctal; two of them lying above area D, and larger than the rest, may possibly be oculars or genitals (Plate 11, fig. 3). Trautschold (1868) figured nearly complete lanterns in this species, but a specimen in the Munich Museum and some fine lanterns that Dr. E. O. Hovey kindly procured for me in Russia, now in the Museum of Comparative Zoology Collections, gave the opportunity to study details and to show somewhat better figures than Trautschold' s. These are seen in text-fig. 208 (p. 184), and Plate 12, figs. 1-8. The lantern is depressed, subtending an angle of about 90 degrees. The pyramids are wide-angled, with curving sides which admitted of relatively long inter- pyramidal muscles. The foramen magnum is moderately deep. The teeth in place extend just above the base of the foramen magnum. The epiphyses are narrow, capping the half- pyramids, and present a glenoid cavity for interlocking with the condyles of the brace. The brace is block-shaped, as in Recent regular Echini. In one choice specimen (Plate 12, figs. 1, 2) a compass with bifid outer end is in place resting on the brace. This is the only good compass known in the Palaeozoic, though I also show one in Pholidechinus brauni and Meeke- chinus elegans. The sides of the pyramid are corrugated for the attachment of interpyramidal muscles. This lantern, as earlier discussed (pp. 81, 82), is very much like the lantern of young cidarids, also young Strongylocentrotus. 266 ROBERT TRACY JACKSON ON ECHINI. *Archaeocidaris agassizi Hall. Plate 13, figs, la-lc, 2^. Archacocidaris agassizi Hall, ISoS, p. 098, Plate 26, figs. la-Id; Loven, lS7i, p. 43; Quenstedt, 1875, p. 373, Plate 75, fig. 11; Keyes, 1894, p. 127, Plate 15, fig. 5; 1895, p. 185, Plate 18, fig. 5; Beede, 1900, p. 48, Plate 8, figs. 6-6e; Klem, 1904, p. 46; Lambert and Tliiery, 1910, p. 124. Archaeocidaris agassizii Jackson, 1896, p. 213. A complete test is unknown, but specimens in Mr. F. Springer's collection add a good deal of information to that previously known. The ambulacra are narrow, plates low, bev- eled strongly under the adradials, pore-pairs uniserial. Interambulacrum with four columns of plates in an area; these plates are hexagonal, the height about equal to the width, basal terrace developed; many secondary tubercles are in a narrow row about the margin of plates. Primary spines are elongate, compressed, contracted above the milled ring, swollen in the lower < third, and from there tapering to the" tip, smooth proximally, above which muricate with small spinulose tubercles arranged quite definitely. In Plate 13, fig. 4, are shown some ovally truncate plates which appear to be genitals. As they are not in place, nothing positive can be affirmed, but, while they do not resemble other known genitals in Echini, it is difficult to see what else they could be. The lantern is of the usual character, pyramids wide-angled, with moderately deep foramen, narrow epiphyses, and typical braces. Burlington Limestone, Lower Carboniferous, Burlington, Iowa, American Museum of Natural History; Museum of Comparative Zoology Collection 3,034-3,038 and 3,191; F. Springer Collection; University of Michigan; Munich Museum; Hannibal, Missouri (Keyes); Upper Coal Measures, Topeka, Kansas (Beede). Archaeocidaris illinoisensis Worthen and Miller. Plate 13, fig. 5. Archaeocidaris illinoisensis Worthon and IMiller, 1883, p. 338, Plate 31, figs, la, II); Keyes, 1895, p. 187; Klem, 1904, p. 49; Lambert and Thiery, 1910, p. 124. Known fragmentarily. Primary spines stout, circular, swollen about the middle, studded with short sharp spinules that are directed outward and slightly distally. Rather close to A. agassizi, but the spines are not flattened and spinules are more pronounced as described. Beach beds of St. Louis Limestone, Lower Carboniferous, near the Illinois Furnace, Hardin County, Illinois, holotype, no. 2,475, of the Illinois State Collection. ARCHAEOCIDARIS. 267 *Archaeocidaris coloradensis nom. nov. Plate 13, fig. 6. Archaeocidarls ornahis Newberry, 1861, p. 116, Plate 1, figs. 2,3, 3a; Keyes, 1895, p. 191; Klem, 1904, p. 54 (non Echinocrinus [Archaeocidaris] ornahts Eichwald, 1860, p. 654, Plate 32, fig. 24; noii Archaeoci- (hiris ornatus White, 1877, p. 104, Plate 6, fig. 7a, Wliite's figure is here referred to A. mucronala, p. 271). Archaeocidaris ornaia Loven, 1874, p. 44; Lambert and Thiery, 1910, p. 124. Known only from fragmentary primary spines. These are 2.5 inches long by 0.36 of an inch in diameter, according to Newberry, but it may be remarked that his figured specimens are much shorter and very fragmentary. The spines are thickly studded with short spinules irregularly arranged, with points directed slightly distally. As Newberry's name ornatus is preoccupied by Eichwald's ornatus (see p. 274), I give a new name to the species. Carboniferous (Coal Measures), Crinoidal Limestone, near junction of the two Colorados, and in same horizon, 60 miles west of that point, near Great Canon of Colorado. Cotype (Plate 13, fig. 6), Columbia University Collection, no. 6,900 G. *Archaeocidaris keokuk Hall. Plate 13, figs. 7a, 7b. Archaeocidaris keokuk Hall, 1858, p. 699, Plate 26, figs. 2a, 2b; Keyes, 1894, p. 128"; Klem, 1904, p. 50. Archaeocidaris worthcnl Zittel, 1879, p. 485, text-fig. 343d (not 343a-343c, which are true worihcni). Archaeocidaris keokuk {Iletcrocidaris) Lambert and Thiery, 1910, p. 124 (their page and plate references are to true .4. keokuk, but this is not Hderocidaris keokuk, see Nomina Nuda. Fragmentarily known. Interambulacral plates low, wide hexagons, basal terrace marked, scrobicule wide, secondary tubercles on narrow rim around the margin. Spines circular in section, of moderate length, muricate, with elongate ridges and spiniform tubercles. Keokuk Limestone, Lower Carboniferous, Warsaw, Illinois, American Museum of Natural History; Clark County, Missouri; Boonville, Missouri (Klem). *Archaeocidaris gracilis Newberry. Plate 13, fig. 8. Archaeocidaris gracilis Newberry, 1861, p. 117, Plate 1, figs. 4, 4a; Klem, 1904, p. 49; Lambert and Thiery, 1910, p. 124. Fragmentarily known. Newberry says that the form of the interambulacral plates is unknown, but a fragment shows a single row of secondary tubercles around the scrobicule. Primary spines straight or curved, slender, tapering, circular in section, surface set with small spinules pointing distally. 268 ROBERT TRACY JACKSON ON ECHINI. For locality Newberry says, "Same as the last," which reads, Crinoidal Limestone [Coal Measures] near junction of the two Colorados, and in same rock, 60 miles west of that point, near the Great Caiion of the Colorado. The specimen figured (Plate 13, fig. 8) , which is undoubt- edly the original of Newberry's Plate 1, fig. 4, bears the locality label "Cafion Diamond River." It is in Columbia University Collection from the Newberry Collection. *Archaeocidaris aculeata -Shumard and Swallow. Plate 13, figs. 9a-9f. Archaeocidaris verneuiliana Swallow and Hawn, 1S5S, p. ISO (non King, 1850, description of Plate 6, figs. 22-24, for which see this memoir, p. 245). Archaeocidaris aculcatus Shumard and Swallow, 1858, p. 22.3; Klem, 1904, p. 45; Lambert and Thiery, 1910, p. 124. Archaeocidaris aculeata Miller, 1889, p. 225; Keyes, 1894, p. 130, Plate 15, fig. 3; 1895, p. 188, Plate 18, fig. 3. Known only from dissociated spines and plates. Interambulacral plates are rather high hexagons, basal terrace marked, secondary tubercles in a narrow border on the margin of the plates. Primary spines are stout, inflated in lower third, circular in section or multiangular, with numerous fine short spinules directed distally. Primary spine 1 to 2 inches in length. The only published figure of the species is that by Keyes, but specimens kindly sent me bj' Dr. Beede are apparentlj^ this species and are so figured. Upper Coal Measures, Valley of Verdigris River, on Santa Fe Road, near Rock Creek, and 25 miles west of Council Grove in valley of Cottonwood Creek; New Point, Jackson County, Missouri; Fort Belknap, Texas (Klem); Topeka, Kansas, Indiana University Collec- tion. Miller (1889) and Miss Klem (1904) ascribe this species to the Permian in part. *Archaeocidaris shumardana Hall. Plate 13, figs. lOa-lOc. Archaeocidaris shumardana Hall, 1858, p. 699, Plate 26, figs. 3a-3d; Keyes, 1894, p. 128. Archaeocidaris cf. shumardana Walcott, 1884, p. 313. Archarocidaris shumardiana Keyes, 1895, p. 186. Archaeocidaris shumardina Keyes, 1895, p. 186. Archaeocidaris shumardanus Klem, 1904, p. 56. Archaeocidaris shiimardi Lambert and Thiery, 1910, p. 124. Known only from plates and spines. Interambulacral plates are hexagonal, wider than high, basal terrace pronounced, secondary tubercles in a narrow row around the margins of the plates. Primary spines small, short, swollen in the lower third, smooth or finely striate proximally, above which muricate with elongate ascending spinules. ARCHAEOCIDARIS. 269 Warsaw Limestone, Lower Carboniferous, Warsaw, Illinois, American Museum of Natural History; Keokuk Limestone, LaGrange, Lewis County, Missouri. An Archaeocidaris sp. that approaches shumardana, Lower Carboniferous, Eureka District, Nevada (Walcott). *Archaeocidaris edgarensis Worthen and Miller. Plate rS, figs, lla-llc, 12-14. Archaeocidaris edgarensis Worthen and Miller, 1883, p. .337, Plate 30, figs. 1.5a-1.5c; Keyes, 1891, p. 245; Klein, 1904, p. 4S; Lambert and Tliiery, 1910, p. 124. Known only from dissociated plates, spines, and jaws. Interambulacral plates, as figured by Worthen and Miller, hexagonal, the height and width about equal. They do not show the basal terrace or scrobicular area clearly; this is probably due to the wear of specimens. Pri- mary spines long, tapering, slightly constricted above the milled ring, smooth proximally, above which thickly set with short spinules pointing distally. Portions of jaws associated with spines of this species are from Manhattan, Kansas. As seen in Plate 13, figs. 12-14, the pyramid is wide-angled, foramen magnum moderately deep, ridges exist laterally for the attachment of interpyramidal muscles. In an inner view (Plate 13, fig. 14) the dental slide is seen, but it does not reach the base of the foramen magnum, a general Palaeozoic character. Upper Coal Measures, Carboniferous, near Baldwinsville, Edgar County, Illinois. Type from this locality given as no. 2,447, in Illinois State Collection; Lower Coal Measures, Des Moines, Iowa; Junietta Group, Manhattan, Kansas, J. W. Beede Collection. *Archaeocidaris newberryi Hambach. Plate 8, fig. 9; Plate 13, figs. 15a, 1.5b. Archaeocidaris newberryi Hambach, 1884, p. 551, Plate D, fig. 1; Keyes, 1894, p. 129; 1895, p. 187; Klem, 1904, p. .53; Lambert and Thiery, 1910, p. 125. Incompletely known. lAmbulacral plates wide, low, with lateral bevel which would pass under the adradials as known in other species. Interambulacral plates hexagonal, wider than high, basal terrace marked, secondary tubercles with some small secondary spines in a narrow row along margin of plates. Primary spines very long up to or exceeding 57 mm. in length, slender, tapering throughout, with numerous fine sharp spinules pointing distally, and falling in definite parallel series. Lower St. Louis Limestone, Lower Carboniferous, St. Louis, Missouri. The holotype from the Hambach Collection is now in Mr. F. Springer's collection, no. 8,119. *Archaeocidaris trudifer White. Plate 13, figs. 16a, 1Gb. Archaeocidaris trudifer White, 1874, p. 17; 1876, p. 89; 1877, p. 104, Plate 6, figs. 8a, 8b; Keyes, 1895, 270 ROBERT TRACY JACKSON ON ECHINI. p. 191; Beede, 1900, p. 47, Plate 8, fig. 10; (?) Girty, 1903, p. 330; Klem, 1904, p. 59; Lambert and Thiery, 1910, p. 125. Archaeocidaris trudifera Miller, 1889, p. 225. Known only fragmentarily. Interambulacral plates hexagonal, height and width about equal. Details of basal terrace, scrobicule, and secondary tubercles are not shown in the origi- nal figures. Primary spines long, fusiform, up to about 120 mm. long. Surface for a short distance above the milled ring apparently smooth, beyond which it is ornamented with numerous minute spinules arranged around the spine in imperfectly spiral lines. Red Wall, Carboniferous (Coal Measures), Camp Apache, Navajo County, Arizona, cotypes in United States National Museum 8,471. Confluence of the Grand and Green Rivers, Utah; Topeka, Kansas; Leadville district, Colorado. Upper Coal Measures, Topeka Limestone, Topeka, Kansas (Beede) . *Archaeocidaris norwoodi Hall. Plate 14, figs. la-Id. Archaeocidaris norwoodi Hall, 1858, p. 701, Plate 26, figs. 5a-5e; Keyes, 1894, p. 129; 1895, p. 188; Klem, 1904, p. 54. Cidarotropus norwoodi Lambert and Thiery, 1910, p. 125. Known only from dissociated plates and spines. Interambulacral plates are hexagonal, or pentagonal in adradial columns, wider than high, basal terrace marked. Scrobicular area sometimes shows obscure radiating plications, secondary tubercles in a narrow row on the margin of the plates. Primary spines are slender, elongate, slightly curving, longitudinally striate below and muricate above, with fine sharply elevated denticles, and at intervals marked by larger thorn-like diverging spinules; intermediate surface cancellate or granulose. Kaskaskia Group, Lower Carboniferous, Chester, Illinois, American Museum of Natural History; Kaskaskia, Illinois; near St. Louis, Missouri (Klem). Archaeocidaris paradoxa (Eichwald). Plate 14, fig. 2. Palaeoechinvs paradoxus Eichwald, 1856, p. 127; 1860, p. 650, Plate 32, fig. 25 (but not fig. 26). Palaechinus paradoxus Stuckenberg, 1898, pp. 230, 343; Klem, 1904, p. 35; (?) Lambert and Thiery, 1910, p. 119. The spine, which is the only recognizable part of Eichwald's figures or description, evidently belongs to an Archaeocidaris. The shaft only is known, which is stout, tapering gradually, with frequent irregularly placed, stout, thorn-like spinules directed distally. What Eichwald figured as a test (his Plate 32, fig. 26) is an unrecognizable mass, but is entirely the wrong shape for an Archaeocidaris. It evidently has no relation to the spine. Upper Carboniferous (Coal Measures), Sterlitamak and Saraninsk, Ural, Russia. ARCHAEOCIDARIS. 271 *Archaeocidaris mucronata Meek and Worthen. Plate 14, figs. 3a, 3b, 4. Archaeocidaris miwronatiis Meek and Worthen, ISfiO, p. 395; 18G6, p. 295, Plate 23, figs. 3a-3e; Klem, 1904, p. 52. Archarocidaris ortiafiis White, 1877, p. 104, Plate (>, fig. 7a (non Echinocrinus [Archaeocklaris] orrudus Eichwald, 1800, for which see p. 274; non Archwocidarls ornatus Newberry, 1801; non Archaro- cidaris ornafa Lambert and Thiery, 1910, p. 124, for which see p. 267). Archai'dcidnria niiivrnnnta Keyes, 1895, p. 188; Lambert and Thit'ry, 1910, p. 124. Known only from dissociated plates and spines. Interambulacral plates hexagonal, wider than high, width 0.5 of an inch, height 0.35 of an inch. Primary spines long, tapering, slightly compressed, or nearly circular in section, and apparentlj^ a little curved near the base, smooth proximally, beyond which they are armed with strong, sharp lateral spinules directed obliquely distally. Length of a primary spine 2.6 inches, greatest diameter of shaft 0.16 of an inch. Chester Group, Lower Carboniferous, Liberty, Randolph County, and Chester, Illinois. The spine figured by Dr. White (1877) as ornatus is apparently referable to this species and is from the Carboniferous (Coal Measures) of Ojo del Oso, near Fort Wingate, New Mexico; this speci- men is in the United States National Museum Collection 8,472. *Archaeocidaris dininnii White. Plate 14, figs. 5a-5b. Archaeocidaris dininnii White, 1880, p. 260, Plate 1, figs. 13-15; 1880a, p. 131, Plate 35, figs. 6a-6c; Keyes, 1894, p. 130, Plate 15, figs. 6a-6c; 1895, p. 190, Plate 18, figs. 6a-6c. Archaeocidaris dininni Klem, 1904, p. 48; Lambert and Thiery, 1910, p. 125. Known only from primary spines which are fusiform, 50 to 60 mm. long, with a greatest diameter of about 5 mm. Above the short plain neck the whole spine is studded with many irregularly disposed spinules, 1 to 2 mm. long, which stand out nearly at right angles to the shaft, except near its apex, where they are directed distally. The spinules are usually more numerous and stronger upon the lower portion of the spines, and upon the middle portion of some of them the spinules are obsolete. The smaller spines are usually more slender or less fusiform than the larger. A striking feature of this species is that the spinules are directed at right angles to the shaft. Uf)per Coal Measures, near Tecumseh, Nebraska, cotype in United States National Museum Collection 8,031. Upper Coal Measures, Kansas City, Missouri; Red Oak, Iowa (Klem). 272 ROBERT TRACY JACKSON ON ECHINI. *Archaeocidaris cratis White. Plate 14, fig. 6. Archacoddaris crafis White, 1876, p. 109; 1880, p. 130, Plate 33, fig. 2a; Keyes, 1895, p. 188; Girty, 1903, p. 331; 1908, p.-llO; Klem, 1904, p. 47; Lambert and Thiery, 1910, p. 125. Archaeocidaris sp. a, Girty, 1908, p. 110. Known only from primary spines, which are slender, tapering, terete, shaft with sharp, distinct, distant spinules, each about 1.5 mm. long and pointing distally. Length of spine about 60 mm., diameter just above milled ring about 4 mm. Differs from mucronata in that the spine is more slender, terete, and spinules more wddely and irregularlj^ distributed. Girty's (1908) species "a" may well be considered an immature spine of an adult, or the spine of a young individual of cratis. Carboniferous (Coal Measures) at confluence of the Grand and Green Rivers, Utah, holo- type from this locality in United States National Museum Collection 8,235 ; Nebraska. Crested Butte district; Leadville district and Ouray, Colorado (Girty). Guadaloupe Point, Guada- loupe Mountains, Texas (Girty). Archaeocidaris acanthifera Trautschold. Plate 14, figs. 7, 8a, 8b. Archaeocidaris acanthifera Trautschold, 1879, p. 7, Plate 2, fig. lu [no number on figure on the Plate]. Archaeocidaris sp. Hind, 1905, p. 529, Plate 25, figs. 1, la. Known only from primary spines, which are slender, tapering, with long irregularly placed spinules pointing moderately distally. The specimens figured by Hind appear referable to this species, although the spinules are not so long as in the type. These specimens are from the Coal Measures of Staffordshire, England, and are apparently the only Palaeozoic Echini yet recorded in Europe from the Coal Measures. Lower Carboniferous, Miatschkowa, Province of Moscow, Russia (Trautschold). Coal Measures, Nettlebank, Staffordshire, England. Archaeocidaris pizzulana Gortani. Plate 10, figs. 12a-12d. Archaeocidaris pizzulana Gortani, 1905, p. 586, Plate 15, figs. 29-33, 36f. Cidarotropus pizzulana Lambert and Thiery, 1910, p. 125. Known from dissociated interambulacral plates and spines. Interambulacral plates are hexagonal, wider than high, with a double ring of secondary tubercles on the margin. Primary spines smooth, cylindrical, with from three to six pairs of opposite spinules. The spinules are stout, thorn-like, and directed distallJ^ This species is the only Palaeozoic echinoid so far known from Italy. Frequent in the calcareous schists. Carboniferous, Forca Pizzul, Carnic Alps. ARCHAEOCIDARIS. 273 *Archaeocidaris megastyla Shumard and Swallow. Plate 14, fig. 13. Archacocidans mcgastylus Shumard and Swallow, 18.58, p. 225; Keyes, 1894, p. 129, Plate 15, figs. 2a, 2b; 1895, p. 189, Plate 18, figs. 2a, 2b; Beede, 1900, p. 49, Plate 8, fig. 7; Klein, 1904, p. 51. Archacocidaris mcgastylis Loven, 1874, p. 44. Cidarotropus megastylus Lambert and Thiery, 1910, p. 125. Known incompletely from plates and spines. Interambulacral plates are large, hexagonal, wider than high, rather thick; scrobicular area very broad, nearly circular; primary tubercle very large, and relatively large secondaries are in a narrow row on margin of plates. Primary spines long, robust, circular in section, length about 3 inches. In the original description Shumard and Swallow say the spine is studded with rather distant granules, or minute short spines. Keyes's interpretation of the species is spines with rather large spinules of which he gives a good figure. As Shumard and Swallow published no figure and their specimens may have been worn, I accept Keyes's version and consider the spinules as relatively large. This char- acter is shown well in a specimen in the United States National Museum which I figure and which in other respects appears referable to this species. Upper Coal Measures, near headwaters of Verdigris River, and in the Valley of Cotton- wood Creek, Kansas; Independence, Jackson County, Missouri; St. Louis Group, St. Louis, Missouri, United States National Museum Collection 43,009, from E. 0. Ulrich Collection (Plate 14, fig. 13); Kansas City (Beede). *Archaeocidaris biangulata Shumard and Swallow. Plate 14, figs. 9a-9d. Archacocidaris biangulafiis Shumard and Swallow, 1858, p. 224; Klem, 1904, p. 47. Archaeocidaris biangulata Loven, 1874, p. 44; Keyes, 1894, p. 130, Plate 1.5, figs, la-le; 1895, p. 189, Plate 18, figs, la-le. Cidarotropus biangulata Lambert and Thiery, 1910, p. 125. Known from dissociated plates and spines. Interambulacral plates hexagonal, wider than high, basal terrace marked, secondary tubercles in a narrow row on the margin of the plates. Shumard and Swallow say areolar surface with radial striae; but such are not visible in speci- mens I have seen. Primary spines are very striking in character. They are smooth for a slight distance above the milled ring; the shaft is terete throughout, but is ornamented with two strongly marked, opposite, wing-like expansions which are in part at least nearly entire in outline, or serrately spinose. Distally the flanges pass into the general spinulose character of the shaft. Aside from the flanges, the shaft is nearly smooth at least proximally, but distally the whole of the shaft is thickly set with relatively long, pointed spinules closely resembling 274 ROBERT TRACY JACKSON ON ECHINI. those of the whole spine in mucronata. Shumard and Swallow say the length of primarj' spines is about 2.5 inches, diameter including spinules 0.1 of an inch. Middle Coal Measures, Lexington, Missouri; Kansas City, Missouri. Huerco Formation, Diablo Mountains, western Texas, United States National Museum Collection. Archaeocidaris ornata (Eichwald). Plate 14, fig. 10. Echinocrlnus ornatus Eichwald, 1S60, p. 654, Plate 32, fig. 24 (non Archaeocidaris ornafus Newberry, 1861, for which see -1. coloradmsi.^, p. 267; non White, 1877, for which see .4. mucronaia, p. 271; non Klem, 1904, p. .54; non Lambert and Thiery, 1910, p. 124, for which see p. 267). Known only from primary spines. Milled ring well developed, above which the shaft tapers to the distal end ; the spine is smooth proximally, but distally thickly set with long spinules directed distally at an acute angle. Lower Carboniferous, Miatschkowa, near Moscow, Russia. *Archaeocidaris ourayensis Girty. Phite 14, fig. 11. Archaeocidaris ourayensis Girty, 190.3, p. 329, Plate 1, fig. 14; Klem, 1904, p. 63; Lambert and Thiery, 1910, p. 12,5. Known only from a primary spine, which has a slender axis slightly over 1 mm. in diameter, densely clothed with relatively large and stout spinules du'ected distally. Base and tip of spine unknown. Hermosa Formation, Carboniferous (Coal Measures), Ouray, Colorado, holotype, United States National Museum Collection 35,369. * Archaeocidaris triplex White. Plate 14, figs. 12a-12c. Archaeocidaris triplex White, 1881, p. xxii, Plate 4, figs. 3a-3c; (?) Girty, 1903, p. 330; Klem, 1904, p. 57; Lambert and Thiery, 1910, p. 125. Practically known only from interambulacral plates and primary spines. Interambulacral plates described by Dr. White as imperfect, small, thin, only probably belonging to the same species as the spines. Primary spines large, strong, subtriangular in cross section; spinules are strong, short, but prominent and rather distant, in three rows, each upon one of the three obtuse angles of the spine. The rows of spinules extend from a little way above the milled ring to the end of the spine ; the rest of the surface is smooth. Carboniferous (Coal Measures), near Taos, New Mexico, cotypes, United States National Museum 9,449; (?) San Juan region. Needle Mountains, Colorado. ARCHAEOCIDARIS. 275 *Archaeocidaris triserialis (M'Coy). Plate 8, fig. 10; Plate 14, fig. 14 Echinocrinus triserialis M'Coy, 1844, p. 173, Plate 26, fig. 1 (on the legend of Plate 26, fig. 1, the printed generic name has been erased and Echinocrinus written in pencil; see footnotes, pp. 275, 277, 280). Archacocidaris triserialis Desor, 1858, p. 155; M'Coy, 1862 (on legend of Plate 26, fig. 1)^; Klem, 1904, p. 58; Lambert and Tliiery, 1910, p. 125. Known only from primary .spines, which are triangular in section, each of the three angular ridges armed with a row of strong tooth-like spinules; intervening space smooth; the sides between the angles are flat and almost equal. Length of specimen that M'Coy figured is one inch eight hnes, diameter one and a half lines; but it was incomplete at each end. . Arenaceous Limestone, Lower Carboniferous, Killycloghy, Lisbellow, County Tyrone, Ireland. Holotype and only recorded specimen in the Science and Arts Museum, Dublin. Dr. Scharff kindly sent me the photograph of this specimen which is here reproduced. *Archaeocidaris triserrata Meek. Plate 14, figs. 15a-15c. Archaeocidaris (?) triserrata Meek, 1872, p. 151, Plate 1, figs. 6a-6c. Archaeocidaris triserrata Miller, 1889, p. 225. Archaeocidaris triserata Keyes, 1895, p. 190. Archaeocidaris triserratm Klem, 1904, p. 58; Lambert and Thiery, 1910, p. 125. Known only from primary spines, which are moderately long, rather slender and usually a little arched near the base, where they are nearly or quite circular in section. Farther up they are compressed, rhombic in section, the lateral margins being sharp and regularly serrated, the little tooth-like projections inchned outward, and toward the apex of the spine. There are probably from 25 to 30 serrations on each side of the spine and from 9 to 12 of them may be counted in the space of half an inch. On the middle of one face of the spine there is a third serrated carina extending about two thirds the length of the spine ; on the other face three or four rows of smaller granules or elongate nodes extend from the shank of the spine, but become obsolete near the middle. Length about 2.30 inches. No entire specimens were found. Upper Coal Measures, near Omaha, Nebraska, cotypes in United States National Museum Collection 6,599; Platte River, Nebraska; Vermilion County, Illinois; Kansas City, Missouri. 'While Archaeocidaris triserialis is printed on the Plate as stated, Dr. Kitchin write.-! me that M'Coy (1862, p. 274) in the list of errata says "Plate 26, fig. 1, read Echinocrinus instead of Archaeocidaris. (See footnotes pp. 277, 280.) 276 ROBERT TRACY JACKSON ON ECHINI. Archaeocidaris rankini Young. Archaeocidaris rankini Young, 18S2, p. 108. Known only from primary spines which bear four rows of spinules or denticles; these are long, thorn-like, and stand out from the shaft at nearly a right angle. This is the only species in which there are four rows of spinules described. No figure has been published. Lower Carboniferous shales, Gillfoot, Carluke, Scotland. Archaeocidaris prisca (Miinster). Cidarites prisons Miinster, 1839, p. 41. Eckinocrimts priscus L. Agassiz, 1841, p. 16. Palaeocidaris prisca L. Agassiz and Desor, lS4r)-'47, p. 340. Echinocrinus prisca d'Orbigny, 18.50, p. lo4. Archaeocidaris prisca Desor, 1858, p. 154. Archaeocidaris priscus Klem, 1904, p. (i3; Lambert and Thiery, 1910, pp. 124, 125. Known only from individual plates and spines. Tubercles sensibly larger than in A. nerei. Primary spines with six granular ridges. No figure has been published. Carboniferous, Regnitzlosau, Belgium. Archaeocidaris wervekei Tornquist. Plate 14, figs. 17a-17f. Archaeocidaris wervekei Tornquist, 1897, p. 778, Plate 21, fig. 4; Plate 22, figs. 1, 2, 3, 9, 10; (?) Fraipont, 1904, p. 11, Plate 1, figs. 6, 7. Archaeocidaris werwekei Lambert and Thiery, 1910, p. 125. Interambulacral plates are hexagonal, or pentagonal in adradial columns, very high, the height equaling or exceeding the width, basal terrace marked, strong radial plications extending from margins of plates inward. Primary tubercles exceptionally high. Primary spines are incomplete, vertically plicate and finely striate. Nodose eminences are on the ridges between plications. Dr. Tornquist gives a striking and very satisfactory restoration, representing a complete test in side view. In it he shows narrow ambulacral areas, and four columns of plates in the interambulacral areas. The whole test is represented as depressed spheroidal, and I believe is quite correct in form. Fusilina Limestone, Lower Carboniferous, Hunsriicken, Alsace, Germany; Lower Car- boniferous, Marbre Noir de Dinant, Belgium (Fraipont). * Archaeocidaris urii (Fleming). Plate 14, figs. 16, 18, 19a-19c, 20a-20c; Plate 15, figs. 1, 2a-2c, 3. Echinus Ure, 1793, p. 318, Plate IG, figs. 7, 8. Cidaris urii Fleming, 1828, p. 478; M'Coy, 1862, (in legend of) Plate 27, fig. 1 (see footnote p. 277). ARCHAEOCIDARIS. 277 Cidaris vdttsfa Phillips, 1836, p. 208; Portlock, 1843, pp. xxvi, 353, Plate 16, fig. 11. Echinocrinus urii L. Agassiz, 1841, p. 16; M'Coy, 1844, p. 174, Plate 27, fig. 1 (the generic name on legend of plate in pencil).' Cidaris hrnhurhcnsis Portlock, 1843, pp. xxvi, 352, Plate 16, figs. lUa-lOd. Echinocrinu.s irtustus M'Coy, 1844, p. 174. Archacocidaris urii Roemer, 1852-54, p. 288, Plate 4, fig. 2; Desor, 1858, p. 154, Plate 21, figs. 11, 12; Baily, 1875, p. Ixviii, Plate 36, figs. 12a, 12b; Keeping, 1876, p. 39, Plate 3, figs. 14-18; Julien, 1896, p. 123, Plate 16, figs. 8-10; Tomquist, 1897, p. 775, Plate 22, figs. 4-7, 11; Fraipont, 1904, p. 11, Plate 1, fig. 5; Lambert and Thiery, 1910, p. 124. Arrharoridaris vriustu Dujardin and Hupe, 1862, p. 466; Lambert and Thiery, 1910, p. 124. Archacocidaris [urii] Young, 1873; 1876. Archacocidaris urci Loven, 1874, p. 43; Neilson, 1895, p. 77, text-fig. 3; Klem, 1904, p. 59. Archacocidaris benburhensis Etheridge, 1888, p. 221. Archaeocidaris gruncri Julien, 1896, p. 125, Plate 16, figs. 11, 12; Klem, 1904, p. 62; Lambert and Thiery, 1910, p. 124. Archacocidaris rcgimontana Parkinson, 1903, p. 365, Plate 15, fig. 13; Lambert and Thiery, 1910, p. 124. Archacocidaris vctustus Klem, 1904, p. 61. This species is widely distributed and represented by some very good material. The most complete portion of a test known to me is from Knock Hill Quarry, Fife, Scotland, and is in the Museum of Practical Geology, London. In this specimen (Plate 15, fig. 1) the ambu- lacrum is narrow, 5.5 mm. in width. The ambulacral plates are low with pore-pairs uniserial. In one area of this specimen it is seen that there are four columns of interambulacral plates, the same number that is found in all other species of the genus where a complete area is known. The interambulacral plates of median columns are low, wide hexagons, and those of adradial columns are pentagonal with a strongly rounded line on the adradial suture. This specimen shows that there are four plates in the basicoronal row of the interambulacrum and also traces of ambulacral and non-ambulacral plates on the peristome (compare Plate 9, figs. 6, 7). The largest interambulacral plate measured is 12 mm. in width by 6.5 mm. in height. The surface characters of the plates are not shown well in this specimen, though they are in others from the same locality. In another specimen from this locality in the Jermyn St. Museum, an interam- bulacral plate measures 13 mm. in width by 9 mm. in height. It also shows primary spines of the usua,l size and character in the species. ' In a copy of M'Coy's Carboniferous Limestone Fossils of Ireland, of the 1S62 edition, seen at the Jermyn St. Museum, London, the name Cidaris urii is printed on the plate as stated (p. 276). In two copies of the 1844 edition, which was privately issued by Sir Richard Griffith, one at the British Museum, and one at the Musemn of Comparative Zoology, the generic name is erased on the plate and changed in jiencil to read Echinocrinus urii. Dr. Bather (1907, p. 453, footnote) says of M'Coy's publication, "In the legend to the lithographed plates the name Archaeocidaris, which had been printed, was erased, and the name Echinocrinus inserted by hand." According to the evidence Dr. Bather is mistaken, and the name erased was doubtless Cidaris, not Archaeocidaris. (See footnotes, pp. 275, 280.) 278 ROBERT TRACY JACKSON ON ECHINI. The imbrication of plates in this species was described by Professor Young, who, I beUeve was the first to show that this character occurred in Archaeocidaris. A specimen from Fermanagh, North Ireland, in the Jermyn St. Museum, no. 7,659 (Plate 14, figs. 20a-c), shows interambulacral plates with prominent tubercle, basal terrace, secondary tubercles, and strong radial plications which extend inward from the margins of the plate ; also a characteristic primary spine with its strong spinules extending distally from elevated ridges. The longest spine measures 64 mm. in length, is longitudinally finely striate and thickly .set with spinules. This specimen is of importance as it is the original holotype of Portlock's Cidaris benburbensis from whose figures mine are taken. Portlock (1843, p. 353) gives the locality as Benburb, boundary of Armagh and Tyrone. This may be the correct locality of this type ; but Fermanagh is the locality given on the old original label in the Jermyn St. Museum. Dr. Kitchin tells me that the matter of the locality for this specimen is open to question. Port- lock may have been mistaken, or some error may have crept into the label before the day of catalogues. A fine specimen from Millstone Neuk, Dunbar, Scotland, in the Jermyn St. Museum, is of interest from the very perfect condition of preservation of the spines. The longest of these, which is not quite complete distally, measures 61 mm. in length. The spinules are beautifully preserved, measure about 1.5 mm. in length, and are set at an angle of about 45° from the axis of the shaft, pointing distally. A specimen from Corwen, Wales, in the Sedgwick Museum, Cambridge, England, shows the surface characters of interambulacral plates very clearly, as figured by Keeping, from whom my figures (Plate 14, figs. 19 a-c) are taken. Of these plates, several show the basal terrace and radial plications extending inward from the margins. A peculiar elongate plate occurs in this specimen (Plate 14, fig. 19c), the nature of which is doubtful. As it has a perforation near one end, it is possibly a genital plate, but genitals are so far not definitely known in the genus (pp. 265, 266, 414). The species Cidaris vetusta Phillips, as suggested by M'Coy (1844), may be reasonably referred to urii as a synonym. Phillips described the spines as rudely muricate, and Portlock described them as having four or more rows of strong prickles. A specimen from the Portlock Collection, from Fermanagh, Ireland, in the Jermyn St. Museum, no. 16,321, bears the old manuscript label Cidaris vetusta. It is doubtless the specimen referred to by Portlock (1843, p. 353) from this locality, but is not the original of his Plate 16, fig. 11. The spines and plates of this specimen are not distinguishable from urii, though in the plates the radial plications are only faintly preserved. Another specimen from the Portlock Collection, from Clogher, County Tyrone, Ireland, in the Jermyn St. Museum, no. 7,768, consists of two interambulacral plates on a slab with other fossils. This specimen is probably the one referred to by Portlock (1843, p. 353) from this locality, and one of the plates corresponds so closely with that figured by ARCHAEOCIDARIS. 279 Portlock in his Plate 16, fig. 11 (here reproduced as Plate 14, fig. 16) that it is probably the original of this figure. This plate is hexagonal, measures 11 mm. in width by 10 mm. in height; has a prominent primary tubercle, basal terrace, and on the border a row of secondary tubercles as figured by Portlock. This is the only published figure ascribed to the species vetusta. Julien's A. gruneri is in all probability simply based on small and probably young speci- mens. It presents no specific differences. A . regimontana Parkinson is also doubtless a synonym. Lower Carboniferous, Fermanagh, North Ireland, Museum of Practical Geology, no. 7,659 (the holotype of Portlock's Cidaris benburbensis. See note above in regard to the locality of this specimen) ; Benburb, Ireland, Trinity College, Dublin ; Knock Hill Quarry, Fife, Scot- land, Museum of Practical Geology, nos. 16,316 to 16,319; Millstone Neuk, Dunbar, Scotland, Museum of Practical Geology 25,507, 25,508; Fermanagh, Ireland, no. 16,321, and Clogher, County Tyrone, Ireland, no. 7,768, Museum of Practical Geology; Hayfod-y-Calch, Corwen, Wales, Sedgwick Museum, Cambridge; Yorkshire, Museum of Comparative Zoology Collec- tion 3,062; near Glasgow, Museum of Comparative Zoology Collection; Bristol, British Museum Collection 75,724; Bunderan; Ballantillich, Rahan's Bay, Ireland, and Derbyshire, England, Sedgwick Museum, Cambridge, England; Craigenglen, Campsie, Scotland (Young); Calderwood Limestone, in neighborhood of East Kilbride, Scotland (Neilson); Ardisiere, France (Julien) ; Hunsrucken, Alsace, Germany (Tornquist) ; Culm, Konigsberg (Parkinson's A. regimontana); Marbre Noir, Dinant, Belgium (Fraipont); Phillips (1836) gives as locali- ties for his Cidaris vetusta, here considered a synonym, Ravenstonedale ; Whitewell, North- umberland; Coalbrookdale and Florence Court. Archaeocidaris halliana (Geinitz). Plate 15, figs. 4a-4c. Kuriilarix halliaiiii.s Geinitz, IStiG, p. til, Plate 5, figs, la, lb, 2a, 2b; Meek, 1872, p. 152, Plate 7, figs. 9a-9(l; Klem, 1904, p. 68. Eocidaris hallanm Miller, 1889, p. 242. Archaeocidaris ludlianus Keyes, 1894, p. 129; 1895, p. 190. (?) Eocidaris halliana Girty, 1903, p. .332. Cidarotropus hallianm Lambert and Thiery, 1910, p. 125. Known only from isolated interambulacral plates and primary spines. Plates hexagonal, wider than high, with basal terrace, as figured by Geinitz, large secondary tubercles in a single row well in from the margin of the plate, and numerous minute granules on the marginal area. Plates small, up to 3 mm., as given by Geinitz. Primary spine slender, terete, tapering from the milled ring to the tip. Vertically finely striate, the elevated ridges being minutely and regularly set with granules. Spines about 7 mm. long. Upper Coal Measures, Nebraska City, Nebraska; Kansas City, Missouri; (?) Leadville district and Crested Butte district, Colorado. 280 ROBERT TRACY JACKSON ON ECHINI. *Archaeocidaris muensteriana (Koninck). Plate 15, figs. 5a-5c. Cidari^ muensterianus Koninck, lS42-'44, p. 35, Plate E, figs. 2a-2(l. Cidarites munsterianm Koninck, 1842-44, [description of] Plate E. Echinocrimis mumterianm (?) M'Coy, 1844, p. 173, Plate 27, fig. 2 (name on legend of plate in pencil).' Cidaris munsferiana L. Agassiz and Desor, 1846-'47, p. 367. Eocidaris munstcrianus Desor, 1858, p. 156. Cidaris elegans M'Coy, 1862, [on legend of] Plate 27.' Lepidocentrus munsierianus Koninck, 1869, p. .546; 1870, p. 260; (non .lulien, 1874, p. 76, for which see Pholidocidaris gatidry i ) . Archacocidaris yniinsirriana Loven, 1874, p. 43; Young, 1876, p. 2.30. Archacocidaris cicgans Etheridge, 1888, p. 221. Archaeoddaris munsterianus Etheridge, 1888, p. 221. Eocidaris milnsterianus Klem, 1904, p. 69. Eocidaris munsteri Lambert and Thiery, 1910, p. 126. Known only from primary spines and incomplete interambulacral plates. Interambulacral plates of doubtful outline, probably hexagonal, primary tubercle with large scrobicular area; a basal terrace is not figured, but it is quite likely worn off. Primary spines spindle-shaped, with numerous vertical denticulate ribs, arranged regularly in parallel series. Lower Carboniferous, Vise, Belgium; Ireland; Beith district, Ayrshire in Scotland (Young) ; Vise, and Tournai, Belgium, British Museum Collection 32,846 and 56,991. Archaeocidaris forbesiana (Koninck). Plate 15, figs. 6a-6e. Cidaris forbesiana Koninck, 1863, p. 574, Plate 4, figs. 1, 2; 1863a, p. 4, Plate 4, figs. l-2a. Eocidaris forbesiana Waagen, 1879-'87, p. 819, Plate 95, figs. 5-16. Permocidaris forbesiana Lambert, 1899a, pp. 39, 47. Eocidaris forbcsianus Klem, 1904, p. 67. Permocidaris forbesi Lambert and Thiery, 1910, p. 127. This species is known from isolated interambulacral plates and spines. The plates are most imperfectly known, and the figures given by Waagen seem to be of quite impossible shapes as regards outline, also the primary tubercle is so peculiar that it differs from that of any known type. With the meager knowledge and doubtful character of the material, it seems to me best to place it in Archaeocidaris rather than make it the basis of a distinct generic type, as does Lambert. The primary spines on which the species was based by de Koninck, are large, ' In a copy of M'Coy's Carboniferous Fossils of Ireland of the 1862 edition, seen at the Jermyn St. Museum, the name Cidaris elegans is printed on the plate a-s stated. In two copies of the 1844 edition, one at the British Museum, and one at the Museum of Comparative Zoology, the name on the plate is erased and changed in pencil to read Echinocrinus munster- amis (see footnotes pp. 275, 277). ARCHAEOCIDARIS. 281 stout, and strongly swollen in the middle. The milled ring is weakly defined, shaft smooth for about the lower third, above which it is ornamented with numerous elevated nodose spinules arranged in quite definite parallel series, of which, judging from the figures, there must be about 14 vertical series on a spine. Productus Limestone, Permian, Katta, Golawali, Chidru, and Bazarwan, India. Archaeocidaris spinoclavata Worthen and Miller. Plate 15, figs. 7a-7h. Archaeocidaris (?) .ip. uiidet. Meek and Worthen, 1873, Plate 24, figs. 13a-13c. Archaeocidaris spinoclaimtiis Wortlien and INIiller, 1SS3, p. 327, Plate 30, figs. 14a-14e; Keyes, 1895, p. 190; Klem, 1904, p. 57. Archaeocidaris spinoclaeafa Miller, 1889, p. 225; Lambert and Thiery, 1910, p. 124. Known only from dissociated interambulacral plates and primary spines. Interambulacral plates he.xagonal, wider than high, with a nodose rim around the margin, the expression of secondary tubercles. The basal terrace is not shown, and the details of the surface are doubtful in Worthen and Miller's figures, but, as they say, the specimens are highly silicified and external markings thereby obscured. Primary spines long, or short, circular in section, contracted above the milled ring, beyond which very soon, or at a considerable distance, the spine expands to a marked degree. If it expands quickly, the spine appears highly swollen, as in Plate 15, figs. 7g, 7h, or this may be much less marked, as in fig. 7f, or the greatest expansion may be near the tip of the spine, as in fig. 7e. Proximally the spine is smooth for a short distance, above which it is thickly clothed with nodose spinules. This species is more variable in the form of the spines than other species of the genus yet known. Middle and Lower Coal Measures, St. Clair and Marshall Counties, Illinois. Cotypes are stated as in the Illinois State Collection 2,404. Archaeocidaris sp. b. Girty. Archaeocidaris sp. b. Girty, 1908, p. 110, Plate 27, figs. 18, ISa. Archaeocidaris sp. h. var. The same, p. 111. Archaeocidaris sp. c. The same, p. 111. Archaeocidaris sp. d. The same, p. 111. Most fragmentary, based on two pieces of spines which Girty thinks resemble A. spino- clavata, and yet are different, so I insert the record at this place pending further knowledge, should such be attained. The piece representing the distal end expands rapidly into an almost knob-like tip, which, with the shaft, Girty says, appears to be ornamented with short spinules. The shaft is about 0.75 mm., and the expanded distal tip about 2 mm. in diameter. To recog- nize a varietal form of a species which itself is so shadowy that Girty gives it no name, does seem 282 ROBERT TRACY JACKSON ON ECHINI. beyond all reason in splitting. Certainly, without injustice to Girty, his variety of b and his species c and d may reasonably be left under his species b, until better material turns up. Delaware Mountain Formation, Carboniferous (Coal Measures), southern Delaware Moun- tains, Texas; and Dark Limestone, Pine Spring, Guadalupe Mountains, Texas. Archaeocidaris clavata (Eichwald). Plate 15, fig. 8. Echinocrinus clavatus Eichwald, 1860, p. 653, Plate 23, fig. 16. Known only from primary spines, which are club-shaped, above the milled ring expanding to the thickest part, which is near the distal apex, tapering to a blunt end ; with marked, sinu- ous, fluted ridges unlike other known species. Carboniferous, Yegorjefsk, in the Province of Kalonga, Russia. Lepidocidaris Meek and Worthen. Lrpldocidaris Meek and Worthen, 1873, p. 482; Loven, 1874, p. 44; Duncan, 1889a, p. 12; Lanil:)ert and Thie'ry, 1910, p. 126. This genus differs very markedlj' from Archaeocidaris, and instead of having many species, as in that genus, there is at present only one species known. The test is high, spheroidal, or perhaps spherical. Ambulacral areas narrow, with two columns of low plates. Ambulacral plates not all alike, but at the mid-zone every third plate is higher and wider than the two inter- mediate and more or less completely abutting against the two smaller plates of the opposite half -area (Plate 17, figs. 2, 3, 12). Interambulacra are wide, with from six to eight columns of large hexagonal plates, or pentagonal in adradial cokmms. The plates are high, rounded, imbricating moderately aborally, and from the center laterally, and over the ambulacrals. Each interambulacral plate has a large perforate primary tubercle with a deep sunken, rather small scrobicular area but no basal terrace. On the high rounded area beyond the scrobicular circle, the surface bears many small secondary tubercles. Spines are long, cylindrical; primary spines are associated with the primary tubercles and minute secondary spines with the secondary tubercles. Oculars and genitals unknown. A typical Palaeozoic inclined lantern exists. Type and only known species L. squamosa of the Lower Carboniferous. *Lepidocidaris squamosa Meek and Worthen. Plate 16, figs. 1-3; Plate 17, figs. 1-14. Koridaris (?) .iqudiiiosa IMeek and Worthen, 1869, p. 79. Eocidaris (?) squamosus Meek and W'orthen, 1873, p. 478, Plate 9, figs. 15a-15g. Lepidocidaris squamosus Meek and Worthen, 1873, p. 478; Jackson, 1896, p. 220, Plate 7, fig. 41; Klein, 1904, p. 72. Lepidechinus squamosus Keyes, 1895, p. 193. Lepidocidaris squammo?us Lambert and Thiery, 1910, p. 126. LEPIDOCIDARIS 283 This striking species is one of the most distinctive of Palaeozoic Echini. Test high, spher- oidal, perhaps nearly or quite spherical. In the type (Plate 16, fig. 1) at the upper part of the specimen, which I take to be about on the line of the mid-zone, the ambulacrum measures 8 mm. in width; the interambulacrum 52 mm. in width. From these measurements the cir- cumference must have been in the neighborhood of 300 mm. and the diameter about 100 mm., a very large sea-urchin, but no complete specimen is known. In the ambulacrum, every third plate is wider and higher than the intermediate, so that in this character the structure is very different from that of i\rchaeocidaris. In the type specimen at the mid-zone (Plate 17, figs. 1, 3) the ambulacral plates are rather irregular in form; while usually all plates cross the half-area, occasional smaller plates may be excluded from the interambulacrum, or the center of the ambu- lacrum, thus making them occluded or demi-plates. Ventrally in the type, and at the mid- zone in younger specimens (Plate 17, figs. 1, 8, 12), all the plates cross the half -area and the large plate of the triplet abuts distinctly against the two smaller plates of the opposite half-area. The ambulacral plates imbricate moderately adorally, and bevel under the adradials. The interambulacrum is very wide, and in the type has eight columns of plates at the mid-zone, but in two smaller specimens (Plate 17, figs. 9, 11) there are only six columns of plates in an area. The interambulacral plates imbricate moderately dorsally and laterally. The character of the tubercles is as described for the genus. The primary spines are cylindrical, with scarcely any perceptible tapering; the largest seen, yet not quite complete, measures 35 mm. in length. The spines are longitudinally finely striate. Small secondary spines occur associated with the secondary tubercles (Plate 17, fig. 10). The pyramids are wide-angled, foramen magnum moderately deep, lateral wings with ridges for the attachment of interpyramidal muscles, all practically the same as in Archaeocidaris. This species is known only from the Lower Burlington Limestone, Lower Carboniferous, Burlington, Iowa. The holotype is in the Museum of Comparative Zoology Collection 3,026; also a number of other specimens are in the same collection. A fine series of twelve specimens is in Mr. Frank Springer's collection. In the type and in part in other specimens, the introduction of the interambulacral columns is shown. As seen in Plate 17, fig. 9, there are four columns of plates in an interambulacral area ventrally, and apparently four in the basicoronal row, as in Archaeocidaris. Passing dorsally, new columns are introduced with a pentagonal plate, and a heptagonal plate lies ventral to the same. On the basis of introduction of columns and also on the direction of imbrication, I have oriented the type as shown in Plate 16, fig. 1, and Plate 17, fig. 1. This is reversed from the position given by Meek and Worthen. 284 ROBERT TRACY JACKSON ON ECHINI. Family LEPIDOCENTRIDAE Lovcn. Lcpidocciitridac (pars) Loven, 1S74, p. 39; (pars) Zittel, 1879, p. 4S2; (pars) Jackson, 1S96, p. 241. Archarocidaridac (pars) Loven, 1S74, p. 39; (pars) Duncan, 1889a, p. 8; (pars) Jackson, 1896, p. 241. Rhoechinidae (pars) Lambert and Thiery, 1910, p. 121. Test spheroidal or flattened horizontally, in the plane of the ambitus circular, pentagonal, or elongate through the axis III, 5. Two columns of plates in an ambulacral area, all primaries, or, in one species, occasional alternate plates slightly occluded. Pore-pairs uniserial, or in one species slightly biserial. Ambulacral plates, imbricating adorally and beveled strongly under the adradials. Interambulacra with numerous, 5 to 14, columns of plates in an area, moder- ately thin to very thin. Primordial interambulacral plates are in the basicoronal row; the interambulacra usually have a very accelerated development, additional columns coming in early and rapidly. Interambulacral plates imbricate strongly aborally and from the center outward and over ambulacrals. The peristome is plated with many rows of ambulacral plates only. Oculars are small, insert; genitals wide, low, with many pores as far as known. Peri- proct with many small angular plates. Apical disc small. Lantern inclined, composed of forty pieces. There are five genera, including two new ones. KoNiNCKOciDARis Dollo and Buisseret. Koninrkocidans Dollo and Buisseret, 1888, p. 959; Duncan, 1889a, p. 9. Pcrischodomus (pars) Tornquist, 1897, p. 725. Lepidcchinus (pars) Lambert and Thiery, 1910, p. 122. Test spheroidal, ambulacra narrow, with two columns of plates in an area. The plates are rather high, and imbricate adorally and strongly bevel under the adambulacrals ; pore-pairs are uniserial. Interambulacra wide, with many (seven to eight) columns of plates in an area ; the plates are nearly rhombic in outline, imbricating strongly aborally and from the center laterally and over the ambulacrals. Base of corona, peristome, and lantern unknown. Oculars small, genitals low, plates of periproct as far as known numerous, angular. This genus differs from Lepidocentrus in that the ambulacral plates are relatively high instead of low. Since for ambulacral plates to be high is a primitive character, it is considered a lower genus than Lepidocentrus and one of the two species, K. silurica, is geologically much older than any known Lepidocentrus. Type species K. cotteaui Dollo and Buisseret from the Lower Carboniferous of Belgium. Kry to the Species of Koninckocidaris. Ambulacra! plates high, two equaling the height of an adambulacral; seven colunms of plates in an interambulacral area .......... A', cotteaui Dollo and Buisseret, p. 285. Ambulacral plates high, three equahng the height of an adambulacral; eight columns of plates in an interambulacral area. A', nilurica sp. nov., p. 285, KONINCKOCIDARIS. 285 Koninckocidaris cotteaui Dollo and Buisseret. Koninckocidaris cotteaui Dollo and Buisseret, 1888, p. 959. Perischodomus [cotteaui] Tornquist, 1897, p. 725. Lepidechinus cotteaui Lambert and Thiery, 1910, p. 122. Known from a fragment of a test and isolated plates. General form unknown, but proba- bly it was spheroidal as is K. silurica. Apical disc and jaws unknown. Ambulacral plates high, two about equaling the height of an adambulacral plate; pore-pairs lie near the next adjacent interambulacrum, two pores to each plate, not on a line, but one higher than the other. Dollo and Buisseret say that the pore nearer the middle of the area is higher up. This is opposed to what I have found in all Palaeozoic species, and probably they had their fragment incorrectly oriented. If we change the orientation, then it would read, pore nearer the middle of the area lower, which is in accord with what I find in Palaeozoic and most modern Echini (p. 57). Ambulacral plates bear numerous secondary tubercles, similar to those on the interambulacra. Interambulacra with seven columns of plates, the central column narrower, plates polygonal, strongly imbricating, and ornamented with secondary tubercles; the adambulacral plates bear in addition marginally a primary perforate tubercle. Described from a single specimen, which has not been figured. Lower Carboniferous, Belgium. *Koninckocidaris silurica sp. nov. Plate 19, fig. 1 ; Plate 20, figs. 5, 6. It was with keen delight that I received this most interesting fossil which Professor H. L. Fairchild most kindly sent me for study. It occurs in the Niagara Limestone, Silurian, and therefore is much earlier geologically than any American echinoid previously known. It also adds much to our knowledge of this very interesting ancient genus. The specimen is an internal view and the plates are beautifully preserved, but one must bear in mind that as it is internal, therefore the direction of imbrication is opposite to what it would be if the specimen were viewed from the exterior (compare text-figs. 32-38, p. 75). Test high, spheroidal, as gathered from the dorsal portion, which alone is preserved. The lower part of the specimen is probably at or near the plane of the mid-zone. Height of specimen 52 mm., width of ambulacrum J at the lower portion 9 mm.; width of the interambulacrum on same line about 43 mm. From these figures the circumference would be about 260 mm. and the diameter in the neighborhood of 83 mm. Ambulacra rather wide, much wider than they would be on the exterior, on account of the lateral beveling of ambulacral plates under (but here over) the adradials. Pore-pairs relatively near the ventral part of each plate, and near the middle of the area, largely due to its being an internal view (compare Plate 20, figs. 9, 10). Ambulacral plates are high, about three equaling the height of an adambulacral plate; in this 286 ROBERT TRACY JACKSON ON ECHINI. respect they are somewhat lower than in A', cotteaui as described, but much higher than in species of Lepidocentrus. A general depressed area surrounds the pore-pair, extending on to the next plate in this internal view. This leaves an elevated ridge between succeeding pore-pairs, and the general superficial appearance resembles the ribs of a turtle in relation to the carapace. ^Imbulacral plates imbricate adorally and bevel strongly under the adambulacrals. (As this is an internal view, the exact opposite appears in the specimen; that is, ambulacral plates in this view imbricate aborally and laterally bevel over the adradials; compare text-fig. 37, p. 75.) The interambulacrum is composed of eight columns of plates, which are nearly rhombic in outline, much as in Lepidocentrus mulleri. A median column is narrower than the others and in both areas this is column 7. The plates imbricate strongly aborallj* and from the center laterally and over the ambulacrals. (As this is an internal view, the exact opposite appears to be the case; that is, interambulacral plates in this view imbricate adorally and under the ambulacrals; compare text-fig. 34, p. 75.) The ventral part of the test is unknown, but dorsally an ocular which is low but exceptionally wide, covers ambulacrum B and in \)SiYi interambulacra A and C on either side. A very low genital is in A and a part of one in C. A small plate between genital A and ocular B may be a displaced plate, but it appears as if the periproct met the interambulacrum in this area like the condition common in Recent Echino- thuriidae (text-fig. 170, p. 149; p. 287). The apical disc in this species is evidently small in pro- portion to the diameter of the test, a progressive character. A few small angular plates lying dorsal to the ocular and genitals are apparently remnants' of the periproct. As the exterior of the plates is unknown, surface characters cannot be given; but thej^ are probably as in K. cotteaui. This choice and beautifully preserved specimen was collected in 1909 by Mr. A. W. Giles from the base of the Rochester Shale, about ten feet above the Irondequot Limestone, in the Niagara Group, Silurian. It was found in these beds in a ravine of the Genesee River in the north part of Rochester, New York. On the slab are fragments of other fossils including a faceted eye of Dalmanites limulurus Green, a typical Niagara Group trilobite. The holotype with two additional fragmentary pieces is in the collections of the University of Rochester, at Rochester, New York, and was loaned to me by my friend. Professor H. L. Fairchild of that institution. Lepidocentrus Miiller. Lepidocentrus Muller, 1857, p. 258; Schiiltze, 1866, p. 123; Loven, 1874, p. 39; A. Agassiz, 1881, pp. 79, 80; Duncan, 1889a, p. 9; .lackson, 1896, p. 222; Lambert and Thiery, 1910, p. 121. Test spheroidal, ambulacral areas narrow throughout, plates low, imbricating adorally and beveled under adambulacrals; pore-pairs uniserial, situated toward the next adjacent interambulacrum, as seen externally, but near the middle of the plate as seen internally. Inter- LEPIDOCENTRUS. 287 ambulacral areas with many (5 to 11) columns of plates which are moderately thin and imbricate strongly aborally and from the center outward and over ambulacrals. Interambulacral plates are rhombic, or more or less hexagonal, and the primordial interambulacral plates are in the basicoronal row. Peristomal plates are unknown in this genus. Mr. Agassiz (1881, p. 79) says that in Lepidocentrus there is no distinct line of division separating the coronal interam- bulacral plates from those belonging to the actinal membrane. He further says that in a remarkably well preserved , specimen from the Lower Burlington Limestone, the ambulacral plates are continued to the very teeth, and the same is the case with the interambulacral plates. Unfortunately Mr. Agassiz did not figure this structure, or mention in what species it occurred. The peristome is not known to me in this genus, but in two other genera of the family, only ambulacral plates exist on the peristome, and such a structure as Mr. Agassiz describes is not known to me in any sea-urchin, fossil or Recent. The genus is not known from the Burlington Limestone excepting as implied by Mr. Agassiz's remarks, and this is the first mention of its occurrence in America. It is on this statement apparently that Duncan (1889a) records the genus from this country. Insert oculars, and genitals with many pores are known in one species, the new L. whitfieldi (Plate 21, fig. 5). The apical disc is proportionately small. Mr. Agassiz (1881, p. 80) says of Lepidocentrus that, "the genital plates bear about the same proportion to the plates of the anal system which we find in some of the recent Echino- thuridae, and there exists at the apical extremity of the coronal plates the same difficulty in defining where the interambulacral plates terminate and the plates of the apical system begin." I do not know the basis of Mr. Agassiz's statement, but in this genus, Lepidocentrus, and in all Palaeozoic Echini that I know (excepting possibly Koninckocidaris silurica, p. 286) the geni- tal and ocular plates, when preserved, are in contact throughout, so that the interambulacra are separated from the periproct by the oculo-genital ring, not in contact with it, as may occur in the Echinothuriidae (text-fig. 170, p. 149; p. 414). Lepidocentrus occurs in the Devonian and Lower Carboniferous; the type species is L. eifelianus Miiller. Key to the Species of Lepidocentnis} Fi\e columns of interambulacral plates in an area . . . . L. rhenanus (Beyrich), p. 288. Eight columns of interambulacral plates in an area . . . . L. drydenensis (Vanu.xem), p. 288. Ten or eleven columns of interambulacral plates in an area, the plates nearly rhombic in form L. miiUeri Schultze, p. 289. Eleven columns of interambulacral plates in an area, the plates small and rounded hexagonal L. whitfieldi sp. no\-., p. 290. Number of columns of interambulacral plates in an area unknown, plates similar to those of milUeri, but thinner and more rounded in outline i. (7/(//o?n/s Miiller, p. 291. ' Three species of Lepidocentrus, namely L. desori, duponti, and gandryi, of which no description has been published, are considered under Nomina Xuda. 288 ROBERT TRACY JACKSON ON ECHINI. *Lepidocentrus rhenanus (Beyrich). Plate 20, fig. 7. Palaechinus rhenanus Beyrich, 1857, p. 4; Quenstedt, 1S75, p. 377, Plate 75, fig. 18.' Lepidocentrus rhenanus Muller, 1857, p. 264, Plate 4, figs. 4-6; Loven,^ 1874, p. 40; Jackson, 1896, p. 244; (pars) Klem, 1904, p. 17; Lambert and Thiery, 1910, p. 122. Test spheroidal, ambulacral plates low. Interambulacrum with five columns of plates which are nearly hexagonal, as seen in area E of Plate 20, fig. 7. The primordial interambulacral plate is in the basicoronal row, and from this it is inferred that the same occurs in other species of the genus. Columns 2, 3, 4 originate in succeeding rows of plates, and column 5 appears in the sixth row. There are powerful jaws in place ventrally. The type and only known speci- men, which is in the Berlin Museum fiir Naturkunde, is an internal mold, and therefore the external character of the plates is unknown. The plates also are probably more hexagonal in outline than they would be on the exterior (compare text-figs. 32-34, p. 75). Devonian, Wipperfiirth, Prussia. *Lepidocentrus drydenensis (Vanuxem). Plate 16, fig. 5; Plate 21, figs. 1-3. Echinus drydenensis Vanuxem, 1842, p. 184. Eocidaris drydenensis Hall, 1868, p. 298; 1870, p. 341; Loven, 1874, p. 43; Klem, 1904, p. 71. Archaeocidaris drydenensis Keyes, 1895, p. 184; Jackson, 1896, p. 214. This species, has never been figured before, and, as indicated in its synonymy, has been variously placed generically. The type, which may be considered the best preserved individual, is on a large slab with the faint impressions of two or more other individuals. The plates them- selves are not preserved, but we have an impression only with the plates entirely dissolved away. This is true of the spines as well as the plates. The best impression is fortunately of the ambulacral areas and of the spines, while the impression of the interambulacral areas is for the most part weak and ill defined. Diameter through J, E, 70 mm., also through H, C. The view, which is ventral, shows a rather deep depression in the oral region. Diameter of the peristome 6 mm. The shape of the test was doubtless spheroidal, or spherical. Ambula- cral areas narrow, plates low, about seven or eight equaling the height of an adambulacral plate. Ambulacral pore-pairs lie near the next adjacent interambulacrum and are on an angle, the outer pore a little higher than the inner; width of an ambulacrum at the mid-zone 14 mm. ' Non Palaeocidaris rhenanus Quenstedt, 1875, p. 377, Plate 7.5, figs. 30-37 (for which in this memoir see Xenocidaris clamgera). ^ Lov(5n writes " L.[epidocenlrus] rhenanus {Palaeocidaris) Beyrich," tlius indicating that Beyrich put the species in Palaeocidaris, which is a mistake, as he put it in Palaechinus. Palaeocidaris rhenantis is Qucnstedt's name, applied to another species; here referred to Xenocidaris clavigera as a synonym, see under lucertae Sedis. LEPIDOCENTRUS. 289 The plates of the interambulacra are marked as impressions, the outlines of which are largely ill defined. There are apparently nine columns of plates in each area, as shown in Plate 21, fig. 1. The spines are marked as depressions or elevations in the shale; they are elongate and slender, acicular, and the clearest impressions are about 8 mm. long. In the shaly sandstones of the Chemung Group, Upper Devonian, Dryden, Tompkins County, New York; holotype and only known specimens (on a single slab) in the New York State Museum Collection 4,200. This species is geologically the oldest known sea-urchin from America, except the Konincko- cidaris silurica herein described. It is not strange that this species has not been figured before, as it is not easy to make out. Professor John M. Clarke kindly sent me the specimen for study, and after careful examination I feel that it should be referred to Lepidocentrus. It certainly has no relation to Archaeocidaris, and does fit' in with the characters of Lepidocentrus. As regards the characters of tubercles and spines, primary and secondary, nothing can be definitely said except that the larger spines would compare well enough with the larger spines of Lepido- centrus. ^Lepidocentrus miilleri Schultze. Plate 19, figs. 2-5; Plate 20, figs. 8-13. Lepidocentrus mullcri Schultze, 1866, p. 124, Plate 13, figs. 1-lf; Loven, 1874, p. 40; Jackson, 1896, p. 223, text-fig. 2; Klem, 1904, p. 17. Lepidocentrus mulleri Lambert and Thiery, 1910, p. 122. Test spheroidal as gathered from the upper part, which alone is preserved. Ambulacra at the lower part of the type specimen about 5 mm. wide; interambulacrum about 65 mm. wide. This would give a circumference of about 350 mm. and diameter of about 111 mm. The ambula- cra are narrow, plates imbricating ventrally, and stronglj' beveled under the adradials. The ambulacra, on account of this bevel, are much wider on the interior of the test than on the exterior, as seen by comparing Plate 20, figs. 9, 10. The pore-pairs on the exterior lie near the adradial plates, but proximally are in about the middle of each ambulacral plate. Interambula- cra are broad, with eleven columns, as shown in Plate 20, fig. 8, area A, the onlj'^ complete area known; but in area C, of the same figure, there are apparently only ten columns. In area A many columns drop out passing dorsally, namely, columns 6, 5, 11, 10, 9, and in the sequence stated. This is a very marked character, indicating reversion or senescence. In my earlier paper (1896, p. 223) this dropping out of columns was by error taken for the coming in of columns, with the result that I oriented the specimen incorrectly, and also incorrectly stated the direction of imbrication (p. 76). The interambulacral plates are thin, nearly rhombic in outline, and imbricate strongly aborally (not orally as I stated before), and from the center outward and over the ambulacra. The beveled edges are shown well in Plate 20, figs. 11, 12. 290 ROBERT TRACY JACKSON ON ECHINI. The interambulacral plates bear one or occasionally two small eccentric perforate primary tubercles with scrobicules, and scattered secondary tubercles. These are shown best in the Munich specimen (Plate 19, figs. 3, 4). The spines are small primaries and secondaries. From the swollen base they taper to the tip and are longitudinally finely striate. This species is known from two good specimens : the type from the Schultze Collection in the Museum of Com- parative Zoology, and a specimen in the Munich Museum, both figured here. The Munich specimen is especially clear in its surface characters, and the interambulacral plates lie very nearly in horizontal rows instead of alternating in adjacent colunms, as usual in Echini. Besides tubercles, it shows a number of spines in place, which are also figured enlarged. Isolated in- terambulacral plates are fairly common in collections. Middle Devonian, Gerolstein, Prussia, two fine specimens, the holotype, from the Schultze Collection, Museum of Comparative Zoology 3,040; and one in the Munich Museum. Spines from the same locality, including those figured by Schultze, Museum of Comparative Zoology Collection 3,043. Isolated plates from same locality. Museum of Comparative Zoology 3,042, 3,045; British Museum E 1,201; Strassburg Museum; Freiburg i. B. Museum; Pelm, Prussia, Museum of Comparative Zoology Collection 3,103-3,105; Rommersheim, near Prum, Prussia, Museum of Comparative Zoology 3,041. *Lepidocentrus whitfieldi sp. nov. Plate 19, figs. 6, 7; Plate 21, figs. 4, 5. Lepidcchiniis rarispinus (pars) Hall, 1S6S, p. 295 (non Plate 9, fig. 10); 1870 (revised edition), p. 340 (non Plate 9, fig. 10); (pars) Jackson, 1896, p. 228; (pars) Klem, 1904, p. 22. Test spheroidal, flattened on the base, probablj' from pressure. Known onlj' from an internal mold. Ambulacral areas narrow throughout, plates low. Eleven columns of small plates in an interambulacral area, the plates strongly rounded in outline. The primordial interambulacral plate is in the basicoronal row, as shown in one area. The oculars, as indicated by an impression, are small, insert ; genitals high, with numerous genital pores, nine or ten in the impressions preserved. Professor Hall included this specimen in his Lepidechinus rari- spinus, here called Hyattechinus rarispinus. It differs from that species in that the ambulacra are narrow throughout, instead of broad and petaloid ventrally, as in rarispinus. This differ- ence Professor Hall was not aware of, as he had only the dorsal part of rarispinus (p. 395). I think it also differs in its spheroidal form, as rarispinus is almost certainly highly flattened, (p. 292) but this might well be only a specific, not a generic difference. This species I name in memory of the late Professor R. P. Whitfield of the American Museum of Natural History. Waverly Group, Lower Carboniferous, Licking County, Ohio. Holotype, and only known specimen, American Museum of Natural History Collection 6,391. HYATTECHINUS. 291 *Lepidocentrus eifelianus Miiller. Plate 20, fig. 14. Lcpidocentrus eifelianus Muller, 1857, p. 258, Plate 3, figs. 1-8; Schultze, 1866, p. 123, Plate 13, figs. 2-2c; Loven, 1874, p. 40; Quenstedt, 1875, p. 374, Plate 75, figs. 14-17; Jackson, 1896, p. 225; Klem, 1904, p. 16; Lambert and Thiery, 1910, p. 122. This species, the type of the genus, is known only from isolated interambulacral plates and spines. The plates are thinner and more rounded in form than those of L. viiilleri, but with similar surface ornaments. The spines are similar to those of mulleri. Devonian, Nohn, and Rommersheim, from the Schultze Collection, Museum of Compara- tive Zoology 3,061, 3,068, 3,069; Berlin Museum fiir Naturkunde, doubtless the types. ToRNQUiSTELLUS Berg. Leptechinus Tornquist, 1897, p. 785 (non Gauthier, Echinides fossiles de la Tunisie, 1869). Tornquistelhw Berg, 1899, p. 77. Known only from interambulacral plates, which are small, thin, imbricating, with sparsely distributed secondary tubercles. While the material is so meager, its affinities are distinctly with Lepidocentrus, from which it differs in having secondary tubercles only. Type species T. gracilis. *Tornquistellus gracilis (Tornquist). Plate 20, fig. 15. Leptechinus gracilis Tornquist, 1897, p. 785, Plate 20, fig. 5. TornquistcUus {gracilis] Berg, 1899, p. 77. Perischodomus gracilis Lambert and Thiery, 1910, p. 122. Known only from isolated interambulacral plates, which are thin, flat, wider than high, slightly angular, but very nearly rhombic in outline. The type specimen measures 8 mm. in width and 5 to 6 mm. in height. The surface is ornamented with about 20 small tubercles. This species has characters which would not fit into any other genus, so that, although so little is known of it, Tornquist was quite justified in establishing a new genus for its reception. Lower Carboniferous, rare at Hunsriicken, Germany; holotype, in the Strassburg Museum Collection. Hyattechinus gen. nov. Test spheroidal or flattened; through the ambitus nearly circular; pentagonal or elongate through an axis passing through an ambulacrum and opposite interambulacrum, which I take to be the axis III, 5. Ambulacra wide, petaloid ventrally, narrow dorsally. Two columns of plates in each ambulacral area, plates relatively high and wide ventrally, lower and narrower 292 ROBERT TRACY JACKSON ON ECHINI. dorsally; pore-pairs uniserial. Interambulacra with in each area numerous (11 to 14) columns of thin, small plates, imbricating aborally and from the center laterally. Primordial interam- bulacra! plates are in the basicoronal row; above this zone additional columns come in rapidly in succeeding rows, showing a very accelerated development. The peristome is small, with ten columns of ambulacral plates only. Oculars small, insert, covering the ambulacra, and in part the interambulacra on either side. Genitals are large and high. The apical disc is small. Periproct, as far as known, with many small, angular plates. The lantern is strongly developed and inclined, as in other Palaeozoic species. The type species is H. beecheri sp. nov. from the Waverly Group of Warren, Pennsylvania. This genus contains one of the most distinctive groups of known fossil Echini, remarkable alike for the specialized ambulacra and interambulacra. The latter attain the largest number of columns of plates known in the class, with a very accelerated development, yet all the plates are retained, since there is no ventral resorption of the corona, and ventrally one can read stages in development marked by the coming in of columns ; dorsally stages in senescence, as marked by the dropping out of columns, without a single break. The flattened form of rarispinus, the strongly pentagonal form of pentagonus and the bilaterally s>Tnmetrical form of beecheri mark these as specialized and highly unusual sea-urchins (p. 225). I take keen pleasure in naming this genus for the late Professor Alpheus Hyatt, one of the finest minds and finest natures that has graced science in this country, to whom I owe all that a pupil can owe to a master. Key to the Species of Hyatiechiniis. Test flattened, nearly circular, eleven to thirteen columns of plates in each interambulacral area H. rarispinus (Hall), p. 292. Test pentagonal, fourteen columns of plates in each interambulacral area //. pentagonus sp. nov., p. 295. Test clypeastriform, flattened on the base, bilaterally symmetrical through an ambulacrum and opposite interambulacrum, eleven columns of plates in each interambulacral area . . H. beecheri sp. nov. p. 297. *Hyattechinus rarispinus (Hall). Plate 21, fig. 6; Plate 22, figs. 1-8; Plate 23, figs. 1-7. Lepideckinus rarispinus (pars) Hall, 1868, p. 295, Plate 9, fig. 10; 1870 (revised edition), p. 340, Plate 9, fig. 10; (pars) Keyes, 1895, p. 192; (pars) Jackson, 1896, p. 226, Plate 7, fig. 42; (pars) Klem, 1904, p. 21; Lambert and Thi^ry, 1910, p. 122. (See p. 290 and footnote, p. 394.) Lepidocentrus rarispinus Meek, 1874, p. 375. This interesting species is known from abundant material. The specimens are all internal or external sandstone molds, very flat, without distortion or separation of plates, which indi- cates that in life the test was much flattened, probably about as in the Recent Phormosoma, or Echinarachnius. Plates are extremely thin; they are for the most part wanting, but some HYATTECHINUS. 293 fragments of plates in Yale Museum specimens are only a small fraction of a millimeter in thickness. The test was nearly circular in outhne but slightly elongated through an ambula- crum and opposite interambulacrum, as in Plate 22, figs. 1, 8, indicating a tendency to the bilateral symmetry which is more marked in H. beecheri (Plate 24, figs. 5-8). The ambulacrum is broad, petaloid ventrally, at its widest part twice the width of the same area at the mid- zone. Ambulacral plates are relatively high, especially ventrally, where one and one-half about equal the height of an adradial plate; dorsally about two and one-half equal the height of an adradial. This shows that the younger ventral plates are higher, which I believe is a more primitive character than that seen in the lower, later built dorsal plates. Pore-pairs are uniserial, in well marked peripodia, and each pair ventrally is on an angle, with the outer pore distinctly higher than the inner. Ambulacral plates bear small secondary tubercles only, and ventrally on the interior (Plate 23, figs. 2, 7) each plate bears a spinose projection that extends into the interior of the test. This spinose projection lies between the inner pore and the middle of the area; it is otherwise known amongst fossils in H. beecheri only, as here described, and amongst Recent Echini can be directly compared with the internal spines that I show in Phyllacanthus and Eucidaris (Plate 3, figs. 12, 13; p. 61). The interambulacra have from 11 to 13 columns of plates at the mid-zone, this range being shown in a single specimen in its several areas (Plate 23, fig. 3). The type specimen, however, (Plate 21, fig. 6), has 11 columns in each of the three areas preserved. The primordial inter- ambulacral plates are in place in the basicoronal row (Plate 23, fig. 1), and new columns come in rapidly, one, or sometimes two columns appearing in each row as added up to the ninth, but later columns may come in more slowly. Dorsally, the adambulacral or even additional columns drop out on approaching the apical disc, indicating senescence. The interambulacral plates bear a small eccentric primary tubercle with secondary tubercles (Plate 23, figs. 5, 6), to which are attached corresponding small spines. The peristome is covered with ten columns of ambula- cral plates only (Plate 23, fig. 1), much as they are in a Recent echinothuriid (text-fig. 43, p. 80). I (1896) first showed these rather imperfectly in rarispinus, and Mortensen thought it made a connection between the two genera, but I cannot agree with this view, as elsewhere discussed (p. 213). The ocular, genital, and periproctal plates are unknown in this species, but they were probably similar to those of pentagonus. A powerful lantern is represented by molds of the parts (Plate 23, fig. 2) in several specimens. The pyramids are wide-angled, and therefore the lantern is inclined, as in other Palaeozoic types. Waverly Group, Lower Carboniferous, Meadville, Pennsylvania; the holotype (Plate 21, fig. 6; Plate 22, fig. 7) is from this locality, and is in the American Museum of Natural History, no. 6,392. Same horizon, Warren, Pennsylvania; a superb series of ten specimens largely col- lected by the late Professor C. E. Beecher, is in the Yale University Museum nos. 324-333; another fine series of six specimens from the same locality is in the New York State Museum no. 1,799. 294 ROBERT TRACY JACKSON ON ECHINI. On account of the exceptional quality and interest of the material, some detailed discussion of the specimens is desirable. The type (Plate 22, fig. 7; Plate 21, fig. 6) is a fine sandstone mold of the dorsal portion of the test (p. 395). It has three ambulacra and interambulacra preserved, and its periphery coincides with the mid-zone. This was the only specimen of this species known to Hall, as his second specimen is considered here as belonging to a distinct species and genus, namely Lepidocentrus whitfieldi, p. 290. In this type specimen the ambulacra are narrow and the plates are relatively narrow and low, as it is a dorsal view. In each interambula- crum there are eleven columns of plates, the eleventh originating above the mid-zone as shown. In each interambulacral area the adambulacral columns drop out in passing dorsally, as a senes- cent character. As it is an external mold, the lettering is reversed from what it would be if seen from the interior (p. 21). The Yale Museum specimen shown in Plate 22, fig. 8, Plate 23, fig. 3, is also a sandstone impression of the dorsal side, but from Warren, Pennsylvania. Being an external mold, there- fore the lettering is reversed, and it is the most nearly complete dorsal area known. The ambu- lacra are narrow and plates relatively narrow and low. Interambulacra A and G have eleven columns of plates each. In area I there are twelve, and in C and E, thirteen columns of plates, thus showing considerable radial variation. In each area the adambulacral columns 1, 2 drop out dorsally, when columns 3, 4 assume an adradial position near the apical disc. Five radial depressed pits appear, seen best in Plate 22, fig. 8, which are evidently due to a forcing up of the test at these points when, in flattening, the lantern came in contact with the dorsal area. A remarkable specimen in the Yale Museum from Warren, Pennsylvania (Plate 22, figs. 1, 2; Plate 23, figs. 1, 2, 7) is one of the most interesting Echini that I have studied. As seen, the specimen consists of counterparts which structurally supplement each other. In one of these (Plate 22, fig. 1; Plate 23, fig. 1) we have for the most part ari external sandstone mold of the ventral side, showing beautifully the impressions of ambulacral, interambulacral, and peri- stomal plates, with sutures, tubercles, and peripodia. With this there is in areas B, C, and imperfectly in D, an internal impression of the dorsal side, molded on the sand which filled the interior of the test. In the reverse counterpart (Plate 22, fig. 2; Plate 23, figs. 2, 7) we have in part in areas J, A, B, and C an external mold of the dorsal side showing tubercles, etc., but for the most part the specimen shows an internal impression of the ventral side molded upon the sand which filled the interior of the test. On this part there are no tubercles or peripodia, as it represents the proximal faces of the plates, but there are impressions of suture lines, pore- pairs, the spinose projections from the interior of ambulacral plates, and the lantern. Such a condition was rendered possible only by a complete solution of the calcareous parts leaving on one side an external and on the other an internal impression of these parts. Considering the structure, it is seen (Plate 23, fig. 1) that the ambulacral plates of the peristome pass directly into those of the corona, and the plates of the corona are as given in the description of the species. irV'ATTECHINUS. 295 In area B the ambulacral plates of the dorsal side are brought into almost immediate contact with those of the ventral side, superposing a narrower upon a wider plate. It is a similar con- tact which I think accounts for the so called roofing plates of Sollas (1899, p. 705) in Palaeo- discus, as elsewhere discussed (p. 251). In the internal view (Plate 23, figs. 2, 7) a series of slanting pits exists between the inner pore and middle of the ambulacrum and represents molds of the spinose processes, which extend into the test as described. The interambulacral areas all show the primordial plate in place as I earlier described it (1896, p. 226). This plate is directly comparable to the similar plate seen ventrally in Bothriocidaris, in the young of all other Echini, and in the adults of types where it has not been removed by resorption in the advance of the peristome. In the second row there are two plates in each area, and in the third row three plates in each interambulacral area (Plate 23, fig. 1). In the fourth row there are four plates, but in area E the fifth column is also represented by a plate in the fourth row, a case of extreme acceleration of development. Columns 6, 7, 8, 9 come in rapidly, a new one being added in each succeeding row as built. The mold of the lantern (Plate 23, fig. 2) shows casts of the five interradial alveolar cavities, each of which was surrounded by two half-pyra- mids, and also casts of the radial spaces between the half -pyramids seen best in areas B and D, which were originally occupied by the interpyramidal muscles. Evidently the lantern was inclined with the usual structure of the Palaeozoic lantern, as in Plate 27, figs. 4-6 (p. 22). Another specimen from the Yale Museum (Plate 22, figs. 3-6) shows certain differences from that just described. The figures 3 and 5 are external molds of the ventral side, with tub- ercles, spines, and peripodia. The figures 4 and 6 are external molds of the dorsal side of the same individual specimens, and also show the surface characters of tubercles, etc. A mold of the interior of the plates is shown in a small portion only of area J. In two areas of figure 4 and in a third area of the counterpart, figure 3, there are eleven columns of plates, as in the type specimen. The impression of ambulacral plates on the peristome is very clear in figure 3, and the mechanical displacement caused by the thrust of the lantern during flattening is clear in figure 4. This material was collected by my friend, the late Professor Beecher, near his old home at Warren, Pennsylvania. It is a pleasure to feel that his rare skill as a collector and preparator is still contributing to the science and the principles that he loved so well. *Hyattechinus pentagonus sp. iiov. Plate 19, fig. S; Plate 24, figs. 1-4; Plate 25, figs. 1^. The material of this species is a slab with several exquisitely preserved specimens which Professor R. S. Breed kindly loaned me, and two additional specimens that I received through the same gentleman. 296 ROBERT TRACY JACKSON ON ECHINI. The test is relatively high, though far from spheroidal, and strongly pentagonal in hori- zontal outline, the apices of the pentagon coinciding with the ambulacral areas. It is the most markedly pentagonal of any known regular sea-urchin. The diameter in different specimens varies from 45 to 60 mm. The ambulacrum is petaloid ventrally, narrow dorsally, 6 mm. wide at the widest part of the petal, which is twice that of the same area at the mid-zone. Ambula- cral plates are wide ventrally, narrower dorsally, and ventrally somewhat higher than they are dorsally. Pore-pairs are uniserial, in peripodia, with ventrally the outer pore set somewhat higher than the inner pore of each pair. Interambulacra are very wide, with fourteen coluimis of plates in each area, the highest number yet known in Echini. The ventral surface is known in only one specimen (Plate 24, fig. 4; Plate 25, fig. 1), but this is verj^ well preserved. It shows that the primordial interambulacral plate is in place in the basicoronal row in each area, two plates in the second row, and three in the third. In area A the fourth column appears in the fourth row, but in areas C, G, and I both columns 4 and 5 appear in the fourth row; and in area E, apparently (partially restored) columns 4, 5, and 6 appear in the fourth row. This is a degree of acceleration of development much ahead of that known in any other type, and may be correlated with the fact that the species attains more columns than are known in any other type. Columns 6 to 11 come in with great rapidity, as shown in the figure, one or more being added in succeeding rows as built. Turning to the dorsal view (Plate 24, figs. 1-3; Plate 25, figs. 3, 4), we see that the columns 13 and 14 come in later and with considerable intervals in the point of introduction, for, as usual in Hyatt's law of acceleration of development, its effect is most marked on the earlier added parts, the later differentiated structures not yet having come within its control. Dorsally, in this remarkable type, we find the dropping out of columns marking reversionary or senescent characters; this is shown in Plate 25, figs. 3, 4. The adam- bulacral columns 1 and 2 drop out, so that 3 and 4 become in further growth adjacent to the ambulacra; also some additional columns drop out, as 10 and 13 in figure 4. The exterior of the plates is marked by numerous closely set tubercles; but I did not distinguish any division into primary and secondary; rather all seem to be of the second class. The peristome is small, and as in H. rarispinus, is plated with ten columns of ambulacral plates only, which appears to be a character of the whole family. Small insert oculars cover the ambulacral areas and laterally in part the interambulacra on either side. Genitals are relatively large and high; no genital pores were recognizable in the sandstone impression. Impressions of a few small angular plates in the periproct show that it is of the usual Palaeozoic character. The lantern and in- ternal characters of the plates are not preserved in known specimens of this species. Waverly Group, Lower Carboniferous, Meadville, Pennsylvania, slab with the holotype in Alleghany College Collection; paratypes in Museum of Comparative Zoology Collection nos. 3,107 and 3,108. HYATTECHINUS. . 297 *Hyattechinus beecheri sp. nov. Plate 24, figs. 5-8; Plate 25, fig. 5; Plate 26. This very remarkable sea-urchin is known from a single specimen, which is an internal sandstone mold, and bears the impress of the proximal faces of the plates only. The test is depressed, dome-shaped, flattened on the ventral aspect, elongate and bilaterally symmetrical through an ambulacrum and opposite interambulacrum, which I take to be the axis III, 5. The specimen is so perfect in form, without distortion or separation of plates, that it may be taken as representing the original shape. Excepting as shown somewhat in Hyattechinus rarispinus, this is the only regular echinoid known with a central periproct, that presents a bilateral symmetry. The oblong form of the Echinometridae through an oblique axis does not represent a true bilateral symmetry. The specimen measures 26 mm. in height, 80 mm. in length through the axis III, 5, and 71 mm. in width through the peristome and at right angles to the longitudinal axis. While the mold represents only the impression of the interior of the test, it is sufficiently perfect so that I am able to represent the specimen in Plate 26 spread out on the Loven method in which each plate was scrutinized with utmost care. At a few points in areas 3 and 4 details could not be made out, and in these areas in part the plates are restored as indicated by dotted lines. The breaks occurring in the figure at the ambitus are due to the mechanical requirements of getting over the edge in a specimen of this shape. Only one ambulacrum is filled in in detail dorsally, and for this the best area was selected. The ambulacra are broad and petaloid ventrally, being 13 mm. in width at the widest part, 9 nun. wide at the ambitus, and 6 mm. wide at about the middle of the dorsal side of the test. The plates are relatively wide and high ventrally, narrow and much lower dorsally. The pore-pairs are uniserial, situated at about the middle of each plate. This is a general character of the interior, and on the exterior they would doubtless have been much nearer to the inter- ambulacra on either side (compare Plate 20, figs. 9, 10). The outer pore of each pair ventrally is higher than the inner pore, as in other species of the genus. Between the inner pore and center of the area a slanting gash-like pit exists in each plate ventrally; these pits, being molds, represent elevated nodose spine-like processes which extended inward from the ambulacral plates (Plate 24, fig. 6), similar to those described in H. rarispinus (p. 293), and again, similar to, though less elongate than the spinose processes from the ambulacral plates of Eucidaris and Phyllacanthus (Plate 3, figs. 12, 13). Such spinose processes from ambulacral plates of the corona are known only in the types mentioned, but Sir Wyville Thomson and Mr. Agassiz described similar internal spines as given off from ambulacral plates of the peristome in Poro- cidaris (p. 61). The interambulacra are very broad, and make up the greater part of the test, as in all 298 ROBERT TRACY JACKSON ON ECHINI. species of the family. There are eleven colunins of plates at the mid-zone in each area; a similar number is seen in some areas of specimens of H. rarispinus, but a greater number exists in H. pentagonus. The primordial interambulacral plates are in place in the basicoronal row in each area, as in other species of the genus. In the second row there are two plates, in the third row three, and in the fourth row four, these being the point of origin of their several col- umns. The fifth column originates in the fifth row in areas 1, 5, and 4, but in the sixth row in areas 2 and 3. Likewise the sixth column originates in the sixth row in areas 1,5, and 4, but in the eighth row in areas 2 and 3. It is to be noticed that a bilateral synunetry is expressed by the rate of coming in of columns five and six; in both they originate a little earlier in the three posterior interambulacra than they do in the anterior two. Too much weight should not be attached to a single observation, but it is in direct accord with what I have found in the sym- metrical bilateral development of other structures in various regular Echini. Columns 7 to 11 come in very soon after the sixth, either in succeeding rows, or very nearly so, and the eleventh column is attained at or just above the ambitus in each area. Area 4 is imperfect and column 10 apparently originates far to the left of the center, as indicated in the restoration. Senescence is strongly marked by the dropping out of columns of interambulacral plates in this specimen; the adambulacrals 1 and 2 in all areas excepting 4 drop out early, so that above that point columns 3 and 4 assume an adradial position. In addition, in area 2, columns 7, 11, 10, and 9 drop out passing dorsally, so that near the apical disc there are only five columns remaining, a structure which is a senescent reversion to a similar character seen as a stage in development in its own youth, and doubtless to the adult condition of some adult ancestral stock from which it was evolved. A similar dropping out of interambulacral columns more or less completely occurs also in the other areas. I would not point to it other than as a representative of the family, but it is noteworthy that Lepidocentrus rhenanus of the Devonian (Plate 20, fig. 7) has a slow, not an accelerated development, and has five columns of interambulacral plates in the adult. The peristome is small as in all of the family, and is evidently plated with ambulacral plates only (as in rarispinus), although in this specimen they are only partially preserved (p. 287). The apical disc is imperfectly known (Plate 25, fig. 5), but something can be gathered from the mold. The genitals are fairly clear and are high and wide. Spaces for the oculars exist, although the impress of these plates is not marked. They are restored to fill spaces between the genitals, thus making them all insert. The outline is preserved of a few small angular peri- proctal plates, and others are restored in the figure by dotted lines. A partial mold of the lan- tern orally shows that this structure existed, and was probably as in other species of the family (Plate27, figs. 4-6). I take pleasure in naming this species in memory of my close friend the late Professor Charles Emerson Beecher, of Yale University, thus linking his name with Hyatt's in a species that is one of the most striking of known Echini. PHOLIDECHINUS. 299 This species, which I make the type of the genus, is represented only by the specimen described, which was collected by the late Dr. F. A. Randall in the Waverly Group, Lower Carboniferous, of Warren, Pennsylvania. Then it came into the hands of the late Professor Beecher, and by him was transferred to the Yale University Museum Collection. Pholidechinus gen. nov. Test high, spheroidal; ambulacra narrow throughout, with two columns of plates in each area. Plates low primaries, all alike, or at the mid-zone alternate plates slightly narrowed at the interambulacral contact, and nearly or quite cut off from that area by the marginal enlargement of their fellows. Pore-pairs are uniserial, or at the mid-zone slightly biserial. Ambulacral plates are thin, imbricate adorally, and bevel strongly under the adradials. Inter- ambulacra wide with many (nine to ten in the species known) columns of plates in an area; the plates are polygonal with very rounded outlines and imbricate strongly dorsally and laterally and over the ambulacrals. Both ambulacral and interambulacral plates bear only secondary tubercles and spines. The primordial interambulacral plates are in the basicoronal row, and succeeding columns come in with a slow, not an accelerated development. Peristome with ambulacral plates only, oculars unknowia, genitals low, each with many pores. Lantern low, inclined. The type and only known species is P. brauni of the Lower Carboniferous. This genus differs from Lepidocentrus in that interambulacral plates bear secondary spines only instead of primary and secondary ; also in that ambulacral plates at the mid-zone are not all alike and pore-pairs are not strictly uniserial as they are in Lepidocentrus. From Hyatt- echinus it differs in that the ambulacra are narrow throughout, instead of broad and petaloid ventrally, and the interambulacra have fewer columns of plates, without marked acceleration of development. *Pholidechinus brauni sp. nov. Text-fig. 207, p. 184; Plate 27, figs. 1-6; Plate 28, figs. 1-10; Plate 32, fig. 6. Of this striking type there is a considerable amount of excellent material. The test is high, spheroidal. In the holotype (Plate 27, fig. 1) the ambulacra measure 5 mm. in width at the mid-zone, and the interambulacra 19 mm. in width. From these measurements the circumference would be about 120 mm. and diameter about 38 mm. The two other specimens figured are somewhat larger. The ambulacra are narrow throughout, with two columns of plates, which are low, imbricating adorally, and laterally strongly beveled under the adambula- crals. The pore-pairs have well marked peripodia. Ventrally the ambulacral plates are pri- maries, all alike, with pore-pairs uniserial. Above this area and at the mid-zone alternate plates differ somewhat, one plate being a pure primary, but enlarged on its outer margin; the 3Q0 ROBERT TRACY JACKSON ON ECHINI. next coincidently being narrowed on its outer margin and nearly or quite cut off from contact with the interambulacrum (Plate 28, figs. 5, 6). When quite cut off it becomes thereby occluded. Concurrently with this alternating difference the pore-pairs at the mid-zone are slightly biserial instead of purely uniserial as seen ventrally. The condition is quite comparable to that shown in Maccoya burlingtonensis (Plate 33, figs. 1,2), and is interesting as a parallel taking on of a similar character in an independent family of Echini. The interambulacra in each area have nine or ten columns of plates which imbricate strongly aborally and from the center outward and over the ambulacrals. Each plate, instead of being angular, is so rounded in outline that each bounding edge represents very nearly the segment of a circle; the longest segment lies on the left of the plate in the left half-area and on the right of the plate in the right half-area (Plate 28, fig. 1). This longest segment is formed by the free edge of the plate itself. Three shorter segments of circles of the same arc occur on each plate, formed by the overlapping of neighboring plates on the inner side. The structure is much like that shown in text-fig. 32, p. 75. The middle column that imbricates laterally in two directions presents a long sweeping curve which surrounds the plate excepting for a second short curve on the adoral border where in contact with the next succeeding ventral plate of its series. The nearest thing to which the form of these plates can be compared, perhaps, is the scale of an ordinary teleost fish. In addition, a peculiar feature known in no other sea-urchin, is a very thin, delicate flange which bounds and is limited to the longest segment of the circle on each plate (Plate 28, fig. 9). This flange therefore exists on the long curves on the left side of the left half-area, and on the long curves on the right-hand side of the right half-area. In the median column the flange borders the long curve, which nearly surrounds the plates of this column. The flange is so thin and delicate that it is usually lost by erosion, but it is found in exceptional cases, or in others the slight depression for its reception may be found, not in the plate to which the flange belongs, but in the next contiguous plates. Separated interambula- cral plates present a strong bevel toward the center and adorally as seen from above. As surface features, ambulacral and interambulacral plates alike have many secondary tubercles which support small acicular spines. There are no tubercles on the flanges described. The peristome has ten columns of low, wide plates which alone cover the area. Ocular plates are not known, but low, wide genitals exist (Plate 28, fig. 10), each of two preserved having eight genital pores. A typical Palaeozoic lantern exists as described below. This species is named for Mr. Frederick Braun of Brooklyn, New York, an ardent and skilful collector, who helped me greatly with the loan of his choicest material. Keokuk Group, Lower Carboniferous, Crawfordsville, Indiana, Museum of Comparative Zoology Collection (five specimens, 3,109-3,114, including the holotype no. 3,109); F. Braun Collection ; American Museum of Natural History ; British Museum of Natural History Col- lection E 10,678 (a specimen given by me, the original of Plate 27, fig. 3 ; Plate 28, figs. 4-6) ; Yale University Museum Collection 321 and 322 (two very complete lanterns). PHOLIDECHINUS. 301 In the holotype (Plate 27, fig. 1; Plate 28, figs. 2, 3, 9) there are nine columns of inter- ambulacral plates in each area at the mid-zone, but a tenth appears dorsally in area C. The beveling of plates is seen below the initial plate of column 8, some spines are in place, and the delicate flanges described as bordering the plates are well preserved. In Mr. Braun's specimen (Plate 27, fig. 2; Plate 28, fig. 1) there are nine columns of plates at the mid-zone in each interambulacral area. The primordial interambulacral plates are in place in the basicoronal row, two plates in the second, three in the third, and four in the fourth row, above which additional columns come in with a moderate, not an accelerated rate of development. Low, wide ambulacral plates cover the peristomal area, and the teeth are in place. The British Museum specimen (Plate 27, fig. 3; Plate 28, figs. 4-6) has ten columns of plates in each interambulacral area, in both originating at or below the mid-zone; ambula- cral detail at the mid-zone is clear. In Yale University Museum there are two lanterns in the D. A. Barrett Collection from Crawfordsville belonging to this species, which are remarkably perfect in preservation, being the best specimens of this structure so far known in any Palaeozoic echinoid. One of these specimens I mentioned in my earlier paper (1896, p. 213), but they have not been described. The two are essentially similar except that one is laterally compressed. The more perfect speci- men is shown in Plate 27, figs. 4-6, and in its relations to the lanterns of other representative Echini in text-fig. 207, p. 184. It is important in itself, also because it is typical of all lanterns known from the Palaeozoic formations, others known differing from it only in trifling details. In side view with teeth in place (Plate 27, fig. 6) it is inclined at a very wide angle of about 90 degrees, the width of the top being very nearly twice the total height from the tips of the teeth. This wide angle, which I have described as inclined, is characteristic of all Palaeozoic lanterns, also of the lanterns of young modern regular Echini. The teeth are grooved, meet in a point ventrally, and dorsally extend to a point just above the base of the foramen magnum, so that they are seen in side view. This is evidently the dorsal limit of the calcified portion of the tooth, beyond which the uncalcified portion would doubtless extend in life as in Strongylocentrotus (Plate 5, figs. 6, 9). The pyramids are very wide-angled, with a rather shallow foramen mag- num. Hollows are on either side of the face view, in which the retractor muscles took origin; the sides of the pyramids are curved, admitting of relatively long interpyramidal muscles. The lateral wings of the pyramid are wide, as in Oligoporus (Plate 50, fig. 11), a resultant of the inclined angle of the lantern. A narrow epiphysis caps each half-pyramid, seen in face view (Plate 27, fig. 6), and in dorsal view (Plate 27, fig. 5), it extends inward, narrowing toward the oesophageal cavity. A typical brace rests on and interlocks with the two adjacent epiphyses as shown in ventral, side, and dorsal views. The brace has the usual condyles and other out- lines of modern regular Echini. In one area, D, of figure 5, a compass exists, slightly displaced, and apparently fragments of two other compasses in areas B and J. The compass is arched 302 ROBERT TRACY JACKSON ON ECHINI. as usual, but shows no bifid distal end; probably it was worn off, but possibly no bifid tip existed, as in some Recent Echini. In the ventral view (Plate 27, fig. 4) the lantern is in part covered with the ambulacral plates of the peristome, which alone fill this area in the species and the family. It is interesting to see how near the lantern of this ancient t3^e is to the character of the lantern in young modern regular Echini, as discussed (p. 181), the similarities being in the inclined angle, shallow foramen, and wide spaces for long interpyramidal muscles. FamUy PALAEECHINIDAE M'Coy. Palaechinidae M'Coy, 1849, p. 253; Lambert and Thlery, 1910, p. 119. Palaeechinidae (pars) Loven, 1874, p. 40. Mdonitidae (pars) Zittel, 1879, p. 484; (pars) Duncan, 1889a, p. 15; Jackson, 1896, p. 239. Archaeocidaridae (pars) Duncan, 1889a, p. 8. Mclonechinidac Lambert, 1899a, p. 53; Lambert and Thiery, 1910, p. 119. Meleehidae Lambert and Thiery, 1910, p. 120. Test high, spheroidal, elliptical, or obovate; ambulacra narrow or wide, with from two to twelve columns of plates in each area at the mid-zone. Two pores to each plate, situated in a peripodium. Pore-pairs uniserial, biserial, or multiserial. Interambulacra wide or rarely narrow, with from three to eleven columns of plates in each area at or dorsal to the mid-zone. Plates moderately to very thick, not imbricating, but ambulacral plates bevel over the adradials on the marginal sutures. The base of the corona is resorbed so that there are two plates in the basicoronal row in each interambulacral area, instead of one plate as in the Lepidocentridae. Tubercles and spines are secondaries only, on all plates of the corona, a feature pointed out by M'Coy. Peristome (known only in Melonechinus, Plate 5G, figs. 7, 8) with many rows of ambulacral and some interradially situated non-ambulacral plates. The apical disc is small, the diameter in the specimens measured being about 13 to 25 % of the diameter of the test, usually it is proportionately smaller than in most Recent regular Echini (pp. 87, 104). Ocu- lars are insert, or rarely one or more to all may be exsert without pores as seen from the exterior. Genitals large, high, with from two to five pores each; a madreporite is rarely recognizable. Periproct with many angular plates. Lantern inclined, rarely preserved, but as far as known, with the typical character of the Palaeozoic. The characters of the ambulacra at the mid-zone are the essential basis for the division of this family into genera and in part into species. The characters of the genera in brief are given in the key (p. 207), and their phylogenetic relations are set forth in the systematic table (p. 209) and in text-fig. 237, p. 231. (See pp. 82, 362, 414.) The family Palaeechinidae was formed by M'Coy (1849) to include Palaeechinus and Melonechinus, the only genera then known. He defined the group clearly, but his earlier name has largelj' given place to Zittel's name IVIelonitidae. M'Coy's genus Palaeechinus included species that have since been generically divided with some resulting confusion, and some new types are generically different from any species known to M 'Coy ; therefore it is necessary at this place to give the history of the subject. PALAEECHINIDAE. 303 Pomel (1869, 1883), the first reviser, included in Palaeechinus those species in which am- bulacral plates are all primaries and pore-pairs are uniserial, and listed as belonging to this genus P. quadriserialis Wright, P. ellipticus M'Coy', and P. elegans M'Coy. Duncan (1889a, p. 3) objected to Pomel's methods and would not accept his genera. I sympathize with his feeling, but cannot see that this is any reason for rejecting Pomel's names. Duncan (1889, p. 205) referred species with this structure (plates all primaries and pore-pairs uniserial) to the genus Rhoechinus of Keeping, which was a mistake, as Rhoechinus (here, p. 394, considered a synonym of Lepidechinus Hall) has imbricate plates, thus differing essentially from Palaeechinus.^ Pomel (1869, 1883) further proposed Maccoya as a genus to include species in which am- bulacral plates are alternately enlarged and reduced toward the exterior border, from which result two vertical series of pore-pairs in each half-area. Expressed in other words, the genus Maccoya is characterized by ambulacral plates, all of which reach the middle of the area, but which are alternately primaries and nearly or quite occluded according as they extend to the interambulacrum or not, and with pore-pairs biserial. Pomel's type of this genus is Maccoya gigas (M'Coy), and his observation (1883) was probably based on Baily's (1874, Plate 3, fig. c) figure which is here reproduced as Plate 47, fig. 2. He also and quite correctly included in his Maccoya the Palaechinus burlingtonensis Meek and Worthen and P. gracilis Meek and Worthen. Pomel's (1883) Eriechinus with Palaechinus sphaericus M'Coy as a type (?) , and his Wrightia (1869) (or Wrightella, 1883), with Palechinus phillipsiae Forbes as a type I do not recognize as distinct from Maccoya, because in both the true sphaericus and phillipsiae the ambulacral details and other structures as known are essentially the same as in Maccoya. In brief, I include in Maccoya the species burlingtonensis Meek and Worthen, intermedia Keeping, phillipsiae Forbes, sphaerica M'Coy (but not the sphaericus of Koninck^), gigas M'Coy, and gracilis Meek and Worthen (p. 312). Duncan in his (1889) discussion included in his Palaeechinus part of the species here referred to Maccoya and two species here referred to Lovenechinus. In his Palaeechinus he included P. gigas (p. 304), P. sphaericus, P. ellipticus , P. intermedius, P. phillipsiae, and suggested, quite correctly, that the two North American species (P. burlingtonensis and P. gracilis) belong in the same generic group. Duncan 's (1889) Palaeechinus ellipticus is not the Palaeechinus ellipticus of M'Coy as stated. ' Palaeechinus ellipticus M'Coy (p. 307, Plate 29, fig. 2; Plate 30, fig. 10) belongs in this genus, as stated by Pomel but the specimen in the Museum of Practical Geology, figured by Duncan (1889, p. 206, fig. viii) as ellipticus is not that species, but is here referred to Lovenechinus lacazei (p. 329, Plate 36, fig. 1). ^ Professor Duncan included in his Rhoechinus, R. irregularis Keeping (here referred to Lepidechinus, p. 396) ; R. elegans (M'Coy); R. quadriserialis (Wright); R. sp., a specimen in the Woodwardian (now called Sedgwick) Museum, Cambridge, England. See footnote p. 394. ^ Pomel's (1883) Eriechinus was evidently based not on M'Coy's sphaericus but on de Koninck's (1869, 1870) figure of what he called Palaechinus sphaericus. Pomel says of Ills genus that it differs by the structure of the ape.x, one of the genitals having only a single pore, and this structure is that shown by de Koninck. De Koninck's observations were incorrect, and his specimen was not Palaechinus sphaericus M'Coy, but is here referred to Lovenechinus lacazei (Julien), where it is fully described (p. 330-334, text-figs. 240-243; Plate 35, fig. 7). 304 ROBERT TRACY JACKSON ON ECHINI. The Palaechinus gigas M'Coy (Plate 46, figs. 1, 2; Plate 47, fig. 2) is referable to Maccoya (p. 321); but Duncan's (1889, p. 206, fig. 1) Palaeechinus gigas is doubtless based on the specimen in the Museum of Practical Geology which is here referred to Lovenechinus anglicus sp. nov., as shown on p. 346; Plate 46, figs. 5, 6; Plate 47, figs. 3-5. The new genus Lovenechinus has four columns of plates in each ambulacral area at the mid-zone, two columns of demi-, and two of occluded in each half-area, but without isolated plates; the pore-pairs are biserial. This genus includes Palaechinus lacazei Julien, Oligoporus mutatus Keyes, 0. nobilis Meek and Worthen, 0. missouriensis Jackson, and the two new species anglicus and septies (p. 326). The genus Oligoporus Meek and Worthen is here used in a restricted sense to include species with four columns of ambulacral plates at the mid-zone, two columns of demi-, and two of occluded plates in each half-area, but with, in addition, scattered isolated plates in the middle of each half-area; pore-pairs are multiserial. This is a structural advance on Lovenechinus, and yet not an attainment to the character of Melonechinus. This genus includes Oligoporus blairi Miller and Gurley, 0. coreyi Meek and Worthen, 0. sulcatus Miller and Gurley, 0. halli sp. nov., and 0. danae Meek and Worthen (the type), (p. 351). Norwood and Owen's genus Melonites, now called Melonechinus Meek and Worthen, includes species of the family in which at the mid-zone there are from six to twelve columns of plates in each ambulacral area consisting of demi-plates, occluded plates, and one to four more or less irregular columns of isolated plates in each half-area. This genus represents the final stage of differential evolution in the family, and the species are based largely oh the number of columns of ambulacral plates which they attain. There has never been any question as to what species should be included in this genus, and it is unnecessary to enumerate them (p. 364). Pomel's (1883) genus Xystria was based on Palaechinus (?) konigii, and is here considered under Incertae Sedis (p. 450). Palaeechinus M'Coy. Palaechinus (pars) M'Coy, 1844, p. 171; Pomel, 1883, p. 114; (pars) Lambert and Thiery, 1910, p. 119. Palaeechinus (pars) Loven, 1874, p. 40 (non Palaeechinus Duncan, 1889, p. 204; 1889a, p. 13, see above pp. 303, 304). Rhoechinus (pars) Duncan, 1889, p. 205; 1889a, p. 14 (non Rhoechinus Keeping, 1876, for which see Lepidechinus, p. 294); (pars) Jackson, 1896, pp. 200, 239; (pars) Tornquist, 1897, p. 754; (pars) Klem, 1904, p. 27; (non Rhoechinus, Lambert and Thiery, 1910, p. 122). Palechimis Tornquist, 1897, p. 735. Test spheroidal or elliptical, ambulacra narrow, with two columns of plates in each area. Ambulacral plates are all primaries alike throughout the area, pore-pairs uniserial. Interam- bulacral areas wide, with from four to six columns of plates in an area at or above the mid- zone, two plates in the basicoronal row in each area. Peristome unknown; oculars insert. PALAEECHINUS. 305 imperforate, at least externally; genitals wide, high, each with from two to five genital pores; periproct plated with manj^ small angular plates; lantern unknown, but doubtless as in other genera of the family. (See pp. 82, 191.) Type species, as selected by Lambert and Thiery (1910, p. 119), P. eUipticus M'Coy, of the Lower Carboniferous of Ireland. This genus includes only part of the species referred to it by M'Coy, and the succeeding history is given under discussion of the family. Duncan (1889a, p. 14) referred the genus (which he called Rhoechinus) to his family Archaeocidaridae because there are two columns of ambulacral plates, but in this he showed an entire lack of appreciation of other characters, which unciuestionably associate the genus with the Palaeechinidae not the Archaeocidaridae. Structurally this genus is the most primitive member of the Palaeechinidae because its ambula- crum is like that found in the young, as a developing stage, in the next three genera of the family ; also it is comparable to the young plates seen dorsally as a localized stage in all other genera of the family as set forth diagrammatically in text-figure 237, p. 231. Key to the Species of Palaeechinus. ^ Four columns of plates in each interambulacral area, test elliptical . P. quadriscrialis Wright, p. 305. Five columns of plates in each interambulacral area, test elliptical . . P. cllipticus M'Coy, p. 307. Five columns of plates in each interambuhicral area, test spheroidal . . P. clegans M'Coy, p. 308. Five or usually six columns of plates in each interambulacral area, test nearly or quite spherical P. (?) minor sp. nov., p. 310. *Palaeechinus quadriscrialis Wriglit. Text-fig. 217, p. 191; Plate 29, fig. 1; Plate 30, figs. 1-7. Pdacchinus quadrlseriaJis Wright, ISfin, pp. 62, 03, and 88, 89, Plate 3, figs, la-lc; Pomel, 1883, p. 114; Lambert and Thiery, 1910, p. 119. PalaecUnus eUipticus Baily, 1865a, p. 217; lS65c, p. 65-67, and 88, Plate 3, figs. 2a, 2b; Quenstedt, 1875, p. 380, Plate 75, fig. 42; (non Palaeechinus eUipticus Duncan, 1889, see Lovenechinus lacazei this memoir, pp. 320, 329, Plate 30, figs. 1, 3). Palaeechinus eUipticus (pars) Loven, 1874, p. 41. Palaeechinus quadriscrialis Loven, 1874, p. 41. Rhoechinus quadriscrialis Duncan, 1889, p. 205; Tornquist, 1897, p. 755; Klem, 1904, p. 30. Rhoechinus quadrangularis (by clerical error) Jackson, 1896, table facing p. 242. ' A number of species which have been referred to the genus Palaeechinus but not known well enough to intercalate in the systematic series, or species of which no description has been published, are taken up later. These are Palaechinus (?) konigii M'Coy, Palaeechinus (?) regnyi nom. nov. ( = P. konincki Julien), Palaechinus robineli Julien, and Rhoechinus sp. (used in the sense of Palaeechinus), considered under Incertae Sedis, p. 450. Also Palaechinus agassizi, P. carpentcri, P. loveni, all of DoUo and Buisseret, considered under Nomina Nuda, p. 457; a.nd Palaechinus sp. Mitchell, considered under Palaeozoic. Forms Incorrectly Referred to Echini, p. 458. 306 ROBERT TRACY JACKSON ON ECHINI. The test is high, elliptical; according to Wright, the type measures 51 mm. in height and 38 mm. in diameter. The British Museum specimen measures about 42 mm. in height and about 34 mm. in diameter through the mid-zone. The ambulacra measure 4.5 mm. in width near the mid-zone, and the interambulacra 17 mm. in width. The ambulacra are narrow with two columns of low plates in each area, all of which meet the middle of the area and the interambulacrum ; about five ambulacral plates equal the height of an adambulacral ; pore-pairs are uniserial. Ambulacral plates bevel over the adradials on the marginal sutures. The interambulacra are wide with four columns of plates at the mid- zone in each area. There are two plates in the basicoronal row, above which the added columns come in early, and there is no loss of parts by senescent growth dorsally. Plates of the test are moderately thick, bearing numerous secondary tubercles only; spines are not known. Oculars are all insert, imperforate, and cover the ambulacra and laterally in part the inter- ambulacra on either side. Genitals are high and wide, each known having three genital pores. This species is similar to ellipticus in form, but differs in having four instead of five columns of interambulacral plates. It is also a much smaller species. Baily (1865a, 18G5c) described a specimen of this species as P. ellipticus, as he felt that the difference in the number of inter- ambulacral columns did not entitle it to specific distinction. A difference in the number of interambulacral columns is a valid differential character, and I think these two species should be maintained. Baily's use of ellipticus for quadriserialis was quoted by Loven, and his figure was copied by Quenstedt as given in the synonymy. Lower Carboniferous Limestone, Middleton, County Cork, Ireland. This is the locality of the type, but I do not know where the specimen is; the specimen figured by Baily is from Betty ville, near Crome, County Limerick, Ireland; the specimen in the British Museum Col- lection E 193 is from Rathkeale, County Limerick, Ireland; Hook Head, Ireland, Museum fiir Naturkunde, Berlin, a cast of this specimen is in the Museum of Comparative Zoology Collection 3,117. The British Museum specimen of this species is well preserved, so that it is represented spread out by the Loven method in Plate 30, fig. 3. It agrees entirely with Wright's descrip- tion and excellent figures of the species. It is preserved without distortion and free from all matrix. In parts the plates are in place, as shown in Plate 30, figs. 1, 3, 4, and at these parts the tubercles are shown. Much of the rest is preserved as an internal mold of the plates by the filling matrix, and in the figures is represented in simple outline; again, part is restored as indicated by dotted lines. There are two plates in the basicoronal row in each interambula- crum and three plates in the second row. In all areas excepting C the initial plate of column 3 is hexagonal as usual, as it is in contact with the initial plate of column 4. But in area C the initial plate of column 3 is pentagonal, as the initial plate of column 4 is in the second row above, instead of the next row. Column 4 originates in the third row in four areas with a pentagonal PALAEECHINUS. 307 plate that impinges on the initial plate of column 3, and in virtue of its one side lacking, a com- pensating side is added to this plate making it hexagonal (p. 68). In area C, however, column 4 originates in the fourth row, with the result that the plate on its ventral border is heptag- onal (partially restored as indicated by dotted lines), and the initial plate of column 3 is pen- tagonal. A similar case is shown in area C of Lovenechinus septies (Plate 45, fig. 1) but it is of rare occurrence, as column 4 almost always originates in the next row after column 3. Column 4 originates on the right of the center in areas A, C, and I, but on the left of the center in areas E and G, with some consequent readjustment of plates (compare Ai-chaeocidaris, Plate 9, figs. 6-8; text-fig. 239 bis, p. 264). Above the fourth row no new columns are added, but all four columns extend to the apical disc where the youngest plates on either side are in contact with the oculars. Two oculars are in place, are widely insert, and imperforate. The three genitals are high, wide, and have each three genital pores. All bear secondary tubercles as shown iii Plate 30, fig. 4, in which the absent plates are restored, as indicated by dotted lines. I am under obligation to Dr. Bather for the photograph of this specimen which is repro- duced on Plate 29, fig. 1. *Palaeechinus ellipticus M'Coy. Plate 29, fig. 2; Plate 30, figs. 8-10. PalaccJdnus ellipticus M'Coy, 1844, p. 172, figs. 3a-3cl; Desor, 1858, p. 1.5S; (?) Tornquist, 1S93, p. 103; (pars) Klem, 1904, p. 31; Lambert and Thiery, 1910, p. 119. Echinocriirus ellipticus d'Orbigny, 1849, p. 126, text-fig. 278. Palaeocidaris (Echinocrinus) ellipticus Vogt, 1854, p. 332, text-figs. 212-214. Non Palaechinus ellipticus Baily, 1865a, p. 217; non 1865e, pp. 65-67, Plate 3, figs. 2a, 2b; non Quenstedt, 1875, p. 380, Plate 75, fig. 42.^ Palaeechinus ellipticus (pars) Loven, 1874, p. 41.^ Non Palaeechinus ellipticus Duncan, 1889, p. 200, text-fig. viii; (see p. 308, and this memoir, pp. 326, 329, where Duncan's specimen is referred to Lovenechinus lacazei). Non Palechinus ellipticus Tornquist, 1897, p. 739; non Maccoya ellipticus Lambert and Thiery, 1910, p. 119 (both of these names are based on Duncan's 1889, p. 200, text-fig. viii, Palaeechinus ellipticus, here referred to Lovenechinus lacazei as above stated). The test is high and elliptical, 51 mm. high, 41 mm. in diameter, width of the ambulacrum at the mid-zone 4 mm., width of the interambulacrum 22.5 mm. About seven and one-half ambulacral plates equal the height of an adambulacral. The observations were made on M'Coy's type specimen in Dublin, which I studied through the kindness of Dr. Scharff, who also kindly supplied me with a cast and the photograph here reproduced. The ambulacra are narrow, composed in each area of two columns of low plates, all of which ' These several references to Palaeechinus ellipticus as a synonym are in regard to a specimen that Baily described as that species but is here referred to P. quadriserialis (pp. 305, 306) . 308 ROBERT TRACY JACKSON ON ECHINI. are primaries, meeting the middle of the area and the interambulacral areas. The pore-pairs are uniserial (Plate 30, fig. 10). M'Coy was quite correct in describing the ambulacral struc- ture as he did, and Duncan (1889, p. 200, text-fig. viii) was mistaken when he claimed for this species the structure of four columns of ambulacral plates and a biserial arrangement of pores in an area. Duncan's specimen is in the Museum of Practical Geology Collection 16,301 ; it has no connection with Palaeechinus ellipticus, and is here described as Lovenechinus lacazei (footnote p. 303; p. 329, Plate 36, figs. 1-3). The interambulacrum of ellipticus has five columns of plates at the mid-zone, as shown in two areas of the type; the adambulacral plates are pentagonal, the others hexagonal. In one area (Plate 30, fig. 8) there are two plates in the basicoronal row, three in the second, four in the third, and five in the fifth row. Ambulacral and interambulacral plates alike have numerous secondary tubercles. The holotype is the only specimen of the species known to me, and the numerous figures in text-books are taken from M'Coy's original figures, since when it has not been published from direct observations. This species is the type of the genus as selected by Lambert and Thiery (1910, p. 119). Lower Carboniferous, Millicent, County Clare, Ireland; holotype in the Griffith Col- lection, Science and Arts Museum, Dublin; a cast of this specimen is in the Museum of Comparative Zoology Collection 3,115. The specimen referred to Palaeechinus ellipticus by Baily (1865a, 1865c) and by Quenstedt who copied Baily's figure, has four columns of interambulacral plates, and is here referred to Palaeechinus quadriserialis. The specimen referred to P. ellipticus by Duncan (1889, p. 200) is here referred to Lovenechinus lacazei (p. 329) as above stated. *Palaeechinus elegans M'Coy. Text-figs. 9, p. 54; 15, p. 59; 237, p. 2.31; Plate 29, figs. 3, 4, 6; Plate 30, fig. 11; Plate 31, figs. 1-5. Palarchbms elegans M'Coy, 1S44, p. 172, Plate 24, figs. 2a-2d; Roenier, lS52-'54, p. 2S7, Plate 4, figs. la-Id; Desor, 1858, p. 158; Baily, 1865, pp. 44, 45; 1865b, pp. 63-65, and 67, Plate 4, figs. A-E; Quenstedt, 1875, p. 380, Plate 75, figs. 39, 43; Baily, 1875, p. Ixviii, Plate 36, figs. lla-Ud; A. Agassiz, 1892, p. 73, Plate 29, fig. 2; Tornquist, 1893, p. 103; Lambert and Thiery, 1910, p. 119. Palaeechinus elegans Loven, 1874, p. 41. Palacchinus [elegans] Etheridge, 1874, p. 311, Plate 3, fig. 3. Rhoeckinns elegans Duncan, 1889, p. 205; Jackson, 1896, table facing p. 242 (but not text- fig. 1, p. 205, and not Plate 7, fig. 40'); Tornquist, 1897, p. 757, Plate 21, figs. 6, 7; Klem, 1904, p. 28; Fraipont, 1904, p. 10, Plate 2, fig. 9. I had the opportunity to study M'Coy's type of this species and also other good specimens as described in some detail below; from all of these the species description is made up. Test ' These figures are referable to Maccoya intermedia, see p. 314. PALAEECHINUS. 309 spheroidal, in the type somewhat crushed, but it measures about 55 mm. in width, the ambula- crum at the mid-zone is 5 mm. in width, the interambulacrum 24 mm. in width. The ambulacra are narrow with two columns of low plates in each area. The plates are all primaries, meeting the interambulacra and the middle of the ambulacra throughout the areas; pore-pairs uniserial. Five or six ambulacral plates equal the height of an adambul- acral. There are five columns of interambulacral plates at the mid-zone in each area in all specimens seen. The ventral portion of the test unknown, but it is doubtless similar to that of Palaeechinus quadriserialis, Plate 30, fig. 3. Ambulacral and interambulacral plates with numerous secondary tubercles which bear small spines, swollen at the base, tapering to the tip, all of similar size and measuring about 1.1 mm. in length. Oculars are insert, imperforate (not with two pores as described by Baily) ; genitals are wide, high, with three or more genital pores each. The periproct in the type is remarkably well preserved, with many small angular plates like those of modern cidarids. The apical disc and periproct figured by Baily (1865b, Plate 4, fig. B) were freely and in some respects incorrectly restored (p. 363). Lower Carboniferous, Hook Head, County Wexford, Ireland; slab with two specimens including the holotype in the Griffith Collection, Science and Arts Museum, Dublin; a cast of this specimen is in the Museum of Comparative Zoology Collection 3,116; same locality, Museum of Practical Geology, London, 6,580; Putig, Germany (Tornquist); Marbre Noir de Dinant, Belgium (Fraipont). The most nearly complete specimen known is M'Coy's type, which is one of two specimens of this species on a slab in the Dublin Museum. I had the pleasure of studying it there and in addition. Dr. Scharff had the great kindness to bring it with him to America in order to give me further opportunity for inspection. Besides the two specimens of Palaeechinus elegans, this choice slab bears a specimen of Perischodomus biserialis (Plate 29, fig. 5). Baily (1865b, p. 63) in his observations says that there are three specimens of Palaeechinus elegans, but this is a mistake, the specimens being as stated. One of the specimens of P. elegans is better than the other, and is the basis of M'Coy's and Baily's observations. Ambulacra D and J extend directly to the oculars, and F is preserved in part. The ambulacra bevel over the adambul- acrals laterally (Plate 31, fig. 3). Literambulacrum E is the most nearly complete and shows five columns of plates, but dorsally, column 5 (Plate 31, fig. 4) drops out before reaching the apical disc. The surface of the plates is in part worn, but I have shown the small secondary tubercles in the parts where such occur. Interambulacra E and I (Plate 31, fig. 4) evidently have each five columns of plates, although one of the adradial columns is partially wanting in each of these areas. Dorsally four oculars are in place, but ocular F is imperfect and not shaded in the figure. The oculars are all insert, covering the ambulacra and laterally in part the interambulacra on either side. The oculars are imperforate, and Baily's (1865b, Plate 4, fig. B) figure showing two pores in each plate was a mistake. I cannot see how it occurred. 310 ROBERT TRACY JACKSON ON ECHINI. It has proved unfortunate as Baily's figure has been extensively copied as a typical apical disc for the Palaeozoic. The genitals are high and wide; E and G have each three pores, but C apparently has four pores and A has five genital pores as seen in Plate 29, figs. 3, 6. By over- sight, in Plate 31, figs. 1 and 4, only three instead of five pores are shown in genital A. A number of periproctal plates are in place, they are polygonal in outline, and doubtless filled the area, as shown by Baily in his reconstruction (pp. 89, 173). The second specimen on this slab is much less coinplete, but shows two columns of ambulacral plates in an area with pore-pairs uniserial, also five columns of plates in one interambulacral area. In the specimen in the Museum of Practical Geology (Plate 30, fig. 11; Plate 31, fig. 5) there are two columns of plates in each ambulacral area with pore-pairs superposed, and five columns of plates in an interambulacral area. A number of spines are in place, as described above, and these are of interest as the only ones yet known in the genus. The specimen from Dinant, Belgium, figured by Fraipont, is of about the same size as the type, and has five columns of plates in each interambulacral area. *Palaeechinus ('!} minor sp. nov. Plate 29, figs. 7, 8; Plate 31, fig. 6; Plate 32, fig. 7. The specimens of this species are all internal sandstone molds, so that surface characters are unknown. Test nearly or quite spherical. The holotype (Plate 29, fig. 7) measures 36 mm. in height, and 35 mm. in diameter, but it is somewhat distorted. The Museum of Comparative Zoology's specimen (Plate 29, fig. 8) is perfect in form and measures 37 mm. both in height and diameter through the mid-zone. It is one of the very few Echini which have been found to be actually spherical. The ambulacra are narrow with two columns of low plates in each area. About five am- bulacral plates equal the height of an adambulacral ; pore-pairs are uniserial. 'Wliile this species is apparently a Palaeechinus, it must be remembered, as I have shown (p. 60), that in Maccoya the plates on the proximal side are all primary in character and pore-pairs are uniserial, while on the external or distal side of the same plates the character is primary, and occluded plates and pore-pairs are biserial (Plate 33, figs. 4, 5, 7, 8). As the views of P. minor are all molds of the interior, it may therefore later prove that the species is really referable to Maccoya, but whichever the genus, the interambulacral features entitle it to specific separation from other species of Palaeechinus and from known American forms of Maccoya. In the holotype there are six columns of interambulacral plates in an area (Plate 31, fig. 6). The figure (partially restored ventrally as shown by dotted lines) is made up from two areas. Dorsally columns 6 and 3 drop out, indicating senescence, and a full grown specimen. The oculars are all insert, and genitals are high, with three pores each. MACCOYA 311 In the Museum of Comparative Zoology's specimen (Plate 29, fig. 8) there are six columns of plates in four interambulacral areas, but only five columns in one area. Oculars are all insert. The apical disc measures 8 mm. in diameter equaling about 21% of the diameter of the test. This is relatively larger than in adults of most species of the family, but is the same proportion as in the young of Lovenechinus missouriensis (p. 342). In Mr. Kirk's speci- men (Plate 32, fig. 7) there are six columns of plates in all five interambulacral areas. This species is the only one in America at present referable to Palaeechinus proper. Base of the Burlington Limestone, Lower Carboniferous, Burlington, Iowa, holotype. University of Chicago Collection 9,054; paratypes, Museum of Comparative Zoology Collec- tion 3,051; E. Kirk Collection, one specimen; F. Springer Collection, two specimens. Maccoya Pomel. MacCoya Pomel, 1S(J9, p. xlvi. Wrightia Pomel, 1S79, p. xlvi (printed by error Wrigthia); Pomel, 1883, p. 115; non Agassiz, 1S62, Acalephs, Diseophora, p. .354. Macroija Pomel, 1883, p. 115; (pars) Lambert and Tliiery, 1910, p. 119. Wrifjhtclla Pomel, 1883, p. 115; Lambert and Thiery, 1910, p. 120. Non Erirchinus Pomel, 1883, p. 114 (see footnotes, pp. 303, 312, 32G). Non Tijphlechimis Neumayr, 1889, p. 3(52, text-fig. 82e; 1890, p. 85 (see footnotes, pp. 312, 32(3). Palarrchinus (pars) Duncan, 1889, p. 205; 1889a, p. 13; (pars) Jackson, 1896, pp. 200, 239 (see this memoir, p. 303). Palacchinm (pars) Kiem, 1904, p. 31; (pars) Lambert and Thiery, 1910, p. 119. Palrchiini.i (pars) Tornquist, 1897, p. 735. The test is spheroidal, ambulacra are narrow, composed of two columns of plates in each area, all plates meet the middle of the area, but at the mid-zone alternate plates are primaries, meeting the interambulacral areas and enlarged marginally, while the plates between are nar- rowed laterally and are nearly or quite cut off from contact with the interambulacra by the marginal enlargement of their fellows (Plate 34, fig. 2). There are two pores to each plate ' and the pore-pairs are biserial. All this is as seen on the exterior; on the interior at the mid- zone and throughout the area all plates cross the half-areas and pore-pairs are uniserial (Plate 33, figs. 4, 5, 7, 8). As seen from the exterior, ventrally the ambulacral plates built in youth as a stage in development, and also young plates dorsally near the apical di.sc, as a localized stage, are all typical primaries crossing the half-areas and with pore-pairs uniserial. This is like the condition characteristic of the mid-zone in the lower genus Palaeechinus. The interambulacra in the known species have in each area from four to eight, or excep- tionally nine columns of plates at or above the mid-zone. The peristome and lanterns are 'Miss Klem (1904, p. 31) .says of Palaccliinu.s [Maecoya], four pores in each ambulacral plate. This is a character not known in the genus nor in any Palaeozoic echinoid. 312 ROBERT TRACY JACKSON ON ECHINI. unknown to me in the genus; but Duncan (1889a, p. 14) says the teeth are grooved. Oculars are all insert, or one or more to all may be exsert, being shut out from contact with the peri- proct by the contact of the associated genitals. Genitals are high and wide with several pores each. The periproct is plated with many small thick angular plates similar to those of a Recent cidarid. The type species of the genus is M. gigas (M'Coy) from the Lower Carboniferous of Europe. Maccoya as a genus occupies a place structurally intermediate as regards the ambulacrum between Palaeechinus and Lovenechinus. The youthful and dorsal localized stages of Maccoya are like Palaeechinus at the mid-zone, and also the character of the plates at the mid-zone of Maccoya is like the secondary youthful and dorsal localized stages of Lovenechinus as set forth in the diagram (text-fig. 237, p. 231). Pomel's Wrightella with Palechinus phillipsiae Forbes as the type I do not consider distinct as it has essentially the same structure of the ambulacrum as Maccoya. Duncan's Palaeechinus used in his restricted sense was intended to cover much the same ground as Pomel's Maccoya, but was more inclusive and embraced some structural types which I refer to Lovenechinus (see pp. 303, 304 and footnote, p. 303). He apparently made a mis- take in taking for Palaeechinus, species which were not covered by M'Coy's original descrip- tion, when there were species that did fall within those limits (Duncan's Rhoechinus). At all events Pomel had priority as first I'eviser of the old genus Palaechinus M'Coy. Key to the Species of Maccoya. Four columns of plates in each interanihulacral area (America) M. burliin/toncnsis (Meek and Worthen), p. 312. Four columns of plates in each interambulacral area (Europe) . . M. hitcrrnrdiu (Keeping), p. 314. Five (?) columns of plates in an interambulacral area .... M. phillipsiae (Forbes), p. 316. Five to six or seven columns of phites in each interambulacral area, test robust, plates of moderate size M. sphaerica (M'Coy), p. 317. Six (?) columns of plates in an interambulacral area, the plates verj' wirwise are as nearly parallel as is possible in a curved test, and there is no imbrication. Keeping was certainly mistaken in saying that MACCOYA. 315 the interambulacral plates overlap from below upwards [imbricate aborally] and that the am- bulacrals overlap in the opposite direction. Except for the bevel on the adradial suture I have seen no imbrication of ambulacral or interambulacral plates in this or any other species in this family. The apical disc is relatively small. Ocular plates may be insert, or in one specimen one ocular is exsert, and in another all oculars are exsert (Plate 33, figs. 11, 12). It is very rare for oculars to be exsert in the family (p. 89). The genitals are wide and high, with several up to five genital pores each. The periproctal plates are unknown. This species seems to be indistinguishable from M. burlingtonensis, but in view of the incomplete knowledge of the structure and the wide geographical separation it seems best to maintain both species. Lower Carboniferous, Hook Head, County Wexford, Ireland, several specimens, including the holotype, Sedgwick Museum, Cambridge, England; four specimens, Munich Museum; two specimens. Trinity College, Dublin; Museum of Practical Geology, London, Collection 16,302; Museum of Comparative Zoology Collection 3,002. Of the Munich specimens of this species, one (Plate 33, figs. 6-9) has four columns of plates in one interambulacral area, and also shows the tubercles clearly. The ambulacral detail is clear on the surface with alternate primary and occluded plates, but in a portion showing the interior the plates at this area are all primary in character with pore-pairs uniserial. Another Munich specimen (Plate 34, fig. 1) is worn, but shows the form of the test quite well. There are four columns of plates in two interambulacral areas. In area C in the middle line dorsally there is in addition a single plate which may be considered a tendency toward an extra column as a progressive variation. The only ocular preserved is insert. In the Sedgwick Museum, Cambridge, England, there are seven specimens, including Keeping's holotype. Four of these have four columns of interambulacral plates in an area. The type specimen (Plate 33, fig. 10, after Keeping) is most interesting. It has two columns of interambulacral plates in place and displaced plates representing a third column. The left adradial column is wanting in the more nearly complete area. The ambulacrum is slightly pulled away from the adradial plates on the marginal suture, and reveals the structure graphi- cally (Plate 34, figs. 2, 3). To understand it one must remember that ambulacral plates bevel over the adradials, so that the marginal sutural face of the adradials is an inclined, not a vertical face. On the exterior or distal aspect of this face four facets appear for the reception of the primary ambulacral plates, and these facets in passing proximally become much narrower. On the proximal face of the suture, between the narrowed facets, are seen other alternating facets which receive the impact of the occluded ambulacral plates. This relation is perfectly clear, for on the exterior of the specimen (Plate 34, fig. 2) ambulacral plates are alternately primary and occluded, and pore-pairs biserial, whereas on the interior, as shown in part (Plate 34, fig. 3), ambulacral plates all cross the half-area and pore-pairs are uniserial. Because of this structure it seems that from a single adradial plate one could determine the genus. 316 ROBERT TRACY JACKSON ON ECHINI. Another of the Cambridge specimens (Plate 33, fig. 11) shows four columns of plates in three interambulacral areas; all five oculars are in place, four are insert as usual in the family, but one is excluded from the periproct by the contact of two genitals. The oculars cover the ambulacra and laterally the interambulacra in part on either side, the youngest interambulacral plates being in contact with the oculars. The genitals are high, wide, and three of them have three pores each ; the others, two or no pores, but the pores were probably obliterated in fossil- ization. Still another specimen (Plate 33, fig. 12) has all the oculars in place, and they are all exsert, this as above stated being the only case seen in the family. It is a variation toward the character which is the dominant one in Mesozoic regular Echini, and the youthful (sometimes adult) character of all Recent regular Echini. The genitals are very wide and high and have three or five pores each. Of the two specimens in Trinity College, Dublin, one has four columns of interambulacral plates in an area and both show the typical character of the ambulacrum. The specimen in the Museum of Practical Geology Collection 16,302, shows the ambula- crum and part of an interambulacrum. It is the original of Duncan's (1889) text-fig. ix, p. 206. Maccoya phillipsiae (Forbes). Plate 33, figs. 13, 14. Pdhrhiiius phillipsiae Forbes, 1S4S, pp. 302, .384, Plate 29, figs. 1, la; Torntiuist, 1897, p. 739. Palacchinus phillipsiae Roeiner, 1852-'54, p. 288; Desor, 1858, p. 159; Quenstedt, 1875, p. 379, Plate 75, fig. 41 ; Klem, 1904, p. 35. Wriyhtia phillipsiae (printed by error Wrigthia) Poinel, 1869, p. xlvi. Palaccchinus phillipsiae Loven, 1874, p. 41; Duncan, 1889, p. 205. WrightcUa phillipsiae Pomel, 1883, p. 115; Lambert and Thiery, 1910, p. 120. Form of test unknown, but probably spheroidal. Ambulacra are narrow, with two columns of low plates, all of which meet the middle of the area, but are alternately primaries which meet the interambulacra and occluded plates which fall far short of the same. Pore-pairs biserial. The specimen is badly preserved, and in Forbes's reconstruction (my Plate 33, fig. 14) the occluded plates are probably too narrow and the pores of the same too near the middle of the area. There are three columns of interambulacral plates in place, and Forbes assumed five columns for the area as shown in the reconstruction. This may or may not be correct. Forbes says, found in conglomerate under the Worcester Beacon by Miss Phillips, and gives the horizon as Silurian. Forbes (1848, p. 302) considered the age of this conglomerate as Caradoc, which is Ordovician (Lower Silurian), but with later study it is found to be Llando- very, which is the base of the Silurian. Whether the specimen described came from this hori- zon is open to doubt. A cast of the type specimen is in the Museum of Practical Geology Collection 16,359. The location of the type I do not know. MACCOYA. 317 This species is the type of Pomel's Wrightella, but its characters fit very well into the genus Maccoya, and there seems no occasion for maintaining a separate genus for its reception. Llandovery, Silurian (?), in conglomerate under the Worcester Beacon, England. *Maccoya sphaerica (M'Coy). Plate 32, figs. 4, 5; Plate 34, figs. 4-10. Palaeckinns sphacriciis M'Coy, 1,S44, p. 172, Plate 24, figs. 5a-5c; Desor, 18.58, p. 1.58; Dujartlin and Hiipe, 18G2, p. 464; Paily, 18(i.5c, p. 89; Keeping, 187(5, p. 38; Smitli, 1001, p. 509; Lambert and Thiery, 1910, p. 119. Non Palaechinus sphacricus Koninck, 1809, p. 540, Plate [not ni!nil)ere(l], fig. 1; non Koninck, 1870, p. 2,59, Plate 7, fig. 1; non Baily, 1S74, p. 41; non Zittel, 1879, p. 484; (pars) Loven, 1874, p. 41; non Neumayr, 1881, p. 151; non A. Agassiz, 1892, p. 73; (pars) Klem, 1904, p. 35.i Palaccchinus sphacricus (pars) Loven, 1874, p. 41; Duncan, 1889, p. 19(i, text-figs, ii-vii. Non Ericchinus sphacricus Pomel, 1883, p. 114.^ Non Ti/phlcchinus sphacricus Neumayr, 1889, p. 362, text-fig. 82c.' Non Palechinus sphacricus Tornquist, 1897, p. 736.' Palaechinus sphericus (pars) Klein, 1904, pp. 34, 35. M'Coy's type of this species is in Dublin (Plate 32, fig. 5; Plate 34, fig. 7). In addition, there are three specimens in the British Museum (Plate 32, fig. 4; Plate 34, figs. 4-6), three in the Sedgwick Museum, Cambridge, England, and one in the Strassburg Museum (Plate 34, figs. 8, 9), all of which are similar and are referred to sphaerica. M'Coy's type is almost spherical, but the height has to be estimated, as the dorsal part is covered by a lump of the original matrix. Of this specimen the circumference at the mid- zone measures 203 mm.; the diameter, in the same plane, 65 mm.; height, approximately 65 mm. ; width of ambulacrum B at the mid-zone, 6 mm. ; width of an interambulacrum about 34 mm. Ambulacra are narrow, rounded up in the center, so that the greatest elevation is in line with tlie curvature of the interambulacra. Ambulacrum B is the only one in which the surface of the plates is shown, other areas being internal molds for the most part. The details of plate structure are difficult to ascertain, but Plate 34, fig. 7, was drawn at the point marked X in Plate 32, fig. 5. As seen here, all ambulacral plates meet the middle of the area, but against the interambulacrum alternate plates are primaries, marginally expanded, and intermediate plates are narrowed and just cut off from contact with the interambulacrum. Pore-pairs are biserial. In the British Museum and other specimens all ambulacral plates usually meet the interambulacrum, though alternate plates are narrowed, and pore-pairs are biserial. ' These several references in the above paragraphs .are all in regard to the specimen described by de Koninck as Palaechinus sphaericus from Kirkby Stephen, Westmoreland, England. The specimen has a very different structure from that of Maccoya sphaerica (M'Coy), and hkewise different from that of the genera Palaeechinus and Maccoya. It is here described aa Looenechinus lacazei (Julien), footnotes, pp. 303, 312, :?26, also pp. 330-334, text-figs. 240-243; Plate 35, fig. 7. 318 ROBERT TRACY JACKSON ON ECHINI. In the type there are six or seven columns of plates in an interambulacral area at the mid-zone. In area A there are six columns, and below the mid-zone the introduction of the fifth and sixth columns is seen, both originating as usual with a pentagonal plate. By this means the specimen was oriented, and it will be noticed that the orientation of my figure is the reverse of that given by M'Coy. In this area. A, the six columns extend dorsally to the covering matrix. In interambulacrum C there are seven columns at the mid-zone. The seventh column consists of four plates only, on one of which the letter C occurs in Plate 32, fig. 5. Ventrally and dorsally to the seventh there are six columns in this area. Areas E and G are imperfect, for the most part internal molds, and the full nunil^er of columns was not ascertained. Interambulacrum I is also an internal mold, but the impress of six columns of plates was clearly ascertained at the mid-zone. In one of the British Museum specimens (E 3,652) there are seven columns of plates in the only .interambulacral area clearly preserved. Of two other British Museum specimens (E 361) on a single slab, one has five columns of plates in all five inter- ambulacral areas (Plate 32, fig. 4), whereas the other has five columns in one area, but six in two other areas. On account of the matrix covering the dorsal area, the features of this part are unknown in the type. In the British Museum specimen (Plate 34, fig. 6), the apical disc is small, the oculars are all insert, genitals wide, low (pores not visible), and the periproct has numerous small angular plates as usual in the Palaeozoic. M'Coy's type is in Trinity College Museum, Dublin, where I studied it through the kind- ness of Professor Joly. There is no locality given, but it bears the label, "Presented by Dr. A. N. Fox, Staff Assistant Surgeon, 1 Nov., 1867." For several years earlier it had been in the possession of the Rev. S. W. Fox. According to Baily (1865c, p. 89) the, specimen was supposed to have come from the limestone of the County Kildare, but that was not certain. A plaster cast of this valuable specimen is in the Museum of Practical Geology in London, and another in the Museum of Comparative Zoology. Confusion has been introduced as to the structure of this species by de Koninck's (1870) description of a specimen which he referred to Palaechinus sphaericus M'Coy. Through the great kindness of my friend Dr. F. A. Bather, the specimen was borrowed from the York Museum, and it proves that the description was incorrect in several particulars. The specimen is not a Maccoya sphaerica, and does not even belong to the genus Maccoya, having a very different ambulacral structure. It is here referred to Lovenechinus lacazei (text-figs. 240-243, p. 331; Plate 35, fig. 7). From this it occurs that statements in regard to the absence of oculars (entirely wrong), structure of the genital plates (also incorrect), and the number of inter- ambulacral columns ascribed to sphaerica, as far as based on de Koninck's observations, are erroneous (footnote, p. 303). Lower Carboniferous, probably County Kildare, Ireland; on the authority of Baily MACCOYA. 319 (1865c, p. 89) this is the probable locaHty of the holotype which is in the collections of Trinity College, Dublin; County Kildare, Ireland, British Museum Collection E 3,652; Waterford, County Wexford, Ireland, British Museum Collection E 361 ; Clitheroe, Lancashire, British Museum Collection E 4,353; Clitheroe, Lancashire, Sedgwick Museum, Cambridge, Collection 17; Hook Head, County Wexford, Ireland, Sedgwick Museum Collection nos. land 5; Whatley, near Frome, Somerset, Strassburg Museum; East Burn, northeast of Stewarton, Scotland (Smith). Taking up the specimens attributed to Maccoya sphaerica, a specimen in the British Museum Collection E 3,652, is from Kildare, and is free from matrix, but not very clearly pre- served. It measures about 50 mm. in height and 52 mm. in diameter. The ambulacra are narrow, about 6 mm. in width at the mid-zone. The interambulacra are about 26 mm. in width. The ambulacral areas are rounded outwardly as in the type, but details of plates are obscure. Pore-pairs are alternate and plates are apparently alternately primary and occluded. In the only good interambulacral area tlicre are seven columns of plates at the mid-zone. This speci- men in structural detail resembles M'Coy's type more closel^y than anj' other specimen I have seen. It also resembles it very closely lithologically, which lends weight to Baily's statement that the type is probablj' from Kildare, which is the source of this individual. In the British Museum Collection E 361, is a limestone slab from Waterford, Ireland, which bears two specimens, one of which is one of the finest of known Palaeozoic Echini. The slab measures 200 mm. long by 125 mm. in width. The more nearly perfect specimen is shown in Plate 32, fig. 4, and Plate 34, figs. 4-6. The test is spheroidal, somewhat crushed laterally. The ambulacra at the mid-zone measure about 7 mm., the interambulacra 25 mm. in width. From these measurements the circumference would be about 160 mm. and the diameter about 50 mm. Ambulacral areas are narrow, with two columns of low plates in each area. All plates meet the middle of the area, and at the mid-zone most of the plates reach the inter- ambulacra, but alternately they are widened and narrowed marginally, the narrowed plates being nearly or occasionally cjuite excluded from contact with the interambulacrum. A few completely occluded plates occur in each of these specimens, though none is shown in Plate 34, fig. 4, the ambulacra in which are drawn somewhat diagrammatically with certain oversights. It was this specimen on which Professor Duncan (1889, p. 200, text-figs, ii-vii) based his ob- servations of the structure of the ambulacrum in this species. In his text-figs, vi, vii, small demi-plates are shown which he letters as a. These plates, having been overlooked, are not shown in my figvu'es, but nine such plates were counted in the best specimen (Plate 32, fig. 4). Such plates are not a character of this species or genus, and are to be considered as excep- tional progressive variants. At the mid-zone the pore-pairs are biserial, the outer pore of one plate being about in line with the inner pore of the next succeeding plate. Dorsally, near the oculars, as a localized stage, all plates are of equal height, meet the interambulacra fully, and 320 ROBERT TRACY JACKSON ON ECHINI. pore-pairs are uniserial. The structure of the ambulacrum at the mid-zone is intermediate between that of Palaeechinus and Maccoya, and tlie alternate plates are not usually so com- pletely cut off from the interambulacrum as was observed in M'Coy's type specimen. The inter- ambulacra are wide (Plate 34, fig. 4), with five columns of plates in each area. In the second specimen, on the same slab, there are five columns in one interambulacral area and six columns in two other areas. The plates are hexagonal, or pentagonal in the adradial colunms. The hexagons are much wider than high, one at the mid-zone measuring 7 mm. in witlth and 5.5 mm. in height. The ventral area is unknown. Ambulacral and interambulaci-al plates bear numerous small secondary tubercles, and in the second specimen there are in place small, tapering spines, swollen at the base, and about 3 mm. in length. All five oculars are in place. They are all insert, cover the ambulacra and laterally the interambulacra in part on either side. The genitals are wide, apparently very low for this family, but the ventral borders seem to be pushed under the interambulacra in part, which accounts for the height, and I think also for the absence of visible genital pores. The periproct is exceptionally perfect, the most nearly complete one known in the family. It differs slightlj' in Plate 34, figs. 4 and 6, but the latter I drew with all care, and it is to be considered a corrected drawing. The plates are numerous, small, angular, rounded up, so that they are higher in the center of each plate than on its sutural margin. They are strikingly close to the character of the same plates in the Recent Cidaridae, and may well stand as representing the typical Palaeozoic periproct, as this is the clearest and most nearly complete specimen in regard to this structure known to me from these ancient formations. This British Museum specimen E 361, is the one from which Duncan (1889, p. 196) described the apical disc with oculars reaching the periproct and with periproctal plates in place. Duncan's second specimen which he says is the same species, and describes as having the oculars excluded from the periproct, may have been the specimen in the British Museum (which was labeled P. sphaericus) which I refer to Lovenechinus lacazei (p. 328; Plate 36, fig. 6). A specimen in the British Museum Collection E 4,353, from Clitheroe, Lancashire, is frag- mentary. The interambulacral plates are very large, 11 mm. wide by 8 mm. high, representing a very large individual. Ambulacral plates are preserved only in fragmentary clusters of connected plates, but they are typically alternately primary and nearly, or in some plates quite, occluded, with pore-pairs biserial. The Strassburg specimen of M. sphaerica from Whatley, consists of ambulacral and more or less dissociated interambulacral plates on a limestone slab. All the ambulacral plates meet the middle of the area, and also the interambulacrum, but alternate plates are enlarged or narrowed marginally (Plate 34, fig. 8), the latter not being quite cut off from interambulacral contact, in this respect being like the British Museum specimen E 361. The pore-pairs are biserial. As seen from within (Plate 34, fig. 9), all ambulacral plates are alike, of equal height, and pore-pairs are uniserial. This is the same relation as is shown between the inner and MACCOYA. 321 outer aspect of ambulacral plates in Maccoya intermedia (Plate 33, figs. 7-9). The larger hexagonal interambulacral plates measure 7 to 7.4 mm. in width, and 5.8 to G mm. in height, so that they are distinctly wider than high. They have numerous secondary tubercles like those of Plate 32, fig. 4. Of the specimens in the Sedgwick Museum, Cambridge, England, that from Clitheroe, no. 17, from the Aitkin Collection, consists of portions of ambulacra and dissociated interambula- cral plates. Of the ambulacrals, all plates cross the half-areas, but alternate plates are partially pinched off marginally, and pore-pairs are biserial, as in Plate 34, fig. 8. Another specimen, from Hook Head, Ireland, no. 1, is the original specimen referred to by Keeping (1876, p. 38). What he described as an ocular plate with two pores, lies above an interambulacrum, though it is not in place. In my opinion it is not an ocular, but a genital, high, as usual, and with three pores, not two. There are some ambulacral plates in place and five columns of inter- ambulacral plates, consisting of one column of pentagonal adradial, and four columns of rather low hexagonal plates. One or more columns are evidently wanting. A third specimen from Hook Head, no. 5, from the Walton Collection, consists of a portion of an interambulacrum, with four columns of plates in place, one column of pentagonal adradial and three columns of low hexagonal plates, which measure about 7.5 mm. in width by 5 mm. in height. * Maccoya gigas (M'Coy). Plate 46, figs. 1-3; Plate 47, figs. 1, 2. Palaechinm gigas M'Coy, 1844, p. 172, Plate 24, figs. 4a-4c; Dujardin and Hupe, 1862, p. 463; Baily, 1874, p. 41, Plate 3, figs, a-d; Quenstedt, 1875, p. 380, Plate 7.5, fig. 40; Keeping, 1876, p. 38, Plate 3, figs. 12, 13; Julien, 1896, p. 130; (pars) Klem, 1904, p. 32. MacCoya gigas Pomel, 1869, p. xlvi. Palaeechinus gigas Loven, 1874, p. 41 (non Duncan, 1889, p. 198, text-fig. i; non Jackson, 1896, p. 204, Plate 7, figs. 38, 39 1). Maccoya gigas Pomel, 1883, p. 115; Lambert and Thiery, 1900, p. 119. Non Palechimis gigas Tornquist, 1897, p. 739, which is based on Duncan's Palaeechinus gigas.' The cotypes of this species which are in the Science and Arts Museum, Dublin, are the basis of M'Coy's and Baily's observations and also of my diagnosis. The whole test is unknown, but the parts which are known indicate that it was high, spheroidal. M'Coy's restored figure is, I think, much too low. Ambulacra are relatively narrow, about 10 mm. in width, and as shown by Baily, are composed of two columns of low plates, all of which meet the middle of the area, but marginally alternate plates are primaries, reaching the interambulacra and enlarged at ' Duncan's and Jackson's observations were apparently both based on the specimen in the Museum of Practical Geology which has a different ambulacral structure from Maccoya, and is here referred to Lovenecldnus anglicus sp. nov. (pp. 304, 346, Plate 46, figs. 5-6; Plate 47, figs. 3-5). 322 ROBERT TRACY JACKSON ON ECHINI. that point, the plates between being narrowed marginally, occluded, and cut out from interam- bulacral contact by the enlargement of their fellows; pore-pairs biserial (Plate 47, fig. 2). This is the typical generic character. On the interior of the test all ambulacral plates are of about equal height and cross the half-area completely, as shown in a specimen from Rahan's Bay, in the Sedgwick Museum, Cambridge, England, no. 3, which also shows that the ambulacral plates bevel over the adambulacrals on the marginal suture, a structure similar to that shown in Maccoya intermedia (Plate 34, figs. 2, 3). The interambulacra of the type (Plate 46, figs. 1, 2) are more nearly complete than those of any other known specimen of the species. This specimen is bent sharply on itself, as on a hinge, so that on one side (Plate 46, fig. 1) is seen an ambulacrum and three adjacent inter- ambulacral columns, and on the reverse side (Plate 46, fig. 2) is seen an adradial pentagonal column and an adjacent column of hexagonal interambulacral plates. This structure is shown clearly in Baily's (1874, Plate 3, figs, a, b) excellent figures. M'Coy in his restoration assumed six columns in an area, which is very likely correct, but there maj' have been fewer, or possibly more than that number. The plates are very large and relatively high in proportion to their width. The pentagonal adambulacral plates measure about 12 mm. in width by 9 mm. in height; the hexagonal plates measure about 14 mm. in width by 9 mm. in height. These plates bear numerous secondary tubercles, as shown better ;n M'Coy's second type (Plate 46, fig. 3). The characters of the ventral and apical areas are unknown. This species is the type of the genus. Arenaceous shale, Lower Carboniferous, Rahan's Bay, Dunkineely, County Donegal, Ireland, cotypes, two specimens, in the Griffith Collection, Science and Arts Museum, Dublin; same locality, Sedgwick Museum, Cambridge, England, Collection 3 and 266; Lower Carbonif- erous shale, Drumlug, County Donegal, Ireland, Trinitj' College Museum, Dublin; Regny, Central France (Julien). The specimen from Drumlug in the Trinity College Museum shows an ambulacrum very clearly, in which all plates meet the middle of the area, but alternate plates are primary and occluded, meeting or being cut off from the interambulacrum ; pore-pairs are biserial, as in M'Coy's type. There is a column of large pentagonal adradial plates in place with the ambu- lacrum and in addition scattered interambulacral plates. One large adradial plate measures 16 mm. in width and 10 mm. in height. The specimen in the Sedgwick Museum Collection no. 3, is an interesting individual of this species from Rahan's Bay, the type locality. This is the original of Keeping's (1876) Plate 3, fig. 13, as stated on the old and somewhat faded label. His figure, however, is somewhat inadequate. The specimen is a slab on which there are portions of an ambulacrum and ten more or less perfect and dissociated interambulacral plates. The ambulacral plates are in three short, but continuous pieces. All plates meet the middle of the area, but alternate plates MACCOYA. 323 meet and are cut off from interambulacral contact (Plate 47, fig. 1); pore-pairs are biserial, as in M'Coy's type. The ambulacral plates bevel strongly over the adradials laterally. Adradial plates on the inclined marginal face show one set of facets distally for articulation with the primary ambulacral plates, but, passing proximally on the inclined face, these facets become narrower and other facets appear between them, so that proximally there are facets for each arnbulacral plate of the area, as explained in detail in the case of Maccoya intermedia (p. 315, Plate 34, figs. 2, 3). The adradial plates of this specimen measure 8 mm. in width and the same in height, hexagonal plates 10 mm. in width and 8 mm. in height. Another specimen from Rahan's Bay, in the Sedgwick Museum, no. 266, consists of two large hexagonal interambu- lacral plates with good tubercles. The larger of these plates measures 12 mm. in width, 9 mm. in height. The size and locality indicate that it belongs to this species. *Maccoya gracilis (Meek and Worthen). Plate 34, figs. 11, 12; Plate 35, figs. 1-3. Palaechinus gracilis Meek and Worthen, 1869, p. 82; 1873, p. 473, Plate 10, fig. 2; Keyes, 1895, p. 180. Palaeechinus gracilis Loven, 1874, p. 41; Duncan, 1889, p. 205. Maccoya gracilis Pomel, 1883, p. 115; Lambert and Thiery, 1910, p. 120. Rhorchinus gracilis Jackson, 1896, p. 201, Plate 7, figs. 36, 37; Torntiuist, 1897, p. 762; Klem, 1904, p. 29. The test is small, high, spheroidal. In the holotype (Plate 35, fig. 1), the width of the ambulacrum at the mid-zone is 5.5 mm. ; width of the interambulacrum about 19.5 mm. From these measurements the circumference would be about 130 mm. and the diameter about 41 mm. The ambulacra are narrow, composed of two columns of plates, all of which meet the middle of the area, but on the margin alternate plates are primaries, meeting and enlarged at the inter- ambulacral contact; the plates between are occluded, marginally narrowed and nearly or quite cut off from interambulacral contact by the enlargement of their fellows. Pore-pairs biserial, the outer pore of the inner pair about in line with the inner pore of the outer pair. Interambulacra are wide, in the holotype (Plate 34, fig. 11) with seven columns of plates at the mid-zone in the only area that is at all complete in this specimen. The adambulacral plates are pentagonal and those of intermediate columns hexagonal. Dorsally, the seventh column is interrupted, there being one row of six plates built, above which the seventh column appears again, with a pentagonal plate having a heptagon lying on its left lower border. Am- bulacral and interambulacral plates alike bear small secondary tubercles. Another specimen from Burlington in the Museum of Comparative Zoology (3,009) is a larger individual, but fragmentary and represented by an internal impression. There are six columns of interambulacral plates at the mid-zone. This specimen was incorrectly referred to burlingtonensis by me (Jackson, 1896, p. 204), as I was misled by the original label which bore that name. 324 ROBERT TRACY JACKSON ON ECHINI. In the Kentucky specimen referred to this species (Plate 34, fig. 12) there are seven columns of interambulacral plates at the mid-zone in each area, but an eighth column originates dorsally in each area, and in area C a ninth column is represented by a few plates near the apical disc. The ventral portion is unknown in the species and the dorsal portion is unknown in the type, but in this specimen, which is an external sandstone mold, impressions of insert oculars and wide, high genitals are visible dorsally (Plate 35, fig. 3). Upper Burlington Limestone, Lower Carboniferous, Burlington, Iowa, holotype, from the Wachsmuth Collection 407, in the Museum of Comparative Zoology Collection 3,052; Lower Burlington Limestone, same locality and Museum 3,009; Waverly Group, Lower Car- boniferous, Menifee County, Kentucky, two specimens on one slab. Museum of Comparative Zoology Collection 3,062. The specimens from Menifee County, Kentucky (Plate 34, fig. 12; Plate 35, figs. 2, 3) are the only ones of this species known excepting those from Burlington as described above. The better of the two is figured. The specimen is an external sandstone mold of the dorsal side, therefore in the figures the orientation is reversed from what it would be if viewed from the exterior. The ambulacra are narrow, 5 mm. in width at the widest part, which is narrower than in the type, but the specimen is smaller, and the interambulacra at the widest part are 14 mm. across. Details of ambulacral plates cannot be ascertained, and as restored by dotted lines in Plate 34, fig. 12, are doubtless given incorrectly, but this figure was drawn in 1895 when I had not studied the type. Interambulacrum A (Plate 34, fig. 12) is the most nearly com- plete area; ventrally there are five columns of plates, above which the sixth and seventh col- umns are introduced with pentagonal plates; farther dorsally, an eighth column comes in at the left of the center with a heptagonal plate on its right ventral border (just the reverse as seen from the exterior). In the several other areas there are seven columns of plates in the most adoral portion preserved, and farther dorsally in each area, an eighth column is introduced with a pentagonal plate. In area C the eighth column appears somewhat earlier than in the other areas, and near the apical disc a ninth column is represented by three small plates. This specimen occurs in the Waverly Group, which is somewhat older than the Burlington Limestone, but I think the specimen is referable to M. gracilis. The only difi'erences seen are the somewhat narrower ambulacra and the eighth column of interambulacral plates dor- sally. The type might have shown this additional column if it were complete to the apical disc. LovENECHiNUS gen. nov. Test spheroidal, ambulacra relatively narrow, with four columns of plates at the mid- zone, which consist of two columns of narrow demi-plates and two columns of wider occluded plates, without intercalated isolated plates. The wide occluded plates are roundly arched upward in the center, and this is the beginning of that dominating prominence of the two LOVENECHINUS. 325 occluded columns, which is a marked feature of the next genus, Oligoporus, and especially most species of Melonechinus. The pore-pairs are biserial in sunken valleys on either side of the ambulacral areas, and the pores are situated in that portion of each plate nearest the next adjacent interambulacrum. All this as seen from the exterior; on the interior (Plate 43, figs. 4, 5) demi- and occluded plates are of about equal width, and pore-pairs lie near the middle line of each half-area, also demi-plates which lie opposite horizontal interambulacral sutures are higher and fan-shaped. Ventrally, next the peristome (Plate 42, fig. 1), ambulacral plates built in youth are all primaries, with pore-pairs uniserial (as in adult Palaeechinus). Above this zone plates are primary and occluded, with pore-pairs biserial (as in adult Maccoya), before attaining the character plates of the genus. Dorsally, a comparable condition occurs next the ocular in the placogenous zone (Plate 42, fig. 3) ; here ambulacral plates are primaries with pore-pairs uniserial. Passing ventrally, the older plates are alternately primary and occluded as localized stages in development, before they acquire the full characters of the genus. Ambulacral plates bevel over the adradials laterally, and bear small secondary tubercles and spines only. The interambulacra are wide, with from four to seven columns of plates in an area at the mid-zone in the known species, composed of pentagonal adradial, and hexagonal median plates. There are ventrally two plates in the basicoronal row, above which additional columns come in as usual in the family (Plate 45, fig. 1). There is no imbrication in ambulacral or inter- ambulacral plates, but the adradials bevel under the ambulacrals laterally. Interambulacral and ambulacral plates bear secondary tubercles and spines only. Peristome and jaws are unknown ; but they were doubtless similar to those of Oligoporus and Melonechinus. Oculars are usually all insert but one or more may be exsert. They are imperforate as seen from the exterior, but indications of a pore on the interior have been seen in some cases (Plate 41, fig. 2; Plate 42, fig. 6). Genitals are high, wide, with from three to five genital pores each; a madreporite has apparently been observed in one specimen (p. 334; text-fig. 243, p. 331). Small angular periproctal plates, as usual in the family, have been seen in one specimen (Plate 42, fig. 6). The type species is Lovenechinus missouriensis (Jackson). This genus differs from Maccoya in that ambulacral plates are demi- and occluded instead of primary and nearly or quite occluded. It differs from Oligoporus in that there are no scattered isolated plates in the middle line of each half-area as in that genus, also as a correlated character in that pore-pairs are biserial instead of multiserial. Lovenechinus in its development passes through a stage like the adult of Maccoya, and Oligoporus in its development passes through a stage like the adult of Lovenechinus (text-fig. 237, p. 231). It is thus a type structurally and phylogenetically inter- mediate between Maccoya and Oligoporus. Lovenechinus includes certain species previously referred to either Palaeechinus or Oligoporus, with two new ones (p. 304). 326 ROBERT TRACY JACKSON ON ECHINI. I take pleasure in naming this genus in memory of the late Sven Loven, past master in the study of the structure of Echini, and whose methods of work have been of incalculable service to me. Key fo the Species of Lovcruchinus. Four columns of plates in an interanibulacral area . . . . . L. lacazci (Julien), p. 326. Five columns of plates in an interambulacral area; occluded and demi-plates meeting in serrated suture L. midafm (Keyes), p. 335. Five columns of plates in an interambulacral area; occluded and demi-plates meeting in interlocking tongue-like suture; test massi\'e ...... L. nobills (Meek and Worthen), p. 335. Five, or in part, occasionally, six columns of plates in an interambulacral area; occluded and demi-plates meeting in tongue-like suture ; test not massive as in Hoti//* . . . i. mmoMriewsw (Jackson), p. 337. Six (or possibly more) columns of plates in an interambulacral area; the plates are very wide and high, but not thick; occluded and tlemi-plates meeting in tongue-like suture . . L. anglicus sp. nov., p. 346. Seven columns of plates in an interambulacral area; occluded and demi-plates meeting in serrate suture L. septics sp. nov., p. 34S. *Lovenechinus lacazei (Julien). Text-figs. 240-243, p. 331; Plate 35, figs. 4-7; Plate 36, figs. 1-8; Plate 38, figs. 1-3. ' Palaechinns sphaericus Koninck, 1869, p. 546, Plate [not numbered], fig. 1 ; 1870, p. 259, Plate 7, fig. 1 ; Baily, 1874, p. 41; Zittel, 1879, p. 484; Neumayr, 1881, p. 151; A. Agassiz, 1892, p. 73. ^ Patarechinus sphaericus (pars) Loven, 1874, p. 41. ^ Eriechinus sphaericus Pomel, 1883, p. 114. ' Typhlechinus sphaericus Neumayr, 1889, p. 362, text-fig. 82e. ^ Palaeechinus ellipticus Duncan, 1889, p. 200, text-fig. viii (non M'Coy, 1844). Palaechinm lacazei Julien, 1890, p. 737; 1896, p. 128, Plate 16, figs. 3-5; Fraipont, 1904, p. 9, Plate 1, fig. 8; Plate 2, figs. 1-6; Klein, 1904, p. 34. ^ Palechinus sphaericus Tornquist, 1897, p. 736. ^ Palechinus ellipticus Tornquist, 1897, p. 739. Palechinus lacazei Tornquist, 1897, p. 742, Plate 20, figs. 1-4; Plate 21, fig. 1. ^ Palaechinus sphericus (pars) Klem, 1904, p. 34. PaUwehirms sp. Fraipont, 1904, p. 9, Plate 2, figs. 7, 8. ^ Maccoya ellipticus Lambert and Thiery, 1910, p. 119, (this is based on Duncan's, 1889, description and figure). Maccoya lacazei Lambert and Thiery, 1910, p. 120. ' These references are all in regard to the specimen from Kirkby Stephen (text-figs. 240-243), described by de Koninck as Palaechinus sphaericus. He stated that the oculars are wanting, a mistake which has proved unfortunate as on this character were based the two genera Eriechinus and Typhlechinus. As these two genera were based on an erroneous ob- servation, and their types are not the species supposed, they are considered out of court as generic names (see footnotes pp. 303, 312). The Palaechinus sphaericus M'Coy (1844, p. 172, Plate 24, figs. 5a-5c) is an entirely distinct species and is here considered as Maccoya sphaerica, pp. 317, 318; Plate 32, figs. 4-5; Plate 34, figs. 4-10. * These references are all in regard to the specimen from Helsington Barrows in the Jermyn St. Museum, described by Duncan (1889, p. 200, text-fig. viii) as Palaeechinus ellipticus; but here referred to Lovenechinus lacazei (footnote p. 303, p. 328, Plate 36, figs. 1-3). The Palaechinus ellipticus M'Coy (1844, p. 172, Plate 24, figs. 3a-3d) is an entirely distinct species and is here considered on p. 307, Plate 29, fig. 2, Plate 30, figs. 8-10. LOVENECHINUS. 327 This species includes specimens of various sizes and many localities, but as the structure in all is essentially the same, as far as known, I consider them a single species. The specimens from Llysfaen (Plate 35, fig. 6; Plate 36, figs. 7, 8) and from Kirkby Stephen (Plate 35, fig. 7; text-figs. 240-243, p. 331) are much larger than the other known specimens. It is possible that these two may prove to be a closely related but distinct species. As they show no structural differences, however, I consider them the same as the smaller specimens. Test spheroidal, or in one case moderately elliptical. Ambulacra narrow, composed in each area of four columns of plates, which are narrow demi- and wider occluded plates in each half-area, pore-pairs in that portion of each plate nearest the next adjacent interambula- crum. Ambulacral plates low, covered with minute tubercles. From six to eight (or in the Kirkby Stephen specimen five and a half) demi-plates equal the height of an adradial. Ven- trally ambulacral plates are primaries with pore-pairs uniserial (Plate 36, fig. 2; Plate 38, fig. 2). The interambulacra in all known specimens have four columns of plates in an area at the mid-zone, the adradials being pentagonal, the median hexagonal. These plates bear minute tubercles and corresponding spines, the latter known only in the Kirkby Stephen specimen. The oculars as far as known (Plate 36, fig. 6 ; text-fig. 243) are small and fully exsert, an excep- tional Palaeozoic character (p. 89). Genitals are wide, high, with from two to four genital pores each (Plate 36, figs. 4, 6, 7; text-fig. 243). This last figure apparently shows a madreporite as observed by Dr. Bather. Periproctal plates are not known. Lower Carboniferous, I'Ardoisiere, Central France, cotypes in the possession of the family of the late Professor Julien; Hunsriicken, Germany (Tornquist) ; Hook Head, County Wexford, Ireland, Science and Arts Museum Collection, Dublin; Helsington Barrows, southwest of Kendal, Westmoreland, Museum of Practical Geology Collection 16,301; Armagh, Ireland, two specimens, British Museum Collection E 10,051, and E 10,052; Llysfaen, near Llandulas, Wales, British Museum Collection E 3,432; Kirkby Stephen, Westmoreland, England, York- shire Philosophical Society Museum Collection; Marbre Noir de Dinant, Belgium (Fraipont). I will consider now the several recorded specimens, with the principal features of each. Professor Julien's types from I'Ardoisiere, as gathered from his figures and description, have four columns of ambulacral plates in an area, wide occluded and narrow demi-plates, which latter alone reach the interambulacra. Two pores are in each plate, situated near the marginal suture. Ambulacral plates are low, six equaling the height of an adambulacral plate. Inter- ambulacra with four columns of plates in an area, the plates being pentagonal in adradial col- ums, and hexagonal in the median columns. According to Julien, the adradial plates measure 6 mm. in width by 5 mm. in height, the hexagonal 7 mm. in width by 5 mm. in height. They do not show any imbrication. Ambulacral and interambulacral plates alike with numerous small granulations, which Julien says do not merit to be called tubercles, but I do not see that 328 ROBERT TRACY JACKSON ON ECHINI. they are at all different from the small secondary or, possibly better, miliary tubercles in other species of this family. The dorsal portion is not figured or described by Julien. Tornquist next took up this species and published a detailed description of his specimens, with excellent figures, including a good restoration of the whole test. He shows (my Plate 38, figs. 1-3) that there are four columns of ambulacral plates as the species character, wide occluded and narrower demi-plates, with pore-pairs in peripodia, biserial, situated near the interambula- crum on the exterior, but near the middle of the half-area on the interior of the test. He shows that ventrally ambulacral plates are primaries, of equal width, with pore-pairs uniserial. There are four columns of interambulacral plates in an area, and both these and the ambulacrals bear small secondary tubercles. In the British Museum there are two specimens clearly referable to this species, from Armagh, Ireland, numbers E 10,052 and E 10,053. In the larger of these (Plate 35, fig. 5; Plate 36, figs. 4, 5) the ambulacrum at the mid-zone measures 7 mm. in width, the interam- bulacrum 16.5 mm. From these measurements the circumference would be about 117.5 mm. and the diameter about 37 mm. The height is about equal to the diameter. The ambulacra are narrow and at the mid-zone have four columns of narrow demi- and wide occluded plates, the pore-pairs are biserial, lying near the next adjacent interambulacrum. Interambulacra are wide, with four columns of plates in two areas, seen dorsally only in area A. The plates are pentagonal adradials and hexagonal in the median columns. A hexagon at the mid-zone measures 5 mm. in width by 4.2 mm. in height. Dorsally, in area A, a genital plate in place is high, wide, with three genital pores. The second specimen from .Armagh (Plate 35, fig. 4; Plate 36, fig. 6) presents the dorsal portion only. The ambulacrum is similar to the last, and there are four columns of plates in three interambulacral areas. The most important feature is the apical disc, unknown in the type specimen. There are three oculars in place which are small and triangular in form, and all are fully exsert. Oculais D and F are as fully excluded as is ocular B (the drawing being not quite correct in this point, as I have recently ascertained by examining the specimen) . The oculars cover the ambulacra and in area E the interambulacra in part on either side as usual. They do not impinge on the interambulacra in areas A, G, and in the left side of C, but this lack is ascribed to local disturbance in fossilization, as I have found so universally that oculars do reach the interambulacra in Echini (p. 86). The genitals are high and wide, and three have three pores each, but one has only two pores. This specimen was labeled " Palaechinus sphaericus" and it is probably the one referred toby Duncan (1889, p. 196) as Palaeechinus sphaericus of which he says that the oculars are all excluded from the periproct (p. 320). A specimen from Helsington Barrows, in the Museum of Practical Geology Collection 16,301 , is ascribed to this species (Plate 36, figs. 1-3). The ambulacra are narrow, with four columns of demi- and occluded plates at the mid-zone, and pore-pairs are biserial. Ventrally, near the LOVENECHINUS. 329 peristomal border, the plates are primaries only, with pore-pairs uniserial, a typical character of this zone in the genus. The interambulacra are somewhat imperfect and a number of plates are restored in area A as indicated by dotted Unes. With this restoration (a space for which exists in the specimen), there are four columns of plates in the area. Both ambulacral and interambulacral plates in part bear small secondary tubercles. This specimen is of impor- tance, as the one from which Duncan (1889, p. 200, text-fig. viii) described the structure of the ambulacrum as that of Palaeechinus ellipticus. He was perhaps misled by the original label, which reads Palaechinus ellipticus M'Coy, but since studying M'Coy's type of ellipticus in Dublin, I know that this was a mistake of identification. The structure of the ambulacrum and the interambulacrum in this specimen differs essentially from that of P. ellipticus (Plate 29, fig. 2; Plate 30, figs. 8-10; p. 308, and footnotes, pp. 303, 326). A specimen from Hook Head, Ireland, in the Science and Arts Museum, Dublin, has the same structure as that last described, with four columns of demi- and occluded ambulacral, and four columns of interambulacral plates. The size is about the same as that of the Hel- sington Barrows specimen. The specimen from Llysfaen in the British Museum Collection E 3,432 (Plate 35, fig. 0; Plate 36, figs. 7, 8) I ascribe to this species with much hesitation, as it differs so much in size and superficial appearance from those previously described. However, as it has the same structural character as typical lacazei, it is placed in this species. The test is large, rounded, and somewhat elliptical, but its actual height is apparently greater from the dorsal dislocation of plates. It is a superb specimen, finely preserved. Width of ambulacrum at the mid-zone, 10 mm., interambulacrum, 30 mm. From these measurements the circumference would be about 200 mm. and diameter about 64 mm. The height as estimated is about 72 mm. and the width, if completed on the left side, about 68 mm. The ambulacra at the mid-zone have four columns of plates which are narrow demi- and wide occUided. Pore-pairs lie near the next adjacent interambulacra. About eight demi-plates at the mid-zone equal the height of an adradial. The ambulacra are high, rounded, and laterally bevel over the ambulacrals on the adradial suture. In ambulacrum B ventrally, as shown in the specimen (not in the drawing), within about 10 mm. of the base, the ambulacral plates are all primaries crossing the half- areas, and pore-pairs are uniserial, as in the ventral area of other species of the genus. There are four columns of plates in an interambulacral area at the mid-zone and throughout most of the vertical extent of the area, as shown in area A. The same is true of area I, and a third area on the reverse side. In area I ventrally (not shown in the drawing), there is a single plate representing the left of the two plates of the basicoronal row, three plates in the second row, and four plates in the third row, as in text-fig. 25, p. 70. The surface of the plates is high, rounded, and covered with small secondary tubercles. A single genital plate dorsally in place is high, wide, with three genital pores. Three other genitals are more or less displaced, and 330 ROBERT TRACY JACKSON ON ECHINI. with pores showing in part only. A small triangular ocular lies next to genital A and is similar in size and appearance to the small oculars shown in text-fig. 243. The specimen from Kirkby Stephen and now in the York Museum, is the original described by de Koninck (1869, 1870) as Palaechinus sphaericus. De Koninck's description and figures have entered much into succeeding publications (footnotes, pp. 303, 317, 326). In considering these, I found discrepancies and characters which I could not account for, and at my request my friend Dr. F. A. Bather, most generously came to my aid, borrowed the specimen, devel- oped it with great pains, and sent me the accompanying photographic figure and drawings (Plate 35, fig. 7; text-figs. 240-243). I w^ould express to him my heartiest thanks for this most friendly and important service. x4.s seen by the figures, the specimen is not a Maccoya sphaerica (M'Coy), and does not belong to that genus, the ambulacra being quite different from those of sphaerica (Plate 34, figs. 4-10) and from M'Coy's description (p. 318). Further, oculars are present, not absent, as claimed by de Koninck, a very important point, for now one can say that they are not absent in any known regular echinoid. The following description is taken from Dr. Bather's critical and painstaking notes. This specimen in character is close to the British Museum specimen from Llysfaen, and it is possible that they may form a species distinct from lacazei, but as they agree with it structurally, they are included here. Dr. Bather writes, "The specimen was originally in the collection of Mr. Edward Wood, F. G. S., of Richmond, Yorkshire, and came as part of the Reed Collection to the Museum of the Yorkshire Philosophical Society. Thence it was kindly lent to me by Mr. Oxley Grabham. It was at once obvious that de Koninck's sketch was not merely diagrammatic, but inaccurate in several important particulars. It was not, however, clear that his statement as to the oculars was incorrect. Indeed, it took me several weeks of careful preparation before all the questions asked me by Dr. Jackson could be answered with the aid of a good lens under the most favourable conditions of illumination. The wisest course, therefore, seems to be to draw up a fresh description of this interesting specimen. "The echinoid lies at the corner of a fragment of pinkish grey limestone, weathering to a darker red of almost haematite tinge. This deep color is now, however, destroyed by the methods of preparation. The matrix can never, I think, have borne to the fossil quite the relations shown in de Koninck's figure. In that figure the upwardly directed ambulacrum is the anterior one, and the left-hand boimdary of the matrix now passes upwards in continuation of its median line. From the angle where the matrix joins this amliulacrum to the limit of the posterior interradius, the length of the exposed test is 60.7 mm. The distance from the same point to the apical pole is 44 mm. The distance from the apical pole to the visible end of the right anterior ambulacrum is about 36 mm., and to the visible end of the adjacent right posterior interradius about 26 mm. It may be inferred from these measurements that the diameter of the test was about 55 mm., and that its height was rather greater. LOVENECHINUS. 331 "In the corona the general relations of the interambulacra and ambulacra are remarkably clear owing to the depression of the adradial sutures, from which the interambulacra rise up in a flattened curve, and the ambulacra in a more prominent sharply rounded curve. in ^3 243 Text-figs. 240-2-13. — Lorenechinus lacnzei (Julien). Lower Carboniferous Limestone, Kirkby Stephen, Westmoreland, England; Yorkshire Philosophical Society Museum Collection. Drawn by A. H. Searle under the direction of F. A. Bather. 240. View of whole specimen, oriented by the madreporic pores in genital 2. X L5. Four columns of plates in each ambulacra! area, pore-pairs biserial. Four columns of plates hi each interambulacral area. Same specimen as photo- graphic figure, Plate 35, fig. 7. 24L Ambulacrum III enlarged X about 4. Plates X, and XX, correspond with the similarly marked plates in text-fig. 240. 242. Ambulacra! detail much enlarged. 243. Dorsal aspect enlarged X about 3.8, showing small triangular exscrt oculars; genitals high, wide, with three genital pores each, excepting genital 3, which has four pores; fine madreporic pores in genital 2, a very rare character to be seen in this family (p. 172). (The lettering of the figures is mine.) 332 ROBERT TRACY JACKSON ON ECHINI. "The ambulacra are composed of four columns of plates. The inner rows meet in a zigzag radial suture. The outer end of each plate, bearing a pore-pair, interlocks with the inner ends of the two adjacent outer plates. The outer plates then expand and bear pore-pairs alternating with those of the inner series. This structure obtains in all visible parts of the ambulacra, from the apical end to the ambitus. The number of ambulacrals corresponding with an inter- ambulacral is 10 to 12, and this relation also appears fairly constant, the ambulacrals being re- duced in height in proportion to the height of the interambulacrals. The fifth interambulacral from the summit, in the left anterior interambulacrum, has a height of 4.5 mm. and a great- est width of 4.7 mm. The total width of the anterior ambulacrum at the adoral margin of this plate is 6.2 mm. ; about five and a half ambulacrals of the outer series abut against it, so that the height of each ambulacral here is about 0.8 mm. "The whole of the poriferous tract of each ambulacrum is depressed, and from it the perradial tract of the ambulacrum rises in a well marked curve. The perradial tract bears miliaries like those of the interambulacrals, from 6 to 8 being on the adradial portion of each ambulacral plate of the inner series, and a single miliary on the adradial tongue of each ambula- cral of the outer series. These miliaries are regularly arranged in diagonal rows, crossing at a constant angle. I am unable to detect on these miliaries any definite mamelon or any perfora- tion, or any suggestion of a scrobicule surrounded by a scrbiocular ring. That is why I call them miliaries and not tubercles. W. H. Baily has described and figured very precisely" all these appearances in the corresponding structures of Palaechinus elegans (Journ. Roy. Geol. Soc. Ireland, vol. 1, p. 63) and P. gigas (op. cit., vol. 4, p. 41), and I should be glad to know whether anyone has ever confirmed his observations. [I have not seen such a structure in any species in this family. — R. T. J.] "The pore-pairs are surrounded by a rim [peripodium], and the interporal septum is well defined and prominent. These structures, however, have been considerably worn in the prepa- ration of the specimen. The pores are not circular. Those of the inner pair are together like the outline of a hen's egg, cut across at the septum, the broader half of the egg being adradial. In the outer pore-pair, the adradial pore is of similar shape, but the outer pore lies close to the margin of the ambulacrum and appears as though compressed thereby into an ellipse with its long axis parallel to the margin. "The adradial suture. Owing to the position of the outer pore-pairs and to the sudden uprise of the adjacent interambulacrals, the ambulacrum, when viewed superficially, seems to dip under the interambulacrum, as in the flexible suture of primitive Cidaridae. The suture was probably not flexible; the abutting plates are very thick, about 3.5 mm. near the ambitus; and the suture-face of the interambulacrum dips towards the perradius [that is, ambulacrals bevel over the adradials, a typical character in this family. — R. T. J.]. The pore-canals of the ambulacra also slant in the same direction. These facts, seen somewhat obscurely at the LOVENECHINUS. 333 broken ends of the two ambulacra on the left, are more than confirmed by a larger specimen of this species in the British Museum (E 3,432) from the Carboniferous Limestone of Llysfaen, near Llandulas [my Plate 36, fig. 6]. This specimen further shows that the adradial suture face of the interambulacrum is denticulate at its outer margin, and faintly grooved over all its surface for the reception of the outer ambulacrals. " The interambulacra are closely united to the genital plates by the proximal plates of their admedian columns. Immediately adjoining these plates the proximal plates of the outer columns make their appearance. The area in which most is preserved is the right anterior, which is visible at least as far as the ambitus and possibly further. Throughout the whole of this there is no evidence for more than four columns, although de Koninck's figure shows a fifth coming in. The plates of the outer columns are pentagonal, and about the ambitus attain a width of 7.4 mm. with a height of 5 mm. Those of the admedian columns are hexagonal aird, at the same level, have a width of 6.4 mm. and a height of 5.1 mm. The total width of the interambulacrum here is about 22.5 mm. "The interambulacrals are covered with miliaries, about four to the square millimeter, showing a slight tendency to be arranged in rows. They bore small radioles of which the remains are seen in the adradial furrows. The plates seem firmly united by vertical sutures. "Remains of minute radioles, with fine longitudinal striation, are preserved in the matrix, especially in the grooves on each side of the ambulacra. "The apical system consists of five genitals (or basals) and five oculars (or radials). "The five oculars are minute triangular plates, squeezed like wedges between the genitals. They are depressed and bear miliaries like those of the genitals. Having with much trouble removed the matrix, I can now see the outlines of all, except the suture separating the anterior ocular from the left anterior genital. The plates are about 1.4 mm. high and about 0.8 mm. wide at the base, or adoral margin. On none can I detect any trace of an ocular pore, nor are the miliaries arranged round any depression where a pore might lie. Of course these pores are notoriously difficult to see in all fossil echinoids, so that it does not appear necessary to adopt the alternative view that the ocular tentacle may have emerged between the ocular and the ambulacrum. [I have not seen a pore on the exterior of an ocular plate in any species of this family. — R. T. J.] Each ocular lies below the level of the adjacent genitals and bends sharply down to meet the ambulacrum almost at right angles. This gives the impression that the ambulacrum passes beneath the ocular and that the union was flexible; but it is difficult to see clearly into the angle. "The genital plates form a closed circlet, with abutting sides about 1.2 mm. long. Four of the genitals are approximately equal in size and form, having a width of about 4 mm., meas- ured from the distal ends of the interbasal sutures, and a height, measured along the interradius, not exceeding 3.5 mm. The outline of these plaies is much more pentagonal than heptagonal; 334 ROBERT TRACY JACKSON ON ECHINI. for, though strictly speaking all have nine sides, some of these sides are very short and almost in a line with one other of those adjoining. The fifth genital (the madreporite) has a width of 3.6 mm. measured at the same level as the other genitals; but, whereas the other genitals scarcely, if at all, increase in width distally, this plate, owing to its greater height, viz., 5.4 mm., attains ultimately a width of 5.7 mm. This plate also is pentagonal in general outline, though it abuts, not only on interambulacrals as do the other genitals, but also on the adjacent ambu- lacra for about 1.5 mm. [I have found so generally in Palaeozoic and later Echini that ocular plates cover the ambulacra and laterally in part the interambulacra on either side (pp. 62, 86) that I think the lack of this latter character in the specimen here described may be due to displacements in fossilization — R. T. J.] "Each genital plate, including the madreporite, is pierced by three rounded and well marked pores; but the left anterior has a fourth pore. In three of the four ordinary genitals, these pores follow a curved line, about 0.8 mm. from the adoral margins. In the right posterior genital the two pores next the anterior margin are close together, and one of them is distal to the other. The pores of the madreporite are placed quite differently, one being near the adapical margin, the other two at irregular distances from the adoral margins. It is probable that the normal position of the pores is presented by the two genitals furthest from the madre- porite, and that the modifications in the two abutting on the madreporite were induced by the proximity of the stone-canal, which may have checked the synometrical development of the gonads. [I have not found such irregularity in other cases in this family (Plate 42, fig. 6). — R. T. J.] The genitals are covered entirely but irregularly by miliaries, four or five to the square millimeter. These form circlets round the gonopores. The madreporite also shows a multitude of punctae, much smaller than either the round pores or the miliaries. These may be the usual minute pores of the madreporite ; but they may be due merely to a looser texture in this plate; that is to say, a larger proportion of stroma. On the latter interpretation the pore next the adapical margin, which is larger, more transversely elongate, and in more of a depression than the others, would have been the hydropore, and probably not a gonopore, as all the round pores doubtless were. "The periproct is, in the fossil, a pentagonal opening, with a diameter of 4 mm., and almost equal sides formed by the genital plates. The side formed by the right anterior genital (the madreporite) is slightly shorter than the others." Of the specimens from Dinant, Belgium, which Fraipont refers to this species, all are moderate sized, with four columns of interambulacral plates in each area, as clearly shown by his excellent photographic figures, which are similar to my Plate 35, figs. 4, 5. He says that the details of the ambulacra could not be ascertained, but, as in every other respect the speci- mens are referable to this species, it seems that they may safely be considered as belonging to L. lacazei. The only other European species in the family which has a spheroidal test and four LOVENECHINUS. 335 columns of interambulacral plates is Maccoya intermedia, and so far this species has been re- corded from Ireland only. The two larger specimens which Fraipont doubtfully refers to lacazei have each four columns of interambulacral plates, and are quite similar in appearance to the Kirkby Stephen specimen (Plate 35, fig. 7), so that I should equally refer them to Loven- echinus lacazei. Lovenechinus mutatus (Keyes). Plate 38, figs. 10, 11. Oligoporus mutatus Keyes, 1S94, p. ll'li, Plate 15, figs. 4a, 4b; 1S95, p. 1S.3, Plate IS, figs. 4a, 4b; Klem, 1904, p. 39; Lambert and Tliiery, 1910, p. 121. Test spheroidal, with strongly marked melon-like ribs. Ambulacra less than half the width of the interambulacra, composed of four columns of low plates at the mid-zone, two of narrow demi-, and two of wide occluded plates. Pore-pairs in peripodia lying in that portion of each plate nearest the marginal portion. Occluded plates elevated in a high, steep, median, rounded ridge. According to Keyes's figure, the occluded plates do not enter between the demi-plates in a tongue-like fashion, as in L. nohilis (Plate 38, fig. 6), but the contact is rather serrate, as in L. septies (Plate 45, fig. 3) and Oligoporus dan.ae (Plate 50, fig. 7). According to his figures also each demi-plate bears one small secondary tubercle and each occluded plate bears three or four of the same in a nearly continuous horizontal row. The interambulacra are wide, steeply rising from the adradial suture, then roundly curved across the area. There are five columns of plates in an area at the mid-zone, the adradial plates are pentagonal, the median hexagonal, their surfaces covered with small secondary tubercles. This species is known only from Dr. Keyes's original description; the holotype is in the private collection of Mr. L. S. Cox of Keokuk, Iowa. It is smaller than, but structurally close to L. nohilis, from which it differs in the single row of tubercles on ambulacral plates, the shape of demi- and occluded plates at their point of contact and the carinate edges on the sides of the interambulacra where they dip down to the adradial sutures. Keokuk Limestone, Lower Carboniferous, Keokuk, Iowa (Keyes). *Lovenechinus nobilis (Meek and Worthen). Plate 37, figs. 1-3; Plate 38, figs. 4-9. Oligoporus )iobiIis Meek and Worthen, 1808, p. 3.58; 1873, p. 47(3, Plate 11, fig. 3; Loven, 1874, p. 42; Keyes, 1895, p. 182; Jackson, 1890, p. 198, Plate (3, fig. 35; Klein, 1904, p. 39; Lambert and Thiery, 1910, p. 121. Mclouupsis nobilis Meek and Wortlien, 1873, p. 478. This fine species is spheroidal in form with elevated melon-like ribs. The plates are the most massive of any known Echini. Measurements of the type as given by Meek and Worthen 336 ROBERT TRACY JACKSON OX ECHINI. (1873) are: height and breadth about 3.73 inches each, width of the ambulacra about 0.6 of an inch, width of the interambulacra about 1.06 inches. The height bf the largest interambul- acral plates is 0.26 of an inch, width 0.40 of an inch, and thickness 0.25 of an inch. In a splendid specimen in the U. S. National Museum (Plate 37, fig. 3), the ambulacra at the mid- zone measure 17 mm. in width, interambulacra 40 mm. The height as estimated is something less than the width. Two specimens in the Museum of Comparative Zoology (Plate 37, figs. 1, 2) have about the same proportions, but one of these (fig. 2) is more massive, the inter- ambulacrum measuring 50 mm. in width. The very thick plates are seen well in these two specimens. The plates in the middle of the ambulacrum and the interambulacrum (Plate 37, fig. 1) measure 7 mm. in thickness, but at the adradial suture they are only 3 mm. thick. The type according to Meek and Worthen is an internal siliceous mold, so that as this was a unique specimen the thickness of the plates was not shown. The ambulacra are relatively broad, with four columns of plates at the mid-zone, two of narrow demi- and two of wide occluded (Plate 38, figs. 5, 6). The demi-plates bevel strongly over the adradials, the occluded medially rise into a high rounded ridge, in the center of which the plates attain a thickness of some 7 mm. The inner face of the test (Plate 38, fig. 9) presents no corresponding elevation, but rather is on one continuous curve of a practically uniform arc throughout. The occluded plates laterally intercalate in a tongue-like end between the elon- gated and narrowed corresponding tongues on the inner ends of the demi-plates. The inner end of the tongue of the demi-plates in some cases is superficially almost or entirely separated from the bodj^ of its plate by the expanded tongues of the occluded plates (Plate 38, fig. 6). Pore-pairs are situated in the outer tongues of the occluded plates and close to the marginal suture in the demi-plates. Occluded plates bear double alternating rows of small secondary tubercles, the outlines of the plates waving in correspondence with their position. The demi- plates bear usually only a single tubercle on the rounded inner end of the tongue. About five or five and a half demi-plates equal the height of an adradial. The interambulacra are roundly curved and laterally dip down to the adradial sutures where they bevel under the ambulacrals (Plate 38, fig. 9). There are five columns of plates at the mid-zone in all areas of the specimens seen. The thickest plates are in the center of the area, and the thinnest are the pentagonal adradials. All five columns extend throughout the areas as far as observed; but the ventral developmental stages are not preserved in known specimens. The surface of the plates is covered with numerous small secondary tubercles, similar "to those of the ambulacral areas. According to Meek and Worthen, the apical disc in the type specimen is similar to that of Melonechinus muUiporus, the ocular and genital plates being much the same. No pores were seen in the oculars, and the genitals, as far as ascertained, had four or five pores each. Two of the genitals have five pores each, one apparently five, and two apparently four pores each. The apical disc is not preserved in specimens I have seen. LOVENECHINUS. 337 The type specimen is from the Burhngton Group, Lower Carboniferous, Calhoun County, Illinois. It is said to be in the Worthen Collection, now in the University of Illinois, at Urbana, Illinois; same locality and horizon. United States National Museum Collection 33,277; Webb City, Missouri, Museum of Comparative Zoology Collection 3,123 and 3,124; Keokuk Lime- stone, Keokuk, Iowa, a fragmentary specimen, American Museum of Natural History Collection 7,245; Boone Formation, Lower Carboniferous, western Arkansas, a specimen collected and loaned me by Dr. G. H. Girty. The National Museum specimen of this species is a magnificent individual, one of the finest Palaeozoic Echini seen (Plate 37, fig. 3; Plate 38, figs. 6, 7). Besides having the typical species characters as described, this specimen shows dorsally a few ambulacral plates which are pri- maries crossing the half-area (Plate 38, fig. 7). This is the same feature of dorsal simplicity seen more completely in L. missouriensis (Plate 42, fig. 3). In the Museum of Comparative Zoology specimen (Plate 37, fig. 1; Plate 38, figs. 8, 9), the test is spheroidal without compression. It shows the massive character of the plates, the lateral beveling of ambulacrals, and ventrally, as an internal mold in part, an impress of the proximal faces of plates. Another specimen in the same museum (Plate 37, fig. 2; Plate 38, figs. 4, 5) is less nearly complete, but represents a larger individual, the interambulacrum being about 50 mm. in width. It shows clearly surface characters, ambulacral detail, and the thickness of plates. Adambulacral plates are rounded in outline on the internal face of the adradial suture as in missouriensis (text-fig. 244). *Lovenechinus missouriensis (Jackson). Text-figs. 11, p. 54; 18, p. 59; 221, p. 193; 237, p. 231; 244, p. 338; Plate 39, figs.l~6; Plate 40, figs. 1-3; Plate 41, figs. 1-3; Plate 42, figs. 1-7; Plate 43, figs. 1-5; Plate 44, figs. 1-5; Plate 46, fig. 4. Oligoponis missotiriensis Jackson, 189G, p. 184, Plate 9, figs. 50-52; 1899, p. 131; A. Agassiz, 1904, p. 80; Klem, 1904, p. .38; Fraipont, 1904, p. 11; Lambert and Thie'ry, 1910, p. 121. This species is represented by a fine series of specimens of various ages. It is largely known from internal or external siliceous molds, but in two examples seen the plates are preserved intact. Test spheroidal, somewhat wider than high. The holotype (text-fig. 244) measures 90 mm. in height. It is somewhat flattened laterally, and measures through the mid-zone 96 mm. in greatest diameter, and 70 mm. in a plane at right angles to this but in the same zone. A very large specimen, but somewhat compressed (Plate 39, figs. 4, 5), measures about 86 mm. in height and 93 mm. in diameter through the plane J, E, also 90 to 99 mm. in other horizontal planes. If the plates were in place, it would measure several millimeters more. A smaller but quite uncompressed specimen (Plate 44, fig. 2) measures 59 mm. in height, 62 mm. in diame- 338 ROBERT TRACY JACKSON ON ECHINI. ter through the mid-zone. Another specimen witli plates all in place (Plate 46, fig. 4), in Mr. F. Springer's collection, measures 91 mm. in height, and 91 mm. in diameter through the mid-zone; the ambulacra at the mid-zone are a little more than 10 mm., and the interambulacra are about 44 mm. in width. The original description was based on an internal mold, and as the external characters differ con- siderably from the internal, certain features given as specific need modi- fication, and both internal and ex- ternal aspects have to be taken into account (p. 60). The ambulacra are relatively narrow, composed of four columns of plates at the mid-zone. As seen fi'om the exterior, the demi-plates are narrow, the occluded wide, with pore-pairs in that portion of each plate nearest the next adjacent in- terambulacrum (Plate 42, figs. 2, 3; Plate 43, fig. 3). The occluded plates are arched up in an elevated median ridge which is formed wholly by the thickening of plates, no corresponding arch existing be- neath them. The occluded plates are produced as tongues laterally, which fit between corresponding tongues extending inward of the adjacent demi-plates. Occluded plates bear two rows of alternating small secondary tubercles, the margins of the plates curving to correspond to the positions of the same. A single tubercle exists on the inner tongue of the demi-plates. While this is the character of plates at the mid-zone and throughout the greater part of the test, vcntrally the plates built in youth, and young plates dorsally near the apical disc are primai'ies, or otherwise simple (Plate 42, figs. 1, 3), as later described in detail. Four or five demi-plates equal the height of an adradial, and demi-plates on the marginal suture bevel over the adradials. Such is the structure on the exterior, but the interior differs markedly. The interior of the ambulacrum 244 Text-fig. 244. — Looenechinns missouriensis (Jackson). Burlington Group, Lower Carboniferous, Webb City, Missouri, Museum of Com- parative Zoology Coll., 3,078 (from R. T. J. Coll.), holotype. Natural size. Four columns of ambulacral plates, demi- and occluded; of about equal width. Demi-plates ojiposite horizontal interambulacral sutures fan-shaped, pore-pairs in middle of half-areas. In area A a sixth column is represented by three plates 6, 6' and the next succeeding aboral plate. .\dambulacral plates rounded on the adradial suture. The smaller figure (a) shows an enlargement, X 2, of ambulacral detail. LOVENECHINUS. 339 presents a continuous curve corresponding with the rest of the test (Plate 39, figs. 4, 5), instead of being arched up in the center. Demi- and occluded plates are of about equal width (Plate 43, fig. 4) instead of the occluded being wider, and pore-pairs are close to the middle line of each half-area instead of lying near to the next adjacent interambulacrum (compare Plate 43, figs. 1, 2, and 3, 4). Demi-plates opposite horizontal interambulacral sutures are broad and fan- shaped. This is not a specific, but a generic and family character, seen also in Melonechinus (Plate 56, fig. 5, p. 360). The interambulacra are wide, and at the mid-zone consist of five columns of plates in each area (Plate 39, fig. 3; Plate 44, figs. 2-4; Plate 46, fig. 4), or occasionally a sixth column in addition may be represented by from one to three or more plates in an area, but in no specimen seen does this extra column occur in all areas (text-fig. 244; Plate 40, fig. 1, area I; Plate 41, fig. 1, areas E, G, I). The adambulacral plates are pentagonal, beveled under the ambulacrals on the adradial suture, this inclined face bearing curved facets for articulation with the demi- plates. Occluded plates on the interior do not touch the adradials, as they do in Maccoya (Plate 34, figs. 2, 3). The median columns are composed of hexagonal plates excepting where new columns come in, and also excepting local irregularities as the plates marked P, in text-fig. 244, and Plate 41, fig. 1. The interambulacral plates are thick, but not nearly so much so, or so massive as in the nearly related Lovenechinus nobilis. The surface is thickly studded with small secondary tubercles like those of the ambulacra, but spines have not been observed in this species. In the interambulacra ventrally there are two plates in the basicoronal row (Plate 43, figs. 1,2), above which there are three plates in the second row, and four in the third, the fifth column coming in later at somewhat variable intervals as shown in the figures cited and on Plate 41, fig. 1. In a relatively young specimen (Plate 39, figs. 1, 2; Plate 40, fig. 1) the fifth column in four areas originates near the mid-zone, but there are many fewer rows than in an adult (Plate 41, fig. 1), in which the same column, appearing at about the same age, is pushed adorally in virtue of the fact that many more rows have been added dorsally. The peristome is unknown. The apical disc is relatively small in the several specimens measured. Its diameter in mature individuals proportionately is about 16 to 18 % of the diameter of the test. In the young individual (Plate 39, fig. 1) the apical disc is proportionately larger, as usual in young Echini, being about 21 % of the diameter of the test. Oculars are usually all insert and cover the ambulacra and laterally the interambulacra in part on either side (Plate 39, figs. 3, 5; Plate 41, fig. 3; Plate 42, fig. 6). As seen in several of the oculars figured, there is on these internal molds an elevated siliceous plug, near the adoral margin, which apparently represents a cast of an ocular pore. An ocular pore has not been seen on the ex- terior in any species of this family, and whether these pores would have reached the surface is doubtful (p. 89). In one case, the young specimen, two oculars are exsert, being excluded from the periproct by the contact of the two adjacent genitals (Plate 41, fig. 2). The geni- 340 ROBERT TRACY JACKSON ON ECHINI. tals are high, wide, and cover the interambulacra in part in each area. They have from three to five genital pores each, situated below the middle in each plate, and arranged in a semi- circle. There is no trace of a madreporite in any specimen seen (Plate 39, figs. 3, 5; Plate 40, fig. 3; Plate 41, fig. 3; Plate 42, fig. 6). In one specimen there are siliceous molds of a few periproctal plates which are small and angular; they were doubtless numerous, as in other species in this family (Plate 42, fig. 6). Jaws are represented only by indistinct impressions in two specimens (Plate 39, fig. 4). Burlington Group, Lower Carboniferous, Webb City, Missouri, Museum of Comparative Zoology Collection, the holotype, 3,078, and seventeen other specimens 3,124 to 3,141 (all from R. T. J. Coll.); British Museum Collection E 9,554, E 10,679; Coriscana, Missouri, Yale University Museum Collection, three specimens; Carterville, Joplin, Missouri, Museum of Comparative Zoology 3,189; F. Braun Collection; Keokuk Limestone, Nauvoo, Illinois, Museum of Comparative Zoology Collection 3,197; Burlington, Iowa, F. Springer Collection 8,126. I have studied altogether some thirty specimens of this species, a richness of material much exceeding that of most known species of Palaeozoic Echini. The more important features of a few of these will now be considered. The Burlington specimen, in Mr. Springer's collection, is a most instructive and important one. It is one of only two specimens so far found outside of Missouri, and with plates all in place (the other specimen being from Nauvoo, Illinois). The Missouri specimens are siliceous molds. The Burlington specimen is excellent for showing surface characters, especially thick- ness of plates, curvature of the ambulacra, and external tubercles (Plate 46, fig. 4). These features in this species are otherwise known only from external molds, as Plate 44, figs. 4, 5. There are five columns of interambulacral plates in each area at the mid-zone, and the ventral development of these areas is well shown, there being, as usual in the family, two plates in the basicoronal row, etc. The most interesting feature of this specimen is the perfection of detail shown in the ambulacrum (Plate 42, figs. 1-4), at the mid-zone, and through- out most of the area the typical species character exists as described (Plate 42, figs. 2, 4), with four columns of plates, pore-pairs biserial. Ventrally, near the peristome (Plate 42, fig. 1), the ambulacral plates are primaries with pore-pairs uniserial, like the genus Palaeechinus. Higher up, the plates are alternately primaries, crossing the half-area and enlarged marginally, with occluded plates, which meet the center of the area, but are cut off from the interambulacra by the enlargement of their fellows. Pore-pairs are biserial, which is like the character in Mac- coya. Still higher we find that the primary plates have failed to reach the middle of the half- area and are here transformed suddenly into demi-plates, thus assuming the generic character which is maintained thence to the ambitus. We see that in the development of this type it repeats the character of the lower genera in its family in a very complete degree, as set forth also diagrammaticallj' in text-figure 237, p. 231. Looking at the dorsal region (Plate42, fig. 3), LOVENECHINUS. 341 we see that the younger plates in the placogenous zone close to the oculars are primaries, pass- ing across the half-areas, and pore-pairs are uniserial. This simple condition of the young, last added plates as a localized stage is directly comparable to the plates built in youth (Plate 42, fig. 1), or the plates throughout the whole area in the simpler genus Palaeechinus (Plate 31, fig. 1). Passing ventrally from the youngest plates to those somewhat older and struc- turally more advanced, although there is some irregularity, we see that at the zone marked X, X, in Plate 42, fig. 3, the plates for a short distance are alternately primaries, which are expanded at the adradial suture, and occluded plates, which are cut off from the adradial suture, but both meet the center of the area, and pore-pairs are biserial. This is like the second stage in development seen ventrally, and like the character of Maccoya, the next lower genus in which ambulacral plates are typically primary and occluded with pore-pairs biserial (Plate 33, fig. 7). Thus, from ventral developing stages and from dorsal localized stages in a single specimen can be read the relations of the type to associated genera in the family. But this is not all that is shown by this wonderful specimen. At the point marked y, in the left half-area (Plate 42, fig. 3), there are two plates, and at y in the right half-area there is one plate, which lie in the middle of the half-areas and do not reach either the adradial suture or the middle of the area. In other words, these are sporadic cases of isolated plates, and the only ones seen in any specimen of the genus. As a character, they represent local progressive variations toward just that addition of plates which is the character of Oligoporus, and is the beginning of the system of isolated plates which is the main feature of Melonechinus. A young specimen in the Museum of Comparative Zoology Collection 3,140, which is only 32 mm. in diameter (Plate 39, figs. 1,2; Plate 40, figs. 1,2; Plate 41, fig. 2), is of much interest as the youngest Palaeozoic echinoid seen. It is so nearly complete that with some restoration, indicated by dotted lines, it is drawn spread out by the Loven method (Plate 40, figs. 1, 2). There are two plates in the basicoronal row as restored, three in the second row, observed or inferred, and four in the third row, as observed or inferred. The fifth column originates in the fifth to the eighth row in four areas, but in area E this column is apparently represented by only a single plate, which is in the thirteenth row. A sixth column is represented by a single plate dorsally in the thirteenth row in area I. As it is a young specimen, there are fewer plates than in an adult, there being only 13 or 14 plates in each adradial column in four of the areas, but in area E, which is much narrower, there are 15 adradial plates on each side. This area in the drawing of the internal mold is restored ventrally, but a good deal of this portion is shown in Plate 40, fig. 2, drawn from a wax cast of the external mold, which therefore supplements for this area the less complete drawing of the interior. In a large adult (Plate 41, fig. 1) there are from 25 to 27 plates in each adradial column, about twice as many as in this young specimen. (Compare studies of the interambulacrum, Jackson, 1899, p. 129.) In a dorsal view of this young specimen (Plate 41, fig. 2) oculars B, J, and probably H, 342 ROBERT TRACY JACKSON ON ECHINI. are broadly insert, but oculars D and F are exsert. This is the only case of exsert oculars seen in the species. Ventrally, the oculars bear each a small siliceous plug, representing a cast of the ocular pore, which probably did not reach the surface, as such have not been seen on the surface in any species of this family. The oculars ventrally cover the ambulacra and laterally the interambulacra in part on either side. The apical disc measures 7 mm. in diameter and is proportionately about 21 % of the diameter of the test. This is proportionately large for the species and family, but evidently, as in Recent Echini, the apical disc is proportionately larger in young individuals (p. 87). In this dorsal view the single plate representing the sixth column is clearly seen in area I. In Mr. Braun's specimen (Plate 39, fig. 3; Plate 41, fig. 3), from Joplin, Missouri, we have an internal siliceous mold about 58 mm. in diameter and nearly spherical. It has five columns of plates in each interambulacral area, but dorsally the fifth column is strung out, and in areas A, C, and E fails to reach the apical disc (Plate 41, fig. 3). Oculars are all insert, and ventrally cover the ambulacra and laterally the interambulacra in part on either side. The genitals are wide, high, and have four or five genital pores each. Dorsally, in this internal mold, ambulacral plates are all primaries, with pore-pairs uniserial; farther ventrally they are primaries and occluded, with pore-pairs biserial (compare Plate 42, fig. 3). A magnificent specimen in the Museum of Comparative Zoology Collection 3,128, from Webb City, Missouri, is one of the finest Palaeozoic Echini seen. It is an internal siliceous mold, and is represented in ventral and dorsal views in Plate 39, figs. 4, 5. The boundary of each plate is represented by a siliceous ridge, so that, if plates have been separated, there is an inde- pendent bounding ridge for each plate, as seen well in area E (Plate 39, fig. 5). Siliceous plugs represent the ambulacral and ocular pores and delicate siliceous tubes the casts of the larger genital pores. The specimen is somewhat compressed, but averages about 95 mm. in diameter. It is not only full grown, but is an old-age individual, as indicated by the dropping out of columns dorsally, as described below. So nearly complete is this specimen that I am able to represent it spread out by the Loven method (Plate 41, fig. 1), with the restoration of only a few plates ventrally, as indicated by dotted lines. The ambulacral plates at the mid-zone are demi- and occluded, with pore-pairs biserial, but situated on either side in the middle line of each half-area (compare Plate 42, figs. 2, 3; Plate 43, figs. 4, 5). The ambulacral plates near the peristome, which were built in youth, and the young plates dorsally, near the ocular, as a localized stage, are primaries, crossing the half-areas, and with pore-pairs uniserial (compare Plate 42, figs. 1, 3). In the interambulacra (Plate 41, fig. 1) there are two plates in the basicoronal row, in part restored, three plates in the second row, also partially restored, four plates in the third row, restored in two areas. The fourth column originates on the left of the center in areas A and C, but apparently (in part restored) to the right of the center in E, G, and I. There are a good LOVENECHINUS. 343 many local peculiarities in this choice specimen which I will now describe. In area A the fifth column originates in a pentagon in the eighth row, with a heptagonal plate on its right ventral border, as usual. In the ninth row there are only four plates, one being a large octag- onal plate, O, whose two extra sides are compensated for by a pentagon above it marked 5, where the fifth column starts again, and another pentagon, P, below it. Octagonal plates are very rare. In ai-ea C the fifth column originates on the left of the center with a pentag- onal plate having a heptagon in its right ventral border. In the eighth row there is an adventitious pentagon, P, in which the side wanting is compensated for by an adjacent heptagonal plate. In areas E and G the fifth column originates in the sixth row, with a heptagon on the right ventral border, as usual, and in area I it originates in the seventh row, with a heptagon on its right ventral border. In areas A and C there are only five columns of plates, but in the other areas a sixth column is represented by one or two plates in each. In areas E and I a sixth column is represented by one pentagonal plate in the thirteenth or the eleventh row, with a heptagon on its left ventral border. In addition, as a peculiarity, in each area an adventitious pentagon, P, lies on the left of pentagon 6, and to compensate for the one side wanting, a heptagonal plate, H, lies on the dorsal border of the pentagons. In area G the sixth column is represented by two plates, a pentagon in row 11 and an overlying hexagonal plate. A heptagonal plate lies on the left ventral border of the initial pentagon. In addition, as a peculiarity, an adventitious pen- tagon, P, lies on the right side of hexagon 6, and to compensate for its one side wanting, a heptagon lies on its dorsal border, as in areas E and I. Passing dorsally, column 5 drops out before reaching the apical disc, as seen more clearly in Plate 42, fig. 6. In Plate 41, fig. 1, the plates are drawn representing the character as seen on the proximal or interior aspect of the test, and here all the adambulacral columns extend to the apical disc. As viewed super- ficially, however, a somewhat different condition occurs, as shown in Plate 39, fig. 5; Plate 42, fig. 6. Taking area I as the best example, on the left and right we see the silicified marginal walls of ambulacra J and H, beveling over interambulacrum I. The plates of the adambulacral columns 1 and 2 are existent, but do not reach the surface, being roofed over, or occluded, by the lateral borders of the plates of columns 4 and 3, which alone meet the ambulacra laterally near the surface of the test. This is explained by the diagram (Plate 42, fig. 7), in which on the interior of the test there are five interambulacral plates, but on the exterior only three, as plates of columns 3 and 4 roof over and shut out plates of columns 2 and 1, which therefore meet the ambulacra proximally, but not distally. A similar shutting out of plates quite probably occurs in such a case as Plate 53, fig. 1. A comparable shutting out of plates by occlusion occurs more or less completely in the other areas of the specimen (Plate 42, fig. 6) . Dorsally all oculars are in place (Plate 39, fig. 5; Plate 42, fig. 6). The apical disc measures about 16 mm. in diameter, being about 16 % of the diameter of the test which is 344 ROBERT TRACY JACKSON ON ECHINI. relatively smaller than in the young individual (p. 342). Oculars are insert, ventrally cover the ambulacra, and laterally the interambulacra in part on either side. Two of the oculars show pores ventrally as indicated by siliceous plugs like those previously described. The genitals are high, wide, with three or four pores each. These are represented by delicate siliceous tubes which were evidently formed as post-mortem linings to the original pores and before the solution of the plates. A few molds of periproctal plates are in place (Plate 42, fig. 6) ; they are small, angular, as in other species in the family, and are the only ones yet known in this genus. Impressions of jaws are imperfectly seen ventrally (Plate 39, fig. 4). A specimen that I gave the British Museum, now no. E 10,679 of its collection, is a large individual measuring 83 mm. in diameter. It is a dark brown, silicified internal mold from near Webb City, Missouri. The ambulacral plates are demi- and occluded at the mid-zone, but primaries only close to the oculars. In three interambulacral areas. A, C, and G, there are six columns of plates. In each the sixth column is represented by a considerable number of plates, more than in any other specimen of the species seen. The exact number could not be ascertained as the point of origin of this column is not preserved in any area, but in area G a sixth column is represented by at least ten plates, above which it drops out, and there are five columns; and far dorsally, another column drops out, so that only four columns are in contact with the apical disc, as in area E of Plate 41, fig. 3. Dorsally molds of the five oculars are in place, all are insert, and ventrally cover the ambulacra and laterally the interambulacra in part on either side. The genitals are high, wide, with several pores each, but details of the pores are imperfect. A specimen from Coriscana, Missouri, in the Yale University Museum Collection, is well preserved and is practically spherical (Plate 44, fig. 2), measuring 59 mm. in height, 62 mm. in diameter through the mid-zone in the view shown, and also 59 mm. in diameter in the same plane, but at right angles to that figured. The specimen is an internal siliceous mold and shows the impression of plates clearly. There are five columns of plates in each interambulacral area at the mid-zone, and in no area is a sixth column developed. The oculars are all in place and insert; genitals are high, wide, with three pores in two of the plates, five pores in two, and in one genital the pores are indistinct. Another specimen from Coriscana in the Yale Collec- tion, no. 313, is very large and somewhat elongated by compression. It measures 114 mm. in height, 101 mm. in diameter through the mid-zone in one plane, and 76 mm. in diameter through a plane at right angles to the same. There are five columns of plates in all the interambulacral areas, though one area is obscure at the mid-zone. The genitals are high, wide, two with three genital pores, two with four, and one with five pores. A choice specimen from Webb City, Missouri, in the Museum of Comparative Zoology Collection, no. 3,125, has on one side a siliceous mold of the interior and on the other a siliceous mold of the exterior, the spaces originally occupied by the plates being hollow (Plate 44, figs. 3, LOVENECHINUS. 345 4). The internal mold has impressions of the proximal faces of the plates, ridges representing bounding lines, and siliceous filaments which are casts of ambulacral pores. The external mold has impressions of the outer or distal faces of plates with tubercles, ridges representing bounding lines of plates, and siliceous filaments which are casts of ambulacral pores as in the inner mold. By studying the internal mold, and wax impressions made from the external mold, it is possible to compare the external and internal characters of the same plates as if seen in optical section (Plate 43, figs. 1-4). The ambulacrum seen from the exterior at the mid- zone (Plate 43, fig. 3) has four columns of plates, wide occluded and narrower demi-plates, with pore-pairs biserial and situated in the outer end of each plate, near the marginal suture. Two rows of tubercles are on the occluded, and a single tubercle on the inner tongue of the demi- plates. On the proximal or inner side of the very same plates (Plate 43, fig. 4), the occluded and demi-plates are of about equal width. Demi-plates opposite horizontal interambulacral sutures are higher and fan-shaped, and pore-pairs lie near the middle of the half-area. This passage of pore-pairs from the middle to the outer margin of the area, in traversing the thick- ness of the plates, is shown graphically in a unique bit of preservation (Plate 43, fig. 5), in which we find proximally a mold of the interior, distally a mold of the exterior, and the pores represented by slender siliceous filaments which traverse the hollow space originally occupied by the plates. It is here seen that the filaments are strongly inclined laterally, being near the middle of the area proximally, but close to the outer margin of each plate distally. Ventrally, in the basicoronal row, as seen from the exterior (Plate 43, fig. 1), the ambulacral plates are all primaries, and pore-pairs uniserial, lying close to the marginal sutures. On the interior at the same zone (Plate 43, fig. 2) the ambulacral plates are also primaries, but the pore-pairs are about in the middle of the half-areas. Passing dorsally, on the exterior at the zone marked X (Plate 43, fig. 1), the plates are alternately primaries, expanded marginally, with those between narrowed and nearly or quite occluded, pore-pairs biserial, as in Maccoya. On the interior at the same zone marked X in Plate 43, fig. 2, the plates are still all primary, with pore-pairs uniserial, again like Maccoya, but as seen from the interior (Plate 33, figs. 8, 9). Again passing dorsally, on the exterior at the zone marked XX (Plate 43, fig. 1) the plates are demi- and oc- cluded, the genus character. On the interior at the same zone, marked XX (Plate 43, fig. 2), the plates are primary and occluded. They have therefore not yet taken on the character as seen on the exterior, but have lagged behind in development, and are like the more ventral plates of the exterior, and again like the typical plates on the exterior of Maccoya. This close comparison that can be drawn by stages in development, as seen in plates ventrally, built in youth, and the greater primitiveness of plates on the proximal side as compared with the distal side, with the simpler characters of lower genera in the family, is all in favor of the re- capitulation theory in evolution. Abundant similar cases are found throughout the Echini, supporting the same view in the strongest way. 346 ROBERT TRACY JACKSON ON ECHINI. In the interambulacra of this choice specimen there are five columns of plates at the mid- zone, in all five areas. Ventrally, there are two plates in the basicoronal row in all areas (Plate 43, figs. 1, 2), three plates in the second, and four plates in the third row. The fourth column originates on the right of the center in areas A, C, and E; but on the left of the center in areas G and I. In area A the fifth column originates as a pentagon in the sixth row, with a heptagon on its right ventral border. In area I the fifth column originates also in the sixth row, but by rare exception the initial plate is tetragonal, and to compensate for its two sides wanting, there is a heptagonal plate, H, on its right ventral border and another on its left border. The inter- ambulacral plates on the exterior (Plate 43, fig. 1), as usual, bear numerous small, secondary tubercles. These tubercles are shown on the external mold (Plate 44, fig. 4), and especially clearly on another external mold (Plate 44, fig. 5). The character of primary plates ventrally, passing into four columns dorsally, as seen from the interior, is shown in Plate 39, fig. 6, and Plate 42, fig. 5. There is, however, some confusion ventrally in area B, and the series is not so complete as in the cases previously described. *Lovenechinus anglicus sp. nov. Plate 46, figs. 5, 6; Plate 47, figs. 3-5. ^ Palacechinus yiyas Duncan, 1S89, p. 198, text-fig. i; Jackson, 1896, p. 204, Plalc 7, figs. 38, 39. ^Non Pcdarchinus giqas M'Coy, 1844; non Baily, 1874, p. 41, Plate 3, figs, a-d; non Keeping, 1876. ^Non PalacccJiinus gigas Loven, 1874, p. 41. ^Non Maccoya gigas Pomel, 1883, p. 115; non Lambert and Thiery, 1910, \>. 11'.). ' Palechinus gigns Tornquist, 1897, p. 739. ' Pcdacchinus gigas (pars) Klein, 1904, p. 32. The specimen on which this species is based, in the Museum of Practical Geology Collec- tion 6,576, is apparently that of which Professor Duncan figured the ambulacrum as Palae- echinus gigas, and I also figured it as the same .species. The structure of the ambulacrum of Palaeechinus gigas, however, is entirely different, being composed of primary and occluded plates, all of which meet the middle of the half-area (Plate 47, figs. 1, 2). The great size of the interambulacral plates, and also the original label, which reads Palaeechinus gigas, probably misled Professor Duncan when he described it. This specimen is in many respects very perfectly jn-eserved, but is somewhat remarkable for the peculiarities by which portions of the test are shown (Plate 46, fig. 5). In interambula- crum A there are five columns of plates in place and a vacancy on the right for at least one additional column, which would be a right pentagonal adradial. Ambulacrum B is represented ' The several references included in these paragraphs are based on the .specimen here described as Lovenechinus anglicus and which Duncan erroneously referred to Palaeechinus gigas, see )ip. 304, 321. ^The several references included in these paragraphs refer to the true Palaechinus gigas of M'Coy, here referred to Maccoya gigas, p. 321. LOVENECHINUS. 347 by the right half-area only. The left half of ambulacrum B and the right portion of inter- ambulacrum C have been pushed under areas A, B so as to be quite hidden from view. Inter- ambulacrum C is represented by the left adradial and one adjacent median column of hex- agonal plates. Of ambulacrum D, the whole width of the area is in place, though not very clearly seen. Interambulacrum E has six columns of plates, as can be seen dorsally, although they are somewhat displaced. A complete test is unknown ; but the shape as preserved indicates that the entire specimen was probably spheroidal, and the size of the plates shows that the whole specimen must have been very large, one of the largest of known Palaeozoic Echini. The height of the specimen as it exists is 115 mm., and the width 150 mm. The half ambulacrum B measures 7.5 mm. in width, therefore the whole area would measure 15 mm. in width. Interambulacrum A measures 55 mm. in width. Adding one adambulacral column on the right (as this at least is wanting), the width on the basis of six columns would be about 65 mm. Assuming 15 mm. for the width of the ambulacrum, and 65 mm. for the interambulacrum, the circumference would be about 400 mm. and the diameter about 127 mm. The interambulacral plates are very large, the pentagonal adradials measuring up to 11 mm. in width by 8 nmi. in height, and the largest hexagonals 12 to 13 mm. in width by 8 mm. in height. With this great areal size they are relatively very thin, about 2 mm. in thickness. The ambidacra are relatively narrow, with four columns of plates throughout most of the area, there being two columns of wider occluded and two of narrower demi-plates in each half- area (Plate 47, figs. 3, 5) The outer ends of the occluded plates fit between the inner tongue- like extensions of the demi-plates as in Lovenechinus missouriensis. The pore-pairs are in well marked peripodia and are biserial in arrangement, each pair situated in the outer ends of the occluded and demi-plates as usual in the genus. About four demi-plates equal the height of an adradial. The extreme ventral part of the test is unknown, but as far ventrally as preserved in area D (Plate 47, fig. 4) all ambulacral plates meet the middle of the half-area, but alternate plates are primaries expanded marginally, and occluded or cut off from interambulacral contact by enlargement of their fellows. This character is like that typical of adult Maccoya (Plate 33, fig. 7), and also like that seen as a second stage in development in Lovenechinus ynissou- riensis (Plate 42, fig. 1). On the reverse side of the specimen are several excellent fragments of ambulacra seen from the interior of the test. In these the demi- and occluded plates are of about equal width, and the pore-pairs lie near the middle of the half-areas, as seen in the in- ternal view of Lovenechinus missouriensis (Plate 43, fig. 4). The interambulacra are very wide, composed of probably six columns in each area. In area A (Plate 47, fig. 3) the left adambulacral column of pentagonal plates is in place, and also four columns of hexagonal plates. I have indicated by dotted lines a right adambulacral column which probably would complete the area. Six columns can be made out dorsally in 348 ROBERT TRACY JACKSON ON ECHINI. area E of the photographic figure (Plate 46, fig. 5). In area A ventrally, the initial plate of column 6 is seen as a pentagon with a heptagon on its left ventral border. Ambulacral and interambulacral plates alike bear small secondary tubercles with some minute tubercles between them. Many small, slender, tapering spines up to about 3 mm. in length are borne on both ambulacral and interambulacral plates (Plate 46, fig. 6). The dorsal part of the test is quite unknown. I am indebted to Dr. Kitchin of the Museum of Practical Geology for the excellent photograph of this species, which he kindly had taken for me by Mr. J. W. Tutcher. Lower Carboniferous, Clitheroe, Lancashire, holotype and only known specimen, Museum of Practical Geology, London, no. 6,576. *Lovenechinus septies sp. nov. Text-fig. 17, p. 59; Plate 44, figs. 0, 7; Plate 45, figs. 1-6. The test is small, spheroidal, with remarkably perfect detail in the type, but crushed. Width of the ambulacra at the mid-zone 8.5 mm., of the interambulacra at the same zone 18 mm. Diameter in the neighborhood of 50 mm., but from the crushing it cannot be very closelj' stated. Ambulacra at the mid-zone and throughout most of the areas with four columns of plates, wide occluded and narrow demi-plates in each half-area (Plate 45, figs. 1, 3). The occluded plates laterally fit into the inner angles of the demi-plates, but with serrate, not tongue-like extensions (as in L. missouriensis, Plate 42, fig. 2). Pore-pairs are biserial, situated in the outer portion of each ambulacral plate. The ambulacral plates bevel over the adradials, and the occluded plates rise in a high arch which is formed by the median thickening of these plates (Plate 45, fig. 5). Ambulacral plates ventrally and dorsally are primaries, as later described in detail. Interambulacra at the mid-zone are wide, with six columns of plates, but a seventh appears dorsally in each area (Plate 45, fig. 1). The adambulacrals are pentagonal, others hexagonal as usual. Two interambulacral plates are in the basicoronal row in most areas, above which additional columns appear as usual in the family, as later described in detail. Small secondary tubercles occur alike on ambulacral and interambulacral plates, but no spines are preserved. Dorsally, the apical disc is crushed in, but can be made out in part. Oculars are insert, imperforate, and adorally cover the ambulacra and laterally in part the interambul- acra on either side. Genitals are high, wide, with two and probably more pores each. This species differs from others of the genus in having more columns of interambulacral plates; also the contact of occluded and demi-plates differs from that of other species excepting L. mutatus, where the interlocking of these plates is closely similar. The holotype is in Mr. Frank Springer's collection and was received by him from Professor G. Hambach of St. Louis, Missouri. Warsaw Group, Lower Carboniferous, Boonville, Missouri; holotype in F. Springer's LOVENECHINUS. 349 Collection, no. 8,115; also three additional incomplete specimens from the same locahty and in the same collection. Although crushed, the type specimen is so completely preserved that I am able to repre- sent it spread out by the Loven method (Plate 45, fig. 1), and it presents interesting develop- mental characters in both ths ambulacra and the interambulacra. Ventrally, for a short distance, ambulacral plates are primaries (Plate 45, fig. 2), above which follow primary and demi-plates immediately, and these again are shortly followed by the typical occluded and demi- plates characteristic of the type. No stage of primary and occluded plates (such as occurs in Lovenechinus missouriensis, Plate 42, fig. 1) was found in this species, and it has apparently been skipped by acceleration of development. This is interesting, for in other characters it is the highest species in the genus, and it is in just such a case of highest species that accel- eration is most marked, as shown by Hyatt. Dorsally, close to the oculars (Plate 45, fig. 4) high, narrow primary plates exist as a localized primitive stage, but, passing adorally, they quickly change into the characteristic four columns of demi- and occluded plates. The interambulacra with only slight reconstruction (indicated by dotted lines) are shown throughout each area (Plate 45, figs. 1,2). Ventrally, in each area (excepting C and G, in which they are restored) there are two plates in the basicoronal row and three plates in the second row. In four areas the initial plate of column 3 is hexagonal as usual, with the initial pentagon of column 4 in the next succeeding row. In area C, however, there are two rows of three plates each before column 4 originates in the fourth row. As a result of this difference, the initial plate of column 3 is pentagonal and the succeeding plate is heptagonal, having one side added to compensate for the one side short in pentagon 4 (p. 68). A similar case of this rare varia- tion of a later introduction of column 4 with coincident modifications in the angles of asso- ciated plates is seen in Palaeechinus quadriserialis (p. 307; Plate 30, fig. 3, area C). In L. septies (Plate 45, fig. 1) the fourth column originates on the right of the center in areas A and E, but on the left of the center in areas C, G, and I. The fifth column originates with a pentagon in the middle in all areas. In areas A and E it originates in the sixth row with a heptagon on the right ventral border, in G and I in the sixth row with a heptagon on the left ventral border, and in area C in the seventh row with a heptagon on the left ventral border. The sixth column originates with a pentagonal plate in the tenth or twelfth row in each area. It originates in areas A and G in the twelfth row on the right of the center with a heptagon on the right or left ventral border; also in area C it originates in the twelfth row, but on the left of the center, and with a heptagon on its right ventral border. In areas E and I the sixth column originates in the tenth row, on the left of the center with a heptagon on the right ventral border. The seventh column originates with a pentagonal plate in the middle of each area, with a heptagonal plate on the ventral border. In three cases, areas E, G, and I, the heptagon lies on the right, and in two cases, areas A and C, on the left of the initial pentagon. The 350 ROBERT TRACY JACKSON ON ECHINI. seventh column originates in the sixteenth to the nineteenth row in the several areas. In area A it appears in the eighteenth row; in C in the nineteenth row; in G in the seventeenth row ; and in areas E and I in the sixteenth row. It is worth noting that in areas E and I, in which the seventh column originates earlier than in the others, column 6 also originates earlier, showing an acceleration in both features. Also, in area C columns 4, 5, and 7 all originate later than in the other areas, showing a corresponding retardation of development in those several features (p. 50). Dorsally in part, as shown in area C, the adambulacral columns 1 and 2 drop out before reaching the apical disc, indicating senescence. Oligoporus Meek and Worthen. Oligoporus Meek and Worthen, 1860a, p. 472; 1866, p. 247; (pars) Loven, 1874, p. 42; Duncan, 1889a, p. 16; (pars) Jackson, 1896, p. 239; (pars) Lambert and Thiery, 1910, p. 121. MeUmopsis Meek and Worthen, 1866, p. 249. The test is spheroidal, with more or less strongly developed melon-like ribs. Ambulacra at the mid-zone with four columns of plates which consist of two columns of wide occluded and two columns of narrower demi-plates, with, in addition, scattered isolated plates in the middle line of each half-area, but not of sufficient frequency to constitute an additional column. The wide occluded plates are roundly arched upward in the center. The pore-pairs are multiserial, one series in each column of demi- and occluded plates, and, in addition, a broken series in the scattered isolated plates. The interambulacra are wide, with from four to nine columns of plates in an area in the known species. The columns are composed of pentagonal adradial and hexagonal median plates. There are ventrally two plates in the basicoronal row, above which additional columns come in as usual in the family. There is no imbrication in ambulacral or interambulacral plates, but the ambulacrals bevel over the adradials laterally. Ambulacral and interambula- cral plates bear small secondary tubercles with small tapering spines. Peristome unknown. Apical disc imperfectly known, but doubtless similar to that in Melonechinus with insert, imperforate oculars and high, wide genitals bearing several genital pores each. Lantern known partially in 0. danae, from which it is seen that the lantern was inclined, pyramids wide-angled with moderately deep foramen magnum as usual in the Perischoechinoida. The type species of the genus is Oligoporus danae (Plate 49, figs. 4, 5; Plate 47, fig. 13; Plate 50, figs. 1-12). Oligoporus differs from Lovenechinus in that it has scattered isolated plates in the ambulacra in addition to the four columns of demi- and occluded plates. Thus it is a step in advance in the evolution of this important structure. In the interambulacra it is also more advanced in that the highest species (danae) has nine columns of interambulacral plates in an area, whereas OLIGOPORUS. 351 in Lovenechinus the highest known species (septies) has only seven columns in an area. Oligo- porus differs from Melonechinus, as in that genus the isolated plates are sufficiently numerous so that they form one or more irregular columns of plates between the demi- and occluded plates in each half-area. Melonechinus is also more evolved in the interambulacrum in that the high- est species has eleven columns of interambulacral plates, whereas the highest species of Oligo- porus has but nine. In the development of the ambulacrum ventrally, Oligoporus passes through a stage in which the plates are like Lovenechinus at the mid-zone; and Melonechinus in the development of its ambulacrum ventrally passes through stages in which the plates are successively like Lovenechinus, then like Oligoporus at the mid-zone (text-fig. 237, p. 231). Thus both the structure in adults and the progressive stages in development luiite these genera into a phylogenetic whole. K('!/ io till' Specii'.f of Oligoporus.^ Interamlnilacra with six or seven columns of plates in an area (in one area of one specimen, as an excep- tional arrested radial variation, there are four columns only); melon-like ribs moderately developed 0. bhtiri Miller and Gurley, p. 3.51. Interambulacra with seven cohunns of plates in an area, or in part with six columns only; melon-like ribs strongly elevated and rounded in outline 0. corcyi Meek and Worthen, p. 353. Interaml)ulacra with seven cohinins of plates in an area; melon-like ribs strongly elevated and sharply rounded; interambulacra dipping steeply down to adradial sutures . 0. sulcatm Miller and Gurley, p. 354. Interambulacra with eight or seven columns of plates in an area . . . 0. kalli sp. nov., p. 355. Intoraiiihulacra with nine columns of plates in an area . . 0. danar (Meek and Worthen), p. 356. *01igoporus blairi Miller and Gurley. Plate 47, figs. 6, 7; Plate 4cS, fig. 1 ; Plate 49, figs. 1-3. Oligoporus hlairi .Miller and Gurley, 1893, p. C, Plate l,figs. 2, G; Plate 2, fig. 7; Lambert and Thiery, 1910, p. 121. Oligoporus bell ul us ]\Iiller and Gurley, 1893, p. 7, Plate 1, fig. 3; Lambert and Thiery, 1910, p. 121. Oligoporus danar (pars) Klem, 1904, p. 37. The test is spheroidal, with moderately developed melon-like ribs. In one specimen (Plate 49, fig. 1) the ambulacra measure 12 mm., the interambulacra 21 mm. in width. In a second specimen (Plate 49, fig. 3) the ambulacra measure 12 mm. and the interambulacra 18 mm. in width. The ambulacra are relatively wide, having at the mid-zone four columns of narrow demi- and wider occluded plates, also some scattered isolated plates in or near the middle of the half- areas. Occluded plates are elevated in a low rounded median ridge; pore-pairs are near the outer border of each ambulacral plate. ' Tlip sjieeies described as Oligoporus {.') mimdiis is considered under Incertae Sedis (p. 450). 352 ROBERT TRACY JACKSON OX ECHINI. Interambulacra broad, elevated in a moderate curve from the adradial sutures, with six or seven columns of plates in each area. In one specimen (Plate 48, fig. 1) one area, A, as an exception is very narrow, and has four columns of interambulacral plates, but the next ad- jacent area, C, has seven columns. This specimen, on account of the four colunms in one area, was considered a distinct species by Miller and Gurley, and described as Oligoporus bellulus. The ventral and dorsal portions are unknown. Ambulacral and interambulacral plates bear small secondary tubercles, and one specimen has numerous small tapering spines about 2.5 mm. in length. The specimen which Miller and Gurley described as the type of Oligoporus bellulus (Plate 47, fig. 6; Plate 48, fig. 1) I feel is nothing more than a very peculiar exception. The ambula- crum and interambulacrum C with seven columns are quite normal for the species. The interambulacrum A is certainly very exceptional in having only four colunms; no such range in two areas of one individual has been seen in any other specimen in this family. It may, however, better be considered an extreme arrested variant in this one area rather than as a basis for a distinct species. I therefore consider bellulus a synonym of blairi. Miller and Gurley do not mention that there are seven columns in one of the areas of this specimen. It has numerous small spines as described above. One of Miller and Gurley's cotypes of blairi (Plate 47, fig. 7; Plate 49, fig. 1) represents a considerable part of the dorsal half of a test. It is perhaps the most characteristic of all the specimens. The interambulacra A and C have each seven columns of plates. Miller and Gurley give six columns as the species character, but do not mention a seventh. I do not quite understand how they overlooked this feature. This specimen shows the surface form of the areas better than any other. A second cotype (Plate 49, fig. 3) represents about the middle of the corona. In area A there are six columns of plates; ventrally the initial pentagonal plate of column five is seen in the middle line and in the second row from the base. The initial pentagon of column six is seen in the sixth row from the base, and a heptagonal plate lies on its right ventral border. In area C there are six columns of plates and indications of a seventh column as shown by the pentagon, with a heptagonal plate on its right ventral border lying 30 nun. from the base of the specimen and with three columns of interambulacral plates lying between this pentagon and ambulacrum D. The third cotype of blairi and the smallest specimen (Plate 49, fig. 2) has six columns of interambulacral plates in area A. The initial plate of the sixth column is a pentagon with a heptagon on its left ventral border and lying just below the middle of the area. In area C there are seven columns of interambulacral plates. The initial plate of column 5 lies 7 mm. above the ventral border of the specimen, and there is a heptagonal plate on its right ventral border. Three rows above appears the initial pentagon of column 6, with a heptagon on its OLIGOPORUS. 353 right ventral border. Three rows higher again appears the initial plate of the seventh column with a heptagon on its left ventral border. This species is very close to Oligoporus coreyi; but apparently differs from that species in the less pronounced and less sharply elevated ribs in the ambulacral areas. It is very distinct from Oligoporus danae of which it was treated as a synonym by Miss Klem. Through the kindness of Professor Stuart Weller, I have been able to study all of the original material and give figures of the same. Keokuk Group, Lower Carboniferous, Boonville, Missouri, cotypes, three specimens in the University of Chicago Collection 6,470; same locality, an additional specimen (which is the holotype of Oligoporus bellulus) University of Chicago Collection 6,190; Keokuk Group, Bono, Lawrence County, Indiana, Museum of Comparative Zoology Collection 3,188. *01igoporus coreyi Meek and Worthen. Plate 47, figs. S, 9; Plate 48, fig. 2. Oligoporus coreyi Meek and Worthen, 1870, p. 34; Loven, 1874, p. 42; Keyes, 1895, p. 183; Jackson, 1896, p. 189, text-fig. 1, p. 191, Plate 6, figs. 25, 28, 29; Klem, 1904, p. 38; Lambert and Thiery, 1910, p. 121. The type specimen is silicified and beautifully preserved as regards form, although not very clear in detailed structure, as it is worn, and the test is somewhat compressed. Test spheroidal, with strong, rounded, elevated melon-like ribs in both ambulacral and interambulacral areas. Height 40 mm. ; this measurement would be greater if the specimen had not been compressed. Diameter at the mid-zone through the plane B, G, 58 mm. ; through the plane D, I, 59 mm. The measurements of the diameter are somewhat exaggerated by the dorso-ventral compression of the test. The ambulacra at the mid-zone measure about 12 mm. in width and the inter- ambulacra about 20 mm. in width. The ambulacra could not be made out satisfactorily at the mid-zone, but in the mid-dor- sal region (Plate 47, fig. 9) there are four columns of demi- and occluded plates. These are of about equal width, a usual character dorsally; but at the mid-zone the occluded are probably wider than the demi-plates. In addition, there are some scattered isolated plates that do not reach either the middle of the area or the interambulacra, which is the generic character. Pore-pairs are in deep sunken furrows, lying in the outer border of each plate. The occluded plates in the middle are arched up in a high rounded melon-like curve. The interambulacra just above the mid-zone have seven columns of plates in areas A, E, and G. In area C there are only six columns, and in I there are six columns, and possibly seven, but this area is not clear dorsally. The seventh column comes in a little dorsal to the mid-zone, and at that zone, strictly speaking, there are six columns of plates in all areas. The 354 ROBERT TRACY JACKSON ON ECHINI. surface ornamentation is not preserved, but it doubtless bore small secondary tubercles as in related species. The ventral portion and apical disc could not be ascertained in the type. A specimen in the Museum of Comparative Zoology Collection 3,008, which I described previously (1896) and referred to this species, is an internal view. Ventrally, the ambulacral plates are not very well preserved, but are seen to cross the half-area. Higher up the plates are in four columns, demi- and occluded, and the pore-pairs are in the middle of the half-areas, as seen on the interior of Lovenechinus. In the interambulacrum there are two plates in the basicoronal row, three plates in the second row, and four in the third. The fifth column conies in in the sixth row, and as seen in another area, the sixth column comes in three rows above the fifth, apparently in the ninth row. This specimen is preserved as calcite plates in a clay matrix, stained yellow with iron. The locality given on the label is Indiana, and Mr. Springer informs me that material of this character occurs at Crawfordsville, so that the specimen probablj^ came from that locality or its vicinity. This species is very close to blairi and sulcatus ; the difference from blairi is its more strongly elevated melon-like ribs, and the difference from sulcatus is the sharply angular character of the interambulacral areas of that species where they dip down to the adradial sutures. These differences are slight, but they are sufficient, it seems, to maintain these as distinct species, although in the number of columns of interambulacral plates they are practically identical. The specimen which I identify as the holotype is in the University of Michigan Collection 147, from the Keokuk beds of Crawfordsville, Indiana. It bears the locality and identification in Meek's handwriting, and corresponds with the original description in essential features, so that it is undoubtedly the type; another specimen from Crawsfordsville is in the British Museum Collection E 4,273. A third specimen, in the Museum of Comparative Zoology Collection 3,008, as described above, and probably from the same locality, is the only other known specimen of the species. *01igoporus sulcatus Miller and Gurley. Plate 47, fig. 10; Plate 48, figs. 3, 4. Oligoporns svlcatvs Miller and Gurley, 1893, p. 8, Plate 1, figs. 4, 5; Lambert and Thiery, 1910, p. 121. Melonites multiporus (pars) Klem, 1904, p. 43. The specimen is silicified, test high, spheroidal, with strongly marked and quite angular melon-like ribs in ambulacral and interambulacral areas. The holotype, which is the only known specimen, measures 53 mm. in height, and 65 mm. in diameter through the mid-zone, in the plane J, E. Ambulacra measure 13.5 mm. in width, interambulacra 25 mm. in width at the mid-zone. The surface detail is corroded and gone in parts, especially ventrally, as seen in the photographic figures, but the essential specific features can be ascertained. The ambulacra at the mid-zone (Plate 47, fig. 10) consist of four columns of narrower demi- and OLIGOPORUS. 355 wider occluded plates, with scattered isolated plates in the middle line of each half-area. The occluded plates are medially rounded up in a high, steeply arcuate ridge. Pore-pairs are in deep, sunken valleys, lying in the outer border of each ambulacral plate. The interambulacra are wide, strongly elevated in melon-like ribs, which outwardly present a gently rounded curve, but laterally dip suddenly down to the adradial sutures. At the mid- zone in each area there are six columns of interambulacral plates, but just above the mid-zone, a seventh column appears in each area. As noted by Miller and Gurley, part of the columns drop out dorsally before reaching the apical- disc, indicating senescence. Traces of insert ocular and genital plates appear dorsally. This species is close to blairi and coreyi, but differs from them, as discussed under coreyi. Its reference to MelonecMnus multiporus as a synonym by Miss Klem is certainly wide of the mark. St. Louis Group, Lower Carboniferous, Hardin County, Kentucky, holotype in LTniversity of Chicago Collection 6,623, whence it was kindly loaned me by Professor Stuart Weller. *01igoporus halli sp. nov. Plate 47, figs. 11, 12; Plate 48, fig. 5. Oligoforus sp. Klem, 1904, Plate 1, figs. 3a, 3b. A large, massive sea-urchin, slightly elliptical in form, with moderately developed melon- like ribs. Height of type 103 mm., diameter through the mid-zone 100 mm., width of ambu- lacra at the mid-zone 18 mm., width of interambulacra 31 mm. Ambulacra near the mid-zone with four columns of demi- and occluded plates, the latter slightly the wider, and with scattered isolated plates in the middle line of each half-area. Oc- cluded plates in the middle are curved outward in a low, rounded arch. Pore-pairs in lateral depressions, are situated in the outer portions of each ambulacral plate. Interambulacra are wide, rounded up from the adradial sutures in a gentle rounded curve. In areas A, C, and I there are eight columns of interambulacral plates at or just above the mid- zone, but in areas E and G there are only seven columns. Areas G and I are not seen in the photographic figure (Plate 48, fig. 5), being on the opposite side of the specimen. In area A (Plate 47, fig. 11) the two ventral rows of plates are restored, as indicated by dotted lines; there are four plates in the third row. The fifth column appears in the sixth row and the sixth column in the eleventh row. The seventh column appears on the left of the center in the fifteenth row. In area C the seventh column appears in the center five rows above the base as far as pre- served, and the eighth column originates at or just above the ambitus. The introduction of these two columns in area C is clearly seen in the photographic figure (Plate 48, fig. 5). The apical disc and dorsal portion of the specimen are wanting. This fine species is known only from the holotype which was kindly loaned me by Mr. 356 ROBERT TRACY JACKSON ON ECHINI. Springer. It is named for the late- Professor James Hall, who did such extensive and valuable work for Palaeontology in America. This species is near to Oligoporus danae, but differs from it in that it has seven or eight instead of nine columns of plates in an interambulacral area; also in that the interambulacra have a much less accelerated development than in danae. Keokuk Group, Lower Carboniferous, Boonville, Missouri, F. Springer Collection 8,114, originally in G. Hambach's collection. Oligoporus danae (Meek anTamids, and braces are of the character usual in all regular Echini. Lepidechinus Hall. ^ Lepidechinus Hall, 1861, p. 1<8; (pars) 1868, p. 295; (pars) 1870, p. 339; (pars) Lovcn, 1874, p. -14; A. Agassiz, 1874, p. 648; non 1892, p. 74; (pars) Duncan, 1889a, p. 12; (pars) Keyes 1895, p. 192; (pars) Jackson, 1896, pp. 226, 242; (pars) Klem, 1904, p. 20; (pars) Lambert and Thiery, 1910, p. 122. ^ Rhoechinus Keeping, 1876, p. 37; (pars) Duncan, 1889, p. 205; 1889a, p. 14; (pars) Jackson, 1896, pp. 200, 239; (pars) Tornquist, 1897, p. 18; (pars) Klem, 1904, p. 27; Lambert and Thiery, 1910, p. 122. Test spheroidal, ambulacra narrow with two columns of plates in each area throughout. Plates all primaries, pore-pairs uniserial or slightly biserial at the mid-zone. The ambulacral plates imbricate moderately adorally, and laterally bevel under the adradials. Interambula- cra are wide at the mid-zone with from four to eight columns of plates in each area in the known species. Interambulacral plates are relatively thick, rounded somewhat in outline, but pentag- onal in adambulacral and hexagonal in median columns. They imbricate aborally and from the center outward and over ambulacrals on the adradial sutures. The primordial interam- bulacral plates are in the basicoronal row (Plate 64, fig. 1). The surface of interambulacral plates bears secondary tubercles only. Peristome with ambulacral plates only. Apical disc as known in one species (Plate 63, figs. 7, 8) with oculars exsert, each with one pore, at least as seen from within, and genitals high and wide, each with one pore. As the apical disc is known only from one specimen, and that one seen from within (Plate 63, figs. 7, 8), it is quite possible that an external view or other species might show other characters. Periproctal plates are unknown. The lantern is inclined, teeth grooved, and pyramids wide-angled, with moderately deep foramen magnum. The type species is Lepidechinus imbricatus Hall, from the Lower Carboniferous of America. ' Lepidechinus as here described includes L. imbricatus Hall, the type, but not L. rarispinus Hall, which is referred to the genus Hyattechinus, p. 292. ^ Rhoechinus as used by Duncan includes R. irregularis Keeping, here referred to Lepidechinus, and also species of Palae- echinus, p. 303. In this he wa-s followed by Jackson (1896), Tornquist (1897), and Miss Klem (1904). Lambert and Thi6ry (1910, p. 123) point out this error in the application of the name Rhoechinus. LEPIDECHINUS. 395 This genus is considered the lowest member of the family, and is very near to Palaeechinus, which is also considered the lowest member of its family. Lepidechinus differs from Palae- echinus chiefly in the fact that the plates are imbricate and that adambulacral plates bevel over the ambulacrals instead of ambulacrals beveling over the adambulacrals, as in Palaeechinus. Hall in describing Lepidechinus, and Keeping in describing Rhoechinus, pointed out the close relations of these genera to Palaeechinus. Unfortunately Professor Hall did not figure Lepid- echinus imbricatus, the type, but did figure what he described as L. rarispinus, which, how- ever, has very different characters and is here referred to Hyattechinus (p. 292). Naturally the understanding of Lepidechinus has been taken largely from rarispinus with the result that the genus Lepidechinus has been misinterpreted. As I have studied and here figure the type, it is hoped that the relations expressed may prove correct. Mr. Agassiz (1874, p. 648) says, "In Hall's genus Lepidechinus the ambulacra and inter- ambulacra lap in opposite directions. Interambulacra in eight rows; outer pentagonal, others hexagonal. Hall, in his Report on the Geology of Iowa, has given us excellent figures of the tubercles, spines, and portions of the ambulacral and interambulacral plates (PI. XXVI)." This reference is quite mistaken, as in the work and Plate referred to. Professor Hall figured Archaeocidaris, not Lepidechinus. Hall never figured Lepidechinus imbricatus, which has eight columns of plates in an interambulacrum, and which is here figured for the first time (Plate 62, fig. 5; Plate 64, fig. 1). Hall, however, did give a figure of Lepidechinus [Hyatte- chinus] rarispinus, which has eleven columns of plates in each interambulacral area (pp. 292, 294; Plate 21, fig. 6; Plate 22, fig. 7). Further, Mr. Agassiz (1892, p. 74) says, "Hall's figures of Lepidechinus in the Geology of Iowa [1858], Vol. I., Part 2, Plate IX, fig. 10,' show three or four plates at the_ actinal edge." Hall's Lepidechinus rarispinus is here referred to Hyatte- chinus rarispinus (pp. 292, 294); his figured specimen, and the only one he did figure (my Plate 21, fig. 6; Plate 22, fig. 7), is a dorsal mold, so that Mr. Agassiz is mistaken in .speaking of it as ventral. In rarispinus there is a single plate at the actinal (or peristomal) border of the test (not "three or four plates") as I show in three species of Hyattechinus (p. 292), as well as in the true Lepidechinus (p. 400). Key to the Species of Lepidechinus. Four columns of plates in an interambulacral area at the mid-zone . . L. irregularis (Keeping), p. 396. Five columns of plates in an interambulacral area at the mid-zone . . L. iowensis sp. nov., p. 397. Six columns of plates in an interambulacral area at the mid-zone . . L. tessellatus sp. nov., p. 397. Eight columns of plates in an interambulacral area at the mid-zone . . L. imbricatus Hall, p. 399. ' This reference is incorrect as Hall's figure of Lepidechinus, which is L. [Hyaltcchinus\ rarispinus was not, published in the Geology of Iowa, but in 1868 (and 1870, revised edition) in the Twentieth Report N. Y. State Cabinet Nal. Hist., Plate 9, fig. 10 (.see this memoir, p. 292). 396 ROBERT TRACY JACKSON ON ECHINI. *Lepidechinus irregularis (Keeping). Plate 63, figs. 1, 2. Rhoechinns irregularis Keeping, 1870, p. 37, Plate 3, figs. 6-8; Zittel, 1879, p. 484; Duncan, 1889, p. 205; Jackson, 1896, p. 200; Tornquist, 1897, p. 754; Klem, 1904, p. 30; Lambert and Thiery, 1910, p. 122. Palaechinus rutoti Julien, 1896, p. 129, Plate 6,fig. 8; Klem, 1904, p. 30; Lambert and Thiery, 1910, p. 119. Palcckinus rutofi Tornquist, 1897, p. 753. Test small, spheroidal, imperfectly known, .\mbulacra are narrow, with two columns of low primary plates which imbricate adorally and laterally bevel under adambulacrals. About four ambulacral plates equal the height of an adambulacral. Pore-pairs are uniserial, situated near the outer border of each plate. Interambulacra are broad, with four columns of plates at or about the ambitus, and in the type a trace of a fifth column, as represented by one plate, exists dorsally (Plate 63, fig. 2). These plates imbricate aborally, and from the center laterally and over the ambulacra on the adradial sutures. Externally they bear small secondary tubercles. Keeping's holotype is a small specimen which I studied at the Sedgwick Museum, Cam- bridge, England. He naturally made it the type of a new genus (Rhoechinus), as it differed so distinctly from Palaeechinus by the imbrication of its plates. Keeping says of his genus Rhoechinus that it differs from Perischodomus and Lepidechinus in the absence of primary tubercles. This is true in regard to Perischodomus, but was a mistake as regards Lepidechinus, for the type of that genus and other species here attributed to it have only secondary tubercles. It is to be remembered however that Hull's Lepidechinus [Hyattechinus] Tarispinus has primary tubercles (p. 293). The species irregr^Zaris is clearly referable to Professor Hall's genus Lepid- echinus, so that Rhoechinus becomes a synonym. Professor Duncan overlooked the character of imbrication and used Rhoechinus to include species which he separated from the old genus Palaechinus M'Coy, as earlier discussed (p. 303). The specimen described as Palaechinus rutoti bj^ Julien with present evidence cannot be distinguished structurally from L. irregularis. As Julien says, it has two columns of low narrow ambulacral plates, imbricating adorally, with pore-pairs uniserial, and four columns of interambulacral plates which imbricate aborally, and bear small secondary tubercles only. Four ambulacral plates are equal in height to a pentagonal adambulacral plate. The hex- agonal interambulacral plates have a width of 5 mm. and a height of 3 mm. Lower Carboniferous, Hook Head, County Wexford, Ireland, holotype in Sedgwick Mu- seum Collection, Cambridge, England, 12; near Savigny-Poil-Fol, central France (this is the holotype of Palaechinus rutoti Julien, and is in the possession of the family of the late Professor Julien). \ LEPIDECHINUS. 397 *Lepidechinus iowensis sp. nov. Plate 62, fig. 1 ; Plate 63, figs. 3, 4. Test spheroidal. Known only from the holotyiae, which measures 52 mm. in diameter. The ambulacra at the mid-zone are 4.5 mm. in width and the interambulacra 28 mm. in width. The ambulacra are narrow, composed of two columns of low primary ambulacral plates alike throughout the area. Pore-pairs are uniserial, situated near the outer border of each plate. Ambulacral plates imbricate slightly adorally and laterally bevel under the adradials (Plate 63, fig. 4). The interambulacra are broad with five columns of plates in each area. The in- terambulacral plates are moderately curved outward, are pentagonal in adambulacral and hexagonal in median columns. They imbricate moderately aborally and from the center outward and over the ambulacrals, on the adradial suture line presenting a curved outline (compare Perischodomus, Plate 64, fig. 3). The ventral portion and apical disc are wanting, but the lantern is in place and partially visible, as seen from above (Plate 63, fig. 3). The lantern is inclined, so that, with the angle presented, one sees a considerable portion of the inner face of the teeth (compare Plate 27, fig. 5). The teeth are grooved, pyramids wide-angled, and foramen magnum moderately deep. Typical braces are in place. The structure of the lantern as far as gathered from the dorsal view is essentially like that of Lepidesthes (Plate 68, figs. 9-14). Upper Burlington Limestone, Lower Carboniferous, Burlington, Iowa. The holotype and only known specimen is in Mr. Frank Springer's collection 8,010. *Lepidechinus tessellatus sp. nov. Plate 62, figs. 2-4; Plate 63, figs. 5-11. Lepidcchinus imbricafus A. Agassiz, 1904, p. 80 (non Lepidechinus {mbricaius Hall, 1S61, p. IS. See p. 299). Test spheroidal. The holotype (Plate 62, fig. 2) measures 50 mm. in diameter. The width of the ambulacrum at the mid-zone is 5 mm., width of the interambulacrum 25 mm. The ambulacra are narrow, composed of two columns of low primary plates in each area. Pore-pairs are uniserial, situated near the outer end of each plate (Plate 63, fig. 6). About five to six and a half ambulacral plates equal the height of an adambulacral. The ambulacral plates imbricate adorally and laterally bevel under the adambulacrals. From this beveling it occurs that the ambulacral plates on the interior of the test (Plate 63, fig. 7) are wider than on the exterior, and also on the interior the pore-pairs are in the middle of the half-areas instead of near the outer end of each plate, as on the exterior (p. 60). The interambulacra are wide, composed of six columns of plates at the mid-zone in each area. In the paratype (Plate 63, fig. 9) the sixth column originates below the raid-zone, and 398 ROBERT TRACY JACKSON ON ECHINI. in the holotj^je (Plate 63, fig. 6) one column, either 5 or 6, drops out dorsally with a pentagonal plate, indicating senescence. The interambulacral plates are curved on the outer surface and bear secondary tubercles only. They are slightly rounded on the suture lines and imbricate moderately aborally and from the center laterally and over the ambulacra. Adambulacral plates are pentagonal, rounded on the adradial sutures (compare Plate 64, fig. 3). The plates of the median columns are hexagonal. On the interior of the test (Plate 62, fig. 3; Plate 63, fig. 7) the interambulacral plates are more sharply angular than on the exterior, and adambula- cral plates are not rounded on the adradial sutures (compare text-figs. 32, 34, p. 75). The ventral portion of the test is unknown. Dorsally, as seen from within (Plate 62, fig. 3; Plate 63, figs. 7, 8) the apical disc is in place; it is small, measuring 9 mm. in diameter. The whole of this specimen is not known but it corresponds nearly enough with the holotype so that it was at least fully as large. On this basis the diameter of the apical disc as seen from within would be proportionately about 18 % (or less) of the diameter of the test. Several ocular and genital plates are in place. The oculars are triangular, fully exsert, each being shut out from the periproct by the contact of the two adjacent genitals, a rare character in the Palaeozoic, but like the dominant Mesozoic and the youthful character of modern Echini (p. 89). Each ocular has one pore near the ventral border, but as this is an internal view, it is not certain that the pore reached the surface (p. 89). The oculars ventrally cover the ambulacra and laterally in part the interambulacra on either side, the young interambulacral plates being in contact with the oculars (pp. 62,86). This is the specimen to which Mr. Agassiz (1904, p. 80) referred as Lepidechinus imbricatus, and in which he noted that the young interambulacral plates origi- nate against the oculars. The genital plates are high and wide, and, as far as preserved, meet in a continuous ring dorsally on the periproctal border. Each genital has a single pore near the middle of the plate. This single pore in each plate is the only case with this character seen in the Palaeozoic, and also the median position is unusual; but it should be remembered that this is an internal view, and in an external view the pores may be nearer the ventral border (p. 171). Genital pores are usually near the ventral border of the plates, but they may be near the middle of the plates in external view, as seen in Dermatodiadema (text-fig. 80, p. 104) and Peltastes wrighti (Plate 4, fig. 7). The plates of the periproct and the lantern are unknown in this species. This species differs from Lepidechinus iowensis in having six instead of five columns of plates in an interambulacral area. It differs from Lepidechinus imbricatus in having six instead of eight columns of plates in an interambulacral area, and also in that the plates are rela- tively larger, heavier, and less strongly imbricating. Upper Burlington Limestone, Lower Carboniferous, Burlington, Iowa, holotype, Museum of Comparative Zoology Collection 3,053; paratype, same museum, no. 3054; second paratype, F. Springer Collection 8,011. LEPIDECHINUS. 399 In the holotype, which is in the Museum of Comparative Zoology, the dorsal half of the test is preserved (Plate 62, fig. 2; Plate 63, figs. 5, 6). In areas A, E, and I the sixth column drops out passing dorsally and terminates in a pentagonal plate, an apex of which points dor- sally; and in areas C and G the fifth column drops out similarly. In area E the fifth column also drops out, but two rows later than column 6. The terminal pentagons in several cases have a heptagonal plate on their dorsal border. These might easily be taken for the point of intro- duction of columns, bat this is the dorsal, not the ventral portion of the test, as shown by the direction of imbrication of plates. This specimen has very clear ambulacral detail which is taken to represent the species character. In Mr. Springer's specimen (Plate 62, fig. 4; Plate 63, figs. 9-11) the plates are worn, but the imbrication is clear, as shown in Plate 63, figs. 10 and 11. Ventrally there are five columns, the sixth coming in below the mid-zone. The ambulacral plates ventrally are all typical primaries with pore-pairs uniserial, but at the mid-zone in this specimen there is a tend- ency for alternate plates to be expanded on the lateral border and the plates between narrowed. There is nothing of this seen in the type, but otherwise the specimen has the characters of this species. The Museum of Comparative Zoology specimen, which is seen from the interior, is very interesting (Plate 62, fig. 3; Plate 63, figs. 7, 8). There are three interambulacral and four ambulacral areas. The interambulacral columns in each area are all continued nearly or quite to the apical disc instead of dropping out dorsally, as in Plate 63, fig. 5. The ambulacral plates and apical disc of this choice specimen are considered above. *Lepidechinus imbricatus Hall. Plate 62, fig. 5; Plate 64, fig. 1. Lepidechinus imbricatus Hall, 1861, p. IS; 1868, p. 296; Loveii, 1874, p. 44; Keyes, 1895, p. 192; Jackson, 1896, pp.226, 242; non A. Agassiz, 1904, p. SO; Klem, 1904, p. 21; Lambert and Thiery, 1910, p. 122. Lepidocentrus imbricatus Meek, 1874, p. 375. Lepidechinus intricatus Pomel, 18S3, p. 113. In the type specimen, which is in the Museum of Comparative Zoology, the test is sphe- roidal. The ambulacra at the mid-zone measure about 3 mm., and the interambulacra about 20 mm. in width. The ambulacra are narrow, composed throughout each area of two columns of low pri- mary plates which imbricate strongly adorally and bevel under the adjacent adambulacrals. Pore-pairs are uniserial, situated near the outer border of each plate. Two and a half ambulacral plates equal the height of an adambulacral. The interambulacra are broad, with, at the mid-zone, eight columns of plates, which are small, curving outward, and rounded on the suture lines. They imbricate strongly aborally and from the center laterally and 400 ROBERT TRACY JACKSON ON ECHINI. over the ambulacrals. The adambulacral plates are pentagonal and rounded on the ad- radial sutures. Plates of the median columns are hexagonal, but the angles of all the.se plates are somewhat rounded, not sharply defined, as in the Palaeechinidae. There are secondary tubercles only on ambulacral and interambulacral plates. No spines are preserved in the type, but in a second specimen in the Museum of Comparative Zoology 3,186, there are fine tapering spines, swollen at the base, and measuring up to about 2.5 mm. in length. Ven- trally, the primordial interambulacral plates are in the basicoronal row, and, passing dorsallj^, new columns are progressively added until the full complement is attained below the mid-zone. Although only partially preserved, it may be safely stated that there are only ambulacral plates on the peristome. The dorsal portion of the test is unknown, and the lantern is not preserved in the type, but in another choice specimen in the Museum of Comparative Zoology no. 3,186, from the Wachsmuth Collection, there is a well preserved lantern in place. The pyramids are wide-angled, with ridges for the attachment of interpyramidal muscles, foramen magnum moderately deep, and teeth grooved, all as usual in Palaeozoic species. Some ambul- acral plates in this specimen are evidently peristomal plates. This species, the type of the genus, differs from the other species in that it has more columns of interambulacral plates, also more strongly imbricate plates, both in the ambulacra and the interambulacra. The specimen which I identify as the type has no original label, but bears the diamond-shaped green ticket that Professor Hall used to indicate types. It agrees entirely with Hall's original description. Moreover, the late Professor Whitfield informed me that the type was in the Barris Collection, which was purchased by Professor Louis Agassiz for the Museum of Comparative Zoology. Professor Hall in his description says that the interam- bulacra] plates imbricate adorally, "the lower edges of each range overlapping those below; while the plates of the ambulacral areas are imbricating in the opposite direction." The fact that he said that interambulacral plates imbricate adorally is ascribed to incorrect orienta- tion, which has led to a similar interpretation in regard to imbrication in some other cases. Indeed, Professor Hall (1868, p. 296) himself said that he had probably given the direction of imbrication incorrectly in his original description. Professor Hall pointed out that the essential difference of the genus (and species) from Palaeechinus is the fact that the plates are imbri- cate. He also considered as a difference the number of columns of interambulacral plates, but this with present understanding is not considered a generic difference. The species differs generically from what Hall described as Lepidechinus rarispinus, which species is here referred to Hyattechinus (p. 292) and figured on Plate 21, fig. 6 and Plates 22, 23. Burlington Limestone, Lower Carboniferous, Burlington, Iowa, holotype. Museum of Comparative Zoology Collection 3,055; another choice specimen especially good for the ventral part of the test, lantern, and spines. Museum of Comparative Zoology Collection 3,186; also a fragmentary specimen in the same museum no. 3,056 (original no. 417). PERISCHODOMUS. 401 Perischodomus M'Coy. Pcrischodomus M'Coy, 1S49, p. 253; 1854, p. 114; Loven, 1874, p. 40; Keeping, 1876, p. 35; (pars) Dun- can, 1889a, p. 10; Jackson, 1896, p. 242; Tornquist, 1897, p. 783; (pars) Klem, 1904, p. IS; (pars) Lambert and Thiery, 1910, p. 122. Tretechinus Tornquist, 1897, p. 784. Test high, spheroidal. Ambulacra are narrow, composed in each area of two columns of low plates. Interambulacra are wide, with many columns of plates (five in the known species) in each area. The interambulacral plates imbricate strongly aborally and from the center laterally and over the ambulacra on the adradial sutures. The primordial interambulacral plates are in place in the basicoronal row. Adambulacral plates bear medium-sized eccentric perforate primary tubercles with secondary tubercles, and median interambulacral plates bear primary and secondary tubercles, or the latter only. Spines are medium-sized primaries and small secondaries. Plates of the peristome are imperfectly known, but apparently are ambula- cral only. Oculars are imperforate, genitals low and wide, with numerous pores. The lantern is known from pyramids which are similar to those of Lepidesthes and other Palaeozoic Echini. The characters of this genus are based on Perischodo7nus biserialis, the type species of the genus and the only one which is well known. This genus differs from Lepidechinus in that the interambulacral plates, in part at least, bear primary with secondary tubercles. It differs from the other genera of the family in that it has two instead of more than two columns of plates in each ambulacral area. In my earlier paper (1896, p. 242) I included Perischodomus in the Lepidocentridae, but with fuller knowledge of Lepidechinus and Perischodomus it seems that both genera belong in the Lepidesthidae as primitive representatives of the family. Kcj/ to the Species of Perischodomus .'^ Five columns of plates in each interambulacral area, test large . . . P. biserialis M.'Coy, p. 401. Five, or perhaps more, columns of plates in each interambulacral area, test small (imperfectly known). P. illinoisensis Worthen and Miller, p. 400. *Perischodoinus biserialis M'Coy. Text-fig. 30, p. 70; Plate 29, fig. 5; Plate 62, figs. 6, 7; Plate 64, figs. 2-8. Perischodomus biserialis M'Coy, 1849, p. 253, text-figs, a-c; 1854, pp. 114, 115, text-figs, a-c; Desor, 1858, p. 157; Quenstedt, 1875, p. 374, Plate 75, fig. 13; Loven, 1874, p. 40; Keeping, 1876, p. 36, Plate 3, figs. 1-5; Jackson, 1896, p. 226; Klem, 1904, p. 18; Lambert and Thiery, 1910, p. 122. Perischodomus bisserialis Pome], 1883, p. 114. Excellent material of this species which I had the opportunity of studying in Dublin, also in London and Cambridge, England, shows the characters of this species well, and is structurally very interesting. M'Coy mentioned two specimens, one in Cambridge, and ' Perischodomus magnus is considered under Incertae Sedis, p. 453. 402 ROBERT TRACY JACKSON ON ECHINI. one in the Griffith Collection in Dublin. Of these, the Cambridge specimen is apparenth' the one from which M'Coy's figures were taken, and is the original of Keeping's (1876) descrip- tion and figures here reproduced as Plate 64, figs. 7, 8. M'Coy's second specimen is doubtless the one in the Griffith Collection in Dublin, here reproduced as Plate 29, fig. 5. The test is spheroidal, and representative measurements are as follows. The Griffith specimen (Plate 29, fig. 5) measures 55 mm. in diameter through the plane J, E, and the inter- ambulacra at the periphery, which is about at the mid-zone, measure 28 mm. in width. On account of crushing, the width of the ambulacra could not be ascertained. The Trinity College specimen, dorsal view (Plate 62, fig. 7), measures about 52 mm. in diameter. The ambulacra at the adoral margin, which is about the mid-zone, measure about 3 mm. in width, and the interambulacra about 25 mm. in width. The Trinity College specimen, internal view (Plate 62, fig. 6), measures about 50 mm. in diameter; the ambulacra at the aboral border, which is about the mid-zone, measure 8.5 mm., and the interambulacra about 19 mm. in width. It is seen from this that the ambulacra on the interior are more than twice as wide as on the exterior. This is due to the strong lateral beveling of ambulacral plates under the adradials. Con- currently the interambulacra are narrower than on the exterior, due largely to the same cause (compare text-figs. 32, 34, p. 75). The ambulacra are narrow, especially dorsally, and are composed of two columns of low plates which imbricate strongly adorally, and laterally bevel strongly under the adradials. Dorsally, all plates are alike, tj'pical primaries with pore-pairs uniserial (Plate 64, fig. 5), but at the mid-zone alternate plates tend to be narrowed laterally, and may in cases even be cut off from contact with the interambulacra. Pore-pairs are situated in the outer portion of each plate, and are dorsally uniserial, but at the mid-zone are somewhat irregularly slightly biserial. On the interior (Plate 64, fig. 2) ambulacral plates are much wider than on the exterior on account of the lateral beveling, and the pore-pairs are near the middle of each plate. The interambu- lacra are wide, composed at the mid-zone of five columns of plates in each area. The plates are relatively thick, and imbricate strongly aborally and from the center laterally and (iver the ambulacrals on the adradial suture. The plates are curved outward on the external surface and rounded on the suture lines. The adambulacral plates are pentagonal, strongly rounded on the adradial sutures. Plates of median columns are rounded polygonal. Each adradial plate bears an eccentric perforate primary tubercle with scrobicule and, in addition, secondary tubercles. Plates of median columns may have a primary tubercle with secondaries, or appar- ently the latter only. Dorsally, in each interambulacrum in the known specimens two columns drop out before reaching the apical disc. Vent rally, the primordial interambulacral plates are in place in the basicoronal row (text-fig. 30, j). 70; Plate 64, fig. 2). There are two plates in the second row, three in the third, four in the foui'th, and five in the fifth row, above which no more column's are added. Primary spines are expanded at the base, tapering, and about 12 mm. long. Secondary spines are similar, but fine and slender, and about 3.5 mm. long. PERISCHODOMUS. 403 The peristome is known only fragmentarily, but some ambulacral plates extend on to this area below the primordial interambulacral plates (Plate 62, fig. 6), indicating' that there are ambulacral plates only on the peristome, as in Lepidesthes (Plate 68, fig. 3). The apical disc is small relatively to the diameter of the test, but I have no measure- ments. Of the apical disc in one specimen one small imperforate ocular is in place, situated between the genitals (Plate 64, fig. 8). The genitals are relatively low and wide, with from five to seven or eight pores each. The lantern is known only from pyramids, which are stout and practically like those of Lepidesthes (Plate 68, figs. 9-12). Lower Carboniferous Limestone, Hook Head, County Wexford, Ireland, cotype, Sedgwick Museum, Cambridge, England; also a second specimen in same museum collection no. 16; coty^pe. Science and Arts Museum Collection, Dublin; Museum of Comparative Zoology Collection 3,071 ; probably the same locality, two specimens, Trinity College, Dublin, Museum Collection; Yorkshire, England, three specimens, British Museum Collection E 10,686 to 10,688; Coplaw, Chtheroe, Lancashire, Museum of Practical Geology 16,310, 16,311; and Ulverstone, Lancashire, the same collection 16,314; Hook Head, Ireland, Museum of Practical Geology 16,315; Clitheroe, Lancashire, Mr. H. L. Hawkins's collection. The structural characters of the species were gathered from various specimens, and I will now consider these with any additional features which they show. The specimen in the Dublin Science and Arts Museum, from the Griffith Collection, which is doubtless M'Coy's second specimen, is the dorsal half of a test (Plate 29, fig. 5). It occurs on a slab from Hook Head, with two specimens of Palaeechinus elegans, one of which is the original of M'Coy's (1844) and Baily's (1865b) figures of that species as earlier described (p. 309). Baily (1865b, p. 63) says of this slab that there are three specimens of Palaeechinus elegans, but one of the three is evidently this Perischodomus. The specimen is somewhat crushed, and the ambulacral areas are largely hidden by the over-riding of the adjacent adradial plates; but some ambul- acral plates are seen. There are five columns of interambulacral plates in each area, the plates showing thin lateral and aboral imbricating edges, also a number of eccentric primary tubercles. The specimen from Hook Head, described by Keeping, which is also one of M'Coj^'s origi- nal specimens, is one of the best specimens of the species known. This choice specimen (Plate 64, figs. 6-8) is in the Sedgwick Museum, Cambridge, England, where I studied it through the kindness of Professor Woods. At the mid-zone, ambulacral plates are alternately somewhat narrowed, but none was seen actually cut off from contact with the interambulacra. Pore- pairs at the mid-zone are slightly biserial. Some of the ambulacral plates show well the lateral bevel (Plate 64, fig. 6). The interambulacra have the typical species character of five columns of plates in each area, the plates bear primary and secondary tubercles (Plate 64, fig. 8), and two of the columns drop out passing dorsally. The apical disc is the best known in the species. There are one small imperforate ocular and four genitals in place. The genitals have from five 404 ROBERT TRACY JACKSON ON ECHINI. to seven pores each, but one of the plates does not show any pores (Plate 64, fig. 8). In the periproct are several plates, not shown in the figure; they are not in contact, and are turned on edge, but apparently belong to this area. They are small and angular, similar to the peri- proctal plates of Lepidesthes (text-fig. 251, p. 428). Another specimen from Hook Head in the Sedgwick Museum, Cambridge, England, no. 16, has interambulacral plates showing pri- mary and secondary tubercles well, and also primary and secondary spines. The primary tubercles are perforate, with clearly defined scrobicules, and are situated eccentrically. There is one primary tubercle to each adradial plate and also on some plates of the median columns. The secondary tubercles are irregularly scattered over the surface of the plates. Primary spines are stout, swollen at the base, and measure about 11 mm. in length. The secondary spines are small, filiform, and measure about 3.5 mm. in length. In Trinity College, Dublin, are two fine specimens of Perischodomus, which I studied through the kindness of Professor John Joly. There is no locality label, but, judging from the lithological character, they are probably from Hook Head, County Wexford. One of these specimens is a dorsal external view (Plate 62, fig. 7; Plate 64, figs. 3-5). The ambulacra are narrow. Dorsally, all plates cross the half-areas, and pore-pairs are uniserial (Plate 64, fig. 5). At the mid-zone (Plate 64, fig. 4), all the plates meet the middle of the area, but some of them are narrowed outwardly and more or less completely shut out from interambulacral contact. As an associated feature, the pore-pairs are somewhat biserial. There are five columns of interambulacral plates at the mid-zone in each area (Plate 64, fig. 3), but dorsally in each area columns 3 and 4 drop out before reaching the apical disc. Columns 1, 5, and 2, however, extend to the upper limits of the areas. The plates are marginally rounded and strongly imbricating. Each adambulacral plate bears an eccentric primary tubercle, and some of the median plates also have a primary tubercle as seen in Plate 62, fig. 7, but they were overlooked in making the drawing of this specimen. The second specimen which I saw in the Trinity College Collection at Dublin is an internal view of the ventral side, and is very choice as the only specimen known in this species showing the ventral portion of the test (Plate 62, fig. 6; Plate 64, fig. 2). The ambulacral plates are very much wider than on the exterior. This is ascribed to the lateral beveling of ambulacral plates under the adradials, but, as the exterior of the ventral side is unknown, it may be in part due to a greater width of the ambulacrum ventrally. As seen in this internal view, the ambulacral plates imbricate aborally and bevel over the adradial plates, the reverse condition of that seen from the exterior (compare text-figs. 32, 34, p. 75). The pore-pairs are about in the middle of each plate, a usual character of the interior (compare Plate 20, fig. 10). Adorally some ambulacral plates extend beyond the primordial interambulacral plates on to the peristomal area and by their position (Plate 62, fig. 6) indicate that ambulacral plates alone existed on the peristome, as in other members of this family and all of the Lepidocentridae PERISCHODOMUS. 405 (p. 284). The interambulacral plates in this internal view (Plate 62, fig. 6; Plate 64, fig. 2) imbricate adorally and from the sides toward the center of each area; .also they bevel under the ambulacral plates on the adradial suture. All this is just the reverse from what is seen from the exterior (compare text-figs. 32, 34, p. 75). The plates of the interambulacral areas are nearly complete, but are restored in part, as indicated by dotted lines (Plate 64, fig. 2). In four areas the primordial interambulacral plate is in place in the basicoronal row (compare text-fig. 30, p. 70), indicating that there is no resorption of the corona by the advance of the peristome, the same being true as far as known of other members of this family, of Bothriocidaris, and all of the Lepidocentridae. In the second row there are two plates in each area. The initial plate of column 2 is represented on the left in this internal view, which is the equivalent of the right as seen from the exterior, and the same method has been adopted in other cases of internal views (Plate 23, fig. 1). In the third row there are three plates, the initial plate of column 3 being in the center. In the fourth row there are four plates in each area. The initial plate of column 4 is on the left of the center in areas A, C, E, and I (which is the equivalent of the right as seen from the exterior) , but in area G the initial plate of column 4 is on the right of the center (which is the equivalent of the left as seen from the exterior). The initial plate of column 5 is in place in the fifth row in areas E and I, and is restored in area C, but areas A and G are not shown above the fourth row. This choice specimen shows that in Perischodomus there is no resorp- tion of the base of the corona, and, as a result, the primordial interambulacral plates are in place. It also shows that succeeding interambulacral columns are added rapidly, one in each succeed- ing row as built, the fifth column coming in immediately after the fourth, indicating a markedly accelerated development, much in excess of anything known in the Palaeechinidae, but similar to that characteristic of many of the Lepidocentridae. This acceleration, with the small number of columns attained, which is five, and the early dropping out of two columns dorsally, marks Perischodomus biserialis as a very peculiar type. In the Museum of PracticaL Geology Collection 16,314, is a specimen of this species from Ulverstone, Lancashire, which consists of ambulacral and interambulacral plates with excellent primary and secondary spines. The longest primary spine, which is not complete distally, measures 13 mm. in length, and 1.5 mm. in diameter in the middle of the shaft. The spines are swollen at the base, nearly cylindrical, but taper gradually to the end; smooth throughout with no spinules. If fine striation existed, as is likely, it is worn off. The secondary spines are slender, and measure about 3 mm. in length. This is the best specimen for the spines that I have seen. In the fragment of the ambulacrum, alternate plates are barely pinched off from interambulacral contact, and pore-pairs are biserial. Another specimen in the same collection, 16,310, from Coplaw, Clitheroe, Lancashire, consists of dissociated interambulacral plates and spines. The tubercles on the plates are very clear. A third specimen in this collection, 16,311, also from Coplaw, consists of interambulacral plates with spines and pyramids of the lantern. 406 ROBERT TRACY JACKSON ON ECHINI. The pyramid is wide-angled with a moderately deep foramen magnum. A half-pyramid seen from within shows the dental slide not quite reaching to the base of the foramen, the usual Palaeozoic character. The pyramids show the suture for the epiphyses, but these structures are wanting. The longest spine measures 12 mm. in length. This specimen is the most instruc- tive for the lantern of any known in this species. The specimen from Hook Head in the Museum of Practical Geology Collection 16,315, .is a crushed test seen from the dorsal side. It shows two genital plates which are wide, rather low, and each has five genital pores. In the British Museum Collection E 10,686 to 10,688, there are three specimens of this species from Yorkshire. The specimens consist of small slabs with interambulacral plates bearing eccentric primarj^ and secondary tubercles. There are fragments of spines, but no ambulacral plates; one of the specimens, E 10,686, has two half-pyramids similar to those of Lepidesthes. One of these as seen from side view shows ridges for the attachment of interpyramidal muscles. Mr. Hawkins's specimen, which he collected at Clitheroe, consists of dissociated interam- bulacral plates, a few ambulacrals, and a very good half-pyramid, which has ridges for the attachment of interpyramidal muscles. All of the English specimens seen are clearly referable to the genus. The plates are thicker and more massive than in the Irish specimens, so that they may represent a distinct species. In their very fragmentary condition it is best to leave them in biserialis. Perischodomus illinoisensis Worthen and ]Miller. Text-fig. 247. Perischodomii.i illinoisensis Worthen and Miller, 1883, p. .33.3, Plate 31, fig. 8; Klem, 1904, p. 20; Lambert and Thiery, 1910, p. 122. Perischodomus {? f) illinoisensis Keyes, 189.5, p. 191. Tretechi nus illinoisensis Tornquist, 1897, p. 784. I have not seen specimens of this species, and the characters are taken from the original description. Ambulacra are narrow, with two columns of low, interlocking plates, with a pore-pair in each plate. 247 Ambulacral plates not uniform in size or shape, though most of them Text-fig. 247. — Peri- schodomus iUinoisensis Seem to have an imperfectly pentagonal outline. The ambulacral plates ort en an i er. jn-^bricate adorally. The interambulacra have five, or perhaps seven Chester Limestone, Lower Carboniferous, Bay City, columns of plates in an area. The plates are very irregular in size, Holotype dorsal view imbricating aborallj' and probably from the Center out Ward; at least the natural size (after Worth- adradial platcs imbricate laterally over the ambulacra. The plates bore en and Miller, 1883, Plate 31, fig. 8). two kinds of spines. The primary tubercle is moderately large, perfo- rate. There is only one plate with a primary tubercle seen, and that is in one of the median columns. Secondary tubercles on adradial plates and perhaps [prob- ably] on others. PERISCHOCIDARIS. 407 Chester Limestone, Lower Carboniferous, Bay City, Pope County, Illinois. The type as stated by Worthen and Miller is in the Illinois State Collection no. 2,483. This species is imperfectly known. The fact that it has two columns of ambulacral plates, and primary and secondary tubercles on interambulacral plates, with other characters, seems quite sufficient to refer it to Perischodomus. Perischocidaris Neumayr. Arvhiuocidaris (pars) Baily, 1874, p. 42; (pars) 1875, p. Ixviii. Perischocidaris Neumayr, 1881, p. 174; Lambert and Tliiery, 1910, p. 120. Proscchinus Pome), 1883, p. 113. Ilomotocchus Sollas, 1892, p. 153. Perischodomus (pars) Duncan, 1889a, p. 10; (pars) Klem, 1904, p. 19. Test spheroidal. Ambulacra broad with six columns of plates at the mid-zone, composed of wide occluded, narrower demi-, and one column of isolated plates in each half-area (Plate 67, fig. 1 ) . The two median columns of occluded plates in the middle line of each area are elevated in rounded ridges similar to those of Melonechinus. Pore-pairs are in sunken valleys, in three vertical series on either side, and lie in the outer portion of the ambulacral plates in each half-area. The interambulacra are wide with in each area five columns of plates at the mid-zone, the plates are high, convex, polygonal, and rounded on the suture lines. Part of the inter- ambulacral plates bear large, elevated, perforate primary tubercles with secondary tubercles, which latter exist on all coronal plates as shown by Baily (1874). The oculars are small, insert, and imperforate, ventrally covering the ambulacra and laterally in part the interambulacra on either side. The genitals are high, wide, and peculiar in that most of them bear a large primary tubercle. Genitals have from three to six pores, each. One of the genitals does not show any pores (Plate 67, fig. 3), but this is ascribed simply to imperfection of the mold, as the only known specimen is an external sandstone mold. This same imperforate plate is also- without a primary tubercle which exists on the four other genitals, but I see no necessary reason for considering it a madreporite as does Sollas. There are no genitals with sixteen pores, as stated by Harte (1865, p. 68) and Neumayr (1881). The ventral half of the test is unknown. This genus differs from the Palaeechinidae in having large perforate primary tubercles on interambulacral plates, also in that the plates are probably imbricate. It differs from other genera of the Lepidesthidae in having melon-like ribs in the ambulacra. It is the first genus of the family systematically considered which has more than two columns of plates in an ambulacral area. Type and only known species, Perischocidaris harteiana (Baily) from the Lower Car- boniferous of Ireland. 408 ROBERT TRACY JACKSON ON ECHINI. *Perischocidaris harteiana (Baily). Plate 65, figs. 1, 2; Plate 67, figs. 1-3. (without name) Harte, 1865, pp. 67-69 and 90, Plate 5, three figs. .Irchacoeidaris sp. Baily, 1865c, p. 90. Archaeocidaris harteiana Baily, 1874, p. 42, Plate 4, figs, a-e; 1875, p. Ixviii. Perigchocidaris hartei Neumayr, 1881, p. 175, Plate 1, fig. 7; Lambert and Thiery, 1910, p. 120. Prosechinns hartciniana Pomel, 1883, pp. 113, 114, Plate [no number], fig. 35. Homotocchus hartii Sollas, 1892, p. 153. Perischodomus hartei Klem, 1904, p. 19. This species is known only from external sandstone molds of the dorsal side of two tests (Plate 65, fig. 1). One of these is nearly complete, the other is a small fragment only. These specimens on a single slab are in Trinity College, Dublin, where I studied them through the kindness of Professor John Joly. A cast is in the Sedgwick Museum, Cambridge, England, 13. The mold of the more complete specimen (Plate 65, fig. I) is a cup-shaped cavity, a plaster cast of which is shown in Plate 65, fig. 2. These indicate a spheroidal test, although the ventral half is unknown. The diameter of the test is about 75 mm. The ambulacra at the mid-zone are about 12 mm., the interambulacra about 35 mm. in width. The ambulacra at the mid-zone have six columns of ambulacra! plates composed of wide occluded, narrow demi-, and one column of isolated plates in each half-area (Plate 67, fig. 1). The plates of the two occluded columns are elevated in a high, rounded melon-like rib that suggests the character in Melonechinus (compare Plate 54, fig. I), a feature not known in any other species of the familj^, although the whole ambulacrum is rounded outward in a rib-like fa.shion in Lepidesthes carinata (Plate 66, fig. 11). The pore-pairs lie in depressed valleys on either side of the ambulacra and are situated near the outer border of each ambulacral plate. A peripodium around each pore-pair is figured by Harte and better by Baily (1874), but I overlooked it in my somewhat hurried examination of the specimen. Baily figures a small tubercle on the inner part of each occluded plate. About fowr ambulacral plates equal the height of an adambulacral, as Harte observed. The interambulacra are broad, with five columns of plates in each area in the zone of the most adoral portion preserved, which I take it is about on the plane of the mid-zone (Plate 65, fig. 1). Dorsally, part of these columns drop out before reaching the apical disc. As shown in Plate 67, fig. 2, column 5 drops out first, then 4, and finally 3, columns 1 and 2 alone reaching the apical disc, and the youngest plates lying in contact with the two oculars on either side. This is the most extreme reduction of columns seen in any member of the Perischoechinoida (p. 211). The interambulacra! plates are high and I'ounded outwardly, being more strongly convex to the exterior than in any sea-urchin that I know. The plates are polygonal, but PROTEROCIDARIS. 409 r rounded on the suture lines, which I thinlv indicates that the plates were more or less imbricating, a family character. However, as the plates are represented only by external molds, it is diffi- cult to be certain on this point. Adambulacral plates are pentagonal, median plates hexag- onal or an equivalent of the same. Part, but not all, of the adambulacral plates bear a large, elevated, perforate primary tubercle which is the largest known in the family, and is surpassed in size in the Palaeozoic only by the Archaeocidaridae. A large tubercle is on the second and fourth adambulacral plates from the apical disc (Plate 67, fig. 2), and this peculiar system seems to hold for each area, as pointed out by Harte and by Baily (1874). The latter also shows numerous secondary tubercles scattered over all the interambulacral plates, and, according to his figures there are primary tubercles on some additional adambulacral plates, also on a few plates of the median columns. The apical disc is in place complete (Plate 67, fig. 3). The oculars are small and imper- forate, and all separate the genitals, meeting the periproct as in Melonechinus, etc. Ventrally, they cover the ambulacra and laterally the interambulacra in part on either side. The genitals are high and wide, almost V-shaped on their adoral termination. Four of the genitals have a prominent primary tubercle, but it is wanting in one, very likely from imperfection of the mold. This is the only case of large primary tubercles on genital plates known in the Palaeozoic, and it is exceptional in any Echini. None of the genitals has sixteen pores, the number given by Harte and Neumayr. Baily pointed out this error and said that there were six pores in four of the genitals arranged in a semicircle, ventral to the large tubercle. In my observations I found six pores in genital C (Plate 67, fig. 3), five pores in genitals A and I, and only three pores in genital G. In genital E no pores were seen, but this may fairly be ascribed to imperfection of the sandstone mold, not to any structural difference of this plate. A similar apparent ab- sence of pores is seen in several genital plates in Plate 53, fig. 1. Harte and Sollas consider the genital without primary tubercle or pores as a madreporite. This may be correct, but in the absence of madreporic pores, which are very rare in the Palaeozoic (p. 172), this view must be taken with qualification. This is one of the most distinct species of Palaeozoic Echini, making no close approach to any other known form. It is from the Lower Carboniferous, yellowish sandstone, Lough Esk, about six miles from Donegal, Ireland. This important and unique specimen is in the collection of Trinity College, Dublin. PROTEROCIDARIS Koniuck (emended). I'rotcruridaris Koninck, 1881, p. .514, Plate S. OUgoporus (pars) Lambert and Tliiery, 1910, p. 121. In describing this genus, de Koninck did not give the characters of the ambulacrum, but his figure of the type and only known species, Proterocidaris giganteus de Koninck, bears a close 410 ROBERT TRACY JACKSON OX ECHIXl. resemblance to the sea-urchins figured by Fraipont (1904) as Oligoporus soreili sp. nov. Frai- pont's specimens, as well as de Koninck's, are from the Carboniferous of Belgium, and on the evidence it seems to me that Oligoporus soreili is a synonym of Proterocidaris giganteus. Act- ing on this opinion, the characters of the genus Proterocidaris are emended by features gathered from Fraipont's publication. Test large, spheroidal, or flattened. The ambulacra are narrow, composed at the mid- zone in each area of four columns of low plates which evidently imbricate adorally and laterally bevel under the adradials. Interambulacra are broad, with many columns of plates at the mid- zone. (According to de Koninck's description, there appear to be 65 columns as a whole, and in P>aipont's figures there are from twelve to thirteen columns in each interambulacral area.) The interambulacral plates imbricate strongly aborally and from the center laterally and over the ambulacrals on the adradial sutures. The interambulacral plates bear each a small primary with secondary tubercles and corresponding spines (Plate 65, fig. 3). The apical disc is known onlj^ from one genital plate (Fraipont, 1904, Plate 5, fig. 1) , which is low and wide. The peristome, periproct, and lantern are unknown. This genus differs from any of the Palaeechinidae by its imbricating plates and also by its primary tubercles and spines. It differs from other genera of the Lepidesthidae by its four columns of ambulacral plates. It has also many more columns of interambulacral plates than are known in other genera of this family. Type and only known species, Proterocidaris giganteus from the Lower Carboniferous of Belgium. Proterocidaris giganteus Koninck (emended). Plate 65, fig. 3; Plate 67, fig.s. 4-7. Protcrocidaru giganteus Koninck, 1S81, p. 514, Plate 8; Klem, 1904, p. 74. Oligoporus soreili Fraipont, 1904, p. 10, Plate 3, fig. 2; Plate 4, figs. 1,2; Plate 5, figs. 1, 2. Oligoporus giganteus Lamliert and Thiery, 1910, p. 121. This species is known from de Koninck's publication and also from the description and fine photographic figures given by Fraipont of what he calls Oligoporus soreili sp. nov., but which I consider a synonym as above discussed. Messrs. Lambert and Thiery (1910), con- sider Fraipont's species soreili a synonym of de Koninck's giganteus, but retain the generic name Oligoporus for the species. The test is apparently spheroidal, though flattened in the specimens figured. De Koninck says that the diameter of his specimen is exactly 30 centimeters, which represents certainly a huge sea-urchin. Basing additional measurements on Fraipont's Plate 4, fig. 1, which he says is natural size, the test is in this case 160 to 170 mm. in diameter. The ambulacra at the mid- zone, as represented by the periphery of the figure cited, measure about 10 mm. in width and the interambulacra about 85 mm. in widtli. PROTEROCIDARIS. 411 The ambulacra are narrow, at the mid-zone composed of four columns which consist of occluded and demi-plates in each half-area (Plate 67, fig. 5). In this figure the plates of the two half-areas are mechanically separated. It is a view from the interior, and the occluded and demi-plates, at least on this side of the test, are of about equal width, and pore-pairs lie near the middle of each half-area, doubtless due to the fact that it is an internal view (com- pare Plate 43, figs. 3, 4). About three ambulacral plates equal the height of an adambulacral, and they bevel under the adambulacrals on the adradial suture (or over them, as seen in internal view). DeKoninck says that in the type the ambulacra are not visible, but, in the flattening of a form with interambulacral plates imbricating laterally, it is mechanically natural that this area should be overridden and thus shut out from external view, as occurs in Perischodomus biseriaUs (Plate 29, fig. 5). The • interambulacra are very broad, with numerous culunuis of plates in each area. De Koninck says that the type appears to have 65 columns; this would be an average of thirteen columns to an area, which is just the number shown here in Plate 67, fig. 4, after Fraipont. In his description, Fraipont says that there are eleven or twelve columns of plates at the middle [mid-zone]. In a section seen from the exterior (Plate 67, fig. 4) at the mid-zone there are seen to be thirteen columns of plates. The thirteenth column originates on the left next to the adradial column. The initial plate of this column is a pentagon, and instead of having a heptagon on its ventral border as usual, the adambulacral plate X has attained an additional compensating side, so that it is hexagonal instead of pentagonal as usual. The interambulacral plates imbricate strongly dorsally and from the center laterally and over the ambulacrals. Each plate bears a small primary tubercle with numerous secondary tubercles. The primary spines are small, slender, and swollen at the base. De Koninck says that the primary spines have a length of 10 mm. and a diameter at the base of 1 mm. The primary spines shown in Fraipont's figures are smaller, apparently about 4 mm. long, but his specimen is much smaller than the type. The secondary spines are more slender than the primaries, and measure about 2 mm. in length, judging from the figure (Plate 67, fig. 7). The interambulacra as seen from within are very interesting for comparison. In this internal view (Plate 65, fig. 3, area I ; Plate 67, figs. 5, 6) the plates imbricate adorallj^ and toward the center from the sides, largely covering up the median column of the area; also the ambul- acral plates laterally bevel over the adambulacrals. All this is just the reverse of what is seen on the exterior, as shown in different areas of Plate 65, fig. 3 (also compare text-figs. 32 to 35, p. 75). Fraipont's fig. 2 on Plate 5 is inverted; that is, the dorsal portion is toward the bottom of the plate. This gives the impression that the direction of imbrication is the reverse of what it really is, but I have corrected the orientation in my copy of part of this figure (Plate 67, fig. 5). In the specimen figured on Plate 65, fig. 3, there are twelve columns of plates in inter- ambulacrum I, and in the view of a section from another specimen (Plate 67, fig. 5) there are twelve columns. In one of Fraipont's figures (his Plate 5, fig. 1) there is a low, wide genital 412 ROBERT TRACY JACKSON ON ECHINI. surmounting the interambulacrum. The pores cannot be distinguished in the photographic figure. This is all that is known of the apical disc in this species. Professor Fraipont saj^s that he had twelve specimens, so that it is evidently quite abundant where found. He referred his specimens to the genus Oligoporus, comparing the ambulacrum with that of my Oligoporus ( = Lovenechinus) missouriensis, see text-fig. 244, p. 338, of this memoir. Structurally, as regards this character, the comparison is apt, but other characters, as imbrication of plates and two kinds of tubercles and spines, remove this species from Oligo- porus or Lovenechinus, and all other genera of the Palaeechinidae as well. The characters are very distinctive and do not fit into any known genus of Palaeozoic Echini except Proterocidaris, as discussed under consideration of the genus. De Koninck's type specimen is from the Lower Carboniferous of Loyers, Belgium. Frai- pont's specimens are from the Lower Carboniferous, Marbre Noir, of Dinant, Belgium. Lepidesthes Meek and Worthen. Lepidesthes Meek and Worthen, ISCSa, p. 522; Lovt'n, 1S74, p. 42; Duncan, lSS9a, p. IG; .Jackson, 1S9G, p. 241; Tornquist, 1897, p. 732; Klem, 1904, p. 24; Lambert and Thiery, 1910, p. 123. Lcpidestcs Neumayr, 1881, p. 151. Hybochinus Worthen and Miller, 1883, p. 331; Duncan, lS89a, pp. 16-18; Tornquistj 1897, p. 732. Test spheroidal, elliptical, or obovate, exceptionally with elevated rounded ribs (L. cari- nata). Ambulacra very wide at the mid-zone, with in each area from eight to sixteen columns of plates, which consist of occluded plates that meet the middle of the area, demi-plates that meet the interambulacra laterally, and, in addition, from two to six regular columns of isolated plates in each half-area. Ambulacral plates are all small, uniform in size, and rhombic or hexag- onal in shape (Plate 70, figs. 1-3), imbricating adorally, and laterally beveled strongly under the adambulacrals. Pore-pairs are in peripodia, situated in each plate above the middle, and in the middle line or more typically nearer the next adjacent interambulacrum than the middle of the plate. Ambulacral plates bear many secondary tubercles and spines but no primaries (Plate 67, figs. 8^ 10). As seen from the interior of the test, ambulacral plates imbricate aborally and bevel over the adradials laterally, and pore-pairs are about in the middle of each plate. For comparison of imbrication seen from the exterior and interior, see text-figs. 32 to 38, p. 75, where these relations are shown. Ventrally, near the peristomal border there are four columns of ambulacral plates as a stage in development (Plate 68, fig. 3), above which area the number of columns increases to the full number at the mid-zone. Dorsally, as a localized stage in devel- opment, there is a very simple condition of the ambulacrum next to the ocular plates (Plate 68, fig. 5; text-fig. 251, p. 428), and from this area passing ventrally toward the mid-zone the number of columns increases to the full specific number. The interambulacra are very narrow at or near the mid-zone with in each area from three to six columns of small plates of uniform size that imbricate strongly aborally and from LEPIDESTHES. 413 the center outward and over the ambulacrals on the adradial sutures. These plates are strongly rounded in outline, and on the exterior bear numerous secondary tubercles with corresponding spines (Plate 67, figs. 8, 13, 14). There are no primary tubercles or primary spines known in the genus. The interambulacral plates as seen from the interior imbricate adorally and from the sides toward the center, and laterally bevel under the ambulacrals, just the reverse of the exterior. On the interior also the plates are much less rounded and more sharply angular than on the exterior, all as shown diagrammatically in text-figs. 32 to 38, p. 75. The primordial interambulacral plates are in place in the basicoronal row (Plate 68, fig. 3), and, as far as known, there are two plates in the second row, three in the third, and four in the fourth as usual in the Perischoechinoida. In later growth a fifth and a sixth column may develop dorsally, as in Plate 66, fig. 10, or one of the four ventral colunans may drop out so that for most of the area there are only three columns, as in Plate 67, fig. 8. The peristome is covered with ambulacral plates only (Plate 68, fig. 3). The apical disc is small proportionately to the diameter of the test, and is rather fully known, oculars are insert, separating the genitals, and with two pores each (Plate 68, fig. 5), which latter is a very unusual character (p. 81) and perhaps represents an individual variant. Or the oculars may be apparently quite imperforate (text-fig. 251, p. 428). The genitals are low and wide, with two or three pores each. In some cases no pores are visible, but this is ascribed to pre- servation and not to a real absence of pores. A madreporite with typical fine pores occurs in two known cases (Plate 68, fig. 5; Plate 71, fig. 1). The periproct is plated with many small, somewhat rounded plates, which are less angular than in the Palaeechinidae (Plate 68, fig. 5; text-fig. 251, p. 428). The lantern is inchned, teeth are grooved, pyramids wide-angled, with a moderately deep foramen magnum, and laterally with ridges for the attachment of interpyramidal muscles. Epiphyses are narrow, capping the half-pyramids. The brace is of the character typical of all regular Echini (Plate 68, figs. 9-14). Mr. Agassiz (1881, p. 79) in discussing the Echinothuriidae, says of the actinal system (peristome) that "from the similarity of its structure in such genera as Archaeocidaris, Pholi- docidaris, and Lepidesthes, we can fairly assume that the abactinal system [oculo-genital plates] corresponds in its general features with that of the Echinothuridae." I think that this statement is hardly justifiable. In the Perischoechinoida there is no evidence of gill cuts for peristomal gills (p. 223), whereas these are present in the Echinothuriidae.' In Archaeocidaris there 'Mr. Agassiz (1874, p. 645) says, "It is well known that the Cidaridae and Echinothuriae have no cuts in the test for the passage of the actinal gills, but we find these cuts directly at the point of contact of the buccal membrane and the teeth, and there the gills make their appearance." (See also A. Agassiz, 1874, pp. 694, 699; 1881, p. 53.) The Ci- daridae have no external or peristomal gills, but do have internal radially situated Stewart's organs (pp. 61, 62, 212). On the other hand the Echinothuriidae do have small peristomal gills, which, as usual in the Centrechinoida (pp. 62, 213), impinge upon the basicoronal plates of the interambulacra thus making slight reentrant cuts (text-figs. 43, 44; pp. 80, 83). The Echinothuriidae in addition to peristomal gills have internal radially situated Stewart's organs (p. 214). 414 ROBERT TRACY JACKSON ON ECHINI. are both ambulacral and non-ambulacral plates on the peristome, while in the Echinothurii- dae there are ambulacral plates only (text-figs. 43, 47, p. 80). I do not know the character of the peristome in Pholidocidaris (p. 433), but presume that like Lepidesthes (Plate 68, fig. 3) it had ambulacral plates only, and in so far both genera may be compared with the Echino- thuriidae as regards the structure of the peristome. I do not see that a similarity of the peristome, where existent, necessarily implies a similarity of the apical disc, as assumed by Mr. Agassiz. The character of the apical disc in the Echinothuriidae is to have oculars all insert, genitals split by secondary sutures, and very commonly for oculars and genitals to be separated, so that the periproct comes in contact with the interambulacra (text-fig. 170, p. 149; also pp. 63, 110, 168, 213). This last feature, of separation, apparently was the most important one in Mr. Agassiz's mind. Mr. Agassiz (1904, p. 30, Plate 11, figs. 1,5,6) shows that in Poroci- daris cobosi the peristome may be covered with ambulacral plates only, yet the apical disc with exsert oculars is very different from that of the Echinothuriidae (p. 110). In the apical disc of the Perischoechinoida oculars are usually all insert (p. 89) ; I know of no case of split genitals, and oculars and genitals form a closed ring in all cases I know of with the possible exception oi Koninckocidaris silurica (pp. 286, 287). I do not know the structure of the apical disc in Archaeocidaris (pp. 265, 266), though I infer from a statement on another page of the work cited, that Mr. Agassiz (1881, p. 80) did. The apical discs in Lepidesthes (text-fig. 251, p. 428; Plate 68, fig. 5), and Pholidocidaris (Plate 73, figs. 3, 4) as far as known do not bear any close resemblance to that structure in the Echinothuriidae. In discussing the anal system, Mr. Agassiz (1881, p. 11) says, "The figures of Bailey, and of Meek and Worthen, and a specimen of Lepidesthes w^hich I have had occasion to examine, would seem to indicate a splitting up of the central plate into a great many smaller plates." Baily (1865b, Plate 4, fig. B) figured the periproctal plates of Palaeechinus elegans (p. 309), but I believe neither he nor any one else has shown a central (suranal) plate in Palaeozoic species (pp. 174, 176). As regards the origin of periproctal plates by splitting in Lepidesthes as assumed by Mr. Agassiz, I can only say that I have seen no evidence for the formation of peri- proctal plates by splitting in this or an}- other genus of Echini. It appears rather that peri- proctal plates originate as separate centers of calcification (text-figs. 59-170). Mr. Agassiz (1874, p. 642) says that the compound ambulacral plates of the Centrechinoida are formed by splitting, whereas they are clearly formed by the coalescence of originally simple plates as discussed (pp. 55, 56). He also considered that interambulacral plates in the Echinothuriidae are derived from the periproct and he assumed the same view for certain of the Perischo- echinoida (A. Agassiz, 1881, p. 80; 1883, p. 33). As Mr. Agassiz (1883, p. 32) expresses it, "The new interambulacral plates are found to be pushing out from the plates of the anal system on each side of the genital plates." Or as he says later (A. Agassiz, 1904, p. 95), "That this remarkable intercalation exists there is not the slightest doubt, and it naturally suggests in old LEPIDESTHES. 415 specimens a flow of anal plates into the interambulacrum, similar to the flow of the ambulacral plates of the corona onto the buccal plates of the actinal system." This view of an entirelj- distinct origin for interambulacral plates from what obtains in all other Echini I believe is a mistake (pp. 63, 110, 287). Mr. Agassiz (1881, p. 95; 1904, pp.96, 104) also says that inter- ambulacral plates in the Echinothuriidae and apparently in the Perischoechinoida are formed by the splitting up of earlier plates. I believe this view is incorrect as discussed (pp. 64, 362). Yet it is possible that coronal plates may originate by splitting, as a peculiar case of the split- ting of ambulacral plates is shown by Dr. Mortensen (1907, text-fig. 27, p. 171) in Sperosoma grimaldi Koehler. This case was overlooked in my discussion of the origin of plates by split- ting (p. 28). In Sperosoma the primary element of each compound ambulacral plate in passing adorally in development becomes divided by a vertical secondary suture, so that as a result there exist two admedian columns of imperforate ambulacral plates. I know of no other similar case in coronal plates, and Messrs. Agassiz and Clark (1909, p. 194) say of this genus, "The existence of a pair of median columns of imperforate ambulacral plates is a feature in which Sperosoma is absolutely unique among all recent regular Echini." I believe the case of Sperosoma and the splitting of genitals and rarely of oculars as discussed (pp. 28, 168-170), are the only cases in Echini in which plates have been shown to be derived by the sphtting of preexistent plates. The type species is Lepidesthes coreyi Meek and Worthen, from the Lower Carboniferous of America. This genus is striking on account of the great development of the ambulacra (com- pare text-figs. 2-14, p. 54) and the relative reduction of the interambulacra. In ambulacral differentiation it is surpassed onlj' by the new genus Meekechinus. The geologically oldest species of Lepidesthes occurs in the Devonian of England, and the geologically highest, also the structurally highest species, is found in the Coal Measures of North America. Kci/ to the Species of Lepidesthes. Eiglit columns of plates in each ambulacral area and three columns of plates in each interambulacral area at the mid-zone L. woiiheni Jackson, p. 41(1. Probably eight or more columns of plates in an ambulacral area, with four known columns of plates in an interambulacral area L. laevis Trautschold, p. 41S. Eight columns of plates in each ambulacral area and fi\e columns of plates in each interambulacral area L. formosa Miller, p. 418. (?) Incompletely known, apparently eiijht columns of plates in an ambulacral area and seven columns of plates in an interambulacral area L. devonicans Whidborne, p. 420. Ten or more columns of plates in an ambulacral area and H\-e columns of plates in an interambulacral area L. spcctabilis (Worthen and Miller), p. 421. Ten or twelve columns of plates in an ambulacral area and six columns of plates in each interambulacral area ............ L. corei/i Meek and Worthen, p. 42:5. 416 ROBERT TRACY JACKSON ON ECHINI. Twelve columns of plates in an ambulacral area and six columns of plates in an interambulacral area; ambulacral areas elevated as prominent ribs . L. carinata sp. nov., p. 424. Sixteen columns of plates in each ambulacral area and four (exceptionally five) columns of plates in each interambulacral area; ambulacra about twice as wide as the interambulacra . . L. coUetti White, p. 425. Sixteen columns of plates in an ambulacral area and four columns of plates in each interambulacral area; ambulacral areas several times as wide as the interambulacral . . . . L. extremis sp. nov., p. 430. Number of columns of ambulacral and interambulacral plates in an area unknown; ambulacral plates proportionately lower and wider than in any other known species of the genus L. caledonica sp. nov., p. 432. *Lepidesthes wortheni Jackson. Plate 66, figs. 1-3; Plate 67, figs. 8-14. Lrpidesthcs wortheni Jackson, 1896, pp. 207, 241, Plate 9, fig. 53; Klem, 1904, p. 26; Lamljert and Thiery, 1910, p. 123. Test elliptical. The type (Plate 66, fig. 3) is a small individual and measures about 36 mm. in height and 27 mm. in width, but it is incomplete laterally, and the diameter, if complete, would be somewhat greater, probably about 30 mm. The ambulacra at the mid-zone measure 9 mm. in width and the interambulacra from 5 to 6 mm. in width. A large and nearly perfect specimen in Mr. Braun's collection (Plate 66, fig. 1) measures about 60 mm. in height and 55 mm. in width through the mid-zone. The ambulacra at the mid-zone measure 15 mm. in width and the interambulacra 9 mm. in width. The ambulacra are wide, with eight columns of plates at the mid-zone. The plates are nearly or quite rhombic in young specimens (Plate 67, fig. 9), but are hexagonal in larger speci- mens (Plate 67, fig. 8). Ventrally, near the peristome there are four columns of plates only, in this character being like the ventral border of Melonechinus (Plate 56, fig. 3). The pores are surrounded by peripodia, and are situated above the middle and nearer the outer border than the median line of each plate, so that from the median line they are situated toward the next adjacent interambulacrum on either side (Plate 67, figs. 8, 10). In small specimens the pores lie nearer the middle line of each plate (Plate 67, fig. 9). The ambulacral plates bear numerous secondary tubercles; they imbricate adorallj^ and laterally bevel strongly under the adradials. The interambulacra are narrow, with, at the mid-zone, three columns of plates which are rounded on the suture lines. The plates imbricate strongly aborally, and the plates of the central column imbricate strongly laterally over the plates of the two adradial columns, and these in turn over the ambulacrals on the adradial sutures (Plate 67, fig. 8). The inter- ambulacral plates bear numerous secondary tubercles and spines, which are slender, swollen at the base, and vertically finely striate (Plate 67, figs. 13, 14). At the ventral portion of the interambulacrum, as far adorally as preserved, there is a row of three plates (Plate 67, fig. 8). Above this row xi fourth column is introduced with a pentagonal plate, but there are only four LEPIDESTHES. 417 plates in this fourth column, above which three columns continue to the dorsal limit of the test (p. 374). A similar short-lived fourth column exists ventrally in other specimens of the species where this area is clearly preserved (Plate 67, fig. 9). There are only three species known in the Perischoechinoida which are characterized by three columns of plates in an interambulacrum at the mid-zone (p. 221). These are: the present species, Melonechinus obovatus sp. nov. (p. 374), and Meekechinus elegans sp. nov. (p. 444). The peristome, apical disc, and periproct are not preserved in the known specimens of this species. The lantern is inclined and pyramids are wide-angled, with a moderately deep foramen magnum. The epiphyses are narrow, capping the half-pyramids (Plate 67, fig. 12). Keokuk Group, Lower Carboniferous, Crawfordsville, Indiana, F. Braun Collection, one specimen; Museum of Comparative Zoology Collection, four specimens, nos. 3,057, 3,171, 3,172, and 3,185; Yale Museum Collection, three specimens, nos. 318, 319, and 320; F. Springer Collection, two specimens, no. 8,018. The holotype, which is in the Boston Society of Natu- ral History Collection 11,601, was without an original label, but from the lithological character and associated fossils, it is in all probability from Crawfordsville or that vicinity. The type as above noted is a small specimen (Plate 66, fig. 3; Plate 67, figs. 9, 12), not much more than half grown. The ambulacral plates at the mid-zone are very nearly rhombic. This is apparently an immature character as in the larger specimen (Plate 66, fig. 1), the plates at the mid-zone are hexagonal, while the younger dorsal plates are more nearly rhombic (p. 426). In the type there are only four columns of ambulacral plates ventrally, as seen in area B, and these are wider and lower than the more dorsal plates. There are three columns of plates in an interambulacral area at the mid-zone and throughout most of the area, but there are four columns of plates ventrally for a short distance in both areas A and C. This specimen on the reverse side has a well preserved pyramid with an epiphysis in place on one side (Plate 67, fig. 12). This is all that is known of the lantern in this species. Mr. Braun's specimen (Plate 66, fig. 1 ; Plate 67, fig. 8) is the largest known in the species and is beautifully preserved. The ambulacral plates are wide hexagons at the mid-zone with pore-pairs set toward the next adjacent interambulacrum on either side. The interambulacral plates show the imbrication well, and in area A, where some plates are absent, the beveled edges are plainly seen. Ventrally, in area A are seen the introduction and dropping out of column 4. The Museum of Comparative Zoology specimen, no. 3,171 (Plate 66, fig. 2; Plate 67, fig. 10), is less nearly complete, but is beautifully preserved. Ambulacral plates at the mid-zone are hexagonal, and the occluded plates are somewhat wider than those of other columns. The pore-pairs are in peripodia and tubercles are very plain. Of the three Yale Museum specimens, one is very small, the youngest specimen of the species seen. Another has the spines well preserved (Plate 67, figs. 13, 14) as described above. 418 ROBERT TRACY JACKSON ON ECHINI. *Lepidesthes laevis Trautschold. Plate 67, figs. 15-18; Plate 68, figs. 1, 2. Lepidesthes laevis Trautschold, 1879, p. 8, two text-figs. This species is known only f ragmentarily ; the form of the complete test is unknown. The ambulacra are wide, but the total number of ambulacra! columns is unknown. There must have been, however, at least eight columns of plates, because in a fragment of an ambula- crum in the Berlin Museum (Plate 68, fig. 1) there are six columns as shown, and the right side of this is composed of hexagonal and therefore not marginal plates. To attain a marginal column and also an equal number of columns in each half-area, it is necessary to add two col- umns to those existent, which would give at least eight columns for the area. Ambulacral plates are thin, imbricate strongly adorally, and bevel under the adradials. They bear small secondary tubercles, and the pore-pairs are situated in the upper half of each plate (Plate 67, fig. 15; Plate 68, figs. 1, 2). On the interior of the test, the pore-pairs are in the lower half of each plate (Plate 67, fig. 16). This same character of pore-pairs in the lower half of the plates internally is seen in Lepidesthes extremis (p. 431; Plate 7, fig. 2). In the interambulacra there are four columns of plates in an area, as shown by Trautschold, but his figures are inverted. The plates are thin, rounded on the suture lines, and imbricate aborally, also from the center laterally and over the ambulacra (Plate 67, figs. 15, 17, 18). On the interior the interambulacral plates are hexagonal in outline instead of rounded, as seen by comparing the outer and inner sides of the same plates in the Berlin specimen (Plate 67, figs. 15, 16). This relation of the interior and exterior of plates in this genus is also shown in text-figs. 32 to 38, p. 75. Lower Carboniferous, Miatschkowa, Province of Moscow, Russia; this is the locality given by Trautschold for the cotypes. Moscow, Russia, two specimens in the Museum fiir Naturkunde, Berhn, which I studied through the kindness of Professor Jaekel. *Lepidesthes formosa Miller. Plate 66, figs. 4-7; Plate 68, figs. 3-14. Lepidesthes formosus Miller, 1879, p. 41, Plate 8, fig. 4; Jackson, 1896, p. 210; Klem, 1904, p. 25; Lambert and Thiery, 1910, p. 123. Of this species I have had the opportunity to study a large series of specimens, including the types which were kindly loaned me from the Chicago University Museum by Professor Weller; also some twenty specimens in Mr. Frank Springer's collection, and two in the United States National Museum. This material not only illustrates the species, but also makes addi- tions to the known generic characters. LEPIDESTHES. 419 Test small, spheroidal or slightly ellipsoidal. The holotype (Plate 66, figs. 4, 5), as esti- mated by restoring the ventral portion, measures about 33 mm. in height. The ambulacra at the mid-zone are 12 mm. in width and the interanibulacra 7 mm. in width. From these measurements the circumference would be about 95 mm. and the diameter about 30 mm. This represents practically a full grown individual, none of the specimens seen being essentially, if at all, larger. The ambulacra are wide, nearly twice as wide as the interambulacra, with, at the mid-zone, eight columns of small, nearly rhombic plates in each area (Plate 68, fig. 4). The plates im- bricate adorally and laterally bevel under the adradials. The pore-pairs lie slightly above the middle of each plate, and either in the median line or a little nearer to the next adjacent inter- ambulacrum. Ventrally, there are four columns of ambulacral plates near the peristomal border, as seen in area B of Plate 68, fig. 3. In other areas of this figure there appear to be fewer columns, but the ambulacral plates are more or less confused and the number of columns cannot be ascertained with assurance. From this zone of four columns the number increases rapidly to the full complement of eight columns, as seen in the aboral portion of ambulacrum B in this figure. Dorsally, next to the ocular plate, in the placogenous zone, there are few plates and apparently only one against the ocular, but one is such an unnatural number for an am- bulacrum that there is probably some plate pushed under, or otherwise obscured. From this small number dorsally, the number of columns increases rapidly, passing ventrally, until the full number is attained. My figures of the type (Plate 68, figs. 4, 5) are so much enlarged that I could not include the whole specimen, but this is compensated for by the photographic figures (Plate 66, figs. 4, 5) and by Miller's excellent figure of the whole specimen. The interambulacra are nari-ow at the mid-zone, with in each area five columns of plates which are small, rounded on the suture line, and imbricate strongly aborally and from the center outward and over the ambulacrals. Ventrally the primordial interambulacral plate is appar- ently in the basicoronal row (Plate 68, fig. 3), with two plates in the second row, three in the third, and four in the fourth row. The point of origin of the fifth column was not definitely ascertained, but it is below the mid-zone, and in the type (Plate 66, fig. 4) there are five columns as far ventrally as the area can be traced. Dorsally the interambulacral plates impinge upon the oculars broadly on either side. The peristome is covered with ambulacral plates onlj- (Plate 68, fig. 3), and this figure represents the best peristomal area known in the family. Teeth are visible in the center of the area. Dorsally in the type, a number of ocular and genital plates are in place (Plate 68, fig. 5). The oculars are relatively large and separate the genitals, meeting the periproct, as usual in the Palaeozoic. Two of the oculars have two pores. This is a very exceptional fact, as pores in ocular plates are very rare in Palaeozoic Echini, and no other case of two pores is defi- nitely known (pp. 89, 413, 435). It is true that Bail}' described two pores in ocular plates of 420 ROBERT TRACY JACKSON ON ECHINI. Palaeechinus elegans, but a further examination of his specimen convinced me that his observa- tion was a mistake, and that the plates are imperforate, as eariier discussed (pp. 309, 363). An accidental pit due to weathering can easily be confused with a structural pore; but there seems no reason for believing that these supposed ocular pores are other than what they appear. The oculars adorally cover the ambulacra and laterally the interambulacra quite widely on either side. The genitals are wide, and almost square in shape, not produced ventrally in an elongate apex as usual in the Palaeechinidae. There are two or three pores to a genital plate, and one of the plates has clearly marked fine madreporic pores, this being one of the few cases in which this structure has been seen in Palaeozoic species (p. 172). On account of the presence of the madreporite, the figures (Plate 66, figs. 4, 5; Plate 68, figs. 4, 5) are oriented correctly by the Loven method, which usually cannot be applied because the true axes are not known. The periproct is covered with many small plates, which are more rounded in outline than in the Palaeechinidae, but are similar to the same plates in Lepidesthes colletti (text-fig. 251, p. 428). The lantern is quite fully known, more so than in any other species of the genus (Plate 68, figs. 9-14). The lantern is inclined as usual in the Palaeozoic. The pyramids are wide-angled, with a moderately deep foramen magnimi; the lateral wings of the pyramids have plicate ridges for the attachment of interpyramidal muscles. The teeth are grooved (Plate 68, fig. 9). The epiphyses are narrow, capping the half-pyramids, and laterally have a glenoid cavity and tubercles for interlocking with the brace as in modern regular Echini. The brace is a block- shaped plate with condyles that fit into the glenoid cavities of the epiphyses (Plate 68, figs. 12- 14), as in all regular Echini, living and fossil (p. 181). Kaskaskia Group, Lower Carboniferous, Pulaski County, Kentucky, holotj'pe and para- type. University of Chicago Collection 6,604; F. Springer Collection 8,042 and other speci- mens; Sloan's Valley, Pulaski County, Kentucky, F. Springer Collection 8,023, 8,027, 8,033, 8,043, and other specimens; United States National Museum Collection 42,349; Chester Group, Lower Carboniferous, Huntsville, Alabama, United States National Museum Collection 37,716. *Lepidesthes devonicans Whidborne. Plate 68, figs. 15-17. Lepidesthes (?) devonicans Whidborne, 1896, p. 376; 1898, p. 200, Plate 24, figs. 1, 2; Plate 25, figs. 3a-3f. Lepidesthes devonicans I/ambert and Thiery, 1910, p. 123. Test very large, form unknown. Whidborne estimated that it was three inches or more in diameter. The specimens in the Museum of Practical Geology consist of two slabs which are counterparts and bear the impressions of plates. One of the specimens, no. 7,160, is a mold of the interior, and the other, no. 7,161, is a mold of the exterior, and bears impressions of tub- LEPIDESTHES. 421 ercles and spines. The details of structure can be ascertained only with difficulty and not with entire certainty. Ambulacral plates are small, about 3 mm. in width, and somewhat wider than high. They are polygonal, imbricating, with, on the exterior, pore-pairs situated in about the middle of each plate. On the internal mold they are situated somewhat nearer the ventral border of the plates (as in Lepidesthes extremis, Plate 71, fig. 2). The ambulacral plates bear small tubercles simi- lar to those of the interambulacral plates. It is difficult to be certain of the number, but there are at least eight columns of ambulacral plates in an area. In the interambulacra there are apparently seven columns in an area, as stated by Whid- borne, and I confirmed this by observation. The plates are small, about 3 mm. in width and height, and bear numerous small tubercles. Whidborne says these are minutely perforate. This is doubtful, though in some of the impressions there seem to be traces of the impression of a perforation. Such are not known otherwise in the genus. The spines are numerous, acicular, swollen at the base, and measure up to 5 mm. in length. The spines as far as seen are all essentially similar, and I saw no second series of smaller spines as Whidborne thought. Of course, in any specimen some spines are smaller than others, but I think that here there is no indication of two series as regards size. The impressions of a stout lantern are present. Whidborne gives a very detailed discussion of this species, and, as he says, it fails to meet entirely the requirements of the genus. The main difference is the apparently minutely per- forate tubercles. It does have characters of Lepidesthes and would not fit in any other genus, so that it seems best to leave it here. The chief interest in the species is that it is from the Devonian, and is thus geologically older than any other known species of the genus, and also it is from Great Britain, being the first of the genus described from that country. The speci- men in general appearance resembles my figure of L. extremis, at least the internal mold does. Pilton Beds, Upper Devonian, North Devon, England, holotype. Museum of Practical Geology Collection 7,160 and 7,161 (these being counterparts); and Croyde, England, in Mr. Whidborne's collection. Lepidesthes spectabilis (Worthen and Miller). Text-figs. 248-250. IIi/bufliinLW spccfahUis Worthen and Miller, 188:3, p. .332, Plate 31, figs. 5a-5d, 6, 7. Ilybochinus (without giving specific name) Duncan, 1889a, pp. 16-18. Lepidesthes spectabilis Keyes, 1895, p. 184; Klem, 1904, p. 26; Lambert and Thiery, 1910, p. 123. Hyhoechinus spectabilis Jackson, 1896, p. 207. This species I have not seen, and its characters are soinewhat ambiguous, as gathered from the description and figures. In regard to the imbrication in this species, Worthen and Miller 422 ROBERT TRACY JACKSON ON ECHINI. saj^ that the ambulacra imbricate aborally and the interambulacra adorally. The imbrication of plates is shown in text-fig. 248, which figure, reversed from the original orientation, also shows a portion of the lantern somewhat displaced, but extending from what Worthen and Miller consider the dorsal side. Professor Duncan's (1889a, p. 16) statement, that Worthen and Miller may have seen the specimen from within, which would account for the direction of imbrication as given, is wide of the mark, because their figure is evidently an ex- ternal view. It is perfectly obvious from their figure that Worthen and Miller ori- ented their specimen incorrectly and that the ambulacra imbricate adorally and the interambulacra aborally as usual in Echini (p. 76). As regards the peculiar protuber- ances, one of which is described as "horn- like " by Worthen and Miller, the}' may well be considered as extreme local distor- tions, and the "horn-like" protuberance even as a foreign adherent body. In its length, lateral position, and structure as shown, it appears to have no relation to anything known in other Echini. Pro- fessor Duncan's (1889a, p. 17) view that these protuberances might be considered as marsupia appears to be quite untenable. The test is subspheroidal, about 20 mm. in diameter, as gathered from the published figure. The ambulacra are wide, with ten columns of plates in an area, Worthen and Miller say that this number may be and probably is exceeded in the equatorial zone; the plates, which are rather wide hexagons, bear each a pore-pair. The ambulacral plates imbricate adorallj^ (see above discussion). Interambulacra with five columns of plates which are small, somewhat rounded on the suture lines, but almost rhombic in outline. Only four columns of interam- bulacral plates exist in the enlarged fragment (text-fig. 249). The interambulacral plates inil)ricate aborally (see above discussion). The surface of the plates as described is covered with small granules for the attachment of spines. Jaws are present. They are shown poorly in text-fig. 248, and better in Worthen and Miller's Plate 31, figs. 6, 7, the latter figure being doubtfully referred to this species by the authors. Chester Limestone, Lower Carboniferous, Prairie du Long Creek, Monroe City, Illinois; same horizon, Pope County, Illinois. Worthen and Miller list the original specimen or speci- mens as in the Ilhnois State Collection, no. 2,481, Text-figs. 2-18-2.50. — Lejiuleslhcs tsjHclabilis (Worthen and Miller). Chester Limestone, Lower Carboniferous, Prairie du Long Creek, Monroe County, Illinois; cotype (after Worthen and Miller, 1883, Plate 31, figs. .5a, 5b, 5c). 24S. Whole test, distorted, a horn-like protuberance extends from one side, lantern protruding; the orientation is reversed from that given by Worthen and Miller. Natural size. 249. Fragment of test, enlarged X 2. 250. Horn-like protuberanee, magnified. LEPIDESTHES. 423 *Lepidesthe3 coreyi Meek and Worthen. Plate 66, figs. 8-10; Plate 68, figs. 18-22. Lepidesthcs coreyi Meek and Worthen, 186Sa, p. 524, text-fig. A on p. 525; 1873, Plate 16, figs. 2a, 2b; A. Agassiz, 1874, pp. 648, 649, text-fig. 4; Trautschold, 1879, p. 9; Keyes, 1895, p. 184; Jackson, 1896, p. 209; Kleni, 1904, p. 25; Lambert and Thiery, 1910, p. 123. Lepidesthcs corveyi (by clerical error) Trautschold, 1879, pp. 8, 9. This species is known from several well preserved specimens, including the type. Test obovate, with ambitus dorsal to the mid-zone, a very unusual character in Echini (p. 32). The holotype (Plate 66, figs. 9, 10) is imperfect dorsally, but the estimated height is about 45 mm. The width is 51 mm., but this is exaggerated, due to lateral compression. The ambulacra at the mid-zone measure 12 to 13 mm. in width and the interambulacra 8.5 to 9 mm. in width. Mr. Braun's specimen (Plate 66, fig. 8) measures 57 mm. in height and 55 mm. in width at the ambitus, but this measurement as in the type is affected by lateral compression; the ambulacra measure 17 mm. in width and the interambulacra 10 to 11 mm. in width. Several other specimens measured give about the same proportionate measurements. The ambulacra are wide, with, in each area at the mid-zone (Plate 68, fig. 18), ten or exceptionally twelve columns of plates, which are rather low, wide hexagons, bearing pore-pairs slightly above the center of each plate and also a little nearer the next adjacent interambula- crum than the middle line of each plate. The interambulacra are narrower than the ambulacra, with six columns of plates in an area at or above the mid-zone. Meek and Worthen speak of there being six or seven columns of interambulacral plates, but I have not observed a seventh in any specimen of this species. The plates are rounded on the suture lines, and imbricate strongly aborally and from the center outward and over the ambulacrals on the adradial sutures. In the Freiburg i. B. specimen, the primordial interambulacral plate is in place in the basicoronal row. There are two plates in the second row and three in the third row, the fourth, fifth, and sixth columns coming in later, at intervals, and the sixth originating usually above the mid-zone. In the type, however, (Plate 66, fig. 10), the sixth column in area A originates below the mid-zone. Ambulacral and interambulacral plates alike bear small secondary tubercles and spines about 1 mm. in length (Plate 68, figs. 19-21). The peristome, apical disc, and periproct are unknown in this species. In the Freiburg specimen there are five dental pyramids in place (Plate 68, fig. 22), similar to those of Lepidesthcs forrnosa. This species is the type of the genus. Keokuk Group, Lower Carboniferous, Crawfordsville, Indiana, University of Michigan Collection, holotype, no. 149; and another larger specimen, no. 150 of the same collection; F. Braun Collection; Museum of Comparative Zoology Collection, two specimens, nos. 3,173, 424 ROBERT TRACY JACKSON ON ECHINI. and 3,174 (from R. T. J. Coll.); Yale University Museum Collection 317; Freiburg i. B., Germany, Museum, one specimen. The holotype in the Michigan Museum has the original label in Meek's handwriting. It is imperfect dorsally but otherwise well preserved, and is well represented in Professor Meek's figure here reproduced as Plate 66, fig. 10. There are ten columns of plates in ambulacral area B, and in interambulacrum A the introduction of the sixth column is seen just below the mid-zone. On the reverse side the tips of a strong lantern are in place. There is also a second specimen of this species in the University of Michigan Collection 150, which I studied through the kindness of Dr. A. G. Ruthven. The second specimen is much larger than the type, measuring 60 mm. in height. In one ambulacral area it has twelve columns of plates and six columns in two interambulacral areas. Mr. Braun's (Plate 66, fig. 8) is the finest specimen of the species seen. It has ten columns of plates in an ambulacral area and six columns of plates in two interambulacral areas. At the zone represented by the figure (Plate 68, fig. 18) only five interambulacral columns were visible. This specimen has numerous secondary tubercles on ambulacral and interambulacral plates and spines about 1 mm. in length. In the University Museum at Freiburg in Breisgau, there is an excellent specimen of this species, which I studied through the kindness of Professor Georg Boehm. It is large, about 60 mm. high, and there are ten columns of ambulacral plates in an area. Ventrally, in one interambulacrum the primordial interambulacral plate is in the basicoronal row, and additional columns come in as above described; also parts of a lantern occur as above described. *Lepidesthes carinata sp. nov. Plate 66, fig. 11. This species is known from one specimen only, which is worn, so that details are not very clear. The test is large, with strongly elevated ambulacral areas recalling the melon-like ribs of Melonechinus, but here the whole area is involved instead of mainly the two median columns of occluded plates as in that genus (Plate 60, fig. 3). This species and Perischocidaris harteiana (p. 408; Plate 65, fig. 1) are the only members of the Lepidesthidae in which melon-like ribs have been observed. The diameter of the test is about 65 mm.; the ambulacrum is about 25 mm. wide, and interambulacrum about 11 mm. in width. There are twelve columns of plates in an ambulacral area and six columns of plates in an interambulacral area at the mid-zone. This species is structurally similar to Lepidesthes coreyi, but differs in the strongly elevated ambulacral areas. Keokuk Group, Lower Carboniferous, Crawfordsville, Indiana; holotype in Museum of Comparative Zoology Collection 3,175. LEPIDESTHES. 425 *Lepidesthes colletti White. Text-figs. 14, p. 54; 21, p. 59; 32-38, p. 75; 251, p. 428; Plate 69, figs. 1-8; Plate 70, figs. 1-5; Plate 71, fig. 1. Lcpidesthes colletti White, 1878, p. 33; 1880a, p. 163, Plate 40, figs. 2a, 2b; 1882, p. 362, Plate 41, figs. 2, 3; Keyes, 1895, p. 184; Jackson, 1896, pp. 210, 241; Klem, 1904, p. 24; Lambert and Thie'n-, 1910, p. 123. This species is one of the most striking known from the Palaeozoic, and I have had the opportunity of studying twelve fine specimens, including one figured by Dr. Wliite (1880a, Plate 40, figs. 2a, 2b) which was kindly loaned me by Professor Stuart Weller; also a fine series formerly in my collection, and now in the Museum of Comparative Zoology, or the British Museum, and specimens in Mr. Braun's and Mr. Springer's collections. The test is high and elliptical. When Dr. White originally described this species in 1878, he had only a single specimen which he says was in a crushed condition, the height about 45 mm. and the diameter considerably less. In Dr. White's second paper in which this species is described, he figured a second specimen which was not the type, and is considerably smaller, as he noted. This second specimen (Plate 69, figs. 2, 3) measures about 37 mm. in height and 35 mm. in width. The ambulacra at the mid-zone measure 12 mm. in width and the inter- ambulacra 5 mm. in width. Another specimen in the Chicago University Museum, which is larger and more nearly complete (Plate 69, fig. 4), measures 47 mm. in height and 40 mm. in width. The ambulacra at the mid-zone measure about 15 mm. in width and the interambulacra 7 mm. in width. A sliglitly larger specimen in the Museum of Comparative Zoology (Plate 69, fig. 6) is somewhat flattened, but otherwise without distortion. It measures about 51 mm. in height and 40 mm. in width at the mid-zone. The ambulacra at the mid-zone measure about 14 mm. in width and the interambulacra 7 mm. in width. A very large specimen in the Museum of Comparative Zoology (Plate 69, fig. 8) measures about 70 mm. in height and 65 mm. in width. The ambulacra measure about 26 mm. in width and the interambulacra 9 mm. in width. These measurements show that the ambulacra are twice as wide as the interambulacra or wider, and are proportionately wider in large specimens than in smaller ones. They are, however, cer- tainly never five or six times as wide, as stated by Dr. White (1880a, p. 163). Tlie ambulacra are very wide, at the mid-zone composed of 16 columns of plates. In no case have I found 18 or 20 coliunns in an area, as Dr. White thought possible. The structure consists of a column of occluded plates extending from the middle of the area outward, a column of demi-plates extending from the interambulacra inward, and six regular columns of isolated plates in each half-area. This is as highly evolved an ambulacral structure as is known in any Palaeozoic sea-urchin excepting Meekechinus elegans, in which there are 20 columns in an area at the mid-zone (Plate 76, fig. 1). The ambulacral plates imbricate strongly adorally and 426 ROBERT TRACY JACKSON ON ECHINI. laterally bevel under the adradial plates. In the smaller specimens the plates are rhombic in outline (Plate 69, figs. 1-6; Plate 70, fig. 1; Plate 71, fig. 1), the individual plates being not at all or very slightly truncated by the succeeding dorsal or ventral plate of their own vertical series. As a result, the plates appear to lie in diagonal series instead of in the usual vertical series. In large specimens, however, the plates by horizontal growth have taken on a hexag- onal form in which the dorsal and ventral borders truncate or are truncated by the next suc- ceeding dorsal or ventral plates of their own vertical series (Plate 69, figs. 7, 8; Plate 70, fig. 3). It is to be observed that a specimen, as Plate 69, fig. 8, in which the plates are hexagonal at the mid-zone, dorsally has rhombic plates, these younger plates as a localized character being like the rhombic plates at the mid-zone in a younger individual. This is the same relative struc- ture as is .shown in the ambulacra of Lepidesthes wortheni (p. 417). The pore-pairs in each plate lie in the dorsal half, as shown by Dr. White, and when the plates are rhombic, lie in about the middle line; but as the plates widen to a hexagonal form, the pores come to lie nearer the next adjacent interambulacrum than the middle of the plate, as usual in Echini (text-figs. 4-14, p. 54). The ambulacral plates bear small secondary tubercles and spines similar to those of the interambulacral areas. Dorsally, next the apical disc (text-fig. 251, p. 428), there are two ambulacral plates in an area. Both of these are primaries, therefore crossing the half-area in which they lie. From this simple condition of primaries only, as a localized stage, the number of columns increases, passing ventrally to the full number charac- teristic of the species, and, in taking on this number, the ambulacrum passes rapidly through a series of stages or phases in which there is the same number of columns that may be found as an adult character in lower species of this genus. These dorsal localized stages in Lepidesthes are directly comparable to the similar stages that I have shown in several species of Melon- echinus (text-fig. 237, p. 231; pp. 366, 380). The interambulacra are narrow, half the width of the ambulacra, or less, and at the mid- zone consist of four columns of plates which extend ventrally and dorsally as far as the areas could be traced in the several specimens. In one area dorsally in one large specimen (Plate 69, fig. 7; Plate 70, fig. 4) a fifth column originates 14 mm. from the top. This is the only case of more than four columns seen in the species. The interambulacral plates are small, rounded on the suture lines, and imbricate strongly aborally, also from the center laterally, and over the ambulacra on the adradial sutures. In Lepidesthes wortheni (Plate 67, figs. 8, 11) and L. for- mosa (Plate 68, figs. 5, 6) there is an odd number of columns of interambulacral plates, and in these the middle column imbricates laterally in two directions, and the plates of this column on the exterior are therefore wider than those of the lateral columns which imbricate laterally only in one direction, to the left in the left half of the area and to the right in the right half of the area. In Lepidesthes colletti with four columns of plates there is obviouslj' no middle column. In this case the plates of colunm 3 are usually wider, and imbricate laterally in two directions, LEPIDESTHES. 427 as in areas 1, 2, 3, and 5 of Plate 71, fig. 1; but sometimes the plates of column 4 are the wider and imbricate laterally in two directions, as in area 4 of this same figure. In a total of 27 interambulacral areas observed, column 3 has the wide plates in eighteen cases and column 4 has the wide plates in nine cases. Why there is this tendency to left-handedness in regard to which column has the wide plates, is without explanation, but it emphasizes the definite- ness of arrangement in trifling details. The interambulacral plates as seen from the interior are angular, and imbricate adorally and from the sides toward the center; also the ambulacral plates in this view bevel over the ambulacrals. This relation of imbrication seen from the exterior and interior is shown diagrammatically in text-figs. 32 to 38, p. 75. Interambulacral plates, like the ambulacrals, bear small secondary tubercles and spines about 1.8 mm. in length. The plates of the peristome are not preserved in any of the specimens, but tips of the dental pyramids and teeth are in place in two cases. The apical disc is in place in two speci- mens (Plate 69, figs. 1, 5; Plate 71, fig. 1; text-fig. 251). In the larger of these specimens (Plate 69, fig. 1) the apical disc measures about 6 mm. in diameter and the specimen about 37 mm. in diameter. The apical disc is therefore relatively small, being proportionately about 16 % of the diameter of the test. A small apical disc is a progressive character (p. 87) and is in keeping with the great development of the ambulacrum which marks this species as a highly specialized type amongst Echini. The oculars are rounded plates, lying above the ambulacral areas and covering the interambulacra in part on either side, seen clearest in areas I and V (text-fig. 251). The genitals are wide, rounded plates. Genital 1, which is partially hidden, shows two pores, and genital 5 has three pores. Genital 2 shows numerous clear, fine madre- poric pores, and by this structure the specimen was oriented by the Loven method. Rather curiously the madreporite does not show any genital pores. Genitals 3 and 4 are in part hidden and do not show any pores. The plates of the periproct are numerous, small, rounded, similar to those oi Lepidesthes formosa (pp. 413, 419). Keokuk Group, Lower Carboniferous, Salem, Washington County, Indiana. This is the locality of Dr. White's holotype, the only one he had when first publishing the species. I do not know where the specimen is; the specimen from the same locality, of which he published a figure in his second paper on this species (1880a), is in the University of Chicago Collection 6,340. The location of the specimen which Dr. Wliite figured in his third paper (1882) I do not know. The same horizon, Bono, Indiana, University of Chicago Collection 6,641 ; Bono, Lawrence County, Indiana, two fine specimens. Museum of Comparative Zoology Collection 3,183, 3,184; Montgomery County, Indiana, Museum of Comparative Zoology Collection 3,176, 3,177, 3,178, and 3,179; British Museum Collection E 10,677; F. Braun Collection, two specimens; Indian Creek, Montgomery County, Indiana, F. Springer Collection 8,102. The specimen figured by Dr. White (1880a), as stated, is in the University of Chicago Collection. As it is not the original single specimen he had when he first published the species, 428 ROBERT TRACY JACKSON ON ECHINI. it is not the typo. It is a small individual (Plate 69, figs. 2, 3), about half grown, and is quite typical in form, but somewhat flattened laterally. The ambulacral plates at the mid-zone and throughout the areas are rhombic, as figured by Dr. White, not yet having taken on any of the dorsal and ventral truncation of the plates seen in older specimens. A second speci- men in the University of Chicago Collection is from another locality (Plate 69, fig. 4). This is a larger individual, more nearly complete and perfect in form excepting for a little lateral flattening. The ambulacral plates at the mid-zone (Plate 70, fig. 2), instead of being rhombic, m b a Text-fig. 2.51. — Lcpidesthcs colletii White. Keokuk Group, Lower Carboniferous, Montgomery County, Indiana. Museum of Comparative Zoology Collection 3,178 (from R. T. J. Coll.). Same specimen as Plate 69, fig. 1; Plate 78, fig. 1. Ambulacra are dorsally simple. Oculars and genitals are broad, low, rounded; madreporic pores in genital 2. are narrow hexagons, having a short dorsal and ventral straight edge, but are not nearly so wide as in those of a large individual (Plate 70, fig. 3). In the interambulacra of this specimen (Plate 69, fig. 4) the plates of column 4 are wide in area A, but in area C the wide plates are in column 3. A fine specimen in the Museum of Comparative Zoology shows a dorsal view somewhat compressed, with a side thrust, but all the plates are in place (Plate 69, fig. 1; Plate 71, fig. 1; text-fig. 251) . The ambulacral plates are rhombic, with 16 columns in each area at the mid-zone. LEPIDESTHES. 429 Dorsally, near the oculars, there are few, even two plates in one row in an area, and the number of columns increases, passing ventrally, as above described. In the interambulacral areas all four columns continue to the apical disc. In four areas, coluimi 3 consists of wide plates imbricating laterally in two directions, and in these areas' there is one column on the left, and two on the right of this wider column. In one area, 4, column 4 consists of wide plates with two columns on the left and one on the right. The apical disc and periproct are well preserved as above described. A specimen in Mr. Braun's collection (Plate 69, fig. 5) is like that just described, a dorsal view and very well preserved. The ambulacral plates are all rhombic. In the interambulacral areas A and E, column 4 is the wider column, and in C and G, 3 is the wider column; area I is doubtful, but apparently column 3 is composed of wide plates. Periproctal plates, similar to those of text-fig. 251, are in place, but the plates of the apical disc were not worked out. A choice specimen in the Museum of Comparative Zoology is perfect in form from the mid-zone up, but ventrally is somewhat imperfect (Plate 69, fig. 6; Plate 70, fig. 1). The am- bulacral plates are for the most part rhombic, but a few at the mid-zone and ventrally have their dorsal and ventral borders more or less completely truncated, forming hexagons. The interambulacral areas are complete, and in both areas A and C, column 3 consists of broad plates imbricating laterally in two directions over the plates of the adjacent columns. Ventrally, the tips of dental pyramids are in place. A specimen which I gave to the British Museum, and now no. E 10,677 of that collection, is similar to the one just described, but somewhat smaller. It is laterally compressed, and measures 39 mm. in height and 34 mm. in width. The ambulacra measure 13 mm. in width and the interambulacra about 5.5 mm. in width. Ambulacral plates are rhombic, with 16 columns in an area at the mid-zone. The interambulacral areas are not very clearly preserved, but there are four columns in two areas. In area A the plates of column 3 are wider, imbricating laterally in two directions, and in area C the plates of column 4 are wider. The tips of five dental pyramids with traces of teeth are seen ventrally. A very large and well preserved specimen in the Museum of Comparative Zoology shows many interesting characters (Plate 69, fig. 7; Plate 70, figs. 3, 4). This specimen measures 70 mm. in height and about 61 mm. in width. The ambulacra are about 24 mm., and the interambulacra about 10 mm. in width. The ambulacra at the mid-zone are relatively wide hexagons, the dorsal and ventral border of each plate impinging on and truncating ventrallj^ or truncated dorsally by, the next succeeding plate of its own series. This is quite distinct from the rhombic plates seen at the same area in younger specimens (Plate 71, fig. 1). Dorsally, however, a different condition exists, for in this region, as seen in the middle line of area J (Plate 69, fig. 7), the ambulacral plates are rhombic in shape, therefore in a large individual, as a localized stage, these younger plates have the character of all the plates of a younger individual. 430 ROBERT TRACY JACKSON ON ECHINI. In interambulacrum A there are four columns of plates from the ventral portion as far as pre- served to the mid-zone; but above the ambitus, and 14 mm. below the top of the specimen, a fifth column appears in the middle of the area and continues to the dorsal part of the test. This is the only case of more than four columns seen in the species. The interambulacral plates are worn, but bear small secondary tubercles and spines (Plate 70, fig. 4). Another very large specimen in the Museum of Comparative Zoology is remarkablj- clearly preserved (Plate 69, fig. 8). The ambulacra are relatively very wide, as shown in the figure and by the measurements given above. The ambulacral plates at the mid-zone are relatively wide hexagons, as in the last specimen described, and, again like it, the plates dorsally are rhombic in outline instead of hexagonal. The interambulacra have four columns of plates in areas A and C, and in both the plates of column 3 are wide, imbricating laterally in two direc- tions. Ambulacral and interambulacral plates show well the numerous small secondarj' tubercles. *Lepidesthes extremis sp. nov. Text-fig. 252, p. 431; Plate 71, fig. 2; Plate 72, figs. 1, 2; Plate 73, figs. 1, 2. This interesting species is represented by two specimens which were kindly loaned me for study by Mr. Eber Hyde, of Lancaster, Ohio, through his son, Mr. J. E. Hyde, of Columbia University. The specimens are internal or external molds in a dark siliceous horn-stone, stained a yellowish brown by iron. This species, while known only from portions of the test, and those not at the mid-zone, yet represents the most massive test and proportionately the widest ambulacra known in this striking genus. The fullest idea of the species is gained from the specimen which is an internal mold. This .specimen, as far as preserved, measures 93 mm. in width by 45 mm. in a plane at right angles to the same. As it is somewhat compressed and not complete on the periphery, it is difficult to estimate the size, but in the ventral view, Plate 72, fig. 1, from the center to the outer limit of ambulacrum H, it measures about 55 mm. From this radial measurement the test when alive probably measured not less than 100 mm. in diameter. One side of the specimen bears an impression of the ventral portion of the test, and on the reverse side a similar impression of parts of the dorsal portion of the test (Plate 72, figs. 1, 2; Plate 71, fig. 2). No portion of the specimen extends far enough to give the character at the mid-zone, and the ventral impression is the most instructive. In this view and at a zone which is much below the mid-zone (Plate 71, fig. 2) ambulacrum B measures 33 mm. in width in a straight line across from its points of aboral contact with interambulacra A and C. Interam- bulacrum C measures at its aboral border (Plate 71, fig. 2) 5 mm. in width; therefore the am- bulacrum at this zone is something more than six times the width of the interambulacrum, a proportion in excess of that known in any other regular sea-urchin, living or fossil, and in this character approached only by Meekechinus elegans (p. 443). The ambulacrum in Echini is LEPIDESTHES. 431 the one most essential feature in their morphology and evolution, and in its great proportionate development to the rest of the test, this species takes rank among the most specialized of known types (p. 234). The ambulacra, as far aborally as known, have 16 columns of plates in each area, and, as stated, are several times the width of the interambulacra instead of about twice the width, as in Lepidesthes colletti. As the impressions of the plates are internal molds, the details are not the same as they would be on the exterior, but the plates are wide and would evidently be hexagonal if the external sutures were visible. The pore-pairs are situated about in the middle line in the lower or adoral half of each plate, a general character of the interior in this genus (compare p. 418, Plate 67, figs. 15, 16). The ambulacral plates evidently imbricate strongly ventrally and laterally bevel under the adradials. The interambulacral areas, as far as preserved, have four columns of plates in an area (Plate 71, fig. 2), and the same is true of the dorsal side (text-fig. 252), as seen, not very clearly, in Plate 72, fig. 2. As there are four columns, both ventrally and dorsally, this is doubtless the character of the area for the parts which are missing, except the ventral developing area near the peristome. The interambulacral plates are all small, about equal in height to the ambulacrals, both ventrally and dor- sally. They imbricate strongly aborally and from the center laterally and over the ambulacra on the adradial sutures. On the exterior, as shown by external molds, the plates are rounded in outline as usual, and bear small secondary tubercles (Plate 73, fig. 1). Crowded masses of spines occur on both specimens as external molds. They are all slender, filiform, swollen at the base, and about 3 to 4 mm. long (Plate 73, fig. 2). Peristomal, apical, and periproctal plates are unknown in this species, but ventrally there are impressions of a powerful lantern. This species has the same number of columns of ambulacral and interambulacral plates as Lepidesthes colletti, but in the extreme width of the ambulacra and narrowness of the interambulacra, it may well be considered the most specialized species of the genus. It is also geologically the highest species of the genus, as it comes from the Coal Measures or Carboniferous proper, whereas all other species are found in the Devonian or Lower Carboniferous. Putnam Hill Limestone, Carboniferous (Coal Measures), New Lexington, Perry County, Ohio, in the collection of Mr. Eber Hyde, holotype no. 524, paratype no. 525. Text-fig. 252. — Lepidesthes extremis sp. nov. Internal mold of the dorsal side, same specimen as Plate 72, fig. 2. Holotype. X about 1.8. 432 ROBERT TRACY JACKSON ON ECHINI. * Lepidesthes caledonica sp. nov. Plate 72, figs. 3-10. In the British Museum and in the Museum of Practical Geology there are a number of small slabs bearing isolated plates of Lepidesthes from Roscobie, Scotland. Excepting the Lepidesthes devonicans from the Devonian, this is the first case of Lepidesthes reported from Great Britain. The number of columns of plates in an area cannot be given, but there is enough detail to warrant considering it distinct from any other known species. Ambulacral plates are low and wide. The surface of the plates, not counting the beveled edges, measures about 2.5 mm. in width by 1.5 mm. in height. The ambulacral plates are strongly imbricating adorally and laterally beveled under the adradials. The plates figui'ed show the dorsal and lateral beveled edges which respectively would underlie the adoral border of the next ambulacral plate dorsally, or the left adambulacral plates of an interambulacral area (compare text-figs. 32, 33, p. 75). The pore-pairs are in peripodia, lying to the right of the centre of the plates as figured. On the surface of each plate there are about five to eight small secondary tubercles. In one of the specimens four columns of ambulacral plates can be counted in place, indicating that there were at least that number in an area. The interambul- acral plates are thin, scale-like, rounded in outline, and imbricate strongly dorsally, as indicated by the wide adoral bevel which would underlie the dorsal border of the next adjacent plate ventrally. The surface of the plates bears numerous small secondary tubercles. Dissociated small secondary spines occur with the plates. These are slender, tapering, expanded at the base, vertically finely striate, and measure from 1.8 to 2 mm. in length. A number of dental pyramids occur with the specimens. These are wide-angled, with a moderately deep foramen magnum. As seen from the interior, the pyramid figured shows part of a grooved tooth in place. Complete teeth seen from the interior and with the tips intact show that the teeth were grooved, as shown by specimens in the British and Jermyn St. Museums. In side view, the pyramids show plicate ridges for the attachment of interpyramidal muscles. Lower Carboniferous Limestone, Roscobie, Dumforline, Fifeshire, Scotland, British Museum Collection E 10,699 to E 10,723, of which the holotype is E 10,710, the other speci- mens being paratypes. Additional paratypes from Roscobie, in the Museum of Practical Geology Collection 16,308 and 16,309. Pholidocidaris Meek and Worthen. PhoUdocidaris Meek and Worthen, 1S69, p. 7S; Loven, 1S74, p. 40; Duncan, 18S9a, p. 18; Jackson, 1896, pp. 210, 241; Tornquist, 1897, p. 76."i; Kleni, 1904, p. 22; Lambert and Thiery, 1910, p. 123. Protocidaris Whidborne, 1S9S, p. 202. Echinocyslis (pars) Lambert and Thiery, 1910, p. US. PHOLIDOCIDARIS. 433 The characters of this genus are taken from the type species, P. irregularis, the only one yet known with any degree of completeness. Test spheroidal. Ambulacra are moderately wide; ventrally the plates are large, and dorsall}^ much smaller, with many columns of plates in each area (six columns in P. irregularis). Interambulacra are wide, with many columns of plates in each area (five to six in P. irregularis). The interambulacral plates are large and scale-like; the adambulacral plates dorsally are much larger than the plates of median columns. They imbricate strongly aborally, from the center outward, and over the ambula- crals on the adradial sutures. Adambulacral plates (dorsally) bear a large, eccentric, perforate primary tubercle, and in addition, numerous small secondary tubercles. Plates of median columns (dorsally) bear secondary tubercles only. Ventrally apparently all interambulacral plates are similar, each with a perforate primary tubercle (pp. 435, 437, 439). Primordial interambulacral plates are in the basicoronal row, above which additional columns originate, passing dorsallj'. The peristome is not known, but probably was covered with ambulacral plates only as in Lepidesthes (p. 413). Mr. Agassiz (1881, p. 79) speaks of the peristome of Pholidocidaris as if it were known, but I believe it has never been described, and if Mr. Agassiz knew what the structure is, he did not describe it. Oculars are doubtful, genitals large, with numerous pores. There is no evidence that the apical disc of Pholidocidaris is structurally comparable to the apical disc of the Echinothuriidae (p. 414), as assumed by Mr. Agassiz (1881, p. 79). The periproct is unknown. The lantern is inclined, and as far as known, with the typical characters of the Perischoechinoida. Type species, Pholidocidaris irregularis (Meek and Worthen) from the Lower Carboniferous of America. Kry to the Species of Pholidocidaris} Six columns of plates in an ambulacral area and six or five columns of plates in each interambulacral area. Ambulacral plates are large ventrally, small dorsally. Interambulacral plates ventrally of about uni- form size; dorsally, adambulacral plates are wider and higher than plates of median columns. Dorsally, primary tubercles are on adambulacral plates only . . .P. irrrgidaris (Meek and Worthen), p. 434. Interambulacral plates, as far as known, are rounded, thin, scale-like, and imbricating, with eccentric primary and secondary tubercles and spines; ambulacral plates are, as far as known, large and rounded, with pore-pairs in the middle of the plates ......../-". tniuis Tornquist, p. 440. Interambulacral plates are polygonal, scale-like, and imbricating, with eccentric primary and secondary tubercles; very imperfectly known; Devonian . . . . . .P. acuaria (Whidborne), p. 441. Apparently six columns of hexagonal plates in an ambulacral area, and apparently three or more columns of plates in an interamliulacral area, with high, wide, adambulacral plates similar to those of P. irregularis; very imperfectly known .......... /'. gaiidri/i (Julien), p. 442. 'In regard to the names Pholidocidaris and P. irregnl(iri)i, attention is called to the names Heteroeidaris and //. keokiik and //. lacrispirui under Nomina Nuda (p. 4.50). 434 ROBERT TRACY JACKSON ON ECHINI. *Pholidocidaris irregularis (Meek and Worthen). Plate 73, figs. 3-7; Plate 74, figs. 1-7; Plate 75, figs. 1-5. Lcpidoccntnts irregularis iSIeek and \Yorthen, 1S69, p. 78. Pholidovidaris irregidaris Meek and Worthen, 1869, p. 78; 1873, p. 512, Plate 15, figs. 4c-4e, 9a-9c; Lo\en, 1874, p. 40; Meek, 1874, p. 375; Keyes, 1895, p. 180; Jackson, 1896, p. 210; Tornquist, 1897, p. 766; Klem, 1904, p. 22; Lambert and Thiery, 1910, p. 123. Pholidocidaris meeki Jackson, 1896, pp. 210, 241, Plate 9, fig. 54; 1899, p. 132; Tornquist, 1S97, p. 766. This species has been incompletely known, but new material adds considerably to the known facts. This leads me to accept Miss Klem's reference of P. meeki. which I described as a new species, to irregularis as a synonym. No one specimen is complete, but the characters are gathered from different specimens which are later considered separately. Test large and spheroidal. In a small specimen, the ventral border of which is about on the plane of the mid-zone (Plate 75, fig. 1), the ambulacra measure G mm. in widtli, the inter- ambulacra 37 nun. in width. Larger specimens greatly exceed this size as gathered from those figured. One imperfect but verj' large specimen (Plate 74, fig. 2) probably was 100 nun. or more in diameter when alive. The ambulacra have apparently only six columns in each area (Plate 73, fig. 3; Plate 74, fig. 1). The ambulacral plates in the ventral and dorsal areas differ very much, a peculiarity known in no other Palaeozoic Echini. Ventrally, the ambulacral plates are very large, the largest measuring about 5 to 8 mm. in width, with the pore-pairs about in the middle of each plate, as seen in area D and partially in area B (Plate 74, fig. 1; also Plate 73, figs. 6, 7). On the other hand, the dorsal ambulacral plates in the same specimen are small, about 3 mm. or less in width, as seen in area B (Plate 74, fig. 1; also see Plate 73, figs. 3, 4). This extreme difference in plates ventrally and dorsally I could hardly have believed if I had not actually found them in place in a single specimen, crushed and distorted enough it is true, yet revealing the facts. There are few plates in the ventral portion of the ambulacrum near the peristomal border (Plate 73, fig. 6), although the exact number cannot be stated, and, passing dorsally, the number of plates and apparently the number of columns increases. The interambulacra are no less remarkable than the ambulacra. At the mid-zone and dorsally (Plate 73, figs. 3, 4) there are five or six columns in each area. The plates are scale- like, rounded on the sutures, and imbricate strongly aborally and from the center laterally and over the ambulacra on the adradial sutures. Dorsally, the plates of the adradial columns are very large, being wide, and in height equaling the height of two or three plates of the median columns (Plate 73, figs. 3, 4). Two large isolated adradial plates (Plate 74, figs. 3, 4) measure respectively 23 mm. in height by 15 mm. in width, and 25 mm. in height by 11 mm. in width. These plates show well the ventral and admedian beveled edges which extended under their PHOLIDOCIDARIS. 435 adjacent fellows when in place. They also show the strongly curved outlines which the suture lines of the several adjacent plates possessed. The dorsal adradial plates bear each a large eccentric perforate primary tubercle with scrobicule, and, in addition, small secondary tubercles (Plate 73, figs. 3, 4; Plate 74, figs. 1-5; Plate 75, figs. 1-3). The plates of the median columns dorsally are strongly rounded on the suture lines, and bear secondary tubercles only (Plate 73, figs. 3, 4; Plate 75, figs. 1-3). The ventral part of the test is known from Meek and Worthen's figure (my Plate 75, fig. 4), also from the interior as seen in part in Plate 74, figs. 1 and 2, and from the exterior as seen in Plate 73, fig. 6.^ This last specimen is somewhat confused and difficult to study, yet it shows important features. The primordial interambulacral plate is in place in the basicoronal row as seen in area C. There are two plates in the second row and three in the third row; thus the third column originates in the third row as usual in the Palaeozoic species in which the base of the corona has not been resorbed in the advance of the peristome (p. 66; text-fig. 30, p. 70). There are three rows of three plates before the fourth column orig- inates in the sixth row. This is very late for the fourth column to develop, the latest of any case figured in this memoir, and it is very likely exceptional for the species, but on this point there are no other specimens as yet known for comparison (p. 439). The fifth column origi- nates in the seventh row (the next row after the introduction of the fourth) and the sixth column originates in the eighth row. These plates all imbricate aborally and from the center laterally and over the ambulacra on the adradial sutures. All interambulacral plates on the ventral side are of about the same size as far as this figure shows (Plate 73, fig. 6), the adradial plates not being strikingly different from plates of the median columns as they are on the dorsal side (Plate 73, figs. 4, 5). The plates of this specimen are much worn, but the adradial plates, and also the median plates bear a large perforate eccentric primary tubercle, doubtless with, in addition, secondary tubercles, but these latter, if they existed, are worn off. There are therefore primary tubercles on all interambulacral plates ventrally, whereas they exist only on adradial interambulacral plates dorsally. The primary spines are rather stout, enlarged at the base, terete. Meek and Worthen say that the larger primary spines attain a length of about an inch and a thickness of 0.1 inch at the base, which is swollen (Plate 75, figs. 4, 5). The largest that I have seen measure about 12 mm. in length (Plate 73, fig. 5). Secondary spines are small, filiform, expanded at the base, and measure about 3 to 4 mm. in length (Plate 75, fig. 4; Plate 73, fig. 5). The apical disc is only partially known, but it is evidently small in proportion to the diameter of the test. A small plate dorsal to ambulacrum B, in Plate 73, fig. 3, has two pores, and may be an ocular (p. 419), but other oculars, if present in any of the specimens, are not recognized. The genitals are high and wide, with many genital pores, up to ten counted in one 1 This figure is of the specimen which I earher described as P. meeki, but which is now referred to irregularis as a synonym, as discussed on p. 438. 436 ROBERT TRACY JACKSON ON ECHINI. plate (Plate 73, figs. 3, 4; Plate 74, fig. 1). The lantern is inclined, teeth are grooved, and p.yramids wide-angled, with moderately deep foramen magnum (Plate 74, figs. 2, 6, 7). Keokuk Group, Lower Carboniferous, Hamilton, and near Nauvoo, Illinois; these are the original localities given by Meek and Worthen. The tj'pe is said to be in the Worthen Col- lection which is now in the University of Illinois, at Urbana, Illinois. Keokuk Group, Keokuk, Iowa, F. Springer Collection 8,015, 8,017; Montgomery County, Indiana, F. Braun Collection, two specimens; Warsaw, Illinois, Museum of Comparative Zoology Collection 3,070; Keokuk, Iowa, two interambulacral plates, Museum of Comparative Zoologj^ Collection 3,182; near Burlington, Iowa, University of Michigan Collection 1,740. Lower Carboniferous, Omer, Michigan, dissociated plates that I collected at that locality. Museum of Comparative Zoology Collection. Up to the present, the best published figure of this species is that given by Meek and Worthen (1873), here reproduced as Plate 75, fig. 3. As they had no genital plates, or other structures to locate the axes, their figure was incorrectly oriented, which led to a wrong im- pression as regards the direction of imbrication (p. 76). The orientation is corrected in my Plate, and therefore shows the imbrication of interambulacral plates as aboral, which is correct. In this figure in area A there are a left adradial and three median columns. The adradial plates are very large and have the eccentric perforate primary tubercles, with numerous small second- aries .characteristic of this column dorsally. The median plates have secondary tubercles only. All the plates are strongly curved outward and rounded on the suture lines as is typical of the species. In ambulacrum B there are four small ambulacral plates, not in place, but typical of the dorsal side. In addition, there is a quantity of small bodies, plates perhaps, which are apparently foreign to the specimen, and are scattered indiscriminately in ambulacral and interambulacral areas. Meek and Worthen's figure of the reverse side of the same speci- men, reproduced as my Plate 75, fig. 4, shows a confused mass of plates from the ventral side. The ambulacral plates are rounded in outline, and much larger than on the dorsal side. The pore-pairs are about in the middle of each plate, and are surrounded by a sunken ring, and the pores are on an elevated prominence, resembling the holes on the back of a button. The inter- ambulacral plates are small, mostly with primary and secondary tubercles, and numerous primary and secondary spines are scattered over the plates. A single primary spine restored, and magnified "about twice," copied from Meek and Worthen (my Plate 75, fig. 5), shows the form well; it is considerably larger than any spine of this species that I have seen. Mr. Braun's specimen, from Montgomery Countj', Indiana (Plate 75, fig. 2; Plate 73, figs. 4, 5), is a small individual, but it is the most nearly complete dorsal view of any specimen known. On accoimt of the lightness and delicacy of the test of this specimen and the next one described (Plate 75, fig. 1; Plate 73, fig. 3), my first impression was that it represented a distinct species, but as the structural characters are the same as in large undoubted specimens PHOLIDOCIDARIS. 437 of irregularis, it seems safest to ascribe the differences to immaturity. In this very choice specimen (Plate 73, fig. 4) ambulacral plates are in place in all five areas. Dorsally in area B, also in area D, there are only four columns. The ambulacral plates are all very small, like the dorsal plates in Plate 74, figs. 1, 2. The interambulacral areas are in part well preserved, and are by all means the most nearly complete of any known specimen. The adradial plates in each area are wide, very high, and about the height of two to three plates of the median columns. The adradials bear an eccentric primary with secondary tubercles, the median plates secondary tubercles only, though for the most part these have been omitted in the drawing. They can be seen, however, in the photographic figure (Plate 75, fig. 2), and are shown in the drawing of another specimen (Plate 73, fig. 3). In interambulacrum A there are six columns of plates, the sixth coming in above the adoral border and represented by only three plates, above which to the apical disc there are only five columns (Plate 73, fig. 4). In area I a sixth column is also represented by some plates. In area C there are only five columns, all of which extend through- out the area as far as preserved. Ai-ea E is doubtful, but there are apparently five columns in area G; of course, ventrally in this area, if preserved, there might have been a sixth column represented as in area A. A genital plate is in place, capping area A. It is a low, broad, rounded plate, with seven pores. Another genital, capping I, but somewhat displaced, shows only five pores. No ocular or periproctal plates were recognized. Another immature specimen, quite similar to that just described, is from Keokuk, Iowa, in Mr. Frank Springer's collection. This specimen (Plate 75, fig. 1; Plate 73, fig. 3) has parts of three ambulacral and two very good interambulacral areas. The adoral border is about on the line of the mid-zone. The ambulacral plates are all small, and in area D there are three columns of plates which evidently belong to the right half-area, and thus represent six columns for the whole area at this zone. In both interambulacral areas the adambulacral plates are wide and very high, equaling the height of two to three median plates in the same zone. It is to be noted that this great size and especially the great elongation of adradial plates are most marked in the three or four more dorsal plates of each column, and near the mid-zone the adra- dial plates are much lower, as seen in Plate 73, figs. 3, 4, and Plate 75, fig. 3. On the ventral side of another specimen (Plate 73, fig. 6) it is seen that in area C the plates of the adradial columns are no wider nor higher than are the plates of median columns. It is evident therefore that this specialization of adradial plates is a character taken on late in development and occurs only in the dorsal portion of the test. The adradial plates have a large eccentric perforate primarj^ tubercle and numerous small secondar)^ tubercles (Plate 73, fig. 3), except that the one youngest plate next the apical disc in each adradial column has not yet acquired the primary tubercle. This is a general character of youthful plates in the placogenous zone (compare text-figs. 92, 94, and 96, pp. 106, 107, 108; Plate 11, fig. 2). The plates of the median columns have secondary tubercles only (Plate 73, fig. 3). There are six columns of interambulacral 438 ROBERT TRACY JACKSON ON ECHINI. plates in the two areas preserved, but in area A column 5 and in area C column 6 drop out early, above which five columns extend to the apical disc. A small plate with two pores above am- bulacrum B may be an ocular; if so, it is the only ocular yet recognized in the genus (p. 433). A genital above interambulacrum A is a large, rounded plate with ten genital pores. A small series of isolated interambulacral plates from near Burlington, Iowa, in the Univer- sity of Michigan Collection, is instructive as they are sharply and clearly preserved (Plate 74, figs. 3-5). The plates are all from adradial columns as indicated by the presence of a primary tubercle on each, and the beveled edges extending adorally and admedially demonstrate that figures 3 and 5 are from a left adradial and figure 4 from a right adradial column. The plates shown in Plate 74, figs. 3, 4, are high and wide, and would belong to the dorsal portion of a test. The plate shown in Plate 74, fig. 5, is low and wide; it could not be a young plate dorsally, as the primary tubercle is well developed, and it may have come from about the mid-zone or farther adorally. A right half-pyramid (Plate 74, fig. 6) indicates that the pyramid was wide-angled with moderately deep foramen magnum; the suture for the epiphyses is seen dorsally. Another half-pja-amid seen from within (Plate 74, fig. 7) shows that the lateral wing is wide, indicating an inclined lantern (compare Plate 12, figs. 6-8). This specimen shows the pyramidal suture and dental slide for the support of the tooth, the sUde failing to reach the base of the foramen magnum as usual in the Perischoechinoida (compare Plate 2, fig. 10). An important, but somewhat imperfectly preserved specimen, showing the ventral part of the test, is in the Museum of Comparative Zoologj*. This specimen is from Warsaw, Illi- nois (Plate 73, figs. 6,7), and I described it (1896, p. 210) as the type of a new species, Pholi- docidaris meeki. I did not then recognize the differences that exist between the ventral and dorsal parts of the test, so that it was felt the differences warranted specific distinction. Miss Klem (1904, p. 23) treated meeki as a synonym of irregularis, and I quite agree with her. In my earlier publication errors were made which require correction. On page 211 (of Jackson, 1896) it is stated that the plates of both ambulacral and interambulacral areas imbricate ador- ally. This is true of the ambulacral plates, but interambulacral plates imbricate aborally, and from the center outward and over ambulacrals on the adradial suture as usual (discussion, p. 76). Another error was made in the drawing of Plate 9, fig. 54, in which I showed column 4 as originating in the fourth row, whereas it does not originate until two rows later, as seen in the present figure. In this specimen (Plate 73, figs. 6, 7) the ambulacral plates are very large, somewhat irregular in shape, but approaching rhombic in form. Dorsally, on the aboral border, some small ambulacral plates appear, having taken on the character typical of all the plates dorsally. The pore-pairs are about in the middle of each plate in a raised area, and resemble the holes in a button. The ambulacral plates imbricate adorally and laterally bevel under the adradials. The specimen is too much worn to show secondary tubercles, which alone probably existed on the ambulacral plates. As briefly considered above, the primordial inter- PHOLIDOCIDARIS. 439 ambulacral plate is in the basicoronal row in area C, and this specimen was one of the first of Palaeozoic Echini in which this structure was discovered. I correlated it with the single plate at the ventral border in Bothriocidaris and the j'oung of Recent Echini (Jackson, 1896, pp.212, 230). In the second row there are two plates as a second stage in development, and in the third row three plates. This is all as usual in the development of all Palaeozoic Echini in which the primordial interambulacral plate is retained (Plate 25, fig. 1), excepting Bothriocidaris, in which there is only a single column of plates in an area (Plate 1, fig. 1). In this area of this specimen of Pholidocidaris there are also three plates in the fourth and fifth rows (Plate 73, fig. 6). In the sixth row there are four plates, column 4 originating on the right of the center as usual. This is, however, a very unusual case in that column 4 originates so late. It ordinarily originates in the next row after the introduction of column 3, so that there is typically in Palaeozoic Echini only one row ventrally with three plates (text-figs. 25, 30, p. 70). In Palaeechinus quadriserialis in area C (Plate 30, fig. 3), and in Lovenechinus septies, also in area C (Plate 45, fig. 1), there are two rows of three plates before the introduction of column 4. Again, in a specimen of Melon- echinus multiporus, figured by Jackson and Jaggar (1896, Plate 3, fig. 12; Plate 4, fig. 18) there are at least four rows of three plates before the introduction of the fourth column (p. 377). These are all rare and exceptional variations of retarded development, and with Pholidocidaris irregularis are the only cases seen in which the fourth column (when existent) did not originate in the next row after the third (p. 349). In Pholidocidaris irregularis (Plate 73, fig. 6) the fifth column originates in the seventh row, immediately after the fourth, and the sixth column origi- nates in the eighth row, immediately after the fifth, thus, as regards these two, showing an accelerated development. The plates of interambulacrum A are so confused that they do not show much structure. Fragments of primary spines occur scattered over the test. A fragment of a large crushed test from Montgomery County, Indiana, in Mr. Braun's collection, is very instructive. This specimen (Plate 74, fig. 1) is a portion of a test extending from the apical disc nearly or quite to the mid-zone. The dorsal plates are seen from the exterior and are not shaded. Part of the dorsal plates are wanting, so that ventral plates are seen from the interior, and are shaded to differentiate them. From this peculiar condition of preservation the ventral plates can be directly compared with the dorsal plates of the same specimen. The ventral ambulacral plates are very large, from 5 to 7 mm. in width, with pore- pairs about in the middle of each plate, as seen well in area D. In contradistinction, the dorsal ambulacral plates are small, about 2 to 3 mm. in width. This extraordinary difference in the ventral and dorsal ambulacral plates (p. 434), is shown more strikingly in this than in any other known specimen. The adradial dorsal interambulacral plates are wide and high as usual. Dorsally, there are two genital plates which are wide and rounded in outline, each with many pores. A fragment of a lantern shows in part a brace, two epiphyses, and part of a pyramid. 440 ROBERT TRACY JACKSON ON ECHINI. Another similar crushed specimen from Keokuk, Iowa, is in Mr. Frank Springer's collec- tion. It is a very large individual (Plate 74, fig. 2) ; the dorsal plates are seen in external view, and, where wanting, part of the ventral plates are seen from the interior and are shaded for contrast. The ambulacral plates are mostly dorsal and therefore small. The adradial plates are large and rounded, as usual, and plates of the median interambulacral columns are smaller. Although fragmentary, it seems that six columns can be made out in each interambulacral area. A lantern in place is strongly inclined with grooved teeth, and the dorsal borders of ten half- pyramids are in place. * Pholidocidaris sp. Plate 74, figs. 8-10. In a small slab from the Kaskaskia Group, Lower Carboniferous, of Pulaski County, Kentucky, in Mr. Frank Springer's collection, there are a number of small plates referable to Pholidocidaris. They may be young specimens of irregularis, or they may be a distinct species, but there is not enough structure to base any conclusions on. The interest of it is that speci- mens of the genus occur in this locality and horizon. The ambulacral plates are typical of those frohi the ventral area in Pholidocidaris and unlike anything known in other genera. Two genital plates are high and wide, with numerous pores. * Pholidocidaris tenuis Tornquist. Plate 72, fig. 11; Plate 74, figs. 12, 13. Pholidocidaris tenuis Tornquist, 1897, p. 767, Plate 21, figs. 8, 9; Plate 22, fig. 8; Klem, 1904, p. 23; Lambert and Thiery, 1910, p. 123. This species was described by Professor Tornquist from numerous interambulacral plates and also primary spines from the Lower Carboniferous of Alsace, Germany. A specimen from the Lower Carboniferous of Lancashire, England, in the Museum of Practical Geology, appears to be referable to this species, and adds something to the known characters and also to the geographical distribution of the species. According to Tornquist's description, the interambulacral plates are all of about equal size, 10 mm. in width, irregularly hexagonal, thin, and strongly imbricating. Each plate bears an eccentric large perforate primary tubercle with numerous secondary tubercles (Plate 74, fig. 12). The primary spines are terete, expanded at the base, nearly smooth, but with fine vertical striae which are seen only on considerable magnification. They measure from 10 to 15 mm. in length (Plate 74, fig. 13). The English specimen consists of a slab 150 nun. long bj' 70 mm. wide, which bears a con- fused mass of dissociated plates, part of which are shown in Plate 72, fig. 11. The interambula- cral plates are thin, scale-like, strongly imbricating. They measure from 8 to 11.5 mm. in PHOLIDOCIDARIS. 441 width, and are of about equal height. The interambulacral plates bear a large eccentric per- forate primary tubercle and secondary tubercles. They are quite similar in size and ornamenta- tion to those described by Tornquist. In addition, some relatively large ambulacral plates occur. These are slightly wider than high; they measure about 8.3 to 11 mm. in width and have a pore-pair nearly or quite in tlie center of the plates. With these large ambulacral plates are some that are very much smaller, which are probably dorsal plates. In the larger plates the pore-pairs are surrounded by a depressed ring, as is typical of the ventral ambulacral plates in Pholidocidaris irregularis. On the reverse side of the slab are found a number of primary and secondary spines. The primary spines are swollen at the base, terete, about 7.2 to 8.4 mm. in length, and marked by very fine longitudinal striae. The secondary spines are much smaller, about 2.8 mm. in length, and show no vertical striae. The number of columns of ambulacral and interambulacral plates in an area is unknown, but from the large number of interambulacral plates in the English specimen, which apparently represents a single individual, there are probably a good many columns in an area. A large stout tooth, which, as far as preserved, measures 28 mm. in length and is 9 mm. wide, bears evidence of a powerful lantern. Lower Carboniferous, Hunsriicken, Alsace, Germany; Coplaw, Clitheroe, Lancashire, Museum of Practical Geology Collection 16,304. This is the first occurrence of Pholidocidaris recorded from the Lower Carboniferous of Great Britain, but it occurs in the Devonian, as seen from the next species described. * Pholidocidaris acuaria (Whidbonie). Plate 74, fig. 11. Eocidaris (?) acuaria Whidborne, 1S96, p. 376. Protocidaris acuaria Whidborne, 1898, p. 203, Plate 25, figs. 1-lb, 2, 2a; Kleni, 1904, p. 75. Protocidaris [acuaria] Sollas, 1899, p. 709. Echinocystis acuaria Lambert and Thiery, 1910, p. 118. This species is known only from external molds, which are confused masses of plates and spines. Judging from the molds, the interambulacral plates are large, from 4 to 5 mm. in width, with an eccentric perforate primary tubercle and scrobicule and secondary tubercles. Whidborne saj'S that the primary tubercle is central, but it is certainly not central in some plates, and I have seen no proof that it is central in any plates. There are impressions of a few large and also small ambulacral plates with a pair of pores in each; in fact it is by the two small plugs representing casts of the pores that these plates are recognized. The spines are primaries and secondaries; the primaries are swollen at the base, otherwise about cylindrical, and the longest that I saw measured about 6 mm. in length, but they were not complete distally. The impressions of a stout lantern are plainly seen in one of the specimens in the Jermyn St. Museum, no. 7,158. 442 ROBERT TRACY JACKSON ON ECHINI. This species is incompletely known, but I think it is clearly referable to the genus Pholido- cidaris. Whidborne made the species the type of a new genus, Protocidaris. I see no need of maintaining this genus, for the species as far as known fits well in the genus Pholidocidaris. It certainly does not belong to the genus Echinocystis [Echinocystites] to w^hich it is referred by Messrs. Lambert and Thiery. Upper Devonian, East of Barnstaple, England, cotypes, Museum of Practical Cleology Collection 7,158, 7,159, and 16,358. The specimen no. 7,158 is the original of Whidborne's Plate 25, fig. 1. Same locality, Oxford Museum (Sollas). Pholidocidaris gaudryi (Julien). Lepidoccntnis munsterianus Julien, 1874, p. 76 (non de Koninck, 1869, p. 546, for which see Archacocidaris mucnsteriand, p. 2S0). Mclonitcs gaudryi Julien, 1890, p. 737. Pholidocidaris gaudryi Julien, 1896, p. 131, Plate 16, figs. 1, 2, 6, 7; Lambert and Thiery, 1910, p. 123. This species is known only from fragmentary molds of plates and spines, and the descrip- tion is gathered from Professor Julien's text and rather indistinct photographic figures. Ambulacra are arranged in six columns, limited on either side by adambulacral plates. The ambulacral plates are hexagonal, 4 mm. wide, and about 1.5 mm. high, with pore-pairs near the ventral border of each plate. They are ornamented on the exterior with a large, and also small tubercles. The number of interambulacral columns is unknown, but a badly preserved specimen led Julien to think that there are at least three columns in the neighborhood of the apex. The interambulacral plates are irregular in form, and Julien says that they are similar in form to those of P. irregularis. Two of the plates which he figures (his Plate 16, figs. 6, 7c) are wide, high, rounded plates, with eccentric primary and secondary tubercles. These from their shape and size are apparently plates from a right adambulacral column similar to what I show in Plate 74, fig. 4. Julien says that the spines are of various sizes, expanded at the base, terete, marked with fine longitudinal striae, and identical with those of Pholidocidaris irregularis. Lower Carboniferous, I'Ardoisiere, central France. The types are in the possession of the family of the late Professor Julien. Meekechinus gen. nov. Test spheroidal, or flattened. The ambulacra are wide at the mid-zone, with in each area numerous columns of small plates (twenty in the known species), and the interambulacra are narrow, with in each area few columns of plates (three in the known species), all plates are of about equal size. A small perforate primary tubercle with scrobicule and secondary tubercles occur on ambulacral and interambulacral plates, also small primary and secondary spines on MEEKECHINUS. 443 ambulacral and interambulacral plates. The peristome and ventral part of the test is frag- mentarily known. Oculars are small, imperforate, covering the ambulacra and laterally in part the interambulacra on either side. Genitals are large, with from two to five genital pores. A well developed madreporite exists in the known specimens. The lantern is inclined, of the usual Palaeozoic character, but the teeth on the outer face are vertically ridged and distally deeply serrate, a unique character. This genus structurally differs from Pholidocidaris in that the dorsal interambulacral plates are uniform in size and character instead of very dissimilar; it differs from all known genera in the serrate teeth. I name this genus for the late Fielding Bradford Meek in recognition of his critical and very accurate work on Palaeozoic Echini. The type and only known species is Meekechinus elegans sp. nov. from the Permian of Kansas. *Meekechinus elegans sp. nov. Plate 75, figs. 6-8; Plate 76, figs. 1-9. This species is known from three excellent specimens that were kindly sent me for study by Professor J. W. Beede of Indiana University. The specimens consist of two very wonder- fully preserved tests, seen in dorsal view, which are very nearly perfect from the apical disc to the ambitus, and a third fragmentary specimen with ventral plates and a fine lantern. The test is spheroidal or flattened, probably the latter, as the tests are now quite flat and without any obvious displacement of plates, and quite circular in outline, as seen from above. Both specimens are very nearly alike in size and preservation, but that selected as the holotype (Plate 75, fig. 6; Plate 76, fig. 1) measures 54 mm. in diameter through the axis III, 5. From the center of the apical disc to the periphery of the test it measures almost exactly 27 mm. radially in every direction, attesting to the absence of any distortion. At the periphery, which I take to be the ambitus, the ambulacra measure 26 mm. in width in a straight line, or about 32 mm. in width following the curve of the periphery. The interambulacra measure 6 mm. in width, and are strikingly uniform in their constant width, from the ambitus to the apical disc. In this respect they surpass any sea-urchin known to me. As seen from the measurements, the ambulacra, measured on the curve of the periphery, are about five times the width of the interambulacra, certainly an extraordinary development for the group, and in all Echini sur- passed in this character only by Lepidesthes extremis, which proportionately has slightly wider ambulacra, as described on page 430. The ambulacra are very wide, and at the ambitus, which is also doubtless the mid-zone, there are twenty columns of plates in an area. Sixteen columns is the highest number known in any other sea-urchin {Lepidesthes colletti and extremis), so that in the number of columns at the ambitus this species surpasses all known Echini. Dorsally, next to the oculars there are 444 ROBERT TRACY JACKSON ON ECHINI. one or two plates in an ambulacral area, from which simple condition as a localized stage, the number of columns of plates increases passing ventrally until the full number is attained at or near the ambitus. The ambulacral plates are low, wide rhombs (Plate 76, figs. 2, 3), and imbri- cate strongly adorally and laterally bevel under the adradials. The pore-pairs are situated in peripodia, and in each plate lie a little nearer the next adjacent interambulacrum than the middle of the plate. Each ambulacral plate bears a small perforate primary tubercle with a scrobicule and also a number of small secondary tubercles. The larger tubercles bear small primary spines which are terete, swollen at the base, and longitudinally finely striate, measur- ing about 3 mm. in length. On account of their small size it might be doubted whether these should be called primary spines, but I do so because they are associated with the larger, minutely perforate tubercles which bear a scrobicule. The secondary spines are similar but smaller, and measure up to about 1 mm. in length. Besides spines, a number of pedicellariae were found in these choice specimens (Plate 76, figs. 8, 9). The pedicellariae are tridentate, similar to the character found in Recent Echini, and measure about 0.5 mm. in length. They are the first pedicellariae found in the Palaeozoic. It is interesting to find that these minute structures of the common typical form, as seen in modern Echini, already existed in these ancient times. Groom (1887) described pedicellariae in the Jurassic Pelanechinus corallinus and these with the pedicellariae of Meekechinus are the only ones known to me so far found as fossils (p. 61). The interambulacra are relatively and actually very narrow as above stated, and as far as preserved, consist of three columns of plates throughout each area. Except Bothriocidaris which has one column, and Miocidaris which has two columns in each area, all Palaeozoic Echini have three or more columns of plates in each interambulacral area. Only rarely are three columns characteristic of the adult, but a three-column stage is typically represented by one row of plates ventrally in species that attain a higher number of columns (text-figs. 25, 30, p. 70). Three Palaeozoic species have three columns as an adult character: the present species, Melonechinus obovatus sp. nov. (p. 374; Plate 53, figs. 9, 10), and Lepidesthes wortheni Jackson (p. 416; Plate 66, figs. 1-3; Plate 67, figs. 8, 9). This last species, however, has four columns ventrally as a stage in early development. The interambulacral plates of Meekechinus elegans are small, rounded on the suture line, about as high as two ambulacral plates, and imbricate strongly aborally and from the center laterally and over the ambulacra on the adradial suture (Plate 76, figs. 4, 5). Each plate typically bears a small, central, perforate primary tubercle with a scrobicule and many small secondary tubercles. On the other hand, in Pholidocidaris the primary tubercles are eccentric, and at least dorsally are wanting on the plates of median columns. The spines of interambulacral plates are small primaries and secondaries, similar to those described in the ambulacra. The plates of the ventral area are known only from a fragment of the test associated with the lantern (Plate 75, fig. 8). Here, MEEKECHINUS. 445 as far as observed, ambulacral and interambulacral plates ai'e essentially the same as they are on the dorsal side. A few ambulacral plates are clinging to the oral face of the lantern and are probably peristomal, but they are not sufficiently continuous or complete to be certain. Am- bulacral plates only on the peristome is the character to be expected, as that is the structure of the allied Lspidesthes (Plate 68, fig. 3). Dorsally, the apical disc is in place and, though not perfect, is well preserved. In the holo- type (Plate 75, fig. 6) the apical disc measures about 9 mm. in diameter through the axis III, 5, which is proportionately about 16 % of the diameter of the test (pp. 87, 362). The oculars are small and imperforate, ventrally covering the ambulacra and laterally in part the inter- ambulacra on either side. The youngest interambulacral plates of each area are in contact with the oculars of either side, as seen best in interambulacrum 2, Plate 76, fig. 6. Oculars II and III are exsert, being shut out from the periproct by the contact of the associated genitals. Ocular V is probably insert, I is wanting, and ocular IV is doubtful. The genitals are all present. Genitals 1, 3, 4, and 5 are all medium sized, rounded in outline, and of about equal size. Each of them bears a small perforate primary tubercle with a scrobicule and smaller secondary tubercles similar to those on interambulacral plates. Genital 1 has five pores, 3 and 4 have four pores each, and genital 5 has three pores. Genital 2, the madreporite, is very much larger than the other genitals. It has numerous fine madreporic pores as in a Recent Cidaris, and is the clearest madreporite seen in the Palaeozoic (p. 172). There are in this plate only two genital pores. It bears no primary but a few secondary tubercles scattered amongst the madre- poric pores just as can be seen in a modern cidarid, as Eucidaris trihuloides (text-figs. 62-69, p. 98). A few small periproctal plates are present, rounded in outline, but somewhat angular, and in general similar to the periproctal plates of other Palaeozoic Echini and modern cidarids (p. 174). An excellent lantern is preserved in one of the specimens (Plate 75, fig. 8; Plate 76, fig. 7). It is inclined, with pyramids wide-angled as in other Palaeozoic Echini. The most important feature of the lantern in this species is the teeth, which are grooved on the inner face, vertically ribbed on the outer face, and, as a very important character, are distally sharply and deeply serrate as seen clearly in two teeth figured (Plate 75, fig. 8; Plate 76, fig. 7). This distal serra- tion is a unique character in Echini as far as I am aware. Taking up the other characters of the lantern, in area E the two half-pyramids are in contact on the line of the interpyramidal suture. In this area it is seen that the foramen magnum is very deep, more so than in any other Palaeozoic species seen (compare Plate 27, figs. 4-6; text-figs. 207-213, p. 184). On the left half-pyramid the suture exists for the epiphysis, but that structure is wanting in this area. The epiphysis is in place, however, capping the right half-pyramid of area C, and above it lies the tip of a somewhat displaced brace. Due to a local twist, the outer side of the left half-pyramid of area A is seen, and this face has plicate ridges for the attachment of interpyram- 446 ROBERT TRACY JACKSON ON ECHINI. idal muscles as usual in regular Echini. Below the pyramid E a distally forked plate occurs, which is apparently a compass out of place (p. 182). The specimens of this species were collected recently by Professor J. W. Beede in the Eskridge shales which are in the base of the Permian, at Grand Summit, Kansas. The three specimens described, including the holotype and two paratypes, are in the collections of the University of Kansas. The first species described in the systematic portion of this memoir is Bothriocidaris archaica sp. nov. from the Ordovician, the oldest formation from which Echini are known, and that species, as I have attempted to show, is most primitive in structure, having features in the adult which are like the young of all succeeding Echini. On the other hand, Meekechinus elegayis, which in the regular series is the last species considered, comes from the highest geological formation, the Permian, in which the order Perischoechinoida is known. Structurally, as regards the development of the ambulacrum and the possession of serrate teeth, it is the most highly evolved of Palaeozoic species, and one of the most specialized of known Echini. As regards the narrow interambulacra composed of only three columns of plates, it is also highly specialized. It makes a fitting climax to the series of remarkable forms which are preserved to us from these ancient geological formations. Incertae Sedis. In this section are considered all described Palaeozoic species of Echini that I cannot reason- ably place in a definite genus, or intercalate in the systematic descriptions. Archaeocidaris konincki Desor. Plate 15, figs. 13a-1.3d. Archaeocidaris konincki Desor, 1858, p. 155, Plate 21, figs. 7-10; Klein, 1904, p. 63. Eocidaris konincki Lambert and Thieiy, 1910, p. 126. Known only from isolated interambulacral plates as figured. Lower Carboniferous, Tournay, Belgium. Archaeocidaris ladina Stache. Plate 15, figs. 15a-15d. Archaeocidaris ladina Stache, 1877, explanation of Plate 5, figs. 11, 12; Lambert and Thiery, 1910, p. 125. Known only from small fragments of spines with rather large spinules. Bellerophonkalke, Lower Carboniferous, St. Martin, South Tyrol. INCERTAE SEDIS. 447 Archaeocidaris scotica Young. Archaeocidaris scotica Young, 1876, p. 230; Smith, 1901, p. 509; Lambert and Thiery, 1910, p. 125. This species is named from jaws, which are probably not generically or specifically recog- nizable. They have not been figured. Lower Carboniferous, Craigenglen, Campsie, Scotland. Archaeocidaris selwyni R. Etheridge, .Ir Plate 15, figs. 9, 10. Archaeocidaris (f) selwyni R. Etheridge, .Jr., lS92a, p. 67, Plate 15, figs. 1-3; Klem, 1904, p. 64. Archaeocidaris sp. R. Etheridge, Jr., 1892a, p. 69, Plate 22, fig. 1; Klem, 1904, p. 65. Archaeocidaris selwyni Lambert and Thiery, 1910, p. 125. « Known from nearly complete tests, but the spines are unknown. What position it would take in relation to other species of the genus if we had the spines, is of course undeterminable. Test large, fully four and a half inches in diameter at the greatest periphery. Ambulacra narrow, plates low. Four columns of plates in an interambulacral area. The interambulacral plates are very large and hexagonal, or in adradial columns pentagonal, and rounded on the adradial suture, with very large primary tubercles. Etheridge says that the plates are in four and perhaps five series. I see no indication of a fifth column in his figures, and as the other species of the genus always have four columns in an area when this is completely known, it is probable that selwyni has also (p. 257) . Powerful jaws are present. Etheridge expresses the view that the two specimens he figures belong to distinct species on account of the shape of the plates, they being proportionately lower in the second and smaller specimen (my Plate 15, fig. 10), to which he does not give any name. Unless further evidence develops, I think they can be safely considered one species, the differences being ascribed to age, as the two differ considerably in size. Upper Marine Series, Permo-Carboniferous, the holotype from Nowra, Shoalhaven River, County St. Vincent; the second specimen from the same horizon, about five miles south of West Maitland, County Northumberland, New South Wales. Archaeocidaris sixi Barrois. Plate 10, figs, lla-llc. Archaeocidaris si.vi Barrois, 1882, p. 320, Plate 16, figs. 5a-5c; Lambert and Thiery, 1910, p. 125. Known fragmentarily only from isolated plates and spines. Interambulacral plates hexagonal; the height equals the width as figured. The spine is compressed, 6 mm. in length, ornamented with fine vertical striae, and with a weakly defined annulus. On account of the small size of the spine in relation to the interambulacral plates, also on account of the 448 ROBERT TRACY JACKSON ON ECHINI. shape of the spine and its weak annulus, this is apparently a secondary, not a primary spine. Secondary spines, as far as known, are alike throughout the genus, and lack the diagnostic char- acter of primary spines; therefore this species cannot be located in its relation to other species of the genus, and is considered here. Lower Carboniferous, Assise de Lena: Ontoria, Sebarga, Spain. Archaeocidaris trautscholdi Toinquist. Plate 15, fig. 14. Archarocidaris irautscholdi Tomquist, 1896, p. 29, Plate 4, fig. 1; Klem, 1904, p. 65. Cidarotropus traiischoldi Lambert and Thiery, 1910, p. 125. Not recognizable. Lower Carboniferous, Miatschkowa, near Moscow, Russia. Archaeocidaris sp. Meek and Hayden. Archaeocidaris sp. Meek anil Hayden, 1859, p. 25; Klem, 1904, p. 66. Coal Measures, Cottonwood Creek, Kansas. Archaeocidaris sp. Meek and Hayden. Archaeocidaris sp. Meek and Hayden, 1S59, p. 25; Klem, 1904, p. 66. Coal Measures, Cottonwood Creek and Manhattan, Kansas. Archaeocidaris sp. Stache. Archaeocidaris sp. Stache, 1877, p. 318, Plate 1, figs. 13-17; Klem, 1904, p. 66. Bellerophonkalke, Lower Carboniferous, South Tyrol. Archaeocidaris (?) sp. Wm-then and Miller. Archaeocidaris {?) sp. Worthen and Miller, 1883, Plate 30, figs. 16a-10c. Good figures of pyramids of a lantern from the Upper Coal Measures of La Salle, Illinois Archaeocidaris sp. R. Etheridge, Jr. Archaeocidaris sp. R. Etheridge, Jr., 1892, p. 213; 1892a, p. 67; Klem, 1904, p. 65. A single interambulacral plate is described as hexagonal with granular margins and central, strong, well marked tubercle. Etheridge remarks that he believes that this is the first announcement of the occurrence of the Palaeechinoida in the Australian Permo-Carboniferous rocks. Gympie Beds, Permo-Carboniferous, Rockhampton District, Australia. INCERTAE SEDIS. 449 Archaeocidaris sp. Julien. Arrhacoridaris sp. Julien, 1896, p. 12(), Plate l(i, figs. 14, 17; Plate 17, figs. 8, 10; Klem, 1904, p. 66. Spines with four ranges of spinules, perhaps referable to A. rankini Young (p. 276). Lower Carboniferous, I'Ardoisiere, France. Archaeocidaris sp. Julien. Archaeocidaris sp. Julien, 1896, p. 126, Plate Ki, figs. 13, 15, 16; Plate 17, fig. 9; Klem, 1904, p. 66. Spines with six ranges of spinules. These spines measure up to 54 mm. in length. Lower Carboniferous, I'Ardoisiere, France. Archaeocidaris (?) sp. Julien. Archaeocidaris sp. Julien, 1896, p. 127, Plate 8, fig. 1; Klem, 1904, p. 65. Lower Carboniferous, Regny; and in the quarries of Saint Germain-Laval, France. Cidarites tennesseae Troost. Cidarites tennesseae Troost, 1850, p. 59; Troost (in Miss Elvira Wood's paper, 1909), p. lOS, Plate 8, figs. 2-5. Archaeocidaris tennesseae Loven, 1874, p. 44; Klem, 1904, p. 64; Lambert and Thiery, 1910, p. 125. Of this species no description is given by Troost, and it is known only from the figures given in Miss Wood's publication of Troost 's Crinoids of Tennessee. The spines are smooth and a hexagonal interambulacral plate is apparently referable to the species. No locality nor geological formation are given. Echinocrinus anceps T. and T. Austin. Echinocriniis anceps (nomen nudum) T. and T. Austin, 1842, p. HI; T. and T. .\ustin, 184.3, p. 207; Bather, 1907, p. 452. Archaeocidaris anceps Etheridge, 1888, p. 221. The description of this species is unrecognizable. The locality and horizon of this species are not given. The Austins (1843) compare Echinocrinus anceps with E. pomum. T. Austin later (ISGO) gave the name Protechinus anceps to a distinct species. As both are very little known, there is a possibility of confusing these names (p. 454). Echinocrinus spinosus T. aufl T. .\ustin. Eckinocrinus spinosus (nomen nudum) T. and T. Austin, 1842, p. Ill; T. and T. Austin, 1843, p. 207; Bather, 1907, p. 452. Archaeocidaris spinosus Etheridge, 1888, p. 221. The description of this species is unrecognizable. The locality and horizon of this species are not given. 450 ROBERT TRACY JACKSON ON ECHINI. Echinocrinus striatus Eichwald. Plate 15, f5g. IG. Echinocrinus striatus Eichwakl, 1S60, p. 654, Plate 33, fig. 17. This species is quite unrecognizable. Known only from a fragmentary spine. Dolomitic Limestone, Bogoslowsk, Ural, Russia. Echinocrinus sp. Austin. Echinocrinus sp. T. Austin, 1848, p. 294. A large species of Echinocrinus [ = Archaeocidaris] is reported by Austin from the Wenlock Limestone, Silurian, of Glidden Hill, which is near Coalbrookdale, in Shropshire. There is no recognizable description, and Dr. Bather, who called my attention to the reference, knows nothing about the specimen (footnote p. 236). Melonitiden Tornquist. Melonitiden Tornquist, 1897, p. 763, Plate 20, fig. 7. A single interambulacral plate is figured by Tornquist. Genus doubtful. Lower Carboniferous, Hunsriicken, Germany. Oligoporus (?) minutus Beede. Plate 43, fig. 6. Oligoporus (?) minutus Beede, 1899, p. 126, Plate 32, fig. 3; 1900, p. 49, Plate 7, fig. 3; Klem, 1904, p. 40. Oligoporus minutus Lambert and Thiery, 1910, p. 121. Test spheroidal. A small specimen, apparently an internal mold, 23 mm. in diameter, has in each ambulacrum four vertical series of pore-pairs. No impressions of plates are shown, so that the character has to be gathered from the shape and pores alone. It is probably refera- ble to the genus Lovenechinus which as described (p. 324) has four columns of plates and therefore four vertical series of pore-pairs at the midzone in each ambulacral area. Deep Creek Limestone, Upper Coal Measures, northeast of Topeka, Kansas. Professor Beede informs me that the type is in the collections of the University of Kansas. *Palaechinus (?) konigii M'Coy. Plate 29, fig. 9; Plate 31, fig. 7. Palaechinus (?) ko7iign M'Coy, 1844, p. 172, Plate 24, figs, la-lc: Desor, 1858, p. 159. Palacchinus koenigii M'Coy, 1844, [legend in pencil on] Plate 24, fig. 1^ ; d'Orbigny, 1850, p. 154. Echinus koenigii M'Coy, 1862, [legend on] Plate 24, fig. 1.^ ' In a copy of M'Coy's Carboniferous Limestone Fossils of Ireland, of tiie 18G2 edition, seen at tlie Jermyn St. Mu- seum, the name Echinus koenigii is printed on tlie plate, as stated. Dr. Kitohin writes me that M'Coy (1862, p. 274) in the errata, referring to this figure says, "read Palaechinus instead of Echinus." In two copies of the 1844 edition, one at the British Museum and one at the Museum of Comparative Zoology, the name on the plate is partially erased and changed in pencil to read, "Palaechinus koenigii." See footnotes pp. 275, 277, 280. INCERTAE SEDIS. 451 Palaechinus konincki [by clerical error] Dujardiu and Hupe, 1802, p. 464; (non Palaeckinus konincki Julien, 1896, p. 130, for which see the next species, Palaeechinus (?) regnyi; non Fraipont, 1904, p. 9, Plate 3, fir. 1); (pars) Klein, 1904, p. 33. Pahwechinus (?) konigi Loven, 1874, p. 11. Xystria koenigii Pomel, 1S83, p. 114. Palaechinus koenigi Julien, 1896, p. 129. Non Palechinus koninchi Tornqiiist, 1897, p. 753, for which see Palaeechinus (?) reqnyl. Palaechinus kihiigl Lambert and Thiery, 1910, p. 119. This species is known only from a few dissociated interambulacral plates with tubercles. It doubtless belongs in the family Palaeechinidae, but nothing further can be said. It certainly has been burdened with names and various .spellings. The specimen is a dark mass (not in a yellow sandstone as Miss Klem states) from the Lower Carboniferous of Rahan's Bay, Dunki- neeley, County Donegal, Ireland. The holotype and only known specimen is in the Griffith Collection, Science and Ai-ts Museum, Dublin, where I saw it through the kindness of Dr. Scharff. The fact that Dujardin and Hupe by error listed this species as P. konincki and that Julien described a new species as P. konincki, has made confusion between this and the next species to which I give a new name. P. (?) konigii is the type and only species referred to Xystria by Pomel, who is the author of that genus. Palaeechinus (?) regnyi nom. nov. Text-fig. 253. Pahin-hinus kunincki (name preoccupied) Julien, 1896, p. 130, Plate 10, fig. 20; Plate 11, fig. 27; (pars) Klem, 1904, p. 33; Fraipont, 1904, p. 9, Plate 3, fig. 1; (non Palaechinus konincki Dujardin and Hupe, 1862, p. 464, for which see Palaechinus (?) konigii, p. 450). Palechinus konincki Tornquist, 1897, p. 753. On account of the confusion in the nomenclature of this species, it seemed best to give it a distinctive name as is done. As the ambulacral structure is unknown, one cannot state to which genus of the Palaeechinidae this species belongs. According to Julien, the specimen is 90 mm. high and approximately 55 mm. wide. This width as given refers only to the width of the interambulacrum, not to what the whole specimen would be if complete. The adambu- 253 Text-fig. 2.53. — Palaeechinus (?) regnyi nom. nov. Lower Carboniferous, R6gny, France. lacral plates are pentagonal, others hexagonal, with numerous Natural size. After Julien, 1896, secondary tubercles, which, Julien says, on the adambulacral ^''''te lO, fig. 20, drawn from his rather indistinct photograiihic plates are arranged in horizontal series. Julien qualifiedly says figure. 452 ROBERT TRACY JACKSON ON ECHINI. that there are apparently five columns of interambulacral plates, and the same number is given by Fraipont, based on his more nearly complete specimen. Julien's best figure shows a column of pentagonal adradial plates and in part smaller hexagonal plates of median columns. A large adambulacral plate measures 14 mm. in width and 8 mm. in height. Fraipont's photographic figure shows an ambulacrum which is about the width of one interambulacral plate. He does not describe the structure but as gathered from his figure it probably has two columns of pri- mary plates, or primary and occluded plates at the mid-zone which structure would refer the species to either the genus Palaeechinus or Maccoya. Lower Carboniferous, Regny, Neronde, Saint-Germain-Laval, central France; type in the possession of the family of the late Professor Julien. Marbre Noir de Dinant, Belgium (Frai- pont). Palaechinus robineti Julien. Palaechimis robineti Julien, 1896, p. 129, Plate 3, fig. 19; Lambert and Thiery, 1910, p. 119. Fragmentarily known. According to Julien, there are two columns of low narrow plates in an ambulacral area, each plate with two pores and 2.5 to 3 mm. wide by 1 mm. high. The ambulacrum therefore is about 5 to 6 mm. wide. The character of the ambulacrum fixes this species as belonging to Palaeechinus proper. Five ambulacral plates equal the height of an adambulacral. Of the interambulacrum there are pentagonal adambulacral and hexagonal median plates, but, as these are dissociated, the number of columns in an area is unknown. According to Julien, adambulacral plates are 5 mm. wide and 3 mm. high, the hexagonals 7 mm. wide by 4 mm. high. These measurements must have been taken from plates of different zones in the area, for hexagonal plates are necessarily nearly or quite the same height as adambul- acral plates in the same horizontal row, and also the height given for adambulacral plates does not equal the 5 mm. which is what Julien says is the height of five ambulacrals which equal the height of an adambulacral. As the number of interambulacral columns is unknown, this species cannot be placed systematically in its relation to other species of Palaeechinus, and if known, it might prove to be the same as some already described species. Julien's figure is too indistinct to be recognizable. Lower Carboniferous, La Varville, Siguret, central France. Holotype in the possession of the family of the late Professor Julien. Palechinus sp. Tornquist. Palechinus sp. Tornquist, 1897, p. 7(53, Plate 20, fig. 6; Klem, 1906, p. 37. A single small pentagonal adambulacral plate is figured; its genus is doubtful. Lower Carboniferous, Hunsriicken, Germany. INCERTAE SEDIS. 453 Palaeocidaris exilis Eichwald. Text-fig. 254. Palaeocidaris crilis Eichwald, 1854, p. 114. Plate 2, fig. 14. Bothriocidaris crilis Eichwald, 1860, p. 656. A small unrecognizable mass. In general, it is spheroidal in form, and has numerous smal elevations which Eichwald considered to be tubercles. Pentamerous Limestone, Silurian, of Talkhof, Russia. Palaeodiscus gothicus Wyville Thomson. Palacodiscus gothicus Wyville Thomson, 1861, p. 118, Plate 4, fig. 8; Lambert andThiery, 1910, p. 117. This species is unrecognizable. Lower Ludlow Flags, Silurian, Leintwardine, Shropshire, England. 254 Text-fig. 254. — Palaeoci- daris exilis Eichwald. Holo- type. Natural size and en- larged. After Eichwald, 1854, Plate 2, fig. 14. Palaeospatangus skiptoni Harte. Palaeospatangus skiptotii Harte, 1869, pp. 135, 136. Described as spatangoid in shape, but no recognizable description is given. All data are doubtful, but Harte thought that the specimen came from the Lower Carboniferous Sandstone of Scotland. Perischodomus magnus Tornquist. Perischodomus magnm Tornquist, 1893, p. 103; Klem, 1904, p. 20. Described from a single interambulacral plate which bears a primary and secondary- tubercles. The status of the species is doubtful as well as its generic reference. Lower Carboniferous, Upper Alsace, Germany. Protoechinus T. Austin. Protoechinus T. Austin, 1860, p. 446; Loven, 1874, p. 42; Jackson, 1896, p. 235; Klem, 1904, p. 75. Palaeechinus (pars) Duncan, 1889a, p. 13. Mcloncchirms (pars) Lambert and Thiery, 1910, p. 120. Ambulacra with two columns of plates, and passing toward the wider portion of the area with intercalated plates added, producing four columns. There is a pore-pair in each plate. The interambulacrum is fragmentarily known, and other parts of the test are unknown. Type species, P. anceps Austin, from the Lower Carboniferous of Ireland. 454 ROBERT TRACY JACKSON ON ECHINI. Protoechinus anceps T. Austin. Protoechinus anceps T. Austin, 1860, p. 446, text-fig.; Loven, 1S74, p. 42 Klem, 1904, p. 75. Proto Echinus Baily, 1865c, p. 66. Meloncehinus anceps Lambert and Thierj', 1910, p. 120. With characters of the genus. This looks Uke a very interesting type, but with present knowledge cannot be definitely located. The ~ ambulacrum passing from two columns of primary plates to four columns in an area, as figured by Austin, suggests the development of an ambulacral area as seen in the Palaeechinidae. Two ambulacral plates equal the height of an adradial. The interambulacrum is represented only by an adradial column as figured by Austin. T. and T. Austin earlier (1842) gave the name Echinocrinus anceps to a distinct species; as both are very little known, there is a possibilitj' of confusing these names (p. 449). Lower Beds of the Carboniferous Limestone, Hook Point, County Wexford, Ireland. Rhoechinus sp. Duncan. Rlwrcliinm sp. Duncan, 1889, p. 205; Klem, 1904, p. 30. Duncan states that the specimen is small and in the Woodwardian [now called Sedgwick] Museum at Cambridge, England. Since he used Rhoechinus for Palaeechinus as here con- sidered (p. 303), this specimen is probably a species of Palaeechinus. Rhoechinus (?) sp. Tornquist. Rhoechitiiis (?) sp. Tornqui.st, 1897, p. 763, Plate 20, fig. 8; Klem, 1904, p. 30. Tornquist figures only an iniperfect plate. Genus quite unrecognizable, but Tornquist uses the generic name Rhoechinus for Palaeechinus as here considered (p. 303). Xenocid^ris Schultze. Xenocidaris Schultze, 1866, p. 126; Loven, 1874, p. 44; Lambert and Thiery, 1910, y>. 126. Eocidaris (pars) I'omel, 1883, p. 113. This genus was based on peculiar spines from the Devonian of the Eifel region, which are not even yet found in association with a test or plates. The base of the spines is known only in a specimen of X. cylindrica as described below. Here the spine has a milled ring and con- cave base for articulation with a tubercle of considerable size. The spines are doubtless refer- able to Echini, and quite possibly to the Cidaroida (pp. 245, 455) but as plates of the test are as yet unknown it seems safest to treat of the genus and species here. Type species, Xenocidaris clavigera Schultze, from the Devonian, of 1^-ussia, INCERTAE SEDIS. 455 *Xenocidaris clavigera Schultze. Text-figs. 256a-256d, p. 455. Xenocidaris clavigera Schultze, 1866, p. 126, Plate 1.3, figs. 3a-3h; Loven, 1874, p. 44; Jackson, 1896, p. 222; Klem, 1904, p. 75; Lambert and Thiery, 1910, p. 126. Pdaeoddaris rkenanus Quenstedt, 1S75, p. 377, Plate 75, figs. 30-37 (figs. 30-32 are doubtful). ' Spines are slender proximally, from this portion enlarging by a gradual tapering, or toward the distal end rapidly expanding into a trumpet-shaped form. Distally, the border and inflated terminal face are set with fluted or spinulose ex- pansions directed distally. These spines are quite unlike anything else known in the Palaeozoic Echini, but recall some of the peculiar cidarid spines occurring in the Meso- zoic. They can well be compared to such spines as those of the Jurassic Tiaris conoideus suevicus (Quenstedt, 1875, Plate 70, figs. 25-28) to which Quenstedt (1875, p. 377) compares them. Eleven specimens in the British Museum measure up to 31 mm. in length, and at the distal expanded tip measure up to 6 mm. in diameter. The spines described by Quenstedt as Palaeocidaris Schultze, 1866, Plate 13, fig. 4b. Museum rhenanus are from Eifel specimens, and his figures 33 to 37 are certainly the same species as Schultze's Xenocidaris clavigera. His figures 30 to 32 are doubtful, but are apparently closely allied as they have a similar club shape, but are not distally truncated with terminal spin- ules in a crown like the spines of clavigera. Middle Devonian, Gerolstein, Prussia, Museum of Comparative Zoology Collection 3,044, some thirty spines, including the cotypes from the Schultze Collection; same locality, British Museum Collection E 1,269, E 398; Munich Museum; Strassburg Museum; Freiburg i. B. Museum; Kerpen, Prussia, Schultze Collection, Museum of Comparative Zoology 3,059. 255 256 Text-fig. 255. — Xenocidaris cylindrica Schultze. Middle Devonian, Gerolstein, Eifel, Prussia. Cotype, enlarged. After of Comparative Zoology Coll., 3,201. Text-fig. 256a-d. — Xenocidaris clavigera Schultze. Middle Devonian, Muhlenberg, near Gerolstein, Eifel, Prussia. Cotypes, en- larged. After Schultze, 1866, Plate 13, figs. 3, 3a, 3c, 3d. Museum of Comparative Zoology Coll., 3,044. Xenocidaris conifera Schliiter. Xenocidaris conifera Schliiter, 1881, p. 213; Klem, 1904, p. 76; Lambert and Thiery, 1910, p. 126. Spines smooth and conical. Middle Devonian, Hillesheimer-Mulde, between Kerpen and Mollenbach, Germany. Professor Steinmann informs me that Schliiter's original specimen or specimens are in the collections at the museum in Bonn. ' See footnotes, p. 288. 456 ROBERT TRACY JACKSOX OX ECHIXI. *Xenocidaris cylindrica Schultze. Text-fig. 255, p. 455. Xeiiocidaris cylindrica Schultze, 1866, p. 126, Plate 13, figs. 4— Ic: Loven, 1874, p. 42; Klem, 1904, p. 76; Lambert and Thiery, 1910, p. 126. The spines are not cylindrical as the name implies, but spindle-shaped, thickly set with small nodose spinules, which are quite regularly spaced. A spine in the Munich Museum measures 55 mm. in length. In the British Museum there are six fragmentary spines, E 10,693 to E 10,698. One of these, E 10,693, is 28 mm. long, incomplete distally, but proximally has an expanded milled ring, which is angular in outline, with the milling nearly worn off, and resembling the base of the spines of Archaeocidaris rossica (Plate 12, figs. 13a-13k). The base of the spine is hollowed for articulation with a tubercle of considerable size. This specimen is important, as it is the only one I know of in the genus which shows the proximal end of the spine. The structure shows that it is typically echinoid, a fact that was not definitely known before. The character of the spines strongly suggests that of Archaeocidaris, but, as that genus has not yet been recorded from the Devonian, it is probably best to leave the species in its present genus, awaiting further knowledge. jMiddle Devonian, Gerolstein, Prussia, cotypes (five spines) from the Schultze Collection, Museum of Comparative Zoology Collection 3,201; British Museum Collection E 10,693 to E 10,698; Munich Museum. Nomina Nuda. Archaeocidaris tirolensis Stache, 1876, p. 260; Klein, 1904, p. 64; Lambert and Thiery, 1910, p. 125. No description of this species has been published. Echinocrinus cidariformis (?) T. and T. Austin, 1842, p. 111. No description of this species has been published. Echinocrinus pomumT. and T. Austin, 1842, p. Ill; 184.3, p. 207. The species Echinocrinus pomum, though not described, is discussed under the consid- eration of Echinocrinus anceps by the Austins (1843). (Seep. 449.) Heterocidaris Hall ; Heterocidaris keokuk Hall ; Heterocidaris laevispina Hall. Miller in his Catalogue of North American Fossils (1897, p. 747) gives the names of the genus Heterocidaris with the two species as listed, and attributes them to Hall's (1861) " Descriptions of New Species of Crinoidea from Investigations of the Iowa Geological Survey." Hall's paper consists of 18 pages in the copies I have seen, but there is a rumor NOMINA NUDA. 457 that a page 19 was separately printed and later issued. On inquiry at Albany and elsewhere, I can find no trace or assurance of this extra page on which I presume these names appear. Miller says that the descriptions are unrecognizable. The generic name Heterocidaris would not hold as it was already given to a sea-urchin by Cotteau (Bull. Soc. Geol. de France, 1860, ser. 2, vol. 7, p. 378). The specimen of Pholidocidaris irregularis Meek and Worthen, shown here on Plate 73, fig. 6, has pasted on the back a label, apparently in Hall's handwriting, bearing the name Heterocidaris laevispina. Also a tablet bearing several plates of Pholido- cidaris irregularis in the University of Michigan Collection, here figured on Plate 74, figs. 3-7, bears in old manuscript the name Heterocidaris laevispina. It is therefore probable that Hall's Heterocidaris laevispina is the same as Meek and Worthen's Pholidocidaris irregularis, but, as it was inadequately described and the paper is unavailable, it is undesirable to resuscitate the name. Messrs. Lambert and Thiery (1910, p. 124) erroneously refer Heterocidaris keokuk Hall to Archaeocidaris keokuk Hall (this memoir, p. 267), but in their citation of page and Plate they refer to the true Archaeocidaris keokuk Hall, which is published in the Report on the Geological Survey of the State of Iowa, 1858, Vol. 1, Part 2, p. 699, Plate 26, figs. 2a, 2b. (By clerical error Lambert and Thiery 's bibliographical reference "956" is to Meek and Worthen, and not to Hall's work, which in their bibliography is no. "635.") Lepidocentrus desori Dollo and Buisseret, 1888, p. 959. No description of this species has been published. Lepidocentrus duponti Dollo and Buisseret, 1888, p. 959. No description of this species has been published. Lepidocentrus gaudryi Dollo and Buisseret, 1888, p. 959. No description of this species has been published. Melonites youngi Young, 1878, p. 225. Melonechinus youngi Smith, 1901, p. 509. Young gives no description of this species of Melonites, but states that plates were first found in Craigenglen (Scotland) and later in other localities. Young attributed the species to Keeping, but I cannot find that Keeping ever published it. Keeping (1876, p. 399) says that Mr. J. Young had mentioned to him traces of Melonites in the Glasgow shales. Smith gives as localities for this species, Beith, Dobry, Muirkirk (Scotland), and says abundant but local. PalaecLinus agassizi Dollo and Buisseret, 1888, p. 959. No description of this species has been published. 458 ROBERT TRACY JACKSON ON ECHINI. Palaechinus carpenteri Dollo and Buisseret, 1888, p. 959. No description of this species has been published. Palaechinus loveni Dollo and Buisseret, 188S, p. 959. No description of this species has been pubhshed. Palaeozoic Forms Incorrectly Referred to Echini. DiscocYSTis Gregory, 1897, p. 131; Klem, 1904, p. 73. Echinodisrits Worthen and Miller, 1S83, p. 3.35, non Echinodiscus Leske, 1778, p. 131; Klem, 1904, p. 73. The name Discocj'stis was given by Gregory to replace the name Echinodiscus, which was preoccupied. Discocystis kaskaskiensis (Hall). Agclncrinus kaskaskiensis Hall, 1858, p. 696, Plate 25, fig. 18. Echinodiscm kaskaskictisis Keyes, 1894, p. 133, Plate 18, fig. 3; Klem, 1904, p. 73. Discocystis optatus (Worthen and Miller). Echinodiscus optafus Worthen and Miller, 1883, p. 336, Plate 31, fig. 9; Klem, 1904, p. 73. Discocystis sampsoni (Miller). Echinodiscus sampsoni Miller, 1891, p. 76, Plate 12, fig. 16; 1892, p. 686, Plate 12, fig. 16; Keyes, 1894, p. 133; Klem 1904, p. 73. The above considered species referred to Discocystis I think are all Cystoidea, but they were referred doubtfully to Echini by Miss Klem (1904, p. 73); and the genus Discocystis also was qualifiedly compared to Echini by Gregory (1897, p. 131). Myriastiches Sollas, 1899, p. 700; Klem, 1904, p. 74; Lambert and Thiery, 1910, p. 117. Genotype, M. gigas Sollas. Myriastiches gigas Sollas, 1899, pp. 700, 701, text-fig. 5; Klem, 1904, p. 74; Lamhert and Thiery, 1910, p. 117. Of doubtful nature; the described material is in the University Museum, Oxford. Palaechinus sp. Mitchell, 1897, p. 258, text-fig.; Klem, 1904, p. 36. Of doubtful nature; Silurian, New South Wales. Spatangopsis Torell, 1869, p. 11; Klem, 1904, p. 75. Genotype, S. costata Torell. Spatangopsis costata Torell, 1869, p. 11; Zittel, 1879, p. 480; Klem, 1904, p. 75. The genus Spatangopsis is not an echinoid, but a medusoid according to Zittel (Grund- ziige der Palaeontologie, p. Ill) and other authorities. BIBLIOGRAPHY. Includes the titles of all works referred to in this memoir. In the text references to the BibUographj' arc indicated by the year of publication. Agassiz, Alexander. 1863 List of the echinoderms sent to different institutions in exchange for other specimens, with annota- tions. Bull. Mus. Comp. Zool., vol. 1, p.- 17-28. 1864. On the embryology of echinoderms. Mem. Amer. Acad. Arts and Sci., new ser., vol. 9, p. 1-30, 4 (unnumbered) pis. 1872-'74. Revision of the Echini. Mem. Mus. Comp. Zool., vol. 3. 1872. Pts. 1 and 2, p. i-xii, 1-378, 49 pis. 1873. Pt. 3, p. 379-628 [+ 1], 28 pis. 1874. Pt. 4, p. 629-762, 69 te.xt-figs., 17 pis. 1881. Report on the Echinoidea, dredged by H. M. S. Challenger during the years 1873-1876. Report of the Scientific Results of the Voyage of H. M. S. Challenger during the years IS73-76, Zoology, vol. 3, pt. 9, p. 1-321, 65 pis. 1883. Reports on the results of dredging. . . .by the U. S. Coast Survey steamer "Blake." Report on the Echini. Mem. Mus. Comp. Zool., vol. 10, no. 1, p. i-viii, 9-94, pi. 1-9, 9a, 10-1.5, 15a-I5c, 16-28. 1892. 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Mag., vol. 2, pp. 217, 218. (This is a preliminary paper to Baily's 1865c.) 1865b. On some new points in the structure of Palaechinus. Journ. Roy. Geol. Soc. Ireland, vol. 1, pt. 1, p. 63-65, and 67, also 88, 89, pi. 4, figs. A-E. (This paper was republished under the same title in Dublin Quart. Journ. Sci., July 1865, vol. 5, p. 261-263, and 265, pi. 7 [= 8], figs. A-E.) 1866c. Some additional notes on the structure of Palaechinus. Journ. Roy. Geol. Soc. Ireland, vol. 1, pt. 1, p. 65-67, and 88, 89, 90, pi. 3, figs. 2a, 2b. (This paper was republished under the same title in Dublin Quart. Journ. Sci , vol. 5, July 1865, p. 263-265, pi. 8 [ = 7], figs. 2a, 2b.) 1874. Remarks on the Palaeozoic Echinidae, Palaechinus, and Archaeocidaris. Journ. Roy. Geol. Soc. Ireland, new ser., vol. 4, p. 40-43, pis. 3, 4. 1875. Figures of characteristic British fossils, with descriptive remarks. London: vol. 1, Palaeozoic, p. i-lxxx, 1-126, 18 te.xt-figs., 42 pis. 1877. Remarks on the fossils. In Mem. 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(.\uthor's edition published in July, 1880; entire volume in 1883.) 474 ROBERT TRACY JACKSON ON ECHINI. White, C. A. {continued). 1881. Report on the Carboniferous invertebrate fossils of New Mexico Report U. S. Geogr. Surv. West of 100th Meridian, vol. 3, supplement, p. i-.\xxviii, pis. 3, 4. 1882. Fossils of the Indiana rocks. Eleventh Ann. Rep. Geol. Surv. Indiana, p. 347—101, pi. 37-55. Wood, Elvira. 1909. .\ critical summary of Troost's unpublished manuscript on the crinoids of Tennessee. Bull. U. S. Nat. Mus., no. 64, p. i-.\ii, 1-150, frontispiece, pi. 1-15. Wortiien. A. H., and Miller, S. A. 1883. Descriptions of new Carboniferous echinoderms. Geol. Surv. Illinois, vol. 7, p. 327-338, pi. 31. Worthen, A. H. See Meek, F. B., and Worthen, A. H. Wright, Joseph. 1865. Description of a new Palaechinus. Journ. Roy. Geol. Soc. Ireland, new ser., vol. 1, pp. 62, 63, and 88, 89, pi. 3, fig. la-lc. (This paper was republished under the same title in Dublin Quart. -Journ. Sci., July 1865, vol. 5, pp. 260, 261, pi. 8 [ = 7], fig. la-lc.) Young, John. 1868. On the Carboniferous fossils collected on the Dunbar coasts by Mr. Robertson and the Secretary. Proc. Nat. Hist. Soc. Glasgow, vol. 1, p. 94 (see also p. 178). 1873. On a Carboniferous genus of echinoderms with overlapping plates. Geol. Mag., dec. 1, vol. 10, p. 301-303. 1876. Notes on Archaeocidaris, a Carboniferous echinoderm, with overlapping plates. Proc. Nat. Hist. Soc. Glasgow, vol. 2, p. 225-231. 1878. On a weathered boulder of Carboniferous Limestone Shale: its probable geological horizon and contained fossils. Proc. Nat. Hist. Soc. Glasgow, vol. 3, p. 224-225. 1882. [On Archaeocidaris raiikinii.] Proc. Nat. Hist. Soc. Glasgow, vol. 5, p. 108. Zittel, K. A. von. 1879. Echinodermata. In Handbuch der Palaeontologies Palaeozoologie. Miinchen und Leipzig: vol. 1, pt. 1, p. 308-560, text-figs. 212-404. INDEX. The index includes all seientific names of sea-urchins or other animals, occurring in this memoir. The accepted names are in Roman type, excepting new ones, which are in heavy-faced type. All synonyms or exceptional spellings are in italics. Personal names are included in the index only where there is discussion of opinions or other special need of comment. The names of those from whom material was borrowed or to whom I am otherwise under obligations are given in the Introduction, as well as throughout the text but are not included in the index. The page-references in heavy faced type indicate the more important pages on which the case in hand is described. abyssicola, Cidaris, 96, 100. acanthifera, Archaeocidaris, 237, 258, 272. Acanthocidaris hastigera, 97. Acrosalenia, 78, 112, 214. aspera, 112, 157. deeorata, 112, 157. hemicidaroides. 111, 112, 113, 157, 164, 174. pseudodecorata, 112, 148, 150, 152. pustulata, 112, 157, 164, spinosa. 111, 112, 157, 174, 175. wiltoni. 111, 112, 157, 175, 176. acuaria, Echinocystis, 441. Eocidaris, 441. acuaria, Pholidocidaris, 236, 443, 441. acuaria, Prolocidaris, 441. aculeata, Archaeocidaris, 237, 238, 258, 268. aculealus, Archaeocidaris, 268. acutus, Echinus, 118, 139, 160. affinis, Cidaris, 61, 91, 92, 95, 96, 97, 99, 100, 141, 140. Echinus, 117, 118, 139, 1.50, 153, 160, 171, 216. Agassiz, Alexander, 10, 11, 18, 28, 31, 32, 33, 34, 52, 56, 57, 61, 63, 64, 65, 69, 71, 74, 79. 81, 82, 84, 85, 86, 88, 89, 101, 102, 104, 110, 111, 113, 116, 117, 121, 127, 128, 167, 168, 170, 174, 175, 176, 177, 185, 187, 189, 198, 210, 214, 215, 217, 218, 219, 223, 287, 297, 361, 362, 363, 378, 395, 398, 413, 414, 415, 433. Agassiz, Alexander, and Clark, H. L., 28, 62, 76, 82, 97, 104, lis, 128, 172, 213, 215, 218, 415. Agassiz, Louis, 15, 33, 257, 400. agassizi, Archaeocidaris, 82, 237, 256, 2.58, 266. Palaechinus, 305, 457. agassizii, Archaeocidaris, 266. Agelacrintis kaskaskiensis, 458. albida, Trigonoeidaris, 159. albus, Strongylocentrotus, 95, 127, 162, 164, 217. Aldrovandus, Ulysses, 10, 244. alexandri, Salmacis, 159. Amblypneustes, 35, 50, 187. formosus, 31, 159. griseus, 159. ovum, 159. pallidus, 159. Ammonites, 56, 232. Anadema, 28. Ananehytes, 52, 92, 150. ovatus, 112, 148. anceps, Archaeocidaris, 449. anceps, Echinocrinus, 258, 449, 454, 456. anceps, Melonechitius, 454. anceps, Protoechinus, 449, 453, 454. anglicus, Lovenechinus, 304, 321, 320, 346. angulosus, Echinu.s, 119, 151, 1.53, 160, 164. annelata, Calmarius, 28. annulatus, Miorocyphus, 166. annulifera, Phyllacanthus, 101, 102, 150, 152, 153. antarcticus, Sterechinus, 120. Antedon, .52. rosaceus, 52. antillarum, Dermatodiadema, 81, 104, 154. anlillarum, Diadema, 28. Arachnoides, 64, 72, 204, 218. placenta, 72. Arbacia, 18, 19, 29, 40, 46, 52, 58, 62, 65, 69, 71, 73, 87, 89, 90, 91, 107, 116, 167, 175, 176, 183, 187, 189, 192, 194, 198, 210, 215, 217, 241. dufresni, 115, 158, 164, 176. lixula, 40, 47, 79, 114, 1,58, 104, 187, 192, 194, 195. nigra, 95, 114, 115, 148, 150, 1.52, 1.53, 1.58, 164, 170, 187, 192, 194, 195. punctulata, 35, 37, 39, 41, 42, 44, 45, 46, 48, 50, 88, 114, 115, 141, 151, 152, 153, 158, 164, 170, 171, 175, 176, 179, 185, 187. spatuhgera, 95, 115, 116, 1.58, 104. stellata, 114, 158. Arbaciidae, 92, 93, 111, 113, 116, 133, 146, 1.50, 158, 203, 209, 214, 215, 218. Archaeocidaridae, 70, 89, 206, 209, 223, 236, 254, 284, 302, 305, 379, 393, 409. Archaeocidaris, 10, IS, 23, 29, 32, 54, 70, 73, 74, 77, 78, 82, 85, 178, 181, 182. 206, 209, 212, 223, 224, 236, 237, 238, 254, 2.55, 256, 201, 270, 275, 277, 278, 280, 282, 283, 289, 307, 361, 363, 395, 407, 413, 414, 450, 456. acanthifera, 237, 258, 272. aculeata, 237, 238, 258, 268. aculealus, 268. agassizi, 82, 237, 256, 2.58, 266. agassizii, 266. anceps, 449. henburhensis, 277. S, biangulata, 237, 258, 273. biangulatus, 273. clavata, 2.59, 282. (475) 476 ROBERT TRACY JACKSON ON ECHINI. Archat'ocidaris (conlinueil) . coloradensis, 237, 258, 267, 274. rratis, 237, 2.W. 272. dininni, 271. dininnii, 237, 258, 271. drydenensis, 288. edgarensis, 237, 258, 269. clegatis, 280. forbesiana, 238, 257, 259, 280. glabrispina, 258, 261. gracilis, 237, 258, 267. gruneri, 277, 279. haUiana, 237, 259, 279. hallianus, 279. harteiana, 408. illinoisensis, 258, 266. keokuk, 258, 259, 267, 457. konincki, 258, 446. ladina, 258, 446. laevis, 262. legrandensis, 256, 258, 260. longispina, 237, 258, 261. megastyla, 237, 258, 273. ttiegastylis, 273. megastylus, 273. mucronata, 258, 267, 271, 272, 274. 77iucrotiatus, 271. muensteriana, 259, 280, 442. miinsteriana, 280. munsterianus, 280. nerei, 2.58, 262. nerii, 262. newberryi, 258, 269. norwoodi, 258, 270. ornata, 258, 267, 271, 274. ornalus, 267, 271, 274. ourayensi.s, 237, 258, 274. paradoxa, 237, 258, 270. pizzulana, 2.58, 272. prisca, 11, 259, 276. priscus. 276. prolei, 262, 263. rankini, 11, 259, 276, 449. regimontana, 277, 279. rossiea, 77, 82, 177, 184, 224, 255, 256, 258, 263, 456. rossica var. schcllwieni, 263, 264. rns-nctis, 263. scotica, 258, 447. selwyni, 238, 258, 447. .shumardana, 258, 268. shumardanus, 268. shumardi, 268. shumardiana, 268. skumardina, 268. sixi, 258, 447. sp., 269, 272, 281, 447, 408, 448, 449. sip. a., 272. sp. b., 237, 259, 281. i-p. 6. var., 281,282. .\rehaeo('idaris (conlin ned) . sp. c, 281. 282. up. d., 281, 282. spinoflavata, 237, 259, 281. spinoclamtus, 281. ■•ipinosiis, 449. stellifera, 261, 262. lenncsseac, 449. tirolensis, 258, 456. trautsohokli, 258, 448. triplex, 237, 258, 274. Iriserata, 275. triserialis, 258, 275. triserrata, 237, 258, 275. triserralus, 275. trudifer, 237, 25s, 269. Irudifera, 270. iirei, 277. urii, 82, 2.50, 257, 259, 276. ri-rneuiliana, 245, 268. vehtsta, 277, 279. vetustus, 277. wervekei, 2.59, 276. wcrwekei, 276. wortheni, 70, 81, 82, 224, 256, 257, 258, 259, 264, 267. archaica, Bothriocidaris, 28, 34, 42, 45, 48, 52, 53, 54, 62, 64, 69, 70, 79, 81, 87, 88, 148, 167, 173, 210, 239, 242, 243, 244, 446. aspera, Aerosalenia, 112, 157. Aspidodiadema, 33. meijerei, 154. nicobaricum, 152, 153, 154. Aspidodiadematidae, 87, 104, 154, 202, 209, 213. Asterias, 52. asteriscus, Centrostcphanus, 109, 156. Asterocidaris minor, 103, 154. Asteroidea, 200, 222. Asthenosoma, 62, 81, 168, 179, 180, 195. hystrix, 156. ijimai, 109, 110, 1.56. owstoni, 156. Astropyga, 56, 74, 109, 110, 174. 189, 202, 213, 214. pulvinata, 74, 76, 109, 156. radiata, 156. allanticus, Leptechitiiis, 121. atlanticus, Toxopneustes, 35, 36, 41, 44, 46, 121, 122, 141, 151, 153, 161, 164, 166, 170. atratus, Colobocentrotus, 35, 45, 147, 163, 217. Aulodonta, 103, 110, 154, 155, 156, 174, 178, 183, 186, 1,87, 202, 209, 212, 214, 220, 221. Au.stin, Thomas, 236, 449, 4.50, 454. Austin, Thomas, and Thomas, Jr., 449, 454, 456. Baciditcs, 56. baculosa, Phyllaoanthus, 99, 101, 102, 148, 150, 152, 153, 192, 194. Baily, W. H., 10, 89, 262, 303, 306, 307, 308, 309, 310, 318. 319, 321, 322. 332, 363, 403, 407, 408, 409, 414, 419. INDEX. 477 Bateson, William, 35, 50. Bather, F. A., 11, 27, 29, 73, 74, 88, 96, 172, 190, 208, 220, 223, 236, 242, 245, 246, 247, 254, 255, 257, 277, 318, 327, 330-334, 387, 450. Beochcr, C. E., 71, 293, 295, 298, 299. beecheri, Hyattechinus, 12, 16, 17, 22, 25, 28, 32, 34, 64, 65, 67, 173, 210, 225, 226, 237, 292, 293, 297. Beede, J. W., 266, 270, 273, 446, 450. Bell, F. J., 175, 195. bellulus, Oligoporus, 351, 352. benburbensis, Archaeocidaris, 277. Cidaris, 277, 278, 279. Beyrich, Ernst, 288. biangulata, Archaeocidaris, 237, 258, 273. bianguhdii, Cidarolropus, 273. biniif/ulatus, Archaeocidaris, 273. bifolor, Salmacis, 159. bigranularis, Stomeohinus, 104, 105, 155. biserialis, Goniocidaris, 100, 102. Perischodomus, 64, 70, 76, 233, 237, 309, 401, 411. bisserialis, Perischodomus, 401. blairi, Eocidaris, 260, 261. blairi, Oligoporus, 304, 351, 3.54, 355. Bothriocidaridae, 201, 209, 238. Bothriocidaris, 10, 12, 23, 31, 34, 46, 53, 57, .'JS, 64, 65, 69, 71, 72, 78, 79, 82, 83, 85, 87, 88, 89, 167, 171, 181, 201, 208, 209, 210, 211, 219, 220, 235, 238, 240, 244, 254, 295, 378, 405, 439, 444. archaica, 28, 34, 42, 45, 48, 52, 53, 54, 62, 64, 69, 70, 79, 81, 87, 88, 148, 167, 173, 210, 239, 242, 243, 244, 446. cxilis, 453. globulus, 10, 34, 87, 88, 210, 239, 240, 243. pahleni, 34, 69, 77, 87, 88, 167, 210, 239, 240, 242, 244. palheni, 242. phaleni, 242. Bothriocidaroida, 201, 209, 211, 238. Bolriocidaris, 238. globulus, 243. pahleni, 242. Botryocidaris, 238. pahleni, 242. Brachiopoda, 71 . brauni, Pholidechinus, 184. 226, 237, 265, 295. Brisaster fragilis, 177. Brissus, 219. Buisseret, A. See Dollo, Louis, and Buisseret, A. burlingtonensis, Maecoya, 54, 55, 59, 60, 227, 229, 230, 231, 300, 303, 312, 314, 315. burlingtonensis, Paliiechiniis, .303, 312. Palaeechinus, 303, 313. Rhoechinus, 313, 323. Bury, Henry, 210. Calamaria, 28. calamaris, Diadema, 28. calamaris, Echinothrix, 28, 109, 155. Calamarius, 28. caledonica, Lepidesthes, 416, 432. Calmarius, 28. annelata, 28. Camarodonta, 30, 116, 1.59, 160, 161, 162, 163, 179, 183, 186, 187, 203, 209, 214, 215. oanaliculata, Goniocidaris, 54, 70, 81, 99, 100, 101, 102, 170, 184. randidum, Diadema, 27. Cannon, G. L., 247, 249. cannoni, Miocidaris, 212, 236, 2.38, 245, 247, 257. carinata, Lepidesthes, 408, 412, 416, 424. carpenteri, Palaechinus, 305, 458. eartieri, Hemicidaris, 154. Caryocrinus, 35. cassiana, Miocidaris, 245. Cassidulus, 92, 150. subquadratus, 148. Centrechinidae, 28, 74, 76, 78, 92, 93, 104, 105, 108, 109, 110, 155, 1.56, 202, 209, 213. Centrechinoida, 27, 28, 30, 55, 59, 60, 61, 62, 64, 78, 79, 83, 84, 85, 87, 91, 92, 93, 94, 95, 96, 102, 103, 109, 113, 116, 129, 133, 146, 152, 154-164, 174, 178, 182, 183, 186, 187, 188, 189, 192, 194, 196, 198, 200, 202, 208, 209, 211, 212, 213. 215, 217, 218, 219, 220, 221, 223, 361,413,414. Centrechinus, 16, 19, 28, .54,-56, 63, 74, 76, 85,91,94, 104, 106, 107, 108, 110, 172, 174, 183, 190, 194, 195. setosus, 28, 76, 104-109, 110, 152, 1.53. 1.55, 164, 167, 168, 170, 183, 185, 191, 192, 213. Centrostephanus, 174. asteriscus, 109, 156. longispinus, 108, 110, 156. rodgersi, 108, 110, 152, 153, 1.56. Cephalopoda, 16, 17, 66, 220. Chadwick, H. C, 35, 179. Chaetodiadema, 76, 109, 110, 174. pallidum, 108, 110, 1.56. chilensis, Fabia, 145. chloroticus, Evechinus, 125, 161, 164. Cidaridae, 55, 60, 61, 62, 78, 79, 81, 82, 87, 88, 95, 100, 101, 103, 172, 192, 195, 201, 209. 211, 236, 240, 245, 248, 257, 320, 3.32, 413. Cidarids, 69, 71, 77, 83, 89, 102, 106, 109, 112, 173, 174, 190, 211, 212, 213, 226, 248, 249, 265, 309, 312. 44.5, 455. cidariformis, Echinocrinus, 258, 456. Cidaris, 17, 29, 32, 34, 51, 52, 55, 58, 64, 65, 66, 73, 82, 83, 86, 96, 179, 182, 190, 192, 214, 221, 260, 277, 445. abyssicola, 96, 100. affinis, 61, 91, 92, 95, 96, 97, 99, 100, 141, 146. benburbensis, 277, 278, 279. cidaris, 96, 100, 182. coronata, 95, 96, 100, 148. deucalionis, 263. elegans, 280. florigemma, 95. forbesiana, 280. glahrispina, 261. keyserlingi, 245. klipsteini, 245. 478 ROBERT TRACY JACKSON ON ECHINI. Cidaris (continued). laevispina, 255, 256. muensteriamis, 280. munsteriana, 280. nerei, 262. panamensis, 34. papillala, 61, 182. rossicus, 263. sceptifera, 95. scrobiculata, 255, 256. urii, 276, 277 vemeuiliana, 245. !)e