MEMOIRS OF THE PFABODY MITSEUM OF NATURAL HT'^TORY VOLUME IV, PART J Brachiopod Genera of the Suborders Orthoidea and Pentameroidea BY CHARLES SCHUCHERT PROFESSOR OF PALEONTOLOGY, FMFRtTUS, YALE UNIVERSITY AND G. ARTHUR COOPER ASSISTANT CURATOR OF STRATIGRAPHIC PALEONTOLOGY U. S. NATIONAL MUSEUM LVX ET VERITAS NEW HAVEN, CONN. 1932 MEMOIRS OF THE PEABODY MUSEUM OF NATURAL HISTORY YALE UNIVERSITY Volume I. Odontornithes: A Monograph on the Extinct Toothed Birds of North America. By Othniel Charles Marsh. Pp. i-ix, 1-201, pis. 1-34, text figs. 1-40. 1880. To be obtained from the Peabody Museum. Price $3. Volume II. Parti. Brachiospongidse : A Memoir on a Group of Silurian Sponges. By Charles Emerson Beecher. Pp. 1-28, pis. 1-6, text figs. 1-4. 1889. To be obtained from the Peabody Museum. Price $1. No more published. Volume III. Part 1. American Mesozoic Mammalia. By George Gaylord Simp- son. Pp. i-xvi, 1-171, pis. 1-32, text figs. 1-62. 1929. To be obtained from the Yale University Press, New Haven, Conn. Price $5. Part 2. A Remarkable Ground Sloth. By Richard Swann Lull. Pp. i-x, 1-20, pis. 1-9, text figs. 1-3. 1929. To be obtained from the Yale University Press, New Haven, Conn. Price $1. Part 3. A Revision of the Ceratopsia or Horned Dinosaurs. By Richard Swann Lull. In preparation. Part 4. The Merycoidodontids, an Extinct Group of Ruminant Mammals. By Malcolm Rutherford Thorpe. In preparation. Volume IV. Part 1. Brachiopod Genera of the Suborders Orthoidea and Pentam- eroidea. By Charles Schuchert and G. Arthur Cooper. Pp. i-xii, 1-270, pis. A and 1-29, text figs. 1-36. 1932. To be obtained from the Peabody Museum. Price $7 (bound in cloth), $6 (bound in paper). Addenda et Corrigenda Date of publication: August 23, 1932. All photographs of specimens figured on the plates were made hy the author, with the assistance of Mr. Percy A. Morris. With the exception of Plate A, and Figure 24 of Plate 1, these photo- graphs are entirely without retouching. Page 57, column 1, line 14 from bottom, for impunctate read ■punctate. Page 80, column 1, delete first two lines. Page 81, column 2, line 7, for Oxarkian read Canadian. Page 84, legend for Fig. 17, next to last line, for lejt read right. Page 88, column 2, lines 14 and 15, delete which is rather strongly convex. Page 109, column 2, lines 4 and 5, delete We refer it doubtfully to our new genus Paurorthis. Page 141, table 14, Aulacophoria should be derived from Schizo- phorta, instead of from Orthotichia. MEMOIRS OF Volume I. Oc Volume II. Pa Nc Volume III. Pa Pa Pa Pa Volume IV. Pa BRACHIOPOD GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA "The esthetic satisfaction to be derived from contemplating the mere variety of animal forms and from tracing the order that runs through all its diversity appeals to a very deep instinct in human nature." David Sharp "There is only one way of seeing things rightly, and that is seeing the whole of them." John Kuskin MEMOIRS OF THE PEABODY MUSEUM OF NATURAL HISTORY VOLUME IV, PART 1 Brachiopod Genera of the Suborders Orthoidea and Pentameroidea BY CHARLES SCHUCHERT PROFESSOR OF PALEONTOLOGY, EMERITUS, YALE UNIVERSITY AND G. ARTHUR COOPER ASSISTANT CURATOR OF STRATIGRAPHIC PALEONTOLOGY U. S. NATIONAL MUSEUM LVXET ■ 1 a-ip.m ■ ■ ^ VERITAS ^ NEV^ HAVEN, CONN. 1932 PRINTED IN THE UNITED STATES OF AMERICA THE TUTTLE, MOREHOUSE & TAYLOR COMPANY NEW HAVEN, CONNECTICUT CONTENTS Page Introduction * Division of work •^ Location of specimens ^ Technique "^ Species lists Genetic lines Acknowledgments Part I. Brief definitions of brachiopod terms " Part II. Principles of morphogenesis or evolution of form 12 12 Need of taxonomy 12 Rise of brachiopod genera 13 The genus concept Paleoecology Homoeomorphy Part III. Morphology of the orthoid shell 18 Morphology of the exterior 18 Orientation of the shell 18 Commissures '" Convexity ' Interareas ^^ Deltidium •■■ 21 Modifications of the delthyrium (deltidium) 21 Function of the deltidium 22 Lateral plates Modifications of the notothyrium (chilidium and chilidial plates) 23 Morphology of the ventral interior Articulation Teeth Ys Sockets -P Dental plates Spondylium ^, History of the term Present views and definitions «j , 29 Septa .; , 30 Musculature 32 Paliial and genital markings -- Pallial markings ^^ » Genital or ovarian markings viii CONTENTS Part III. Morphology of the orthoid shell — Cont. Page Morphology of the dorsal interior 34 Notothyrial platform 34 Denticles and sockets 34 Denticles 34 Sockets 34 Cardinalia 34 Cardinal process 34 Brachiophores or brachial apparatus 37 Cruralium and pseudocruralium 39 Septa 39 Musculature 39 Pallial markings 40 Microstructure of the shell 41 Exopuncts 42 EndopunctEC 42 Value of punctation in taxonomy 42 Old-age characters 42 Part IV. The genera of the suborder Orthoidea 43 Superfamily Orthacea 43 Family Nisusiidae 43 Nisusia 44 Jamesella 46 Family Protorthidje 46 Protorthis 46 Loferia 47 Family Billingsellidas 48 BUlingsella 48 Family Eoorthidje 50 Witnanella 50 Eoorthis 51 Otusia 52 Bohemiella 52 Oligomys 53 Family Finkelnburgiidse, nov 54 Orusia ^ 54 Finkelnburgia 55 Family Plectorthidas 56 Subfamily Plectorthinx 57 Plectorthis 57 Hebertella 59 Mimella 61 Schizofhorella 62 Doleroldes 63 Subfamily Cyclocoeliinas 64 Cyclocaelia 64 Subfamily Platystrophiins 64 Platystrofhia 64; Mcewanella 69 CONTENTS >X Supcrfamily Orthacca — Cont. Page Family Plcctorthidx — Cont. • 70 Subfamily Orthostrophiinae _„ Orthostrophia -. O. dart(F, nov 71 Family Skenidiidas yi Skentdio'tdes ^2 S. hillingsi -o Sken'ui'tum 73 Family Orthida: _- Subfamily Orthinse ^5 Orthis n-j Cyrtonotella n-j Nicolella yg Glossorthis ^9 Paurorthis gQ Archtrorthis g. Subfamily Productorthinae gj Panderina g2 Productorthis r.. Subfamily Angusticardiniinae g^ Angusticardinia „, Subfamily Taffiinae g5 Taffia Z'ZZZZ'Z'. 85 Eostrofbomena „- Subfamily Hesperorthinae g5 Hesperorthis g^ Schizoramma gg S. gotlandica, nov gg Dolerorthis og Subfamily Glyptorthinae gg Glyptorthis ni Eridorthis g2 Ptychofleurella o, P. matapedia, nov 93 Family Dinorthidae o, Dinorthis nc Dinorthis s. s qc Plasiomys ge Retrorsirostra g, Pionorthis g, Marionella ' g_ Valcourea gg V. magna, nov _„ Multicostella gg Auitinella jqq Palceoitrophomena .qq Planidorsa iqi P. hella jQj P. crassicostella, nov Family Porambonitidae jq2 Porambonites Iqc Noetlingia , ^, .._, 105 Family Lycophonidae jq5 Lycophoria X CONTENTS Part IV. The genera of the suborder Orthoidea — Cont. Page Superfamily Clitambonacea 107 Family Deltatretida: 107 Deltatreta 108 Pomatotrema Ulrich and Cooper, nov 109 P. muralis Ulrich and Cooper, nov 109 Polytoechia 110 Family Clitambonitidx 110 Subfamily Plectellinae 112 Plectella 112 Ingrla 112 Subfamily Clitambonitins 113 Clitambonites 113 Vellamo 114 Clinambon, nov 115 Estlandia 115 Hemifronites 115 Pahlenella 117 Afomatella 117 Subfamily Gonambonitinas 118 Gonambonites 11° Superfamily Dalmanellacea H' Family Dalmanellidas 11° Dalmanella 120 Carinijerella 1^2 Aulacella 122 Proschixofhoria *2j Levenea ^^^ Heterorthina 124 Family Wattsellidae 124 Wattsella 125 Resserella 126 Horderleyella 1^7 Mendacella ^^' Idiorthis 128 Parmorthis ^2o P. crassicostata, nov 129 Fascicostella ^29 Family Bilobitidae 1^0 Bilobites 130 Family Mystrophoridae 1-^1 Mystrophora '-^^ Kayserella l-'^ Family Rhipidomellidae ^•'•^ Rhifidomella '•'■' Perdkocardtma '•'-' Platyorthis 1^^ Thiemella 1^^ Family Heterorthidas '•^" Subfamily Heterorthinae j;;' Heterorthh 1-^' Subfamily Harknessellinae J;;^ Harknessella |^° Reuschella |^^ Smeathenella CONTENTS xi Superfamily Dalmancllacea — Cont. Page Family Schlzophoriid.-c 139 Subfamily Schizophoriinae 140 P'tonodema 141 Schizophoria 143 Orthot'tch'ta 144 Aulacofhoria 145 Subfamily Enteletinx 145 Enteletei 146 Parenteletes 147 Enieletina 148 Enteletella 148 Enteletoides 148 Subfamily Isorthinae 149 Isorthis 149 Family Linoporellidx 150 Ltnoporella 150 Orthotrofta 152 Family Tropidoleptidae, nov 152 Tropidoleftus 152 Part V. The genera of the suborder Pentameroidea 154 Superfamily Syntrophiacea 154 Family Clarkellids 155 Syntrofh'to'uies 155 Syntrofhina 155 Clarkella 156 Yangtzeella 157 Family Syntrophiids 158 Syntrofhta 158 Swantonia 159 Family Huenellidae 159 Huenella 159 Huenellina 160 Superfamily Pentameracea 161 Technique 162 Morphology of the Pentameracea 162 Exterior 162 Ventricosity 162 Ornamentation 162 Interareas 163 Ventral interior 163 Delthyrium 163 Deltidial cover 163 Deltidial plates 163 Spondylium 164 Dorsal interior 164 Septa 164 Musculature 164 Cardinalia 164 xii CONTENTS Part V. The genera of the suborder Pentameroidea — Cont. Superfamily Pentameracea — Cont. Page Generic and evolutionary trends 165 Loss of interareas and development of a planarea 165 Rostration 165 Reversion of normal convexity 165 Trilobation 165 Ornamentation 165 Loss of the ventral septum 166 Development of a cruralium 166 Parallel trends 166 Summary 166 Family Camerellidx 166 Camerella 167 Branconia 168 Parastrofhtnella 169 Anastrofhia 169 Metacamerella 170 Family Pentameridas 170 Subfamily Gypidulinse 171 Clorinda 171 Barrandella 173 Gyfidula 173 S'teberella : 175 Pentamerella 176 Zdimir 177 Subfamily Pentamerinae 177 Lasves 177 Pentamerus 177 Pentameroides 179 Lissocoelina 179 C afelliniella 179 Holorhynchus 180 HarpidiuTn 180 Costatffi 180 Rhifidium 180 Conchidium 181 Brooksina 183 Cymbidium 183 Platymerella 184 V'trgiana 185 Family Stricklandidas 186 Stricklandta 187 Appendix 189 ? Superfamily Rhynchonellacea 189 Rhynchocamara 189 Liocoelia 189 Index 257 INTRODUCTION Brachiopods have interested the senior author since the days of his early youth, and by 1886, while assistant to E. O. Ulrich in Cincinnati, he had a private collection of them so large and well arranged that it led James Hall to transport collection and collector to Albany to assist him in the work then in preparation, "An Introduction to the Study of the Genera of Palasozoic Brachiopoda" (1892-1894). This recognition stimulated Schuchert all the more to add to his collection, which he has continued to do ever since, with the exception of the twelve years spent as curator at the United States National Museum, where each official is expected to devote his entire attention to the care and increase of the Government collections. The first great addition was made in 1884, when the brachiopods gathered by E. O. Ulrich were obtained, and later through purchase came the Ernst H. V'aupel and other smaller collections from the Ohio Valley. At Albany was purchased the John M. Clarke collection of Middle Devonian fossils. All the non-brachiopod material in these pur- chases was exchanged with the dealers Braun of New York and Krantz of Bonn, Germany, for European brachiopods, with the permission of the authorities of the National Museum. Upon taking up the chair of paleontology at Yale University in 1904, the stimulus to acquire more brachiopods was renewed. The Marshall collection of English Jurassic Telotremata was pur- chased in 1909 through S. S. Buckman, who relabeled the material carefully, especially with regard to stratigraphic horizons; and later the private collection of Fred Braun of New York, also rich in European species, which that dealer had long been accumulating. Largest of all the acquisitions, however, enriching the Schuchert Collection by many thousands of fine Paleozoic specimens, was the private collection of D. K. Greger, most of which that devoted gatherer of brachiopods had assem- bled, during a long life, from the Mississippi Valley and the Southwest. With this material came an extensive library of brachiopod literature, which, together with that already in the Peabody Museum, forms the still growing Schuchert-Greger Brachiopod Library of that institution. From these statements it is apparent that the authors of the present work had access to one of the largest brachiopod collections known. For Cambrian material, without which no genetic study of brachiopod genera can have a secure foundation, we had the privilege of access to the most exten- sive collections known from rocks of this period, made by Charles D. Walcott and forming one of the great treasures of the United States National Museum. At Washington the curatorial duties of the senior author made it impossible to take up any protracted paleontological studies, due to the rapid growth and constant need for rearrangement of the National Museum collections; but he did bring to final form a card catalogue of brachiopod genera and species which he had begun at Cincinnati, and which appeared in print in 1897 as Bulle- tin 87 of the United States Geological Survey. With his transfer to Yale, the hope of working up his collections was reborn, but here again, due to university and administrative duties, it soon became evident that if anything worth while was to be done, it must come through a colleague who could devote his entire time to a restudy of the genera. For many years, however, neither adequate scholarships nor museum appointments could be found, but, with the hope still in mind, the senior author laid aside savings out of his salary and out of royalties accruing from the sale of his text- books. Finally, in 1925, there came to Yale a post-graduate student, the junior author, who soon showed that he possessed the necessary qualifications and inclination to undertake the work. In the beginning privat-dozent and professor emeritus built "castles in Spain," contemplating a revision of all the brachiopod genera, but caution led them to start with the most primitive of the articulate forms, the orthoids. What an undertaking the original intention would have been is seen in Schuchert and LeVene's "Generum et Genotyporum Index et Bibliographia" of 1929, which lists about 700 valid genera, of which 456 are Paleozoic, 177 Mesozoic, and 74 Cenozoic-Recent. The senior author had also become suspicious that all was not correct in regard to the origin of the Telotremata as held by Beecher (1891) and Schuchert (1897), and in regard to the supposed primordial significance of the deltidium in the Protremata, and of what had been called the pro- GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA deltidium. Yatsu (1902, 1905) had shown that there was no "third plate" in embryonic shells and that therefore the formation of a deltidium could not have started in this way. This agam raised the question: Of what significance in classification is the deltidium? In the meantime Walcott's classic "Cambrian Brachiopoda" (1912) had shown that among these early forms there were no Telotremata {Swantonia is not of this order, as was then believed), and that the few forms regarded as Strophomenacea were probably orthoids. After we had started our studies came a penetratmg little book entitled "Brachiopod Morphology and Genera (Recent and Tertiary)" by that clear- thinking author, the late J. Allan Thomson, which made still more apparent the necessity for a revision of the Paleozoic articulate brachiopods. As is well known, studies along fundamental lines take much more time than generic revision, and as time wore on our Spanish castles showed signs of shrinkage, and we had to limit ourselves to Paleozoic genera. Along this line we were proceeding nicely when the call came for the junior author to take up a position as assistant curator in the United States National Museum, and we had to write "finis" to our work with only one-half of the Protremata done and the Telotremata not more than started. However, we have an abundance of results, showing that the Orthacea, or rather the greater division that we are calling Orthoidea, contains the primary stock from which all the articulate brachiopods have arisen! This means not only that the Pentameracea and the Stroph- omenacea have evolved out of the Orthacea, but that the order Telotremata had its origin here as well. We have indicated these beginnings and regret that conditions prevent our proving the genetic lines. However, a need once pointed out will soon be taken up by others, though we hope that it may be our privilege to follow these leads further, either through independent work by the junior author, or through studies at Yale by another budding paleontologist. Table of the groupings discussed in the following pages Total New Orders Suborders (Orthoidea and Pentameroidea) Superfamilies Families Subfamilies Genera and subgenera Superfamilies Orthacea Clitambonacea Dalmanellacea Syntrophiacea Pentameracea Families 11 (6 new) 2(1 " ) 9 (6 " ) 3 (2 " ) 3 2 2 5 28 20 135 Subfamilies 10 (7 new) 3 (2 " ) 5 (2 « ) 0 2 2 2 15 11 35 103 32 Genera- Subgenera (53 (15 new) 13 ( 4 " ) (37 (10 " ) 2 " ) 4 ( 24 ( ) Old 2 3 13 9 100 Genera known in 1929 !» I 26 28 (15 new) 20 (11 new) 135 (35 new) 78 Division of Work. — The senior author, as intimated above, made it financially possible for his co-laborer to devote himself wholly for two years to this study of the Brachiopoda, and also turned over to him for use his collection and library. He then sketched out the plan of work and passed on the brachiopod lore accumulated during a lifetime, and stimulated by association with E. O. Ulrich, James Hall, John M. Clarke, and Charles E. Beecher. The bulk of the detailed results were thus attained by the junior author, under the guidance of the senior author and of our colleague. Professor Carl O. Dunbar, who was at the time himself engaged in a revision of the Pennsylvanian brachiopods of the Nebraska region. The study so carried on has led to departures from the older lore as radical as were those of Hall and Clarke between the years 1 890 and 1 895. These results, we are fully aware, will in turn have to face the test of new facts brought to light by later workers. Nevertheless we are content to INTRODUCTION 3 send them forth as a contribution to the evolutionary history of the brachiopods — a study the importance of which was well seen by Charles Darwin when he unavailingly appealed to the lead- ing specialist of his time, Thomas Davidson, to take up the brachiopods from the standpoint of descent through evolution. Location of Specimens. — As indicated above, this memoir is based mainly on material in the Schuchert Brachiopod Collection in the Peabody Museum of Yale University. Specimens from this collection have the letter S preceding the catalogue number (e. g., S 12). Specimens from other collections in the Peabody Museum have the letters Y.P.M. following the catalogue number (e. g., 120 Y.P.M.). Technique. — In any critical revision of brachiopod genera it is paramount to know every detail of the shells with which one is dealing. It is not difficult to find brachiopods in the field showing the exterior, as free specimens or as molds in the rock; it is far more difficult to find free valves with the interiors cleaned by nature, or as sharp molds. In the Schuchert Collection by far the majority of the specimens are free individuals showing the exterior in good detail. This is a con- sequence of the collection's being a composite one, built up of choice material brought together by many skilled collectors. To be sure, there are great numbers of interiors in the collection, but not of every genus. Accordingly, it became necessary to prepare interiors for a large number of the genera that we studied. We were able, by various methods, to obtain the essential details of the internal anatomy of every genus with which we worked. There is really no reason why, given plenty of material, the interior of every genus of brachiopods should not be known. The following methods were used in securing the internal anatomy of the brachiopod shell: (1) etching, (2) cleaning with needles, (3) burning, and (4) serial sectioning (Zugmayer process). ( 1 ) When specimens of dissociated valves are filled with shale or clay, the internal surface may be obtained by placing on the specimen lumps of potash and allowing the chemical to deliquesce. The potash then attacks the shale and loosens it. This method is not always satisfactory, however, because the potash usually attacks the substance of the shell, producing a white efflorescence which can be got rid of only by the use of dilute acid. This means the loss of some details of the surface, and is undesirable when other methods can be pursued. When shells are silicified they can as a rule be freed from the limestone by dissolving the matrix in dilute hydrochloric acid. Care must be taken, however, to use exceedingly dilute acid, otherwise the shell itself may be attacked or be disrupted by violent ebullition of gas. It is not usual to find shells properly silicified, but when one does, they show the internal characters exceedingly well (2) More laborious, but more satisfactory by far, than the use of potash is the careful manipu- lation of a needle accompanied by a dentists' drill. With this combination the interior of most brachiopods can be obtained. We found it most satisfactory to cement the specimen in plaster of paris and then excavate the interior with a needle, carrying on all the manipulations under a binoc- ular microscope. This method requires time and patience, but when one has prepared successfully the interior of a rare shell, he is amply repaid for the expenditure of one-half to a day's tirne. In some instances we took specimens in which both valves were in contact and secured the interior of one or the other valve, depending on which we wanted. If the ventral interior was desired, the dorsal valve was ground off, the remaining one cemented in plaster and excavated by the use of a needle. The majority of the interiors figured in this memoir were prepared by the use of a needle. (3) John M. Clarke and S. S. Buckman were the pioneers in the use of the method of burn- ing or calcining the shells of brachiopods. This method was used by the latter to good advantage in his splendid treatise on the Burma brachiopods. Buckman heated the shells to redness and dropped them into water, the shell spalling off in the process. We found, however, that the drop- ping of the shells into water was disastrous to the specimen in certain types of matrices. It is more satisfactory to heat the shells in air to redness, and, after cooling, to scrape the shell off with a sharp needle The process of heating softens the shell and allows it to be removed rather easily. Natural casts of interiors made by this method, and internal molds in general, have their limitations; they are excellent for details of the muscle-scars, but it is usually difficult to get sharp impressions of the cardinalia, which are best seen in free valves. ^-«.,- '^•LICR/ \^ 4 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA (4) We used serial sectioning as a last resort, since this method usually means the total loss of the specimen. We developed a method, however, in the study of the pentameroids, by which we were able to save a thin section of the shell as a result of serial sectioning. This method is described in the chapter on the Pentameroidea, as we were not obliged to use it to any great extent in the study of the Orthoidea; it is of great aid among the rhynchonellids, spiriferids, and terebratulids, where the interiors are of hard cemented rock. Species Lists.— Throughout our paper we have listed under each genus the known species which we believe have the structure of the genus as exhibited by the genotype. In some instances our lists are complete, in others they are quite incomplete. Among the Cambrian brachiopods, for example, there are a goodly number of species left unplaced because there was no hint of the internal structure in the specimens available to us, and we would not place any of the species unless we could feel reasonably sure that our reference was correct. Our lists are incomplete also because we made no systematic attempt to track down in the literature all of the species of the Orthoidea and Pentam- eroidea. Although this might have been desirable, we would doubtless have had to leave a large number of these unplaced, too, because of lack of material or inadequate descriptions and figures, particularly in the older literature. Therefore we placed only the species that came under our direct observation. Some familiar species we left out because we could not establish any substantial generic reference for them. An example is "Orthis" eminens, which has been placed by various writers in Rhipdomella or Dalmanella, but belongs to neither. It appears to be closest to Idtorthis in internal structure but varies too notably to be placed there. It appeared to us better to admit honestly that we did not know what to do in certain instances. After us there will be many readjustments and many new genera will ultimately gather home the flocks we have left to wander. Genetic Lines. — A note regarding the diagrams of generic evolution which appear in the dis- cussion of the families is necessary. These diagrams are designed to be suggestive and not final. There is probably only one family described by us whose evolution has been fairly well established, and that is the Schizophoriidas. But there are difficulties even here: we do not know the Middle Ordovician (Chazy) progenitor of the group, nor do we know definitely the origin of Enteletes, whether directly from Schizophoria or indirectly from that genus through Orthotichia. Our family Plectorthids can be traced to Finkelnburgia and possibly to Orusia, but the antecedents beyond those genera we can not find. It must be borne in mind that the late Cambrian and early Ordovician brachiopods are virtually unknown and that here occur the genera that bridge the genetic gaps between the Middle Ordovician (Chazy) and the Upper Cambrian. It might be suggested, too, that the Chazy brachiopods are none too well known. Accordingly we find ourselves without sure footings among the primitive brachiopods upon which to build our evolutional structure. It is there- fore unwise at this time to link the known Cambrian forms with those of later time. We might again cite an example. It has been maintained by some writers that the genus Billings- ella gave rise to the Strophomenacea, and Eostro-phomena, a Lower Ordovician shell, has been indicated as the forerunner of that great division. We now see that Eostrofhomena is an orthid. Furthermore, genuine Strophomenacea are not known till Middle Ordovician (Chazy) time. At most, what we are trying to do is to show that the indicated evolution is structurally possible, but we can not say that our scheme represents the actual course of evolution. Our genera are, there- fore, rather iceloplasmic types^ having the structure capable of producing the evolution indicated, but this does not necessarily define the actual course of brachiopod genesis. Acknowledgments. — During the progress of our work it has often been necessary to call on specialists and others in order to see more material and to obtain advice along various lines. _ Every- where we turned for assistance we have been aided very cheerfully. Miss Helen M. Muir-Wood of the British Museum kindly loaned us an excellent series of specimens of the genus Schizophorella; Doctor A. H. Westergaard of the Geological Survey of Sweden, at Stockholm, sent us for study specimens of "Dalmanella" testudinaria which enabled us to make the astonishing discovery that the chosen type of Dalmanella was different generically from all other American species referred to this genus except one. Doctor Westergaard also loaned us an exquisite series of Orthis punctata showing ^ Kirk, Amer. Jour. Sci. (5), vol. 18, 1929, p. 345. INTRODUCTION 5 the interior from youthful to old-age stages, which made it possible for us to understand the devel- opment of the pseudospondylium. Doctor Chester A. Reeds of the American Museum of Natural History sent us the Conrad types of Pionodevia, and through Doctor H. W. McGerrigle of Dart- mouth College we were able to study a good series of Clarkella, Syntro-phina, and Finkelnburgia from the Phillipsburg region of Quebec. Doctor E. M. Kindle of the Geological Survey of Canada permitted the study of Billings' types of Orthis electra, now the types of our genus Archceorthis. Professor Paul H. Dunn of Miami University helped us with material of Eridorthis. Through Pro- fessor Percy E. Raymond, the Museum of Comparative Zoology of Harvard College loaned us a collection of European brachiopods particularly rich in representatives from the vicinity of Lenin- grad. Doctor B. B. Bancroft of the Sedgwick Museum, Cambridge, England, and Doctor A. Opilc of the University of Tartu, Estonia, sent us very perfect and interesting specimens of their new genera which are not well known in this country, and many of these specimens have been figured by us. Through the courtesy of Doctors R. S. Bassler and C. E. Resser of the United States National Museum the junior author was allowed to study the incomparable collection of Cambrian brachiopods there preserved. Doctor E. O. Ulrich of the United States Geological Survey and the National Museum loaned us some of his choice specimens of the very rare genera Mcewanella, Deltatreta, and Finkelnburgia. Through Miss Winifred Goldring of the New York State Museum we had access to specimens of the strange genus Australina, which we were able to determine definitely as a representative of some division of brachiopods other than the Dalmanellidas where the senior author had previously placed it. To Miss Clara Mae LeVene we are deeply grateful for her patient and exhaustive labors on our manuscript, to Mr. Percy A. Morris for assistance in the preparation of the photographs and figures, and to Doctor J. B. Knight, research associate in the Peabody Museum, for the generous loan of instruments used in preparing fossils. Finally, special thanks are due our colleague. Professor Carl O. Dunbar, who has been our patient advisor all through this work, and on whom the junior author has tried out most of our diffi- culties relating to the interpretation of the structural details that we have observed. PART I. BRIEF DEFINITIONS OF BRACHIOPOD TERMS' Adductor muscles (pi. A, figs. 4, 13). — ^The muscles that close the shell. In the Protremata and Telotremata these muscles are inserted in the ventral valve, one on either side of the central axis, between the diductors {q. v.). In passing to the dorsal valve they divide into four, and pro- duce in that shell the two pairs of principal scars known as the anterior and posterior Adductor SCARS. Adjustor muscles. — Included in the pedicle muscles, q. v. Adventitious deposit (pi. A, figs. 7, 10).— Extra fibrous shell substance deposited by the mantle on the inside of the shell, filling up cavities and irregularities of the surface. By deposition of such adventitious shell in the umbonal cavities, the dental plates may be obliterated. Anacline. — See Pt. Ill, Interareas, and t. fig. 1. Anterior. — ^That portion of the shell in front of the hinge region, away from the beaks. Apical plate (pi. 23, fig. 9). — A small flat structure situated in the apex of the delthyrium and flush with the interareas. To the under side of it probably was attached the pedicle. This plate is not a relict of the deltidium. Apsacline. — See Pt. Ill, Interareas, and t. fig. 1. Area. — See Interarea. Articulation. — ^The locking together of the two valves, effected in the main by the teeth of the ventral valve moving in sockets in the dorsal valve, but further assisted by the brachial parts (brachiophores) articulating with the ventral valve, at least in orthoid genera. Brachial valve. — See Dorsal valve. Brack idia. — Calcareous brachial supports in the spire- and loop-bearing brachiopods. Brachiophores (pi. A, fig. 7; pi. 4, fig. 29).— Plates that bound the notothyrial cavity (q. v.) in the orthids. More commonly known as socket-plates or brachial apparatus. In a few genera, Brachiophore processes are seen to extend from the distal extremity of the brachiophores, and to them were undoubtedly attached the brachia. The brachiophores may be the homologues of the crural bases of the rhynchonellids, but in any event are more primitive than the latter. Brachiophore supports (pi. A, fig. 14j pi. 1 1, fig. 26).— Plates attached to the dorsal face of the brachiophore, and used to strengthen the latter. Known in Hebertella and Schizofhoria. Camera. — See Cella. Cardinal angles. — ^Angles formed at each of the extremities of the hinge between it and the forward extension of the shell. Cardinal area. — See Interarea and Palintrope. Cardinal process (pi. A, figs. 4, 6, 10).— A median unpaired process, lying immediately on the inner side of the dorsal umbo, and serving for the attachment of the diductor muscles. Cardinalia. — Processes near the posterior or cardinal margin in the interior of the dorsal valve, connected with articulation, muscle attachment, and attachment of brachial supports. (After Thomson 1927.) Catacline. — See Pt. Ill, Interareas, and t. fig. 1. Cella (pi. 24, figs. 21, 26). — A small, inverted, V-shaped chamber beneath or ventrad to the ventral median septum of Parenteletes. A similar structure occurs in Dayia, Cyclosfira, and Camarium (Merista). Chilidial plates (pi. 16, fig. 22). — Discrete plates, one on either side of the notothyrium, and partially closing it. 1 For further discussion of many of these terms, see Part III, Morphology. DEFINITIONS OF BRACHIOPOD TERMS 7 Chilidium (pi. A, figs. 4, 6, 7). — ^The covering of the notothyrium, the dorsal equivalent of the ventral deltidium. Commissure. — The boundary line between the anterior and lateral margins of the valves. It may lie in a single plane or be flexed to a greater or less degree. See Plane of commissure and Rectimarginate. Convexity. — In describing the convexity of a brachiopod, the dorsal valve is named first; this is for the sake of making comparisons always in the same direction, namely, from dorsal above to ventral below. For example, Valcourea defecta would be spoken of as convexo-concave and Orthis callactis would be termed concavo-convex. See Resupinate. CosTA (cost/-e). — A coarse radial rib on the external surface. When a species has costx that are simple and distant it may be called Paucicostate; when the costas are numerous and increase by bifurcation or implantation it is Multicostate; and when they are bundled into fascicles the term Fascicostate may be employed. Costas may be angular, subangular, or rounded. Costella (costell^). — Fine external ribs such as those of Dalmanella. The same prefixes may be used in connection with this term as with costje. Extremely fine radial lines, such as those on the costs of Orthis s. s., may be termed Parvicostell^. Crura (sing. Crus). — Processes in the dorsal valve of the Telotremata to which are attached the fleshy brachia or the brachidia. See Brachiophore. Crural fossette (pi. A, fig. 15). — An excavation on the inner face of the tooth of the ventral valve which receives the postero-ventral edge of the dorsal brachiophore in the articulation of the shell. Crural plate. — A general term commonly applied to the brachial processes of the orthids, strophomenids, and rhynchonellids, without regard to detailed structure, function, or homologies. Cruralium (pi. 18, fig. 17; pi. 25, fig. 24). — The dorsal equivalent of the ventral spondylium {q. v.). When the brachial lamellse unite with a median septum, the whole structure is called a Cruralium simplex; when the brachial plates remain discrete, do not have the muscle attachments, and do not unite with a median septum (as in Porambonites), the structure is known as a Discrete CRURALIUM; and when there is no median septum, it is called a Sessile cruralium. Deltarium. — See Deltidial plates. Delthyrial cavity (pi. A, fig. 15). — The ventral umbonal cavity bounded by the dental plates. Delthyrium. — The triangular aperture which transects the ventral interarea medially, and through some portion of which the pedicle passes. It has also been termed the Fissure. The del- thyrium may or may not be closed by a deltidium or deltidial plates. Its equivalent in the dorsal valve is the notothyrium {q. v.). Deltidial plates. — In Telotremata, two plates growing medially from the walls of the del- thyrium after neanic growth. These often unite medially, closing the delthyrium more or less com- pletely. When united, they make a Deltarium = symphytium of Buckman, pseudodeltidium of Schuchert (not Bronn or Walcott). In Protremata, similar plates are at times developed and these are called Lateral plates (q.v.). Deltidium (pi. A, fig. 2). — An independent, more or less strongly arched plate in the ventral palintrope or cardinal area in many Protremata, growing from the apex toward the hinge-line and partly or completely covering the delthyrium. It is always delimited from the mterarea by grooves. It is characteristic of primitive shells, and is formed by a flap of the ventral mantle. Dental plates or Dental lamell.^ (pi. A, fig. 5).— Vertical or nearly vertical plates associ- ated with the teeth of the ventral valve, usually uniting the palintrope to the floor of the valve, and bounding the delthyrial cavity. They are separated from the walls of the shell by the umbonal cavi- ties When the latter have been filled by adventitious shell, the dental plates become Obsolete. A spedal type of dental plates seen in Hesferorthis, etc., is called Receding dental plates. These are ridges along the ventral surface of the palintrope, which finally reach the mner surface of the valve at the posterior of the shell only. g GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA Dental sockets. — Excavations in the dorsal cardinal margin in which the teeth of the ventral valve articulate. The inner wall of the socket is bounded by the brachiophore in orthids. Acces- sory DENTAL SOCKETS, SCe SoCKETS. Denticles (pi. 10, fig. 21). — Small processes on the posterior surface of the dorsal socket which are inserted into the accessory sockets in the ventral teeth. DiDucTOR MUSCLES (pi. A, fig. 13). — The muscles that open the valves. In the Protremata and Telotremata the principal pair has the larger end attached to the ventral valve near the anterior edge of the visceral area, while the other end has its insertion on the posterior portion of the cardinal process. There is another pair of small accessory diductor muscles, but scars of these are seldom shown in fossil shells. When present they are situated just behind the adductor impressions. Dorsal direction. — Toward the dorsal valve, at right angles to the plane of the commissure. Dorsal median ridge (pi. A, figs. 7, 8). — A low axial thickening on the dorsal interior of most orthids and rhynchonellids. See Median septum. Dorsal valve. — Usually the smaller valve, and the one to which the brachia are always attached. Brachial and Socket valve are other terms less often used. In this work we shall stand by the biological usage of naming the two valves ventral and dorsal, a position that is fortified by the work of Conklin,* which showed, from the orientation of the embryo, that the valves are truly ventral and dorsal, although in nature the adult shells usually have the ventral valve uppermost. Duplex spondylium. — See Spondylium simplex. Endopunct/E. — See Punct;e. Euseptum (pi. A, fig. 5). — ^A ventral median ridge formed by the inner shell layers rising into a septum that usually makes the seat of attachment for the adductor muscles. ExopuNCT^E. — See Punct^. Fascicostate. — See Cost^. Fascicostellate. — See Costell^. Fil;e. — Fine elevated concentric lines. Fold (pi. 24, figs. 5, 19). — A broad median external undulation or plica that may be situated on either the dorsal or ventral valve, hence dorsal or ventral fold. It is more commonly on the dorsal valve. Its counterpart is Sulcus, q. v. Foramen. — See Pedicle foramen. FossETTE. — See Crural fossette. FuLCRAL PLATES (pi. A, fig. 14). — Small concave plates attached to the outside wall of the brachiophore support or brachiophore and the inner wall of the shell. These serve to define the sockets and strengthen the brachiophore supports. Genital markings (pi. 12, figs. 20, 24). — Radial markings, ridges, or pits within the posterior portion of the visceral space, indicating the position and extent of the genitalia, and used for the attachment of muscles which fix the ovarian bodies. Best seen on the ventral shell. Gerontic. — Signifying old age. Hinge-line. — ^The line along which articulation takes place. Hypercline. — See Pt. Ill, Interareas, and t. fig. 1. Interarea. — The posterior plane or curved surface lying between the apex and the line of valve junction. Formerly called cardinal area. Lateral areas. — The parts of the valves on either side of the median axis or on either side of the fold and sulcus. ^ Conldin, E. G., The embryology of a brachiopod, Terebratulma seftentrionalis Couthouy. Proc. Amer. Philos. Soc., vol. 41, 1902, pp. 41-76. DEFINITIONS OF BRACHIOPOD TERMS 9 Lateral plates (pi. A, fig. 11). — External marginal plates restricting the delthyrium and seen only in certain orthids and pentamerids. These discrete plates appear to be formed in exactly the same manner as deltidial plates, q. v. Lateral septa. — See Median septum. Median ridge. — See Dorsal median ridge, and Median septum. Median septum (pi. A, figs. 5, 9). — ^A longitudinal vertical plate between the ventral muscles. Lateral septa are rarely developed between the muscles of the same valve, but are more often present when spondylia are developed. Multicostate. — See Cost;e. Muscle impressions. — Marks of muscle attachment on the shell, further subdivided as follows: Muscle-scar, a more or less well defined area representing the final or last muscle attach- ment. Muscle-track., the path due to the forward migration of the muscles during growth. Myophore (pi. 9, figs. 3, 20; pi. 17, figs. 31, 32). — The rugose surface of muscle attachment on the cardinal process. See Shaft. Neanic. — Signifying youth, or the stage in which specific characters begin to develop. Nepionic. — Designating the smooth-shell stage succeeding the protegulum. NoTOTHYRiAL PLATFORM (pi. 1, fig. 23). — The thickened shell matter in the umbonal interior of the dorsal valve between the brachiophore plates. It is the seat of diductor muscle attachment in primitive brachiopods not yet possessing a cardinal process; in other shells it is the place where the vertical cardinal process arises. This platform has also been called the Pseudocruralium. Notothyrium and Notothyrial cavity. — The dorsal counterpart of the ventral delthyrium and delthyrial cavity, q. v. Orthocline. — See Pt. Ill, Interareas, and t. fig. 1. Palintrope. — The antero-ventrally or antero-dorsally directed shelf developed at the posterior end of the dorsal and ventral valves due to the progressive migration of the hinge margin in its growth. Formerly called cardinal area. Pallial sinuses (pi. A, fig. 13). — Extensions of the coelomic cavity into the mantle. Impres- sions of these sinuses are not uncommonly seen on the inside of the shell as low ridges or shallow furrows. There are usually two main trunks in the ventral valve and three in the dorsal. PARVICOSTELLiE. See COSTELL^^ Paucicostate. — See Cost;e. Pedicle callist (pi. A, fig. 13). — A callus of shell substance at the internal apex of the ventral valve between the dental lamellas, to which the postero- ventral surface of the pedicle was attached. Pedicle foramen (pi. A, fig. 3). — A small or large round perforation at the apex or elsewhere through the deltidium for the protrusion of a small pedicle; with age, it may become large by abra- sion. When this foramen is absent, the pedicle emerges between the deltidium and chilidium, or these coverings may completely close the delthyrium and notothyrium, in which case there is no known functional pedicle. Pedicle muscles. — ^The muscles that retract the pedicle. In the Protremata and Telotremata, one pair originates on the ventral valve at points just outside of and behind the diductors, and another pair on the dorsal valve behind the posterior adductors (not known in orthids), while the opposite ends of both pairs are attached to the pedicle. These muscles are also called the Adjus- tors. (For adjustor scars, see pi. A, figs. 12, 13, 15.) Besides these, there is an unpaired muscle, not known in early brachiopods, lying at the base of the pedicle, attaching it closely to the ventral valve. Plane of commissure. — The plane passing through the anterior commissure and the hinge- line. Platform. — This term should be retained as first proposed, for the elevated and thickened muscle trace in the trimerellids. See Notothyrial platform. 10 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA Plica. — See Fold. Plicate (pi. 24, fig. 5). — Used of a shell that has undulations affecting both the interior and outer surfaces. The primary ornamentation is superposed over the plications. Example: Enteletes. Posterior region. — That portion of the shell back of the transverse axis and toward the beak, or apex. Procline. — See Pt. Ill, Interareas, and t. fig. 1. Protegulum. — The initial shell of all brachiopods. Pseudocruralium. — See Notothyrial platform. Pseudoresupinate. — See Resupinate. PsEUDOspoNDYLiuM (pi. A, fig. 11} pi. 18, figs. 13, 14, 18, 24). — ^A callus resembling a spondylium, developed in some shells beneath the muscles of the ventral valve and confluent with the inner lower surfaces of the dental lamellse. Examples: Glossorthis, Finkelnburgia, Linoporella. Punct;e. — Any minute perforations of the test. Punctse are here divided into two kinds: Endopunct;e, perforations of the internal shell layer, never reaching the exterior unless the thin outer shell layer is abraded; these are the typical punctse of the terebratulids and the Dalmanellacea. ExopuNCT^, perforations that indent the external surface of the shell but do not pass through to the interior; well developed in Paurorthis, Hebertellay and Valcourea. Recti marginate. — Having a straight anterior commissure. Resupinate. — ^A condition wherein the relative convexity of the two valves is reversed, the convex ventral valve of the early growth stages becoming concave and the concave dorsal valve becoming strongly convex, producing thereby a convexo-concave shell. Strictly speaking, this con- dition has never been attained by the orthids, but a few genera simulate it closely, e. g., Valcourea, Dinorthis, Plcesiomys, Hebertella, etc. This latter condition might be called Pseudoresupinate. Rostrate. — Having a long beak, produced by narrowing of the hinge-line, as in Conchidium, Cycloccelia, etc. Septum. — See Dorsal median ridge, and Median septum. Sessile cruralium. — See Cruralium. Sessile spondylium. — ^A spondylium which rests directly on the floor of the valve without the support of a median ridge. Nearly attained in Pahlenella. See Spondyloid. Shaft. — The stalk or shaft of the cardinal process, which bears the myophore or seat of diductor muscle attachment. Sinus. — See Sulcus. Sockets. — ^There are sockets for articulation in both valves. In the ventral, there are two kinds associated closely with the teeth: Accessory dental sockets, on the outside of the teeth; and Crural fossettes or sockets {q. v.) on their inner sides. See t. fig. 2. Spondylium (pi. A, fig. 9). — A spoon-shaped plate, terminating more or less freely, located in the apex of the ventral valve of various stocks of articulate brachiopods. This plate serves as the seat of attachment of the muscles. It is supported by a more or less elevated, long or short, median septum. Kozlowski has recently shown (1929) that the spondylium may be divided into three different types, as follows: Spondylium discretum (pi. 14, fig. 20). — Here the dental plates do not converge and unite medially, but extend directly to the floor of the valve. Strictly speaking, this is not a spondylium, but such a condition of the dental plates is primitive and deserves a designation. This structure occurs mostly in orthids. Also see pi. 18, figs. 13, 14, 1 8. Spondylium duplex. — See below. Spondylium simplex (pi. 7, fig. 31). — ^A term applied by Kozlowski to the type of spondy- lium in CUtambonites, in which the dental plates and the vertical septum are united into a single piece. This type is in contrast to the Spondylium duplex of Pentamerus (pi. 25, fig. 43), which is composed of two pieces each of which is borne on a basal septum. See Pseudospondylium. DEFINITIONS OF BRACHIOPOD TERMS 1 1 Spondyloid (pi. 14, fig. 8). — In this condition the dental plates are so thickened on their inner basal sides that the added testaceous deposit grows together and simulates a spondylium. This con- trasts with the pseudospondylium, which is formed by a callous thickening on the floor of the valve. Example : Porambonttes. Stride. — Interspaces between costx and costellae. This term has been much abused and its current use for a radial rib is incorrect. Sulcus (sulcate). — A median depression in the convexity of the shell, the opposite of a fold or plica. Replaces the term sinus. Teeth (pi. A, fig. 2). — The two articulating processes of the ventral valve. There are also accessory small teeth in the dorsal valve in many brachiopods, which are here called Denticles {q.v.). Transverse axis. — A line through the widest part of the shell from left to right. Umbonal cavities. — Chambers separating the dental lamellas from the walls of the valve. Uniplicate. — A term applied to the anterior commissure when there is a fold in the dorsal valve opposed by a sulcus in the ventral valve. Unisulcate is the reverse condition. Unisulcate. — See Uniplicate. Ventral dental sockets (pi. A, fig. 2). — Small sockets in the teeth of the ventral valve next to the hinge margin. Into these articulate small denticles on the outer wall of the dental socket. They are also called Accessory dental sockets. See Denticles. Ventral valve. — The shell situated on the ventral side of the animal, and in articulate forms having the teeth on each side of the delthyrium. Usually the larger and deeper of the two valves. Pedicle and Dental are other names applied to it. PART 11. PRINCIPLES OF MORPHOGENESIS OR EVOLUTION OF FORM NEED OF TAXONOMY W. T. Caiman, keeper of zoology in the British Museum of Natural History, says in his recent address, "The Taxonomic Outlook in Zoology,"^ that in the days of Linnasus (1758) an experi- enced zoologist might have known the 4370 living species then named, but that to-day, when over 700,000 kinds have been described, the great need for the specialist is apparent. "Nothing can alto- gether replace that instinctive perception of affinity that comes from lifelong study" (p. 280). He goes on to say, however, that there can no longer be a "complete description" of a species, as was thought necessary by the older systematists, but that what should be done is to enumerate the essen- tial characters in which it differs from similar forms of the genus. This address stimulated the editor of Nature to remark:" "There is a strong tendency to deprecate the value of taxonomy and to ignore its claims to a fair share of the attentions of scientific workers." All animal life is subject to change as soon as the normal conditions of the environment change, and, according to Baker,^ "there is evidence that this change may not always be a matter of long years but may take place in the space of five or ten years. This statement is abundantly supported by experimental evidence. The necessity for giving names to these incipient species [ecological variants] is, therefore, obvious." The same writer has shown elsewhere that molluscs in streams and lakes change into different forms in the course of one or two human generations, due to alterations in the environment brought about by man. If subspecies change thus quickly, paleontologists with their superabundance of time as recorded in the geological formations must expect generic change from zone to zone, and yet no genus should be made on the basis of age alone, since a genus is to be founded on the ensemble of characters as seen in one or more species. It is said that Linnsus described in the tenth edition of his "Systema Naturae" (1758) one genus and forty-two species of Homoptera. Now Professor Z. P. Metcalf, a student of this "small order of insects," estimates that there are known no fewer than 30,000 species distributed in 500 genera, and it is thought that there will eventually be three times as many. In this group, therefore, and in many others, it is only too plain that there can be no stability in nomenclature for a long time to come. It does not further our subject, however, to complain of the multiplicity of Nature and the consequent difficulties in attempting to give the proper name to each species under the Rules of Zoological Nomenclature. It is the duty of every systematist to do the best he can and let the future take care of itself. In other words, the grumblings of the anatomists, physiologists, experimental biologists, etc., are all beside the mark, since the systematist is only trying to classify what he sees in an ever changing world, namely, to determine the lines of organic descent to the uttermost ramifica- tions possible in a stated classification that is, after all, more or less artificial. Morphology will grow and taxonomy will change as long as there are great numbers of unknown species and genera. RISE OF BRACHIOPOD GENERA Fabius Columna in 1616, and Martin Lister in 1678, were the first to describe brachiopods, calling them Conchie anomice. Grundler in 1774 was the first to give a good illustration of a living brachiopod, TerebratuUna cafut-serfentis. Cuvier in 1792 and 1802 distinguished brachiopods from Acephala, and in 1805 gave the class its name Brachiopoda. In 1818 Lamarck knew 5 genera, ^Science, n. ser., vol. 72, 1930, pp. 280-284; Nature, vol. 126, 1930, pp. 440-444. -Nature, vol. 126, 1930, pp. 461-462. ^ F. C. Baker, On Genus and Species Making. Science, n. ser,, vol. 72, 1930, pp. 37-39. PRINCIPLES OF MORPHOGENESIS 13 including among them the operculate coral Calceola. In 1849 King recognized 49 genera in 16 families, and Bronn in 1862 listed 51 genera. Hall and Clarke (1892-1894) added 58 new genera and in their "Handbook of Brachiopoda" recognized 325 genera or subgenera. By 1913 (Zittel- Eastman) this number had grown to about 450, Buckman alone added 5 1 new genera of rhynchonel- lids in 1914 and now there are at least 750 genera or subgenera in good standing, in addition to some 200 rejected names, many of which will doubtless be revived. Of these 750 genera, one only belongs with certainty to the order Palxotremata, 50 (7 7o) are Atremata, 47 (6%) Neotremata, 234 (31 %) Protremata, and 41 7 (55%) Telotremata. The Paleozoic has about 500 genera, the Mesozoic about 180, and the Cenozoic-Recent about 75. THE GENUS CONCEPT "To no human question is there an immutable and final answer." — Bather 1927 All paleontologists working to discern the genetic relations of fossils should study the Presidential Address of F. A. Bather entitled "Fossils and Life," which was delivered before Section C of the British Association for the Advancement of Science in 1920.'' This masterly address deals chiefly with the philosophy of ancient life in relation to that now living under the influence of the environ- ment— the leading influence making for genetic evolution as interpreted by both paleontologist and neontologist. Bather says: Like Botany and Zoology, Paleontology describes the external and internal form and structure of animals and plants; and on this description it bases, first, a systematic classification of its material; secondly, those broader inductions of comparative anatomy which constitute morphology, or the science of form. Arising out of these studies are the questions of relation — real or apparent kinship, lines of descent, the how and why of evolution — the answers to which reflect their light back on our morphological and classificatory systems (p. 61). Paleontologists, unlike neontologists, are concerned with the "concept of time ... an orderly and related succession, coextensive, in theory at least, with the whole history of life on this planet" Since the days of Linnasus, biologists have sought for "natural classifications," but back of 1 859 these were more arbitrary than natural j with the publication of Darwin's "Origin of Species," how- ever, it has become more and more possible to determine the genetic relationships, or the "blood-red clue of natural afiinity." Descent, then, is not a corollary of succession. Or, to broaden the statement, history is not the same as evolu- tion. History is a succession of events. Evolution means that each event has sprung from the preceding one. Not that the preceding event was the active cause of its successor, but that it was a necessary condition of it. For the evolutionary biologist, a species contains in itself and its environment the possibility of producing its successor (pp. 66-67). [Not until we pursue the "line upon line" method of paleontology shall we] have linked species into lineages, can we group them into genera; not until we have unravelled the strands by which genus is connected with genus can we draw the limits of families. Not till that has been accomplished can we see how the lines of descent diverge or converge, so as to warrant the establishment of Orders (p. 70). It is undoubtedly true, as Gertrude Elles points out in her paper, "Evolutional Paleontology,"' that species and genera have too often been made by paleontologists in the laboratory without the knowledge of the field, since the strata entombing fossils are the only preserved environments. Furthermore, only too often are species and genera made on the basis of chronogenesis and not on a proved genesis, and most of our genera with large assemblages of species are polyphyletic in origin. The modern taxonomic trend based on evolution, as used by vertebrate paleontologists, is well illustrated in the paper by H. F. Osborn bearing the title "Final Conclusions on the Evolution, Phylogeny, and Classification of the Proboscidea. "" His viewpoint as to the value of the divisions appears, however, to be an extreme one. Here we learn that Lydekker in 1886 knew of but two * Rept. British Assoc. Adv. Sci., Cardiff meeting, 1920, pp. 61-86. ° Rept. British Assoc. Adv. Sci., Liverpool meeting, 1923, pp. 83-107. •Proc. Amer. Philos. See., vol. 64, 1925, pp. 17-35. 14 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA genera, Elefhas and Mastodon, and that not so long ago all vertebrate paleontologists regarded the Proboscldea as a monophyletic line. Osborn, according to his "phylogenetic classification," now divides the "phylum" Proboscidea into at least 1 6 races or lines of descent. These he arranges into 4 orders, 5 families, 13 subfamilies, 28 genera, and 290 species. A. Morley Davies in his preface to Sahni's very careful study of the terebratulids of the British Chalk,^ a work wrought out along the best modern lines, has the following to say regarding the most reliable generic characters in brachiopods: Internal structure has proved the surest clue to relationship. Beecher and others based the classification of the "long-looped" Terebratellids primarily on the form and development of the brachial loop; Buckman relied on the muscle-scars in the case of the Jurassic Terebratulids and RhynchoneUids . . . Once the definite clue has been found by which distinct stocks can be separated, it becomes possible to discriminate among the multitude of seem- ingly random external variations. Some of them, either individually or, more frequently, in particular associations, are definitely correlated with internal features and become diagnostic for particular stocks: they belong to what Douville has termed . . . "static characters," persisting or slowly changing as evolution proceeds. Others are among Douville's . . . "progressive characters," which go through a series of stages often parallel in different stocks. It is these parallel developments, sometimes predominating over the static characters so as to eflrect a "convergence" or homoeomorphy between different stocks that are most apt to mislead in hasty determination. Yet these progres- sive characters may sometimes remain relatively static for a period, so as to serve provisionally or partially as diagnostic characters (pp. v-vi). When Sahni began his work there were 14 Upper Cretaceous species usually referred to Tere- bratula. It now appears that not one of them belongs to this genus, his memoir defining 57 forms in 12 new genera! This shows what must be done nowadays if one's work is to stand. One must lift the covers and delve into the insides for the most reliable characters, and then carefully examine the covers to discover the homoeomorphs and the significance of the slight changes in external form. Among students of brachiopods, none has more clearly set forth the best present methods of determining the genus from the genetic standpoint than J. Allan Thomson.* He says: Biological classification is now an attemft to exfress the degrees of affinity through descent [italics ours]. Instead of as few characters as possible being used to decide upon the generic position of a species, as many characters as are necessary are employed to determine the relationship to other species. It has frequently been found that the species grouped together under the older wide genera form homoeomorphous series of polyphyletic origin — u e., their similarity of form has been attained through different lines of dissimilar ancestors (p. 120). According to Buckman (1918),® Generic division will not be complete for scientific purposes until all polyphyletic series are rightly separated. If the criteria now obtained are insufficient for this work of separation, it is the task for future observers to note and to utilize other characters (pp. 133-134). Thomson says, further: Another change in the conception of the genus has resulted from a fixation of procedure in regard to the law of priority ... It has become necessary to define each group not only by an assemblage of characters originally proposed for it by its author, but by a type within the group with which the aflBnity of other members may be compared — a species by a type specimen or holotype, a genus by a type species or genotype, a family by a type genus (p. 121). The genetic work of paleontologists must be guided, however, by the results of the neontolo- gists, who have the living organisms to study in their entirety, but even among them the demonstra- tion of lines of descent lies with the experimentalist. The work of these biologists shows how deceptive "blood-relationship" may actually be. In this connection we may summarize here the con- ' M. R. Sahni, The Terebratulids of the British Chalk, Men. Paleontog. See., 1927, 1929. * Brachiopod Morphology and Genera (Recent and Tertiary). N. Zealand Bd. Sci. and Art, Manual No. 7, 1927. ° The Brachiopoda of the Namyau Beds, Northern Shan States, Burma. Geol. Surv. India, Pal. Indica, new ser., vol. 3, mem. 2, 1917 (1918). PRINCIPLES OF MORPHOGENESIS 15 elusions of the experimental ecologists H. M. Hall and F. E. Clements, as related in their monu- mental monograph of 1923.'" The basis of this monograph lies in "experimental and quantitative methods," and the experi- ments to be conclusive "must extend over a long period of time." The authors have taken many species of sagebrush and transplanted them from one environment into another, for example, inter- changing one growing in a dry environment with another at home under wet conditions, and this divergence in habitats gives opportunity for convergence in characteristics that "may sometimes become practical identity." From these experiments they got results that led them to revise the species of three genera as follows: "Species" up to 1923 Present divisions Genus "Sections" "Major species" "Sub-species" "Minor variations" Artemisia Chrysothamnus Atriplex 125 88 103 4 4 ? 2 12 47 29 40 37 120 44 70 Let us look a little more deeply into what these experimentalists have to teach us: Taxonomy in biology, as the word indicates, is the science of arrangement or system, and hence classification [of the organic entities. Naming of species is but the first step in taxonomy, and even their description is not the whole of this science, since taxonomy should mean the placing of the species in nature's lines of descent; or, stated in another way, the placing of them so as to give] the best possible record of evolution and relationship. . . Absolute adherence to phylogeny should be the basis of classification. . . Evolution is the process and phylogeny the record of descent. Taxonomy leans most heavily upon morphology, and should bring to its aid not only histology and physiology, but ecology and genetics as well. Indeed, if it [taxonomy] is to reflect evolution as accurately as it should, it must regard physiological adjust- ment as the basic process, and morphological and histological adaptations as the measurable results. This means that the taxonomist of the future will think in terms of evolutionary processes, and will learn to treat his morphological criteria as dynamic rather than static. [Quantitative ecology] traces the evolution of new forms in minute detail, in so far as they arise through adaptation or variation, and consequently furnishes the only direct evidence of relationship by descent. It affords the sole method of testing the manifold assumptions of existing taxonomy, and provides the foundation upon which an objective and permanent taxonomy may be reared. At present, however, taxonomy is more often the artificial expression of organic evolution; arti- ficial because the science is as yet mainly one of personal opinions, and naturally this is more true in paleontology than it is in neontology. A natural classification must maintain as well as reveal the different degrees of relationship as expres- sions of different stages of evolution, and it can do this most accurately with genera, sections, species, and variads, where the lines of evolution are still in a condition conducive to experimental study . . . Since the limits of orders and families are determined by genera, and the limits of genera by species, the whole problem resolves itself into a statistical and experimental study of species and their evolution (pp. 3-6). It is now common practice to regard the genus merely as a concept, and often as an artificial one. Regarding this. Hall and Clements say: This is doubtless true for those who regard the genus merely as a pigeon-hole, chiefly convenient for the filing of new species. Such a view has its justification in the usual practice of making genera. . . It is not sup- "The Phylogcnetic Method in Taxonomy. The North American Species of Artemisia, Chrysothamnus and Atriflex. Carnegie Institution of Washington, Pub. No. 326. See also Clements, An Ecologic View of the Species Question. Amer. Nat., vol. 42, 1908, pp. 253-264. jg GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA ported by the evidence drawn from the methods of evolution or the record of phylogeny. To the student of evolu- tion, the genus represents a certain characteristic portion of the line or field of specialization, and its existence is as definite as that of the species which constitute it. . . As a consequence of the unrestricted play of personal opinion, not infrequently aided by bias or carelessness, present-day taxonomy contains genera of every possible quality. Many of these disappear completely when the test of evolution is applied to them (p. 6). In paleontology, more than in neontology, genera are not uniform in differentiation; one stock will vary but little while another will do so extensively. The genera with but one or a few species will practically take care of themselves, but in those with a great number of species it is desirable to name all the determined genetic lines, since evolution is the -process and taxonomy the method of marking the phylogenetic lines of descent. It is clearly recognized that the making of new genera is purely a matter of personal judgment at the present time. . . There is no general agreement as to criteria, methods, or results, and the importance of evolution as the one safe guide is rarely if ever considered. A knowledge of the genus as a whole, especially when it includes exotic species, is too often lacking, and little or no thought is given to the phylogeny of the genus and its sections in relation to genera of the same evolutionary stock. . . It would seem desirable to make rather more of sections as records ... of specialization within the generic stock (pp. 8-9). PALEOECOLOGY The paleontologist should study his specimens and species primarily in the light of the field to learn their environmental and biotic relations, since the strata of their entombment are the habitats of fossils. We hold with Clements" that the habitat is the motive force in the life processes of plants and animals, both as individuals and as communities. . . Ecology deals primarily with processes and is inherently and universally dynamic . . . quantitative in method, beginning with the habitat in which measurements are relatively simple, and running through individual and community responses in which they are difficult (p. 369). As paleontologists we should, therefore, be primarily interested in the habitat of fossils and of our faunas, since organisms are "the end forms of responsive processes." In other words, it is seem- ingly the environment in the main that brings on the organic responses seen in the development and evolution of fossils; accordingly the paleoecology and the time factor of our faunas should be con- sidered in addition to the morphology, in the making of species and genera. In paleontology we have, it is true, but a small part of the whole of the habitat or of the bios, and therefore it is "puzzling to understand how the demands of ecology can be met in a field where processes have ceased. The readiest answer, and a fairly complete one, is afforded by the princtple of uniformity of processes [italics ours], the use of which has made modern geology possible" (p. 370). Accordingly we paleontologists can learn much from the ecologists, but most from the entombing strata and the associations of the fossil faunas. In the paleoecology of marine faunas, the main factors in conditioning the local biotas are the interrelations of temperature, currents, nearness to shore, and nature of sea bottom; depth, chemical content, and clarity of water; prevalence of bottom-living flora, nature of floating life, presence of oxygen as food, penetration of sunlight, and interaction of life. HOMCEOMORPHY What has long been known as "parallel development" has had but scant notice among Anierican students of brachiopod ancestry, Beecher'" being about the only one to pay attention to this principle of mimicry of outer form; among the ammonites, however, the factor has been used a great deal since its introduction by Alpheus Hyatt and its later acceptance by J. P. Smith, In Europe, Buck- " Scope and Significance of Paleo-ecology. Bull. Geol. See. America, vol. 29, 1918, pp. 369-374. ^''C. E. Beecher, Some Correlations of Ontogeny and Phylogeny in the Brachiopoda. Amer. Nat., vol. 27, 1893, pp. 599-604. PRINCIPLES OF MORPHOGENESIS 17 man" applied the principle of parallel development for many years and chiefly among the Jurassic ammonites and brachiopods; and his work and now ours show that external characters are not always reliable for generic and phyletic studies. The application of this principle to the genera heretofore described often plays havoc not only with them, but above all with our notions of their genetic relationships. This phenomenon of parallel development Buckman has called homceomorphy. It refers to species nearly alike so far as superficial appearance is concerned, but unlike when particular structural details are closely examined. It is the phenomenon of similarity in general with dissimilarity in details, [or] the tendency of different genetic stocks to pass, quite independently, through similar phases of development . . . There is a ten- dency among Jurassic Brachiopoda for independent non-plicate species to become multiplicate . . . and in the Rhynchonellida: for the multiplicate (costate) to become spinous {A catithothyris) , and in certain cases a spinous species may, with age, retrogress to lose spines. The various species of different stocks may cither produce these developmental characters more or less con- temporaneously, in which case such forms are called isochronous homaeomorfhs, or they may produce the char- acters at different dates — a later form simulating an earlier one — in which case they are called heterochronous homoeomorfhs (1901, pp. 231-233). In the Jurassic of the Cotteswold district of England, in rocks of about the same date, there are five independent developments of the same character among species of Terebratulidas. In nearly related stocks much excuse may be made for errors in identification, but when such errors occur in different families whose internal structural details are quite distinct, then "the confusion of two species of these families under one name becomes serious" (p. 239). In fact, among the Jurassic terebra- tulids and rhynchonellids, homoeomorphs are so common that "they may be said to form veritable 'traps' in the matter of identification" (p. 262). Extraordinary homoeomorphs occur among the lobate terebratulids {Pygofe, Antinomia, Pygites) and these are set forth by Buckman in his paper of 1906. When Hall and Clarke prepared their monograph, the perplexing feature of homoeomorphy was not clearly understood, consequently some genera were made on the basis of external form alone. It has been found in the present work that no one character, either internal or more especially external, can be relied upon in the identification of a genus, but that a genus must be character- ized by a combination of these features. Among the orthids there have been convergences either toward other orthid families or toward the external form of other groups. Productorthis of the early Ordovician may be cited as a conspicuous example in its resemblance to the productids of the late Paleozoic. There are three genera that ape the external form of Skenidium (Skenidioides, Mystropkora, Hesperorthis? (merope)), and no fewer than seven have the external form of Piono- dema (Mimella, Hemipronites, Doleroides, Deltatreta (some species)), Finkelnburgia, Schizopho- rella, and early Schizophoria). _ . Of interest in this connection is the accelerated development of the Ordovician Clitambonitids of European Russia. Early species took the outside features of Strophomena, as in Gonambonites, and some of these later aped forms of Hebertella. Hemipronites is much like a Pionodema, and Vellamo has essentially the form of Hesperorthis. This diverse development occurred in a relatively short time, and the whole stock perished with the Ordovician. "S. S. Buckman, Homitomorphy among Jurassic Brachiopoda. Proc. Cotteswold Nat. Field Club, vol. 13, 1901, ^^' Brachiopod Homoeomorphy: Pygofe, Antinomia, PygiUs. Quart. Jour. Geol. Soc., London, vol. 62, 1906, pp. 433-454. Brachiopod Homoeomorphy: Sfirijer glaber. Ibid., vol. 64, 1908, pp. 27-33. PART III. MORPHOLOGY OF THE ORTHOID SHELL The Orthoidea, to be described in more detail later, are a prolific stock of primitive Protremata which persisted, according to our present knowledge, from the middle part of the Lower Cambrian to the end of Permian time. They are rather primitive brachiopods, certainly the simplest of the "articulate" stocks, and are characterized by more or less strongly developed interareas in both valves. Primitively the delthyrium was covered by a deltidium, but in later progressive stocks this structure was lost and the delthyrium was unmodified except for lateral plates or sporadic apical plates. This large suborder may be divided into two superfamilies on the basis of the shell structure, whether fibrous and endopunctate or fibrous and impunctate. With these superfamily characters go definite internal structures that are described in later pages. Most of the Orthoidea described in this volume were, at one time or another, classified under the generic term Orthis. Even as late as 1892, the date of Hall and Clarke's great revision, not more than thirteen orthoid genera were in use. These authors showed that Orthis (1828) had become a "dump" genus, and that it embraced at least 13 groups of shells with orthoid features, besides 10 other old or new genera with related forms. They further blazed the way toward a correct genetic understanding of Paleozoic genera, a method that has been followed ever since and one that has guided us in turn in the present revision. Schuchert (1897) regarded Hall and Clarke's "groups of Orthis" as genera, and in the Bibliography of Schuchert and LeVene (1929) there are catalogued as of July 1928 52 accepted genera of orthoids. In the present volume we recognize 103 genera or subgenera (29 are new) constituting the superfamilies Orthacea, Clitambonacea, and Dalmanellacea. These statistics show also the accelerated rate at which students of brachiopods are discerning the genetic ramifications of the orthoids. We believe that the present arrangement shows genetic relationships more clearly than here- tofore and that our classification is more natural. But it can not be final as yet, and as years pass, greater refinement in paleontologic and stratigraphic practice, along with the making of additional collections from old fields and new, will enlarge our knowledge and tend to alter our scheme. We hope, however, that our fundamentals are sound and that time will fill in either the details of the scheme here presented or that of nature, since after all we are striving to learn nature's ways of creation. MORPHOLOGY OF THE EXTERIOR ORIENTATION OF THE SHELL In citing directions in or on a brachiopod shell, we shall use the terms dorsal and ventral, and anterior and posterior (pedicle end). When discussing or figuring the posterior of a brachiopod, we place the ventral valve down, as this is the correct biological orientation in the living animal. The terms ventral and dorsal, therefore, indicate the vertical direction. In other words, these directions are at right angles to the plane of the commissure, which is oriented by us in a horizontal direction. The terms anterior and posterior, on the other hand, define directions parallel to the plane of the commissure. These terms can therefore be applied with precision regardless of orientation, and thus enable us to avoid the ambiguous terms front and back, down and up. In describing the shape, contour, or profile of a shell, the writers are following Thomson and Buckman in giving preference to the dorsal valve. This leads to the use of an unfamiliar term for some shells which are commonly called concavo-convex, as Dinorthis; according to our usage this is a convexo-concave shell. No matter which valve be taken for reference, any precise nomenclature must recognize a convexo-concave stage as well as a concavo-convex one. For the sake of uniformity, then, we have adopted the scheme used by these two authorities on Mesozoic, Cenozoic, and Recent brachiopods. MORPHOLOGY OF THE ORTHOID SHELL 19 COMMISSURES When viewed from the front or anterior, the line of contact of the two valves is primitively straight but in most derived genera it is more or less undulated. The same holds for the lateral com- missure. Buckman has analyzed the flexures of the anterior commissure in Mesozoic rhynchonellids and terebratulids, and has named nine stages developed out of the primitive straight or recti marginate one. Among the orthids only two of the Buckman stages can be recognized, namely the sulcate and uniplicate ones; both of these are the simplest of the modifications of the rectimarginate stage. In the sulcate stage there is a single sulcus in the dorsal valve and a fold in the ventral, as seen m Aula- cella, Enteletim, or Parenteletes. This type of commissure is rather uncommon. In the uniplicate stage, these conditions are reversed and there is a single fold on the dorsal valve and a sulcus m the ventral one. This is the more common condition and is well exhibited by Platystrofhia. The conclusion that a fold and sulcus, or what amounts to trilobation, may have been developed to facilitate the passage of the incurrent and excurrent streams of water used in the aeration of the mantle and for food-gathering was advanced by Orton. The tendency to develop a fold and sulcus is inherent in most Orthoidea but in some of them, for example Hebertella, there is little stability as to which valve shall receive the sulcus. In some stocks there is a marked reversion of the fold and sulcus, as in Eridorthis and Thiemella. The common condition of the lateral commissure is a more or less strong flexmg toward the dorsal valve. This is true of Valcourea and many other forms. The primitive condition is an unflexed lateral commissure. A ventrally flexed lateral commissure is rare. CONVEXITY In the Cambrian, most shells are normally biconvex, as is the first shell growth in all brachi- opods, and here it is also usual to find the ventral shell the more convex and the deeper. In later forms the reverse condition is, however, of common occurrence and is independently originated over and over again. . . ,. , . ,• • 4.1- * *u» The great majority of brachiopods, viewed in longitudinal section, are biconvex, in that the external curvature of both valves is convex, with the ventral the more so and therefore deeper than the dorsal This common condition is called the lenticular or biconvex phase hy Buckman. More rarely it is the dorsal shell that is more convex, and such forms can be described as dorsi-biconvex. Resupination is a condition that has been reported commonly in the orthids, but unfortunately the term has never been defined with any precision. Some authors regard brachiopod shells as resupinate when the ventral valve has less volume than the dorsal. Atryfa wou d be a fine example of such "resupination," but this is not what the term apparently meant originally. As applied by Hall and Clarke, it embraces such shells as have the ventral valve concave and the dorsal convex. According to this view, Valcourea and Strofhomena would be two unrelated examples of resupina- tion However, there is a marked difference between Valcourea (and all other convexo-concave orthids) and Strofhomena in the manner of the reversion of convexity in the ventral valve. In the young stages of Strofhomena the ventral shell is convex and may carry a low fold; the dorsal valve is essentially flat and may be gently sulcate. In later stages, however, the dorsal shell becomes strongly ventricose and the ventral one deeply concave. In the convexo-concave stage of the orthids, on the other hand, as in Valcourea and Hebertella, the dorsal valve never has the incipient flat stage as seen in Strofhomena, and this affords an easy external way of distinguishing a stropho- menid from an orthid homoeomorph. Consequently in this work we are restricting the term resufma- tion to the condition seen in Strofhomena and its allies, while the other condition among orthids may be called fseudoresupination. ^ Orton. J. H., On Ciliary Mechanisms in Brachiopods and some Polycha:tes. Jour. Marine Biol. Assoc., U. K., new ser., vol. 10, 1914, pp. 283-311. 20 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA INTERAREAS According to Buckman/ the word area should be retained for general use, but interarea might be used for "the area lying between the apex and the posterior line of valve-j unction — the cardinal margin when there is a hinge, but at any rate the posterior margin." Interareas, or what were formerly called cardinal areas, are usual in most of the members of the order Protremata and especially in its most primitive stock, the Orthoidea. True interareas are VEA/T/f'AL ^j -■" Anacline OrthocUne ApsacUne CatacUne Proclcne (^ ^ fx f^ypercline An a c line Ort/iocCcne Apsacllne DORSAL Fig. 1. — Diagram to show various positions of the interarea with relation to the plane of the commissure. at times also well developed in the Telotremata, but never in the Atremata or Neotremata. When a similar structure is present in the two last named orders it is due to holoperipheral growth or to subsequent internal thickenings of the posterior margins, and in both these instances it is termed a ■pseudointerarea. The interareas are, then, the plane or slightly curved surfaces of the palintrope, which is a shelf, growing antero-ventrally or antero-dorsally according to the valve, and representing the pro- gressive growth track of the hinge-line or plane of articulation. In describing or discussing the inter- Brach. Namyau beds, 1917 (1918), p. 453. MORPHOLOGY OF THE ORTHOID SHELL 21 area the writers consider the length of said surface to be in the same direction as the length of the valve. The width of the interarea is the width of the hinge-line. The length of the interarea is thus seen to be perpendicular to the width or perpendicular to the hinge-line. Ordinarily the ventral interarea has the greater length. An interarea may be plane or more or less markedly concave. It may be striated parallel to its length or to its width or both, most interareas having striae of one kind or another. The hori- zontal stris are commonly growth-lines; the vertical stria: may represent the growth tracks of acci- dental irregularities that make, as it were, "teeth" along the hinge-line. More uncommonly the interareas may be ornamented by fine elevated lines passing obliquely across the surface from the beak, as in Polytcechia and Deltatreta (see pi. 6, fig. 14). Another important feature of the interareas is their inclination from the plane of the commis- sure, which may cause important modifications of the muscle marks, the dental plates, and the cardinalia. When studying the inclination of the interareas the ventral valve is placed by us below the horizontal and the beaks (arbitrarily) on the observer's left. The plane of the commissure is then horizontal and the dorsal valve is up. In this orientation the following positions may be observed : Ventral valve: (a) anacline — ^between vertical and horizontal — ex. Orthis s. s.; (b) orthocline — interarea horizontal — ex. Archceorthis ; (c) apsacline — between horizontal and vertical, 90°-180° from erect vertical — ex. Hesperorthis; (d) catacUne — bent down 90" from the horizontal — ex. some species of Clitambonites; (e) frocUne — interarea bent more than 180° from the vertical — ex. Din- orthis {Retrorsirostra). In the dorsal valve the anacline, orthocline, and apsacline conditions are recognized, and in addition, a hyfercline condition in which the interarea is rotated in an anterior direction more than 90° (see t. fig. 1). This nomenclature has the advantage of enabling the observer to state precisely the average or individual inclination of either interarea, or, if he desires to be more specific, the number of degrees subtended by the interareas or by either interarea and the horizontal. For example, one may define a certain interarea as 30° apsacline. Of interest in connection with the interareas is the presence of a narrow triangular space on either side of the delthyrium or notothyrium. One side and the base of the triangle are formed by the delthyrial margin and the hinge-line, respectively. The other side is defined by a line running from a point outside of the tooth in the ventral valve, or the socket in the dorsal valve, to the beak. This line represents a suture marking the progressive growth of the tooth and socket; it is the line between new shell deposited on the tooth or in the socket against the old shell of the hinge-line during the growth of the valve. In the Strophomenacea a similar triangular space has been called the "secondary area," but in these shells, Derbyia, Orthotetes, etc., the outer or suture line is never correlated with either the teeth or the sockets. It is very doubtful, then, if this better known and much discussed secondary area of the Strophomenacea can be homologized with the similar-appearing structure in the orthoids. DELTIDIUM Modifications of the delthyrium (deltidium). — ^The deltidial covering has for many years received much attention from taxonomists, and the kind of covering is now thought to deter- mine the order to which the different groups of brachiopod shells belong. The deltidium' is an arch ^According to Hall and Clarke (Pal. N. Y., vol. 8, pt. I, p. 189, footnote), "The term deltidium was proposed by von Buch for the triangular flate which, in many articulate genera, covers more or less completely the space between the outer margins of the dental ridges. This plate he describes a composed of two pieces which may either completely sur- round the foramen {deltidium amflectens), bound it on its lower side {deltidium sectans), or the parts may be separated for their entire length by the foramen (deltidium discretum). These component parts of the deltidium take their origm from the margins of the triangular cavity beneath the beak, but in some genera, particularly in Strofhomena, Sfirijer, and their allies, there is still another form of shelly plate which grows from the apex downward, and to this the term peudo- deltidium was applied by Bronn. Among recent writers there has been considerable laxity in the use of these terms and it is very doubtful if they can be applied with precision." _ , , .,. In the present work we have preferred to follow Hall and Clarke, Beecher, and others in using the tenn deltidium, applying it, however, to the cover composed of a single piece which restricts the delthyrium of the Protremata. 22 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA built with its piers against the angle formed by the palintrope and the dental plates. The piers are usually buttressed against the track or axis formed by the growth of the tooth (see D, PI. A). The attachment is strengthened by adventitious deposit laid over the whole inner surface and along the sides of the dental plates and usually concealing all sutural contacts. Externally the interarea and deltidium appear to be continuous and a suture is rarely visible unless the deltidium is somewhat broken in. Shells having lateral plates in addition to the deltidium, however, commonly show suture lines (see below) where the lateral plates overlap the deltidium. In the Clitambonitidas, and in the older Billingsellidse as well, it is not uncommon for the deltidium to carry an axial stif- fening along its inner surface. Hence the deltidium is a distinct plate, deposited in all probability by the mantle, and is not a part of the palintrope or cardinal area. It is therefore not homologous with the homoeodeltidium, as stated by Walcott and Schuchert — a change of opinion of very great significance in classification. In Hesferorthis the deltidium, or "apical plate" as it has commonly been called, is truly an arch as in Clkambonitesy and its piers are also built against the sides of the delthyrial cavity. Since Hesferorthis commonly has in addition lateral plates or extensions of the palintrope over the margins of the delthyrium, the deltidium rarely is arched above the interarea. On the other hand, the del- tidium of Hesferorthis has been homologized with the apical plate of Sfirifer. In S. arenosus and many other forms of the genus Sfirifer {sensu lato), the so called apical plate is a callosity filling the back end of the notothyrial cavity but rarely if ever attaining the level of the interarea. It is not a deltidium as in Hesferorthis, and accordingly the various structures are not homologous, although they probably serve the same purpose, that of pedicle attachment. In a few of the punctate orthids or Dalmanellacea, apical plates have been noticed. They are especially well developed in the Schizophoriidas and Parmorthis, but in no wise do they suggest a deltidium as defined above. The structure is not an arch, but a flat plate flush with the interareas. In Pionodema the anterior portion of the plate is concentrically lined and bevelled sharply below the level of the interarea, suggesting that this small structure served the same purpose as the pedicle callist. Mystrofhora areola also has a flat plate, but here it lies beneath the level of the interarea. The deltidium may or may not be perforate at the apex. In some species of Billingsella there is a minute perforation barely large enough for the passage of a hair. The resemblance of this apical foramen to that seen in the Strophomenacea has been urged as one link of kinship between the two groups. There are, however, no other anatomical or stratigraphic grounds to support this supposi- tion, but whenever this apical foramen occurs it is assumed either to have functioned for the protru- sion of a pedicle, reduced in these forms to a mere thread, like the byssal threads in some lamelli- branchs, or to have furnished passage for the anus. In one large group of shells, the Clitambonacea (pi. A, fig. 3), there is commonly a conspicu- ous apical foramen which undoubtedly served for the passage of the pedicle. Distinct scars of pedi- cle attachment have been observed in Deltatreta on the under surface of the deltidium and the floor of the delthyrial cavity. In CUtambonites and Estlandia marginata, the foramen is sealed at matur- ity by shell substance deposited within it (see pi. 8, fig. 8). This clearly means the loss of a func- tional pedicle in later mature life. Function of the deltidium. — The function of the deltidium in all brachiopods is as yet not clearly understood, since our studies do not embrace the Strophomenacea wherein this structure has its best development. From the fact that the deltidium is present in the oldest and most primitive forms, we must conclude, however, that it had some functional importance to the brachiopod possess- ing it. In the geologically younger species, where the deltidium is almost uniformly absent, life without the structure was obviously possible. At first it appears difiicult to understand the value of a structure such as the well developed deltidia in the Strophomenacea, where, by their forward growth, the pedicle opening will be constricted more and more, and in some forms no opening at all will be left for the protrusion of the pedicle {Strofheodonta of the strophomenids) ; whereas in other genera the deltidium has been resorbed or not allowed to grow by a thickened and therefore more vital pedicle. This problem is of great interest, and may be solved when the great line of deltidium-bearing shells, the Strophomenacea, are restudied from this point of view. Among the Orthacea we note, however, the interesting fact that in some species of Valcourea MORPHOLOGY OF THE ORTHOID SHELL 23 the deltidium is a normal and well developed structure, while in other species of the same genus it is completely wanting. In the species without a deltidium, there is a well marked pedicle callist or triangular scar which probably is the seat of pedicle attachment (see pi. 10, figs. 19, 22, 24). This evidence appears to indicate that there is some connection between the deltidium and the pedicle. It has been observed, further, that in orthid species having a deltidium, such as I lesperorthts tricen- aria, Dinorthis szveetieyi, Valcourea, and others, the adductor and diductor muscles occupy the entire delthyrial cavity, but in these forms there is no vestige of a pedicle attachment. Hence if there was a functional pedicle, it must have been attached to some portion of the deltidium. These observations were anticipated years ago by Winchell and Schuchert,* who clearly expressed the same views, as follows: In all the species of Orth'u observed when a pedicle muscle [callist] is present a deltidium is absent; but where this plate is developed the muscle [callist] is rudimentary. This evidence leads the writers to the conclu- sion that the pedicle muscle is attached to the bottom of the valve in the apex of the delthyrium when the del- tidium is wanting, but when it is developed the muscle is then more or less attached to the deltidium. In shells provided with a deltidium of the Deltatreta type, and having a prominent foramen, it has been determined, at least for Deltatreta, that there is a well marked, even conspicuous pedicle callist in the apex of the delthyrial chamber. Punctate shells are usually not provided with a del- tidium, but when an apical plate is present, as in Pionodema, its position in the valve and the lack of any visible scar of pedicle attachment on the floor of the delthyrial cavity make it clear that this plate served for pedicle attachment. Lateral plates. — In many genera, especially those of the Orthidas, there are lateral plates which restrict the delthyrium. Such are prominent in Glossorthis (pi. 4, fig. 28) and Ptycho- pleurella (pi. 6, fig. 9). Perhaps the best example is Glossorthis, in one specimen of which the right plate has been fractured slightly, emphasizing the suture line. In Hesferorthis the ventral shell has not only these lateral plates but likewise a small deltidium, showing the close connection of these different parts of the same structure secreted by the mantle. These lateral plates strongly resemble deltidial plates in manner of growth and position. They apparently grow inward and forward by marginal deposits of the mantle, since they show successive growth-lines as in deltidial plates. They have, however, never been observed to meet and in this respect are not unlike incomplete deltidial plates of certain of the Rhynchonellacea and Terebratulacea. These observations appear to us very important as bearing on the source of deltidial plates in the order Telotremata. Less well developed plates of the sort described above occur also among the Pentameracea and they have received considerable attention by other students of brachiopods. Hall and Clarke describe them in several genera, and Kozlowski has gone so far as to place the Pentameracea in the Telotre- mata because of their presence in that group. MODIFICATIONS OF THE NOTOTHYRIUM (CHILIDIUM AND CHILIDIAL PLATES) In the dorsal valve the covering of the notothyrium is termed the chilidium. This structure is usually absent in post-Cambrian orthids but occurs sporadically (probably as re-developments) or in modified form in later genera. The usual type, seen in Vellamo, is a convex, perforate arch built with its piers against the margin of the notothyrium and with the suture line of contact commonly healed or grown over so that the interarea and chilidium appear continuous. The outer surface of the arch is marked by convex lines or thin lamella; of growth. On the under side of the chilidium the attachments to the sides of the notothyrium are strengthened by callus deposit laid over the points of contact and along the walls of the notothyrial cavity. In many species of the Clitambonitidas the posterior surface or edge of the cardinal process is attached to the under or anterior surface of the chilidium. This type of notothyrial covering occurs throughout the Billingsellidas and Clitambom- tidse. It is also present in some genera of the hesperorthid tribe. *GeoI. Minnesota, vol. 3, pt. 1, 1895, p. 422. 24 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA In endopunctate orthid shells modifications of either the delthyrium or notothyrium are of uncommon occurrence. Only one instance of a true deltidium is known and that is in Kayserella, which stands unplaced in the Orthacea. A chilidium is also known in Heterorthis (see pi. 20, fig. 19). It is strongly convex and arches over the carinate postero-dorsal portion of the carinate median lobe of the cardinal process. The only suggestion we can offer in regard to its function is that it serves for protection of the muscle attachments on the cardinal process. Of unusual form and interest is the umbrelliform chilidium of Productorthis (see pi. 4, figs. 15, 16). This plate has its origin in the growth of two discrete plates along the notothyrial margins. These grow dorsally to the dorsal surface of the cardinal process, uniting and expanding into a cir- cular plate that hangs over the distal end of the process like a small umbrella. Here again the function may be one of protection for the muscle attachments. In some orthids there occurs an incomplete chilidium or what may be termed chilidial plates. These are low plates projecting posteriorly from the notothyrial margins and forming low walls on each side of the notothyrium. These chilidial plates are commonly thickened by callus deposit on the interior and cemented firmly by the spread of callus covering all points of contact. Chilidial plates characterize Nicolella, Deltatreta, Pomatotrema, and Planidorsa. From the above, it appears that the sporadic reappearance of the chilidium and chilidial plates in late forms cannot have a direct genetic linkage with the older genera wherein these structures are of continuous development 5 the former suggest, rather, atavistic returns of primitive features. In the Clitambonitidse, where the normal genera possess a chilidium, its persistent absence in other forms must mean a progressive change of considerable significance in establishing species and genera, for example in A-pofnatella. MORPHOLOGY OF THE VENTRAL INTERIOR ARTICULATION Genuine articulation is well developed in all of the Orthacea of the Cambrian, and its origin is therefore to be sought in the most primitive order, Atremata. In the orders Atremata and Neotremata the two valves are held in apposition by muscles, but in the Protremata and Telotremata they are held in place by teeth and sockets and sometimes by other articular devices that together act as hinges. In many forms the brachial process or brachio- phore also assists in locking the valves, since it articulates with the tooth of the ventral valve and helps to prevent lateral motion. It will be shown directly that the tooth of the ventral valve in Orthacea bears a socket on its posterior surface and a fossette on its inner and dorsal face. The accessory socket of the posterior surface receives a small denticle on the outside of the socket of the ventral valve and the tooth fits into the dorsal socket. On the other hand, it will be shown in the discussion of sockets that the crural fossette receives the carinate or expanded posterior edge of the brachiophore. Accordingly, we see that the brachiophore is as important in the articulation of these brachiopod shells as the tooth ^n.Q socket. Teeth. — ^Teeth and dental plates are unknown in the inarticulate orders Atremata and Neotre- mata, but ventral teeth with their dorsal sockets are present in all the Cambrian genera of the Orthacea, though they appear to be very rudimentary in Kutorgina, thought to be the most primitive genus of the articulate order Protremata. On the other hand, dental plates are not always present in the older Cambrian genera (absent in Nisusia), but later on they make their appearance and are then usually present.** , r , • c u a } The teeth or articulating apophyses are situated immediately laterad of the margins ot the del- thyrium, or, in a few genera, at the intersection of the delthyrial margin and the hinge-line. When viewed from the posterior, the teeth are roughly triangular in outline, the apex of the triangle being "sWalcott, Camb. Brach., 1912, p. 310. MORPHOLOGY OF THE ORTHOID SHELL 25 directed toward the delthyrium. The teeth are considered to extend vcntrally as far as the ridge bounding the crural fossette (see below) on the inner face of the tooth. The progressive forward growth of the tooth produces a thickening under the palintropc along the dclthyrial margin, and likewise a triangular area on each side of that margin. This small area is bounded on the inside by the delthyrial margin and on the outside by a more or less well defined suture line connecting the beak with a small socket in the outside, posterior face of the tooth. These narrow triangular spaces usually can be easily differentiated from the remainder of the interarea by the difference in color. In punc- tate shells the palintrope is punctate, but the dental area is impunctate. In a cross section of the palintrope the triangular area is also three-sided and is supported by the dental lamells. The teeth of Rhipdomella vary from the usual type described above. They form the margin of the umbonal cavity and rest on the floor of the valve with little or no supporting plates, and project dorso-laterally as blunt points. Sockets. — There are sockets in both valves which receive articulating processes from the oppo- site shell. In the ventral valve there are two kinds of sockets associated closely with the teeth: (1) Accessory dental sockets in the postero-lateral face of the teeth (see t. fig. 2); and (2) a crural fossette or crural socket" on the inner face of the teeth. 1 . The accessory socket in the tooth of the ventral valve is located in the postero-lateral sur- face of the tooth next to the hinge-line and on the outside margin. It articulates with a small SfS Y^r.. 2. — Diagram showing the ventral interarea of Vdcourea, with accessory sockets in the teeth {Acs). apophysis or tooth situated on the outside of the dental socket of the dorsal valve. This accessory articulation is common throughout the Orthoidea and in strophomenids, spiriferids, etc. 2. The crural fossette is a more or less deep groove, usually defined by an oblique ridge, located in the inner face of the tooth just ventrad of the delthyrial margin. Not uncommonly the fossette is not sunk deeply beneath the surface of the tooth but is made by a small subtriangular plane sur- face defined by an oblique ridge, a type frequently seen in the family Orthidx. The function of the crural fossette is to articulate with the carinate, or, in some genera, expanded postero-ventral edge or face of the brachiophore. In Hesperorthis the fossette is small, but the oblique ridge defining it is prominent. In the articulation of this shell each tooth fits into the socket which is located outside the brachiophore; it extends into the socket a distance equal to the plane or sunken face of the crural fossette, while the oblique ridge rests on the posterior or postero-ventral edge of the brachio- phore. It follows then that the brachial apparatus of the orthid plays an important role in the artic- ulation of the shell, a role no less important than that of the teeth and sockets. The crural fossette of Parmorthis (pi. 21, fig. 10) is developed to a remarkable degree. It is an elongate, deep groove extending nearly to the floor of the valve and terminating in a deep socket. The oblique groove receives the postero-ventral edge of the crural base, and the socket at the end of the groove articulates with a slight expansion near its ventral extremity. Dental plates. — Beneath each tooth there are as a rule two more or less strong plates divid- ing the space beneath the overhanging palintrope into three chambers, two lateral ones and a central one the delthyrial cavity. These plates have been most commonly termed "dental lamellae" or "dental plates." Thomas calls them "delthyrial supporting plates" and Fredericks speaks of them as "lamellae apicales." We prefer to remain by the older terms "dental plates" or "dental lamella." "The term crural fossette ("fossette crurale") was used first by Kozlowski (Bib. Univ. Lib. Polon^, A, 1927, fasc. 17, p. 8, fig. 1). 26 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA Thomas'' challenged the general conception that these plates gave support to the teeth. He says: As to the function of these plates there appears to be greater reason for regarding them as supports of the areal portion of the pedicle valve than as dental supports. It is certain . . . that they are not in all cases directly con- nected with the teeth, while the connection in other instances need not necessarily imply the primary function of dental support. The delthyrium is situated at the weakest part of the areal portion and the plates are developed along its lateral margins and adjacent to the deltidium. This is just the position where one would expect a strength- ening of the valve to take place. It seems preferable, therefore, to call such "dental" lamellae the "delthyrial supporting-plates." It has been the experience of the writers that in the Orthacea these lamellse are invariably con- nected with the teeth. Thomas' argument that these plates support the palintrope at its weakest point is not conclusive, because there are many strophomenoid genera with long interareas that have no dental supports reaching the floor of the valve, and among the orthids the oldest known genus, Nisusia, is also without them. Furthermore, shells having broad overhanging palintropes, such as Hesferorthis and Plectorthis, are commonly provided with thin, delicate, short dental plates of the receding type, but shells with shorter palintropes and more ventricose valves have ponderous dental plates, e. g., Orthis s. s. and Archworthis. Where these lamellae are present, they undoubtedly serve the double function of supporting the teeth and the palintrope. Dental plates may be described as receding when they extend as ridges under the teeth and the palintrope, and finally attain the floor of the valve near the apex only. Examples of this type are common in American orthids such as Hesferorthis and Schizoramma. The dental plates are called advancing when they slope forward and are continued around the lateral margins of the muscle field as more or less well defined ridges, as in Schizofhoria. The term obsolete may be applied to dental plates when the lateral umbonal cavities have been so filled by adventitious testaceous matter as to obliterate them. This condition is common in old shells, so that the observer, if he does not take the precaution to examine interiors of young specimens of the same species, may fall into the error of describing the particular species or genus as possessing no dental plates. Such an error was made by Hall and Clarke* in their definition of Orthorhynchula, which they say is without dental plates. The young of that genus, however, are provided with well developed receding dental lamellas. SPONDYLIUM History of the term. — The term spondylium (from the Greek word for vertebra) was pro- posed by Hall and Clarke (1892) in their discussion of Clitambonites. They say of Pronites adscendens Pander:® On the interior of the valve the dental lameUae are very strongly developed, converging and uniting in the median line before reaching the bottom of the valve; thus forming a spondylium [a spoon-shaped plate], which with the deltidium encloses a conical subrostral vault . . . This term will apply with equal propriety to the similar plate existing in the pedicle-valve of other brachiopods, e. g. . . . PentameruSy Camarella [Camarophona, etc.]. Later these authors add:^" It has become evident since the introduction of the term that these processes in the two valves [spondylium and cruralium of Pentamerus] , though similar in aspect, are similar neither in origin nor function, and it becomes necessary to modify the application of this term. Hence it is proposed to restrict the term spondylium to the plate existing on the pedicle- valve, and to the plates of the brachial valve, whether united or discrete, the name cruralium will be applied. . The spondylium is an area of muscular implantation. In its early or incipient condition it is evident that it originates from the convergence and coalescence of the dental lamellae, and forms a receptacle for the proximal portion of the pedicle, and for the . . . pedicle muscles. In Clitambonites and Pentamerus, where it attains its 'Mem. Geol. Surv. Gt. Brit., Pal., vol. 1, pt. 2, 1910, pp. 100-101. «PaI. N. Y., vol. 8, pt. 2, 1893, p. 181. •Op. cit., pt. 1, p. 234. "Pt. 2, pp. 331-332, 335 footnote, 341. MORPHOLOGY OF THE ORTHOID SHELL 27 greatest development, it bears all the muscles of the valve, the central adductor, and the lateral diductor scars being often clearly defined, while the posterior portion of the plate is still reserved for the attachment of the pedicle, if functional. In the pentamcroids the median septum of the pedicle-valve supporting the spondylium, is formed in a similar manner by a continuation and coalescence of the dental plates, and wherever the median supporting septum exists in this group, it will probably be found to have this composition. Median and lateral septa, however, in the valves of the Brachiopoda, have a highly diverse origin in different cases. In most instances, except where bear- ing spondylia, they are evidently of muscular origin and surfaces of muscular attachment, as shown in Spirifrrina. [The spondylium is of early manifestation.] It appears in a highly developed state in conjunction with the unmodified deltidium [delthyrium], first in Protorthis, of the Cambrian, then in PoLytoechla, Syntrofhia, Clitam- honites and ScenUium, of the early [Ordovician] and later Silurian and of the Devonian. Schuchert" formerly held that the spondylium probably had its origin in an excessive deposit of testaceous matter about the bases of the powerful adductors, diductors, and pedicle muscles. Growth of the individual necessitates the progressive anterior movement of the muscles, and when these are large there is but little or no space left between or outside of them for the viscera and genitalia, which are therefore crowded farther and farther anteriorly. This condition naturally produces constant pressure of the genitalia against the anterior base of the forming spondylium, and since pressure causes resorption or diverts testaceous deposition, it follows that these organs will gradually produce cavities for their relief beneath this plate. This explanation certainly appears true for the platform in the atrematous Trimerellidas, but it is now apparent that this is not the way the thin, plated, true spondylia of the Clitambonitidae and Pentameridas were made. It is clear that what Schuchert was describing is the origin of what is now called the sessile spondylium (spondylium discretum) so well developed in the Billingsellidas. Hall and Clarke give the correct function for the true spondylium when they say that it is "an area of muscular implantation," originating from the convergence and coalescence of the dental lamellae, and uniting either with a median septum or with the floor of the shell, but their further statements about its making, in connection with the deltidium, a pedicle-sheath that had its first stimulus of growth in the prodeltidium is, as we now see, wrong. What Schuchert and the older students of brachiopods did not see is that the muscle-bearing platforms of the Trimerellidas, the spondylia of Clitatnbonites and Pentamerus, and the cella of Merista are not homologous structures, since, as we now know, all are of independent origin, arising in different ways though functioning more or less alike. Sessile spondylia (spondylia discreta) are common in the Middle and Upper Cambrian, but true spondylia are not present until late in Cambrian time and are chiefly character- istic of the Ordovician and Silurian, while the cella type of muscle plate is of Middle and Upper Ordovician origin, appearing first in Cyclospra and Dayia. What Schuchert in 1 897 regarded as the primitive attached spondylium (= sessile spondylium), Walcott in 1912 called the pseudospondylium, and states that it occurs in PNisusia, "Billingsella" Eoorthisy Finkelnburgia (free in front) and Huenella (free at the sides). The sessile or "pseudo- spondylium" of Eoorthis appears in Orthis ... of the Ordovician and later faunas, probably as a reversion from a free spondylium [here Walcott, as we now know, is clearly wrong]. On the line of descent to Protorthis the pseudospondylium becomes a free spondylium and continues on through Syntrophia and Clarkella into the Ordovician and Silurian Pentameridas and Clitambonitidx.'^ Walcott asked Ulrich to comment on the kinds of spondylia, and this he did as follows." The term spondylium, he says, applies only to the t}'pical free or medially supported umbonal camera or spoon . . . and corresponds to a ventral muscular area which is raised above the floor of the valve and formed by the convergence and union of the dental plates . . . The manner in which the spondylium is att.iched to the bottom of the valve is so variable that the feature does not seem to be of more than generic consequence. Ulrich then points out the great variability in a number of genera. " Bull. 87, U. S. Geol. Surv., 1897, pp. 99-101. "Walcott, 1912, p. 307. "In Walcott, 1912, p. 308. 28 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA Kozlowski^* has recently made a detailed study of the spondylium in the Pentameracea and has reached some striking conclusions. He has shown that there are two types of spondylia in the Pen- tameracea as constituted by Schuchert in 1913. One of these types, in which the spondylium is sup- ported by a simple septum, he calls the "spondylium simplex," and thinks it originated from a pseudospondylium by lateral crowding of the ovarian bodies and consequent resorption of the deposit forming the base of the pseudospondylium. The Clitambonacea are characterized by this type of spondylium and Kozlowski finds this superfamily to be more closely related to the orthids than to Pentamerus. In his conclusion we concur, having arrived at the same opinion independently but using different criteria as well, namely, the pallial and ovarian impressions and the cardinalia. Characteristic of the second group — the Pentameracea — is the "spondylium duplex." In this type the spondylium is elevated on a septum which is clearly composed of two parts thought to have been produced by the squeezing of the two dental plates together due to lateral pressure from the ovarian bodies. This type of spondylium is common to the pentamerids, but the cardinalia are much more characteristic than is a spondylium duplex which is known in many other genera. Present views and definitions. — We may add that the dental lamellse and the floor of the valve between them are in many genera bound together by a chamber called the delthyrial cavity, and in most of the orthids the floor of this space is the seat of attachment of the various muscles. In other orthid genera the dental plates converge and unite with each other medially on the floor of the valve, or are supported by, and fused with, a median ridge before attaining the shell floor, forming an elevated spoon-shaped structure. It is to the latter type of muscle platform that the term spondylium is now restricted. In rare instances among the oldest brachiopods {Protorthis) the dental lamellae unite to form a freely suspended spoon without the intervention of a median septum. These various types of muscle platforms may be briefly defined as follows: 1 . Discrete spondylium. — This, Kozlowski's spondylium discretum, designates the condition in which the dental lamellae converge but slightly and extend directly to the floor of the valve, but never become united. This type of muscle cavity occurs in PoramboniteSy Huenella, and a host of other orthid shells. It is not a spondylium, but the structure does suggest how the genuine spondylia arose, namely, by the complete convergence and union of the dental plates above the floor of the valve, and their support by a median septum standing on the floor of the shell. 2. Pseudospondylium. — This term was introduced by Walcott^^ for "the rudimentary spon- dylia attached directly to the inner surface of the valve, as in BilUngsella." In all the species listed under the genus BilUngsella in the present work the dental lamellae do not converge but extend vertically to the floor of the valve and conform in all essentials to the type of muscle platform called by Kozlowski the spondylium discretum. In some species, however, the anterior portion of the muscular area is elevated on a callosity. This shell thickening within the delthyrial cavity and its connection with the dental lamellae make it appear that these plates are united but in reality they are discrete, resting on the floor of the valve. It is to this type of muscle platform that we propose to limit the term pseudospondylium. Pseudospondylia are excellently exhibited in Glossorthis (see pi. 4, figs. 9, 12), Linoporella (see pi. 18, figs. 13, 14, 18), and Finkelnburgia (see pi. 13, figs. 6, 11). In these genera the callus beneath the muscle attachments is extended from the floor to the inside face of the dental lamellas. Anteriorly the callus is thick and in Finkelnburgia is extended forward as a low ridge. Specimens of Linoforella kindly loaned by Dr. A. H. Westergaard of Stockholm show a complete series from the young stage with discrete dental lamellae and unthickened muscle attachments to a condition in which a typical pseudospondylium with a median ridge has been developed. This series is illustrated on plate 18. Pseudospondylia are naturally more developed in old or late mature shells, as young individuals do not deposit much callus. Since, therefore, the pseudospondylium is a feature of mature shells only, it is obvious that it has a minor value in the taxonomy of the Brachiopoda. "Pal. Polonica, 1919, pp. 122-125. "Camb. Brach., 1912, p. 295. MORPHOLOGY OF THE ORTHOID SHELL 29 3. Spondyloid. — This term is suggested for a structure simulating a spondylium in form but produced by the deposition of adventitious testaceous substance on and about the clental lamellx, swelling them laterally until union is effected. This type of structure is well exhibited in Poram- bowtes. In late mature or gcrontic individuals of this genus, adventitious shell has been deposited on the dental lamellx and on the floor of the valve in front of them, and may be produced forward sufiicicntlv to simulate a median ridge or septum. In old shells this structure so closely resembles the spondylium in pentamerids that Noetling and subsequent workers have been misled into the con- ception that the thickening of the dental plates in Poratnbonites is a spondylium. 4. Free spondylium. — Here the dental plates converge and unite to form a spoonlike muscle platform that hangs suspended in the ventral valve, since it is not supported by a median septum as in the typical forms of spondylia. This type is exceedingly rare, being known in Protorthts of the Middle Cambrian, and the younger genera Holorhynchus and Cymbidium. 5. Spondylium or spondylium simplex. — The term spondylium has customarily been applied to the spoon-shaped muscle platform formed by the convergence and union of^^the dental lamellx with a median septum, regardless of the structure of the septum. Kozlowski, however, has shown that the spondylia are differently constructed and are therefore polyphyletic m origin, and has consequently defined several kinds. He shows that the spondylium in Skenidmrn and Cluatnr- bonhes is different in origin from that in the Pentameracea. In Clitambonites the spondylium is formed by the union of the dental plates with a simple median septum (euseptum), the whole forming one piece. To this type of spondylium he gave the name spondylium simplex. As this is the simplest type of supported spondylium, we may call it spondylium and drop the limiting word simplex. , , ^,. , • • j j • i The spondylium is one of the characteristics of the subfamily Clitambonitidae and is seen also in the Syntrophiidas and Skenidiidx. 6. Duplex spondylium. — The above type of spondylium is in decided contrast to that in the Pentameracea (Conchidium), in which the supporting septum is actually double and composed of two united vertical plates. To the latter type Kozlowski has applied the name spondylium duplex, and it appears not to occur in any genus of the Orthoidea," but is characteristic of the pentamerids, where it is associated with a definite kind of crural apparatus. ■ r . j In its evolution, Kozlowski believes that the spondylium simplex has arisen from the pseudo- spondylium. The genital organs are thought to have resorbed the testaceous deposit about the base of the dental plates and under the muscle attachments. The resorption has gone so far as to leave a rather narrow septum underneath the seat of muscle attachments. This is, in its essentials, precisely the same idea held by Schuchert in 1897.^^ The spondylium duplex, however is considered by Kozlowski to have originated in a different manner, i. e., through lateral crowding by the sexual glands, which has pushed the dental lamellx gradually together. SEPTA Vertical septa are usually not conspicuous features of the ventral interior, but in sonie genera rather prominent ridges or septa are developed in connection with the musculature, deltidium, and pedicle attachment. Such ridges usually have value in species making but only rarely have they generic significance. . . , ,, In shells wherein a pedicle callist or a deltidium is present, it is not uncommon tor a small septum to extend forward from the callist or deltidium for a short distance, dividing the back ends of the diductor scars. This ridge is commonly formed in old shells and is the result of deposition of adventitious shell between the muscles. Such septa are, however, of little taxonomic significance. " 1929, pp. 122-126. ..,.,, l j . "The writers have not seen a thin section of the spondylium of EnteUtella and lU detailed nature can not be deter- mined from Likarev's figure. "Op. cit., pp. 99-100. 30 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA In shells having a spondylium there may be other septa in addition to the median one. This is true notably of Gonambonites, Clarkella, and Y angtzeella, in which the lateral septa probably play a secondary part in the support of the spondylium. In Clarkella the several lateral accessory septa appear to be excessively developed ovarian ridges, and such are common among the syntrophiids and orthids. A few genera that have greatly developed diductor scars, especially where these wrap about and enclose the adductor scars, as in Rhipidomella, may develop short, low septa in front of the latter impressions j the septa then separate the antero-median extremities of the diductor scars. These septa are also of no special taxonomic value. A few genera are provided with a broad and low median ridge which, besides dividing the diductor impressions, also bears the adductor muscles. This type of septal ridge is seen best in the Schizophoriida;, in which its development can be traced from a low ridge in Pionodema and the early Schizophorias to a high septum in Orthotichia and Enteletes. The same kind of median septum is also seen in certain stocks of the dalmanellids such as Cariniferella, and in these the ridges likewise bear the adductor muscles. There is an interesting development of a median ventral septum in Parenteletes. Here the dental plates are similar to those of Enteletes, but are more widely divergent. The median septum, however, has its origin a short distance anterior to the apex, continuing forward and increasing in height in the same direction. But at the anterior end of the septum, just at the point of origin of the shell fold (ventral), there is a small inverted V-shaped chamber, here termed the cella;^^ this chamber evidently acts as an arch to support the dorsal septum above the inner sulcus produced by the external fold. The septum in this genus is further distinguished from that of Enteletes by its expanded, rounded dorsal edge. MUSCULATURE By means of muscles, brachiopods are enabled to open and close their valves, and to a limited extent to protrude and retract the pedicle and to adjust the position of the shell. In the articulates there are three sets of muscles: the diductor s, which by contraction open the valves; the adductors, which by contraction close the valves; and the fedicle muscles (including the adjustors), which by contraction "serve to alter or adjust the position of the animal as a whole by turning it in various directions.""" All of the muscles traverse the visceral area. The attachment of the muscles to the valves leaves as a rule well marked scars. The diductor muscles which attach to the cardinal process are distinguished as the principal or anterior diductors, while the much smaller pair are the accessory or posterior diductors; the latter are seen in the ventral valve as two small scars posterior to the adductors. In most fossil brachiopods it is only the anterior diductor and the adductor scars that are impressed. According to Thomson,^^ The pedicle-muscles serve partly to attach the pedicle to the shell, and partly to retract the pedicle or to allow the shell to erect itself. They consist of an unpaired muscle [unknown as such in orthids, where they are always paired] running from the ventral side of the pedicle to the ventral valve, where it determines a transversely elon- gated scar lying posteriorly of the other scars, and, in addition, two pairs known as the dorsal and ventral adjustors. The dorsal adjustors are attached to the hinge-plates or other similar structures in the dorsal valve . . ., while the ventral adjustors form two scars in the ventral valve lying outside the adductor-scars. The pedicle-muscles, not being relatively very strong, do not form strong scars, the median unpaired muscle making the strongest. In the Orthacea the ventral muscle field begins small, and progressively expands in later forms. Hall and Clarke saw this long ago, saying:^* The entire muscular system on the ventral side of the body, is, in primitive forms, inserted upon the base of the pedicle [delthyrial] -cavity. This is apparent from a study of such a shell as Orthts callactis, where it is per- fectly clear that no muscular bands were attached to the pedicle-valve outside the limits of this strong and con- ^° From the Latin word for small or extra room. ^"Parker and Haswell, Text-book of Zoology, vol. 1, 1921, p. 358. "Brach. Morph., 1927, p. 30. "Pal. N. Y., vol. 8, pt. 2, 1894, p. 338. MORPHOLOGY OF THE ORTHOID SHELL 31 densed posterior area, which is but a sessile spondylium. [In some shells] the original contents of the pedicle-cavity may be represented by enormous muscles whose scars extend almost to the anterior margin of the valve, as in Hipfarionyx and Rhip'uiomella. The detail of the muscle-scars in brachiopods is usually difficult to determine, because of the poor preservation of the inner surface, or because of the thinness of the shells. Furthermore, as the animals grow larger, the muscles migrate forward, leaving more or less elongate tracks widening toward the front. On the other hand, the tracks do not always afford the detail of the muscles because the mantle later on covers up the posterior portions of the tracks as migration continues. It is therefore not uncommon that a doubt may be entertained as to what the actual marks represent in detail. To cover these various aspects of the muscle-scars we deem it best to adopt the following terms from Buckman :"* 1 . "Muscle-mark — any mark indicating muscular attachment. 2. "Muscle-track — the course down the valve shown by successive muscular attachments. 3. "Muscle-scar — more or less defined areas representing the ultimate muscular attachment." In general, when the preservation is favorable, four sets of muscle impressions may be distin- guished in the ventral valve, as follows: (1) the principal or anterior diductors, (2) the adductors, (3) the adjustors, (4) the accessory or posterior diductors. Of these four sets, the first three are usually visible, but the accessory diductor scars are rarely to be seen, and in some instances the adjustors are not determinable. ( 1 ) Anterior diductor muscles. — ^The diductor impressions are always the largest scars in the ventral valve. In the early impunctate genera, their tracks and scars are usually rather narrow and straight, and never enclose the adductor impressions at the front. In later genera, such as Dinorthis s. s.y DinorMs (Pl^siomys), and Rhipdomella, the diductor scars enclose the adductors. (2) Adductor muscles. — When the actual adductor scars are visible they are usually hemi- elliptical in outline and separated by a low ridge which may be single or double. In some genera the adductor impressions can not be determined, but their position is marked by a linear track. In a few genera, such as Platystrofhia, Mcewanella, and Productorthis, there is no trace whatever of either the scars of attachment or the tracks. In other genera (Schizophoria, Orthotichia, and the Entele- tinae), the adductor muscles were borne on a high vertical septum, consequently their marks are not recorded on the shell. (3) Adjustor or pedicle muscles. — These muscles are not always clearly defined on the valves and have not been determined in all genera of the Orthoidea. The position of these mus- cles, however, is always outside of, and posterior to, the diductors at the base of the dental lamella, or they are more or less elevated on the side of these plates. As would be expected from their posi- tion, these muscle impressions are usually elongate, and rarely present the semi-flabellate outlme commonly seen in the diductor scars. The adjustor scars are the most variable of the muscle mark- ings and in this work little reliance has been placed on them in the establishment of genera. Where good material is available, however, they may be of some use in the making of speaes. In their great work on brachiopod genera. Hall and Clarke"' have applied two terms to the set of muscles we are calling adjustors. In the text of the discussion of DinorMs they refer to these muscles as "adjustors," but in the legend to Plate V, where this genus is figured, they call them "posterior diductor muscles." We prefer to use the term adjustor muscles because these scars have precisely the same position in reference to the diductors as in Recent genera. Furthermore, the pos- terior diductor scars in living forms are located just posterior to the adductor impressions. Similarly situated scars can occasionally be seen in Paleozoic shells and their presence lends support to our usage of the term adjustor. It must be admitted that similarity of position does not prove homology of the muscle-scars in question, but in the absence of more definite knowledge of the soft parts of these Paleozoic shells, we can only compare the structures in the older genera with those in the Recent forms. "Brach. Namyau beds, 1917 ( 191 8), p. 90. »« Pal. N. Y., vol. 8, pt. 1, p. 195. 32 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA (4) Accessory diductor muscles. — In Laqueus and other terebratulid genera there is an accessory diductor set located immediately posterior to the adductor scars. From the presence of such marks in these recent shells it might be suspected that homologous muscle patches should occupy a similar position in the orthids. Such scars have been described by Sardeson^' in Dalman- ella emacerata, in which they are located just posterior to the adductors and between them and the pedicle callist, hence in precisely the same position as in Laqueus. In shells like Dinorthis sub- quadrata and Rhifidomella there may be a corresponding set of muscles associated with the adduc- tors, although these were not clearly seen by us. According to our interpretation of this set of muscles as adjustors, there can be no correspond- ing set of scars on the dorsal valve, since these muscles are here attached to the pedicle. PALLIAL AND GENITAL MARKINGS The mantle (or certain of its layers) is folded upon itself at various points, enclosing cavities, or fallial sinuses, which contain the body fluids and frequently portions of the genital organs. Pallial markings. — The markings of the main pallial trunks are often seen in the Cambrian Figs. 3-5. — Ventral pallial and ovarian markings. 3, Orthis s. s.; this type characterizes the family Orthidae as here described. 4, Schizoramma, in which the markings are essentially the same as in Orthis and Hesferorthis. 5, Dinorthis (PUsiomys) subquairata (Hall) ; notice here that the main trunks extend from the anterior of the diductor scars. In the ventral valve the pallial trunks in all orthoid brachiopods begin at the same place. genera of Atremata {Obolus, Lingulobolus, Lingulella, Bicia, Dicellomus) and of Neotremata {Obol- ella, Acrothele, Botsfordia, Acrotreta). Accordingly these markings should be common among the Cambrian Orthacea, and so they are. They are especially well seen in BilUngsella. In living brachiopods, according to Thomson,^' The pallial sinuses take the form of great trunks, entering the mantle-lobes from the coelome, and generally branching repeatedly towards the margins. The disposition of the sinuses varies considerably in different groups, and it is probable that these characters have at least super family value in classification. The pallial marks have been found of great use in defining some of our families. In general, the pallial markings of the ventral valve are rather simple as regards the main trunks. Their general arrangement is foreshadowed in the Cambrian genera of the Billingsellidas in the two prominent trunks which radiate antero-laterally from the forward ends of the diductor scars, thence splitting and diverging toward the front and sides of the valve. One of the anterior branches of each trunk swings dorsally and runs nearly parallel to the lateral margin. The other, shorter branches extend inward and may or may not unite near the mid-line of the valve (see pi. 1, fig. 10). ^'Amer. GeoL, vol. 19, 1897, p. 93. »«Brach. Morph., 1927, p. 11. MORPHOLOGY OF THE ORTHOID SHELL 33 In the Orthidas and Clitambonitidas the two main trunks are clearly visible but instead of being widely divergent as in the Billingsellidas they extend forward near the mid-line of the shell, being separated by a narrow septum only (t. figs. 3, 4; pi. 2, fig. 18; pi. 6, figs. 18, 26; pi. 8, fig. 10). At the front they may or may not diverge. In the higher families of the Orthoidea such as the Dinorthidse and Plectorthida: the pallial markings are more complicated but of the same fundamental pattern. In Plcesiomys (see t. fig. 5; pi. 10, fig. 26) the main trunks are short and rather narrow, diverging from the antero-lateral extremities of the diductor scars. The main trunks branch almost immediately after their inception, sending one trunk posteriorly and another antero-medially. Each of these subsidiary trunks gives rise to minor branches toward the front and sides of the shell. In Mhnella (see t. fig. 6; pi. 12, fig. 20) the general arrangement of the trunks is on the plan of D'morthis, but details of the branching are different. Orthostrophia (t. fig. 7) has the two diverg- ent trunks of the Dinorthidx which branch into subsidiary trunks, but in addition has one or two minor trunks extending forward from the adductor tracks. These may actually be branches from the main trunks in front of the diductor impressions, but no lines of contact were observed. In the Dalmanellidaj the two main trunks are visible in many of the genera, extending forward from the diductor impressions. The branching of the main trunks into subsidiary rami is more or less complicated (t. fig. 8; pi. 17, fig. 22). ^^.<«'< Figs. 6-8. — Ventral pallial markings. 6, Mimella mdonica (Willard). 7, Orthostrofhia aff. O. Urofhomenoides (Hall), in which they are of the same type as those of Dinorthis and Mimella. 8, Dalmanella ignota Sardeson: Pc, pedicle callist; da'», accessory diductor scars; /la!;, adjustor scars; i4» Centralblatt f. Min., etc., 1922, pp. I23-12+. *° (Translation) The original specimen of Platystrofhia bijorata, which is located in Berlin, is a fairly well pre- served example about 22 mm. wide, 16 mm. long, and 13.5 mm. thick. In the sinus [sulcus] it has 5 equally strong, undivided ribs, on the fold there are 6. On the lateral slopes one counts 9 sharp ribs on both valves. About the beak all of the ribs disappear; the sulcus in that place is shallow and at the delthyrium it passes into a small swell- ing, on which the ribs do not go; so far as can be seen, these appear simultaneously and none is favored. On the fold the ribs develop by division. this study he stated that the "presumptions arc strongly in favor of its [the genotype of Platystrofhia^ being biplicate [bicostate]" (p. 18), and accordingly he restricted the term bijorata to bicostate forms. If Dietrich's determination of the genotype as a tricostate form be correct, it is evident that the bicostate species of "bijorata," will need to be replaced by new specific names. Cumings' splendid monograph on the "Morpho- genesis of Platystrofhia" gave the first careful analysis of the genus. It was in this paper that the supposed marked difference between the American and Euro- pean Platystrophias was first indicated. These ideas have been extended and elaborated by McEwan. Ac- cording to Cumings, three divisions may be distin- guished as follows: (1) uniplicate, (2) triplicate, and (3) biplicate. Since in the present work the term plication is restricted to a major undulation of the shell, such as a fold or the undulations of the shell in Enteletes, we here suggest that these terms be altered to unicostate, tricostate, and bicostate. The three groups may then be defined as follows: ( 1 ) Unicostate grouf. — Here there is one costa in the sulcus and two on the fold at the end of nepi- onic development, and this condition continues then throughout life. (2) Tricostate grouf. — In this group the primary costa of the unicostate condition remains unmodified throughout life, but in the early neanic stage a costa is implanted on each slope of the sulcus. Simultane- ously the two primary costae of the fold bifurcate. The tricostate group is merely a modification of the unicostate one but it is convenient to continue the two groups for their stratigraphic value. Under the tri- costate group McEwan recognizes three subgroups: (a) low-fold, (b) high-fold, and (c) Ponderosa. (3) Bicostate grouf. — Here the median costa of the sulcus of the early neanic shell bifurcates, and simultaneously a costa is implanted between the two primary costa: of the fold. Four subgroups are recog- nized by McEwan but a comprehensive study of the European forms would doubtless demonstrate the need for more subdivisions. Cumings has shown that the nepionic shell of Platystrofhia has the sulcus in the dorsal fold and a fold on the ventral. In the neanic stage, however, the costs bounding the sulcus become elevated to form a fold, and with this change the median rib of the dorsal valve is depressed to form the lone costa of unicostate shells. From this fundamental unicostate type the tricostate and bicostate groups have evolved. According to McEwan and Cumings, divergence of the tricostate and bicostate stocks must have taken place in early Ordovician or Upper Cambrian time, this because the modification of "the plications [costae] of the fold and sinus [sulcus] takes place before the shell has reached a length of 1 mm." The tricostate group is thought to have diverged "from the ancestral 68 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA stock much later than the biplicate [bicostate] group" as the uniph'cate condition remains till the shell has reached a length of 1.2 to 1.5 mm. (p. 391). Platystrophia first appears in the European Middle Ordovician (Kuckers formation, Brandschiefer) in well organized bicostate and tricostate species, P. dentata and P. lynx. In the Schuchert Collection is a lot of P. dentata labelled as "probably Echinosphjerites lime- stone." If this horizon be correct, it pushes the first appearance down into lower Middle Ordovician or about the time of the Black River in America. These Echinosphasrites limestone specimens are bicostate forms and would not afford notable support to McEwan's theory concerning the origin of the group. The Kuckers may be upper Black River in age but is more probably lower Trenton. According to McEwan (p. 400), "a uniplicate species was found to occur in the Jewe [Trenton], and one specimen was found which occurred in the Upper Ordovician (F^) [Lyckholm or uppermost Trenton]." The unicostate group therefore appears to be a rather insignificant assemblage. Both Cumings and McEwan maintain that the bicostate group is the dominant one in Europe. The foreign species are so poorly known that it is as yet difficult to know what is the dominant group in the way of species, but it is a fact that after their origin the bicostate group held the European ground occupied from the Middle Ordovician into the Middle Silurian. In North America, however, the situation is just the reverse, the tricostate group holding its ground through- out the Middle Ordovician and into the Middle Silurian. McEwan's table (pp. 402-404) indicates the first appearance of Platystrophia in North America in Black River time (Decorah shale of Wisconsin). Hall and Clarke,-^ Winchell and Schuchert,^^ Ruedemann,^^ and Schuchert^* all have reported Platystrophias from the Chazy but neither McEwan nor Cumings^^ could find any evidence to substantiate these reports. There may also be some doubt as to the precise age of the form P. extensa McEwan, said to have come from the Decorah shale. This formation has recently been sub- jected to a critical survey by G. Marshall Kay,"^ who comes to the conclusion that the upper or Ion member of the Decorah is actually lower Trenton in age. Kay reports P. extensa McEwan from this member (upper Ion, Prasopora faunule. Church, Iowa) in company with P. trentonensis McEwan. This is the only ho- rizon from which he has Platystrophia and it is there- fore probable that the specimen from the Decorah of Wisconsin is from the Ion member and actually lower " Pal. N. Y., vol. 8, pt. 1, 1892, p. 202. " Geo!. Minn., vol. 3, pt. 1, 1895, p. 456. ^^N. Y. State Mus., Bull. 49, 1902, p. 25. " U. S. Geol. Surv., Bull. 87, 1897, p. 309. ^''Amer. Jour. Sci. (4), vol. 15, 1903, p. S. ^*Jour. Geol., vol. 37, 1929, pp. 639-671. Trenton in age. This would be in agreement with the remarks made above on the European section. In North America the first Platystrophias to appear, barring the one mentioned above, are bicostate forms in the lower Trenton and one unicostate species, P. uniplicata McEwan. If the so called Black River form be considered lower Trenton in age, then the three groups appear in North America almost simul- taneously and fully standardized. The unicostate group is represented here by one known species only. After lower Trenton time the bicostate group is, so far, unknown in the American Ordovician until the topmost formation, the Gamachian (Ellis Bay) of Anticosti, is reached. From this time on the bicostate group is dominant in American Silurian strata and a very few rare tricostate individuals are to be found in the early Silurian. A recent table of suggested correlations by Ulrich^^ places the Wierland group, of which the Echino- sphasrites limestone and Kuckers are a part, as upper Chazy in age. This would make the European species antedate the American forms and would be rather sug- gestive, although not final evidence, of a European origin of the genus. The important point, however, is not the precise age of their appearance but the fact that, when Platystrophias are present for the first time, the three groups come together, fully organized. The appearance of Platystrophia in early Trenton, Black River, or late Chazy time is suggestive that the ancestor of the group should be sought in rocks of Chazy age. McEwan has prophesied a unicostate pro- genitor, from which the bicostate group diverged first, followed by the tricostate group. Both Cumings and McEwan have turned to the Upper Cambrian faunas as being the possible source of Platystrophia. Cumings found in Orusia lenticularis (Wahlenberg) "a form that possesses in the adult practically all of the nepionic characters of Platystro- phia" (p. 5). Taking only external characters, this form could have evolved the exterior of Platystrophia, and internally it has subparallel, discrete brachiophore plates, not unlike those of Platystrophia. It is possible, of course, that crowding of the bases of these toward each other could produce the condition seen in Platy- strophia, but the early appearance of Orusia and its short stratigraphic range make it unlikely that it is the direct ancestor, although it may have been in the general line. Cumings has also pointed out the external similari- ties between Platystrophia and Plectorthis, as follows (pp. 11-12): In some respects the adult Plectorthis plicatella resem- bles the neanic Platystrophia. If the fold and sinus be disregarded (and in some Trenton forms of Platystrophia these are surprisingly inconspicuous), the neanic Platy- strophia is almost a Plectorthis. There is little doubt that " U. S. Nat. Mus., Proc., vol. 76, 1930, p. 73. GENERA OF THE SUBORDER ORTHOIDEA 69 when the nepionic shell of PUctorthis is discovered it will be found to be quite indistinguishable from the nepionic shell of Pljtsstrof.) thakil trifida Salter 1865 ? O. (D.) annamitica Mansuy Placed from the literature American Species of PljEsiomys Dinorthis Columbia WWson 1927 D . if higenia CWiW'mgs) 1862 D. meedsi (Winchell and Schuchert) 1892 D. meedsi arctica Schuchert 1900 D. meedsi germana (Winchell and Schuchert) 1892 D.rockymontana'^'^soxi 1927 D . subquadrata (Yi^V^) 1847 D. transversa Willard 1928 D.ulrichi Foerste 1909 Orthis anticostiensis Shaler 1887 GENERA OF THE SUBORDER ORTHOIDEA 95 European Species of Pl/Esiomys Orthis {Dinorthis) fabellulum carrickensis Rccd 1917 O. infijia Salter O. fore at a McCoy 1 846 O. Solaris von Buch O. (D.) striato-costata Salter 186S O. \d.) suiJirisa Salter 1865 American Species of Retrorsirostra Dinorthis carUyi (Hall) 1860 D. carleyi insolens Focrste 1909 D.retrorsa (Salter) 1858 Anticosti Species of Pionorthis Dinorthis carUtona Twenhofel 1928 Orthis sola Billings 1 866 Distinguishing characters. — Dinorthis is char- acterized chiefly by its convexo-concave profile, svib- quadrate and anteriorly bilobate plan of the ventral muscle field, Or?/;w-type brachiophores, crenulated myophore, and pallial sinuses. From Orthis s. s. it differs in its contour, musculature, and pallial markings. Discussion. — Dinorthis of the Dinorthidae and Dolerorthis of the Orthidas form an interesting homoe- omorphic pair, but the two can be distinguished readily by their internal features. The ventral musculature of Dolerorthis is hke that of Orthis s. s. and never has the adjuster scars developed to any marked degree. The most important difference, however, is to be seen in the pallial and ovarian impressions of the two genera (see pi. 5, fig. 20, and compare with pi. 10, fig. 24). The pallial marks of Dolerorthis are of the Orthis type, two subparallel trunks given of? from the anterior ends of the diductor tracks and extending directly anteriorly. They are separated by a narrow septal ridge and bound ovarian impressions which occupy nearly the entire sur- face of the interior lateral spaces. In Dinorthis, on the other hand, the ventral pallial trunks are divergent and the ovarian impressions are greatly reduced. There is so much variation in the external contour and profile of Dinorthis that several distinct subgenera may be separated as follows: 1 . D. fectinella — Dinorthis s. s. 2. D. suhquadrata — Plwsiomys 3. D. carleyi — Retrorsirostra 4. D. sola — Pionorthis (1) Dinorthis fectinella (pi. 9, figs. 2, 5) is the type of the genus and as well of the subgenus Dinor- this s. s. Its costate exterior differentiates it from the multicostellate exterior of Plwsiomys. (2) D. suhquadrata (pi. 9, figs. 3, 20; t. fig. 5) t}'pifies the subgenus that was designated Plirsiomys by Hall and Clarke, and embraces the many species listed above. (3) The D. carleyi subgenus (pi. 9, figs. 21-23), now termed Retrorsirostra, is characterized by its strongly prodine ventral palintrope and deeply concave ventral valve. In the ventral valve the muscle field is square in front and commonly elevated on a thicken- ing of adventitious shell. The tribe is common in the Upper Ordovician of North America and Europe. (4) Another subgenus of Dinorthis is characterized by Orthis sola Billings, referred by Twenhofel"' to Rhipuiontella; it may be called Pionorthis (Gr. fion, fat) in allusion to its biconvex profile (see pi. 9, figs. 4, 6-9, t. fig. 18). D. carletotia Twenhofel and an unnamed species from the Upper Ordovician of the Bighorn formation belong to this subgenus. The name Dinorthis was used first by Hall and Clarke, who considered its most important characters to be its reversed convexity, subquadrate muscle im- pressions, deltidium, and peculiar cardinal process. The same authors also proposed Plcesiomys, which fol- lows the description of Dinorthis and is characterized by having muscle-scars like those of Dinorthis but an exterior similar to that of Hebertella. It is clear from Fig. 18. — Diagram showing the ventral musculature of Dinorthis {Pionorthis) sola (Billings). their descriptions that they had O. calligramma in mind when differentiating Dinorthis and were more mindful of the external resemblances of Plirsiomys to Hebertella than of the internal similarities of Dinorthis and PlcFsiomys. In the description of the interiors of Dinorthis and Plirsiomys, Hall and Clarke emphasized exactly the same features for both of their genera. This is also borne out by later studies. In an examination of a large series of these shells the present writers have been unable to find any internal generic distinction between the two. Therefore to individualize Dinorthis and Plwsiomys external characters only are available. But here, too, there is considerable diflficulty in the way of a precise definition of the two genera. It was the con- tention of W^inchell and Schuchert that there was a gradation between the external characters of Orthis fectinella (genotype of Dinorthis) and O. suhquad- rata (genotype of Plirsiomys). This contention they embodied in the following words :^*' "Mem. 154, Geol. Surv. Canada, 1927 ( 1928), p. 181, pi. 17, figs. 10, 11. "Geol. Minn., vol. 3, pt. 1, 1895, p. 421. 96 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA From O. ■pectinella we pass to the variety sweeneyi, which is a local variation of it. Associated with the latter are specimens in which the strong plications begin to divide near the anterior margin. At the base of the Galena shales the strong simple, plicated forms become rarer, while those with more numerous striae prevail. Upon reaching the strata containing Clitambonites diversa Shaler, provision- ally known to the survey as Galena shales, the numerously striated form, here described as O. meedsi, is the only one found. Ascending into the Galena formation for thirty or forty feet more we find O. meedsi still exhibiting a tend- ency to increase the number of its striae [costella:] , and finally assuming characters (variety germana) which attain their greatest development in O. subquadrata of the Hudson River [Richmond] formation. The change from O. fec- tifiella to O. subquadrata is thus completed. Consistent with this argument they placed Plcesiomys in the synonymy of Dinorthis and the same relationship was continued by Schuchert in 1897 in the Synopsis of American Fossil Brachiopoda.®^ It is, however, doubtful that such a gradation between D. fectinella and Plcesiomys subquadrata actually exists because in the Chazy (possibly Black River) of Tennessee the Dinorthis s. s. of that region are already showing a marked divergence toward Pla-siomys. The D. trans- versa, D. quadriflicata, and D. interstriata all have interstitial or dichotomous ribs. Furthermore, the P. ifhigenia of the Black River and P. ulrichi of the Trenton are well established species agreeing in form and ornamentation with O. subquadrata. There is still another angle at which this problem of the distinction between Dinorthis and Plcpsiomys may be viewed, and that is, that D. fectinella and related forms represent the culmination of a trend which went in the direction of simplification of ribs. If this be true, some early Pleeslomys would be the ancestor of Dinorthis. The value of the name Pleeslomys has been variously interpreted. In 1911 Raymond^^ found it "best to retain the name Pleeslomys for the present, and apply it to such forms as O. subquadrata and O. flatys [our Multlcostella, described farther on], in which the sur- face is not coarsely plicated, but is covered with stria- tions [costellas] which increase by bifurcation and im- plantation." In 1913 Schuchert^^ regarded Dinorthis as a subgenus of Pleeslomys, but later reversed the relationship. It has been shown above that internally Dinorthis and Pleeslomys are inseparable and should be regarded as forming a single genus. But if the suggested evolu- tional trend toward the simplification of ribs in Dinor- this be correct, it would be desirable to continue the usefulness of Pleeslomys by considering it as of sub- generic value. On nomenclatorial grounds Dinorthis " Bull. 87, U. S. Geol. Surv., pp. 215, 308. ^*Ann. Carnegie Mus., vol. 7, no. 2, p. 239. ^° Zittel-Eastman Text-book of Paleontology, 2d ed., vol. 1, p. 382. has priority over Pleeslomys because it appears first in HaU and Clarke's book. Accordingly we suggest that Dinorthis be the generic name under which the whole assemblage of these related brachiopods be designated, and to bring out the tribal relationships we propose that this genus be divided into five subgenera as defined on an earlier page. Subgenus MARIONELLA Bancroft 1928 PI. 8, figs. 16-18 Bancroft, Mem. and Proc. Manchester Lit. and Philos. Soc, vol. 72, 1928, p. 181, pi. 2, figs. 13-16. Genoholotype. — M. typa Bancroft 1928. Description. Exterior. — Dinorthids of medium or small size, subelliptical in outline; hinge-line straight; cardinal extremities rounded. Lateral profile con- vexo-plane to convexo-concave. Anterior commissure faintly sulcate, sulcus shallow; ventral fold low, ob- scure. Interareas short, the ventral one apsacline, the dorsal orthocline. Ornamentation finely multicostellate. Ventral Interior. — Delthyrial cavity shallow; dental plates strong, flaring, continued as ridges about the periphery of the muscle field. Muscle area and pallial markings as in Dinorthis. Dorsal interior. — Notothyrial platform shallow; brachiophores of the Orthls type as developed in Dinor- this; cardinal process slender, simple, adductor field small. Geologic range. — The single known species, M. typa, comes from the Middle Ordovician of Wales. Distinguishing characters. — Marlonella differs from Dinorthis s. s. and Pleeslomys only in its much finer ornamentation and "the frequent, but not uni- versal, presence of a narrow (sharply pinched up) ventral mesial fold" (p. 181). Discussion. — The genus was first placed by its nomenclator as a member of his subfamily Harkness- ellinse, but this reference appears to us wrong. Ban- croft in a letter tells us that he now believes Marlonella to be a subgenus of Dinorthis. Specimens sent by him to us are molds of the interior and exterior which give no information regarding the structure of the shell, whether punctate or impunctate. Our presumption is in favor of an impunctate test because the other internal features fit best with this type of shell, for example the simple cardinal process. Characteristic dinorthid features occur in both valves as follows: In the ventral, the musculature and pallial markings are typical (see pi. 8, fig. 17). The car- dinalia are of the orthoid type. In young forms the crenulated myophore of the cardinal process is not visible in the narrow slots of the internal molds. Marlonella is a parallel development to Pleeslomys, evolved probably from the British Dinorthis. No shells exactly like it are known in North America, and be- cause of this independent development it deserves at least subgeneric designation. GENERA OF THE SUBORDER ORTHOIDEA 97 Genus VALCOUREA Raymond 1911 PI. 10, figs. 16, 19-23, 27-29; t. fig. 2 Raymond, Ann. Carnegie Mus., vol. 7, 1911, p. 239, pi. 35, figs. 15-19, pi. 36, fig. 1, t. fig. 12. Genoholotype. — Pltrslomys strophomeno'utes Ray- mond 1905, Amcr. Jour. Sci. (4), vol. 20, p. 370. Description. Exterior. — Shells strophomcnoid, hingc-linc wide and straight, cardinal margins rarely suhmucronate or subauriculate, commonly deflected; lateral profile strongly convexo-concave, anterior com- missure broadly uniplicate or faintly sulcate ; dorsal sulcus shallow, in some species obsolete at the front; ventral palintrope broadly triangular, apsacline to cata- cline or slightly procline, delthyrium open or closed by a deltidium; dorsal intcrarea shorter than the ventral one, orthocline or apsacline, notothyrium partially or completely covered by a chilidium; ornamentation multicostellate, fine elevated threads in the interspaces and crossing the ribs. Test fibrous, internally impunc- tate; costells e.xopunctate. Ventral interior. — Delthyrial cavity shallow, teeth strong, with accessory sockets (see t. fig. 2) ; crural fossettes oblique, deep; dental supports strong in young shells, obsolete in adults; cavities deep in juvenile individuals. Muscle area subpentagonal, wider and moderately bilobed in front; diductor tracks oblique, elongate, expanded in front; adductor tracks small, thin, elongate; adductor scars elongate, semielliptical; adductor track linear, enclosed by the diductor scars in front; adjuster scars prominent, situated at the base of the dental plates, as in Dinorthis. A small septum may be located at the base of the pedicle callist, or of the deposit under the deltidium. When a deltidium is absent a well marked pedicle callist is present. Pal- lial markings similar to those of Dinorthis, umbo-lateral spaces marked by radiating elevated ridges indicating the position of the ovarian bodies. Dorsal interior. — Notothyrial cavity shallow; car- dinalia confined to about the anterior one-fifth of the length of the valve, brachiophores forming the margin of the notothyrium, short, supported by a shell thick- ening beneath; sockets shallow, bounded by the slop- ing face of the brachiophore and an accessory tooth on the hinge margin. Cardinal process large, having a prominent shaft, and a crenulated myophore as in Dinorthis; chilidium present; median ridge short, extending as a rule approximately one-third the length of the shell. Geologic range. — Middle Ordovician (Stones River to Black River), chiefly of North America. American Species Dinorthis deflecta (Conrad) 1843 D.loricula (Hall and Clarke) 1892 D. recta (Conrad) 1843 D. (Valcourea) strofhomenoides (Raymond) 190 5 Strofhomena venlrocarinata Butts 1926 Valcourea magna, n. sp. European Species ? Orthis grandis (Portlock) Discussion. — The name Valcourea was proposed by Raymond for "impunctate orthids with reversed valves, strophomenoid habit, well developed deltidium, simple cardinal process, and finely striated [costellate] surface." It was differentiated from Plrrsiomys by its finer ornamentation and the presence of a deltidium throughout life. These orthids form a very remark- able convergence toward the genus Strofhomenn. They may be distinguished therefrom, however, by several important features: Externally they may be distinguished by the resupinate form of Strophomena and the lack of this feature in Valcourea. It is true that the relative convexity of the valves in the latter is reversed, that is, the dorsal valve has the greater con- vexity. However this may be, the umbo of the dorsal valve of Valcourea is always convex and there is a distinct beak. In Strophomena, on the other hand, there is scarcely any dorsal beak and the umbo is decidedly concave. It is the change in convexity from concave at the umbo to convex in the middle and front of the dorsal valve that distinguishes Stropho- mena. The external convergence of Valcourea toward Strophomena is not carried to such a degree that the shell is resupinate, although its convexity is reversed. In the ventral valve the subquadrate plan of the mus- cle field of Valcourea readily separates the two genera. The forked cardinal process of Strophomena is a fea- ture unknown in any orthid. Another difference is to be seen in the deltidia, that of Strophomena being apically perforate and that of Valcourea lacking a foramen. Internally, Valcourea and Dinorthis (Pla-siomys) are very similar, differing only in minor details of the pallial markings and ventral musculature. In the latter the diductor scars, although distinctly lobate in front, are not so strongly lobate as those of Valcourea, and the adjustors are much less divergent. The most notable difference, however, occurs in connection with the adductor impressions. Between the diductors of Valcourea is a double ridge which extends to the apex of the reentrant between the diductor lobes. Upon this double ridge are situated the adductor scars which together form an elongate suboval or lanceolate im- pression, widest toward the front. The adductor scars of Valcourea are situated at the front of the mus- cle field; this contrasts with the same impressions in PliFsiomys, which are located in the middle or at the back end of the field. The pallial impressions of Valcourea are much like those of Pla-siomys, but differ in being less distinctly impressed and less broken up into subsidiary rami along the front margins. Fur- thermore, the ovarian radial ridges are larger and more distinctly marked in Valcourea. 98 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA Valcourea appears to be best represented in the lower Middle Ordovician of North America where the genus first appears in the Stones River group and is last seen in the Black River. The range is short but the species are widely spread during this time. In Europe one species, Orthis grandis Portlock, seems to belong in this association. Valcourea is contemporaneous with D'm- orthts s. 5. and PlmsiomySy but dies out long before either of these tribes. Valcourea magna, n. sp. PI. 10, figs. 16, 20, 27-29 Shell large, wider than long, convexo-concave to convexo-plane ; dorsal valve having a faint sulcus which is lost toward the front of the shell. Ventral interarea long, strongly apsacline. Delthyrium open. Surface multicostellate, costellae crossed by fine ele- vated concentric lines. Ventral muscle field bilobed in front, pentagonal in outline. Pallial marks as in Dinorthis. This species is the largest of all the Valcoureas and, so far as known, does not have a deltidium. The con- cavity of the dorsal valve is less than is usual in other species. Measurements of the holotype, Cat. No. 779, Schu- chert Collection, Yale University: Length Thickness 37 mm. 17 mm. Width 47 mm. Horizon and locality. — Ordovician (Simpson), Criner Hills, Oklahoma. Genus MULTICOSTELLA Schuchert and Cooper 1931 (Lat. multiy many; costella, small rib) PI. 8, figs. 19,22,23,27; pi. 15, fig. 12 Schuchert and Cooper, Amer. Jour. Scl. (5), vol. 22, 1931, p. 244. Genoholotype. — Orthis ( ? ) saffordi Hall and Clarke 1892, Pal. N. Y., vol. 8, pt. 1, pp. 217, 340, pi. 5 A, figs. 38-40. Description. Exterior. — Shells semielliptical, an- terior margin convex; lateral margins convex or straight; hinge-line wide and straight; cardinal ex- tremities angular or obtuse; lateral profile subequally biconvex; anterior commissure rectimarginate to sul- cate; fold and sulcus absent, faintly or strongly devel- oped; interareas nearly equal in length, the ventral usually slightly longer; ventral interarea apsacline, umbo convex, delthyrium open; dorsal interarea or- thocline or faintly anacline; umbo very gently con- vex, usually sulcate. Ornamentation multicostellate, interspaces with elevated growth-lines; test fibrous, impunctate. Ventral interior. — Delthyrial cavity shallow; teeth small; dental plates strong, advancing; small pedicle callist in the apex; crural fossettes shallow, oblique; muscle area nearly one-third the length of the shell, about as long as wide, bilobed in front; diductor scars elongate, expanded in front; adductor tracks elon- gate; adjustor scars short, narrow, slightly expanded in front. Elevated, elongate oblique lines occupy the lateral spaces next the cardinal cavities. Dorsal interior. — Notothyrial cavity shallow; sock- ets shallow; brachiophores stout, blunt, supported by the thickening of the notothyrial platform; cardinal process stout, myophore crenulated ; median ridge low, short; adductors not deeply impressed on the shell. Geologic range. — Middle Ordovician (Chazy) of North America. Species Dinorthis flatys (Billings) 1859 Plusiomys brevis Willard 1928 P. «/07z^d/?ibonites, as Kozlowski has done, but the dorsal valve shows departures too radical to justify this reference. In the dorsal valve, the cardinalia and the muscle impressions are the chief structures of interest. The "cardinal process," so called, consists of four distinct parts: (1) the chilidium, (2) the cardinal process proper, (3) the brachiophores, and (4) the "crura." ( 1 ) The chilidium is a thick, subcircular plate lying over the base of the cardinal process, bent ventrally on both sides and also dorsally at the front end. It reminds one much of the chilidium of Productorthis (see pi. 4, fig. 16). (2) The cardinal process is more difficult to indi- vidualize. At its front end it is crescentic or lunate in outline, and extends under the ventral beak. In some specimens its ventral surface bears a rather deep groove. However, the process is clearly single, never being bifid at its front end as in Derbyia, Triflesia, or Meekella. In other words, the cardinal process, although much modified, is essentially orthoid. (3) The brachiophores show as obscure plates con- vergent toward the roof of the valve and possibly uniting at their bases. These have been crowded together and have fused with the cardinal process at their posterior extremities (see pi. 14, fig. 30). The sockets are deep and excavated into the shell substance, and bear a small tooth on the outer wall above the excavation noted before. The curved posterior ex- tremity of the tooth fits into the excavation and the small tooth articulates with the deep socket on the posterior face of the ventral tooth. (4) The crura are long processes extending from the brachiophores on each side of the cardinal process. They have a slight hollow groove toward their dorsal ends and appear as folded plates. The taxonomic position of Lyco-phoria is difficult to express. Lahusen would place it with the Stropho- menida:, but in current classification it has been allied doubtfully with the Parastrophininas. Kozlowski^^ would place it with Poramhonites in the Porambon- itidffi. Strophomenoid affinities are said to occur chiefly in the cardinal process, which, according to Hall and ClarkeV^ section is described as strongly bifid. How- ever, the writers have been unable to find a bifid car- dinal process. If Hall and Clarke's section were cut obliquely across the beaks so as to bevel the cardinal process transversely or obliquely, the tapering of the antero-ventral ends of the ventral groove would give the appearance of a bifid process. None of Lahusen's figures, nor any of the excellent interiors in the Schu- chert Collection, give any indication of a bifid cardinal process. Hall and Clarke thought that the cardinal structure allied this genus to the orthids but that "Atrypa micella adds to these orthoid features the cardinal process of a streptorhynchoid, like Triplecia and Mimtdus, thus presenting another point of tan- gency between these shells and the pentameroids." Schuchert has persistently classified this genus, al- though with a query, with the pentamerids, allying it always with the Parastrophininas. However, there is no pentamerid feature in Lycophoria save the biconvex valves. Kozlowski placed it in the Porambonitidas, referring this family to the Telotremata. It has been shown — and a glance at the figures (pi. 14) will con- firm this — that no characteristic pentameroid features occur in Lycophoria. Furthermore, we have removed the Porambonitidae from this association and referred them to the Orthacea. The same reference, but as a separate family, is suggested for the genus now under consideration, since it shows orthoid features in the ventral interarea, open delthyrium, and discrete dental plates. In the dorsal valve it has been shown that the cardinalia have developed, by growing inwardly and fusing with the brachiophore supports, a cardinal process and chilidium. These features could be devel- oped by the lateral crowding of such cardinalia as are seen in Productorthis, but we are not holding that Lycophoria descended from that genus. The dorsal valve of Lycophoria thus forms a con- vergence toward the type of cardinalia so well known in Meekella and Derbyia of the Carboniferous. Lyco- phoria and Productorthis of the Middle Ordovician are therefore two stocks that have independently orig- inated characters that are re-developed in other stocks {Derbyia and Productus) in the Pennsylvanian. "Pal. Polonica, vol. 1, 1929, p. 131. " Pal. N. Y., vol. 8, pt. 2, pi. 62, fig. 5 3. GENERA OF THE SUBORDER ORTHOIDEA 107 Superfamily CLITAMBONACEA Schuchert 1929, emended Specializing and terminal Orthoidca probably devel- oped out of the Billingsellidx and retaining as a rule the primitive features of deltidia and chilidia, here much enlarged, and developing either pseudospondylia (more rarely) or spondylia simplex. Test impunctate. deltidia and cardinalia as in Vellamo and Clitambomtes, but without spondylia and with discrete chilidial plates that may be remnants of chilidia. This small family of two genera is restricted to the Lower Ordovician (upper Canadian) of the United Tables Of European origin and later dispersion to North America Clitambonitin^ Vellamo DELTATRETIDiE Polytoechia Deltatreta Pomatotrema » Estlandia Hemipronites BiLLINGSELLIDiE Ingria Range, throughout the Ordovician and rarely in the Silurian. The phylogeny of this small superfamily is believed to be as shown in Table 8. Family DELTATRETID^E Schuchert and Cooper 1931 Primitive Clitambonacea that probably originated in the Billingsellid.-c. Largely orthoid in expression, with States and Canada. It forms a morphologic connec- tion between the orthids on the one hand and the large family Clitambonitida on the other. It includes the two genera Deltatreta Ulrich and Pomatotrema Ulrich and Cooper. Internally the dorsal valves are like those of Vellamo and Clitambonites, but there are chilidial plates instead of a chilidium. In the ventral valve there is no spon- dylium, but in old shells of both genera there may be a pseudospondylium. The deltidium of both genera is exactly like that of the Clitambonitidae. 108 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA Genus DELTATRETA Ulrich 1926 PI. 6, figs. 10, 11, 13-15, 19, 30 Ulrich in Butts, Geol. Surv. Alabama, Spec. Rept. No. 14, 1926, p. 100, pi. 18, figs. 1-+ (no description and no type selected). Syn. Deltorthis Ulrich, in Poulsen, Meddel. om Gronland, Bd. 70, 1927, p. 297, pi. 20, fig. 4. Genolectotype. — Deltatreta fillistriata Butts 1926. Description. Exterior. — Resembling Pionodema externally, subelliptical, hinge-line straight, usually nar- rower than the greatest shell width; cardinal extremi- ties obtusely subangular. Lateral profile unequally biconvex, the ventral valve usually having the greater convexity. Anterior commissure rectimarginate. Ven- tral palintrope long, curved, moderately or strongly apsacline; delthyrium closed by a strongly convex deltidium which is perforated near the apex by a large foramen like that of Vellamo and the Clitambonitidas. Dorsal interarea short, anacline ; notothyrium partially closed by discrete chilidial plates. Surface multicostel- late; shell structure fibrous, impunctate. Ventral interior. — Delthyrial cavity deep ; teeth strong, crural fossettes prominent ; dental plates strong, extending directly to the floor of the valve, defined by deep umbonal cavities. Muscle marks strongly im- pressed on a callus which grows under them and spreads about the inner bases of the dental plates, form- ing an incomplete pseudospondylium. Diductor scars elongate, situated near the base of the dental plates, not extended in front of the adductor track, the latter occupying the space between the adductor impressions and in mature and old shells continued forward as a low ridge. Lateral spaces marked by subreniform ovarian impressions as in Orthis s. s. and Hemipronites. Dorsal interior. — Cardinalia confined to the vicinity of the hinge. Cardinal process orthoid, a linear septum. Brachiophores orthoid, supported by lateral swellings of adventitious shell exactly as in Vellamo and Clitambonites. Geologic range. — Lower Ordovician (Canadian) of the United States and Canada. Species D. fillistriata Butts 1926 D. two n. spp. Ulrich MS. Billingsella dice Walcott 1 90 5 Discussion. — The combination of characters in this interesting early Ordovician shell unites features seen in the later Orthidx and the Clitambonitidae. Orthoid features are the ovarian markings and discrete dental plates, clitambonitoid ones are the apically per- forate deltidium and the brachiophores supported by lateral extensions of adventitious shell as in Vellamo. We will first describe in detail the morphology of this genus and then discuss the peculiar nomenclatorial questions involved in the choice of a genotype. Ex- ternally Deltatreta has the appearance of Pionodema in both outline and profile and the shell is finely multi- costeUate with the ventral palintrope curved and apsa- cline as in the last named genus. The ventral inter- area is marked by fine, elevated lines parallel to the posterior margins of the shell. Such elevated lines are also characteristic of Polytcechia. Deltatreta is unlike the later Pionodema in its possession of a perforate del- tidium. Such a deltidium is unknown so far in any of the punctate shells such as Pionodema. The deltidium of Deltatreta is strongly convex, decidedly like the same feature in Clitambonites, and unlike that of Bill- ingsella since the apical foramen is very large, and not uncommonly the foraminal margins are flexed or turned outward. In the ventral valve the musculature and pseudo- spondylium are variable, depending on the age of the specimen. In young shells there is no vestige of a pseudospondylium, the adductor and diductor tracks are rather wide, and there is a prominent pedicle callist showing the inner attachment of the pedicle (see pi. 6, fig. 30). In old shells the space occupied by the adductor field is strongly elevated on adventitious shell, which is not uncommonly prolonged forward to the middle of the valve as a thick median ridge (pi. 6, figs. 13, 15). The latter may nearly completely cover the floor of the delthyrial cavity, forming a rather prominent pseudospondylium. The floor of the lateral spaces and umbonal cavities is marked by low radiating ridges, probably markings of the attachments of the ovarian bodies. In one specimen (see pi. 6, fig. 15) the ovarian impressions are subreniform as in Orthis s. s. or Hesferorthis. The palintrope and dental plates are not unlike the corresponding structures in Hes-per- orthis. The palintrope is long, and the teeth are placed considerably outside of the delthyrial margins. They are rather small but have prominent crural fossettes. The deltidium develops as in Hesperorthis and Clitam- bonites. It is an arch built with its sides attached to the ventral surface of the narrow flange of the palin- trope which overhangs the delthyrial margins. The scar of pedicle attachment is not confined to the callist at the apex but can be traced on the sides of the dental plates and on the ventral surface of the deltidium; on the dental plates its margin runs from the antero- lateral extremity of the callist as a low ridge in an antero-dorsal direction, nearly parallel with the sloping edge of the dental plates, and then passes to the ventral surface of the deltidium some distance behind the foramen. The dorsal valve is usually less convex than the ventral and has a shorter palintrope, and the notothy- rial cavity is rather narrow and shallow. The noto- thyrial platform is inconspicuous and bears a simple cardinal process which is typically orthoid. The brach- GENERA OF THE SUBORDER ORTHOIDEA 109 iophores arc likewise orthoid and supported by adven- titious shell deposited on their inner and dorsal surfaces. This adventitious material may be extended laterally for some distance (see D. dice (VValcott)) so that the cardinalia of Deltatretn resemble markedly those of Citttimhonites. The median ridge given off from the notothyrial platform extends nearly to the middle of the valves as is usual in orthoid shells. The sockets are the cavities formed by the envelopment of the brachiophores by adventitious substance and by the sloping outer face of the brachiophore. Deitatreta, like many orthoid brachiopods, possesses an incomplete chilidium composed of two discrete chi- lidial plates. These are low, extending from the mar- gins of the notothyrium, and covering its sides; they are extensions built on the edges of the brachiophores along the delthyrial margins and strengthened by ad- ventitious shell deposited on the inner face of the brachiophores. The specimens at hand are not favor- ably enough preserved to show the dorsal musculature. This discussion of the genus Deitatreta has been pre- pared from material loaned by Dr. E. O. Ulrich and the U. S. National Museum. The specimens are all from the upper Canadian of Alabama and Oklahoma. We also include in this genus Billingsella dice Walcott, found in the drift near St. Albans, Vermont, which is very close to Deitatreta^ n. sp. (see pi. 6, fig. 1 5 ) from northeast of the Wichita Mountains, Oklahoma. Turning now to the nomenclatorial difficulties, the first use of the name Deitatreta was by Ulrich in a faunal list prepared by him for Purdue and Miser,'* the name appearing as "Deitatreta of. electro Billings" and unaccompanied by any description or figure. This citation is then clearly a nonien nudum and as such has no standing.*" In 1926 Butts used Deitatreta as of Ulrich for shells collected in Alabama and Oklahoma. Two species were named and both were accompanied by illustrations and one of these has a short statement of Ulrich's conception of the genus. Since no type has ever been definitely designated for Deitatreta, we here select D. fillistriata Butts. This species has been chosen instead of D. elegantula Butts which appears first in the report (page 99, in reference to the figures on plate 18), because there is no characterization of the latter species and in addition it is clearly not typical of Deitatreta according to the views expressed for D. fdlistriata and as indicated by Ulrich's specimens in Washington. In the plate legend (pi. 18, figs. 1-4 [p. 100]) for D. fillistriata, Butts states clearly that "U. S. Geol. Surv., Geol. Folio 202, 1916, p. 5. "* The recent Schuchert-LeVcne catalogue cites the geno- type as "D. cf. electra Ulrich ? = Orlhis electra Billings," with bibliographic reference to the last named species. Since, however, the name as written by Ulrich is a doubt- ful identification, it can not be held that he intended to place Billings' species as the type of the genus. See Rules of Nomenclature, Art. 30, 11 e^. Ulrich has proposed the name Deitatreta for shells having a deltidium. Hence D. elegantula, which has no deltidium, is obviously not typical of Deitatreta as conceived of by Ulrich. We refer it doubtfully to our new genus Paurorthis. Genus POMATOTREMA Ulrich and Cooper, n. gen. (Gr. pntna, cover; trciiia, hole) PI. 16, figs. 14-16, 18-21,23, 26,31 Genoholotype. — p. muralis Ulrich and Cooper, n. sp. Discussion. — On plate 16 the writers figure a shell that has been known as "Orthisina" grandcrva, being first referred to Orthisina and later by Hall and Clarke to Billingsella. However, it does not belong to either of these, but is of a new genus which is not uncommon in the Lower Ordovician. We had provisionally re- ferred this species to Tafia, but after the junior author had studied interiors of the genotype, Taffia planocon- vexa, at Washington (unfigured by Butts) he found that the group under consideration {Pomatotrema) has nothing to do with Taffia as figured by Butts. It was Ulrich's intention to affix the name Taffia to shells similar to if not identical with O. granda-va, but in the Alabama report unfortunately T. flanoconvexa was figured and this of course fixed the genotype of Taffia. Being so established, Taffia is not congeneric with Orthisina gratidcrva, but is a member of the family Orthidas. The junior author discovered the differences be- tween Taffia and Pomatotrema after he had removed from Yale University to the U. S. National Museum, and after he had agreed to collaborate with Dr. E. O. Ulrich in a study of the Ozarkian and Canadian brachiopods. Consequently the new genus Pomato- tre7na born by the above union bears as its author the names Ulrich and Cooper, with P. muralis Ulrich and Cooper as the genoholotype. The following discus- sion of the genus is based on material in the U. S. National Museum and the figured specimens from Newfoundland in the Schuchcrt Collection at Yale. Distinguishing characters. — Plano-convex in lateral profile, with perforate deltidium, chilidial plates, and subparallel or vcntrally convergent dental plates. Pomatotrema muralis Ulrich and Cooper, n. sp. PI. 16, figs. 15, 20 Shell small or medium-sized, nearly as wide as long, cardinal extremities nearly a right angle. Ventral interarea long; delthyrium longer than wide, covered by a prominent convex deltidium which is apically per- forate as in Vellamo. Notothyrium partially closed by chilidial plates. Surface multicostellate, about 3 cos- tellae to 1 millimeter at the front of the shell. Dental no GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA lamellx strong, adductor field lanceolate, enclosed by the subcrescentic diductor scars. Pallial trunks as in Orthis s. s.y bounding subreniform ovarian impressions. In the dorsal valve the brachiophores are simple rods supported by lateral growths of shell substance as in Vellamo. Cardinal process simple. Adductor field subflabellate, anterior scars the smaller. Inside the margin of the valve is a thick subperipheral rim. To this genus also belong Orthisina grandwva Bill- ings, and Clkatnhonites semiconvexus Poulsen. Horizon and locality. — Ordovician (Cana- dian), SW 14 Sec. 2, T 1 S, R 1 W, about 4 miles east of Hennepin, Oklahoma. Genus POLYTCECHIA Hall and Clarke 1892 Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 1892, p. 239, pi. 7a, figs. 26-30, t. figs. II, 12. Horn. Waagenia Hall 1889. Genoholotype. — Hemifronkes aficalis Whitfield 1886, Bull. Amer. Mus. Nat. Hist., vol. 1, p. 300, pi. 24, figs. 1-5 (non Orthis? apicalis Billings). Original description. — Shell small, subtrihedral in contour. Hinge-line straight, about equaling the diameter of the shell. Pedicle-valve with a high, nearly vertical car- dinal area marked with oblique striations parallel to the lateral margins. Delthyrium covered by a narrow, convex plate; the presence of a foramen not determined. On the interior the dental lamellx are widely separated, and descend along the umbonal cavity for a short distance verti- cally, thence bending sharply inward and meeting at a low angle in the median line; thus forming, with the del- tidium, a conspicuous subrostral vault. This inner spoon- shaped plate, spondylium, is supported by a stout median septum, and two smaller lateral septa, which meet it at the lines of angulation ; the former of these extends for the entire length of the plate, while the latter is free from the accessory septa near its anterior edge. The umbonal cavity of the valve is thus divided into five chambers, and in the lateral chambers there is still another septum, lower than the rest and not extending to the spondylium. The brachial valve is shallow and depressed-convex, with a nar- row cardinal area. The delthyrium is very broad with a partially developed covering, the dental sockets are widely separated, the crural plates narrow and nearly parallel to the hinge-line. The cardinal process is simple, linear and quite prominent, and at its union with the crural plates is a subtriangular thickening which is supported by a low median septum. Surface covered with fine, elevated, radiat- ing strise, without evidence of median fold and sinus. Geologic RANGE. — Canadian (Beekmantown) and Chazy? American Species Hemifronites aficalis Whitfield 1886 Polytoechia symmetrica Butts 1926 P. ? oaiensi's Butts 1926 Discussion. — The multicamerate apical ventral in- terior of this genus is characteristic of it, and shows a convergence toward similar structures in Gonambo- nkes s. s. and Clarkella. The genus, however, can not be placed in the Syntrophiidae because of its ex- ternal sculpture, shape of the valves, and internal features of the dorsal valve, which are evidently close to those of Vellamo or Deltatreta. The presence of a deltidium and chilidium are further features linking this genus intimately with the Clitambonitids or Deltatretidas. Polytcechia appears to be an American genus only, and for this reason its relationships to Clkanibonkes s. s. are doubtful. The ensemble of the shell ornamenta- tion, profile, etc., suggest Deltatreta, and were it not for the multicamerate apex it would definitely belong here. The position we assign the genus is tentative. Family CLITAMBONITID^ Winchell and Schuchert 1893 Divergent specialized Protremata derived out of the Orthacea, having well developed deltidia and chilidia. In the ventral valve the muscles are usually borne on a spondylium simplex. In the dorsal valve the car- dinalia are of the Orthis type, supported by lateral ad- ventitious shell growths. Ventral, pallial, and ovarian markings essentially the same as those of Orthis. Discussion. — The members of the Clitambonitidas, prolific in variable individuals, have until recently been classified in the Pentameracea, but Schuchert in 1929^' made them the basis of a new superfamily, the Clitam- bonacea. This he did when he saw that the stock had a genetic origin independent of the Pentameracea, as is plainly shown by their different type of spondylia. It is true that the Clitambonacea are closely related to the Orthidje, but as they are not in the main line of brachiopod evolution, being a specializing stock, and since most of them have peculiar deltidia and as a rule strong chilidia, and more or less well developed simple spondylia, it is thought advisable to recognize these tendencies and to separate them from the Orthacea as another superfamily, Clitambonacea. Ventral valve, — The chief points of departure from the usual orthid structure in the Clitambonitidas are in the spondylium and deltidium. The former is a spoon-shaped plate usually resting on the floor of the valve at its posterior end but elevated on a median septum at the front end. In a few forms the septum may be absent or may be so abbreviated as to be virtu- ally absent. The junction of the upper ends of the spondylium with the under side of the palintrope is marked by a distinct suture at which the growth-lines of the inner surface of the spondylium end abruptly. The delthyrial margin is an elongate triangular area "Foss. Cat., Pars 42, p. 15. GENERA OF THE SUBORDER ORTHOIDEA 111 sloping slightly vcntro-latcrally and formed by the inside surface of the teeth as they grow forward. It is to this surface that the deltidium is attached. The upper surface of the spondylium is marked by concentric lines which represent periods of no growth. Superimposed upon these are longitudinal lines which are taken to represent traces of muscle markings. No signs of muscle attachment occur anywhere in the valve except on the inner surface of the spondylium, making it evident therefore that the function of this structure was one of muscle anchorage. However, the indi- vidual muscle marks have not been ascertained. In one specimen were seen four divergent longitudinal ridges on the upper surface of the spondylium. The strongest two are at the bottom, with the other ones some distance on the sides. It is deduced from the above that the adductor muscles occupied the central part of the spondylium over the median septum, and the diductors and adjus- ters were located laterally on the sides of the spondy- lium. The scar of attachment of the pedicle was probably located at the rear. In Hemipronites a slight callosity at the apex indicates the scar of pedicle attach- ment. Deltatreta has a prominent pedicle callist at the apex. The septum consists of a single piece (euseptum) and is usually prolonged to the middle of the valve or beyond. To this ensemble, consisting of a spoon- shaped plate and euseptum, Kozlowski*" has applied the name spondylium simplex. The deltidium of the Clitambonitids is always a convex plate covering the delthyrium. Its sides are buttressed against the upper surface of the ridge formed by the growth of the teeth and are strengthened further by deposits of callus on the sides of the teeth (pi. 7, fig. 14), and in some forms on the walls of the spon- dylium. The upper layer of shell on the interarea appears to be continuous with the outer surface of the deltidium. The deltidium in these forms is usually perforate and far less commonly imperforate. Usually there is a large foramen located near the apex, which suggests that this opening functioned, at least during life, as the pedicle opening. However, in some genera and species the apical foramen was sealed at or before maturity, thus doing away with this structural feature (see pi. 8, fig. 8). The apical foramen is especially well devel- oped in VellamOy and here appears to have persisted throughout life, at least in the Trenton forms. Many of the specimens from Minnesota have an elevated flange or rim about the foramen and from its position at the apex it would appear that the deltidium is formed around the pedicle by the mantle. In the ventral valve the ovarian and pallial impres- sions, where visible, are similar to those of Orth'ts and Hesperorthis. As shown in Hemipronites, a pallial " Pal. Polonica, vol. I, 1929, p. 124. sinus extends from the outside extremities of the spon- dylium in the position of the diductor tracks, along each side of the median ridge, bending abruptly later- ally at the front (see pi. 8, fig. 10). These sinuses bound subreniform ovarian areas marked by elevated wavy ridges exactly as in the Orthidx. In some genera these markings arc much less distinct but in all they strongly suggest direct affiliation with the Orthidae. Dorsal valve. — The pattern of the dorsal valve in most of the genera is alike. The brachiophores are flattened or rodlike divergent plates forming the mar- gins of the notothyrium and extending laterally into the visceral cavity of the valve. In adults the noto- thyrial platform is greatly thickened by adventitious shell which extends laterally onto the inner surfaces of the brachiophores and around their dorsal surface. This gives the brachiophores the appearance of being laterally prolonged plates when actually they are short and hidden within the extra testaceous matter. The postero-lateral face of the brachiophore, however, always forms the inner wall of the socket, while on the outer wall a denticle can usually be seen which articulates with the socket in the postero-lateral face of the tooth. The chilidium is formed similarly to the deltidium. It is a convexly arched plate built against the sides of the notothyrium and cemented by deposition of adven- titious shell under the brachiophores and under the anterior side of the chilidium. Not infrequently adven- titious deposit is laid on the ventral surface of the brachiophore and continuously with the front margin of the chilidium. The precise function of the chilidium is, at present, not known, but it may be of use to the animal in protecting the muscle attachments on the myophore of the cardinal process. The musculature of the dorsal valve is like that seen in Orthis, Hesperorthis, and other orthids. There are four adductor scars divided centrally by a stout median ridge and horizontally by ridges at right angles to the median elevation. The median ridge, its lateral ridge, and the continuous curved ridges of the chilidium and lateral thickenings under the anterior face of the brach- iophores give the appearance of an anchor when the shell is inverted with the posterior toward the observer. This analogy was emphasized by Pahlen*^ in his study of the genus Orthisina. The anterior pair of adduc- tors are not uncommonly divisible into two separate scars, as in Productorthis. The cardinal process is simple, orthoid in structure, and may be continued posteriorly so as to unite with the inner surface of the chilidium. Pallial markings are usually not clearly visible in the dorsal valve. Most shells show two oblique ridges extending antero-later- ally from the adductor pits to a little beyond the anterior margins of the muscle field. " Mem. .Acad. Imp. Sci. St. Petersb., 7th ser., vol. 24, no. 8, 1877, p. 7. 112 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA Affinities. — Kozlowski^* has recently shown that the spondylium of the pentamerids, such as Pentamerus and Conchidium, has had a different origin from that of Clitambonites and Skenidium. The polyphyletic origin of this peculiar muscle platform is clear evidence of the separate origin of the two groups. Kozlowski has pointed out that the prominent development of the deltidium and chilidium in the Chtambonitidffi sug- gests a close relationship with the Strophomenacea, but the presence of an orthoid cardinal process and other features of the Orthidas shows stronger affinities with the Orthacea. Before Kozlowski's paper appeared in this country, the writers had also come to the conclu- sion that the Clitambonitidas were primitive, aberrant, and specialized orthids. We saw this relationship in the ventral pallial and ovarian markings, the simple septum-like cardinal process, the Orthis-Wke brachio- phores and adductor field. We now know that the presence or absence of a deltidium or chilidium is in itself not of such great significance as was formerly believed, since these features appear spasmodically in many of the later genera and families. However, an important feature of the clitambonitid deltidium is its prominent apical foramen. Kozlowski has looked with probable correctness to the Billingsellidae as the ances- tral stock of the Clitambonitidas, and in this we agree, since the Billingsellida: have all the necessary structures by which such a transition could be accomplished. The writers also independently of Kozlowski saw the need of separating Skenidium and its allies from the Clitambonitidae. Skenidium has no deltidium or chilidium, and in this respect is very specialized. The internal differences, and especially the nature of the cardinalia, link Skenidium with the Plectorthids rather than with the Clitambonitidae. Skenidioides, probable progenitor of Skenidium, undoubtedly originated in Finkelnburgioy since the cardinalia of the latter are like those of the Plectorthidae. Opik*^ also points out that the Clitambonitidae in structure are foreign to the Pentameracea and places them close to his new family Plectambonitidae. The latter family has, however, a closer relationship to the Orthacea than to the Strophomenacea, as is shown in the nature of the cardinal process and other internal structures, and it is not unlikely that more study will establish it as an aberrant and terminal division of the Orthacea. We consider the Clitambonitidae as special developments, probably from the same ances- tral stock, namely, the billingsellids. The large family Clitambonitidae is divisible into the following subfamilies: Plectellinae Opik Clitambonitina Schuchert and Cooper Gonambonitinse Schuchert and Cooper "Op. cit., pp. 122-125. *' Acta et Comment. Univ. Tartuensis, A, vo]. 17, pt. 1, 1930, p. 60. Subfamily PLECTELLIN^ Opik 1930 The Plectellinae are primitive Clitambonitidae with- out spondylia, which apparently gave rise to the special- izing Clitambonitidae of Europe. In his splendid monograph on the Estonian Pro- tremata, Opik*^ has proposed a family Plectambonitidae which he defines as follows (in translation) : This family embraces all Strophomenacea with a simple cardinal process and strophomenoid habit, thus with con- cavo-convex or convexo-concave shells. All others, with a double cardinal process, form the family Strophomenidse. Heretofore the members of our new family have been combined with the Rafinesquinina: and were placed near the beginning of this subfamily. Opik divides his new family into three subfamilies, the Plectellinje, Plectambonitinae, and Sowerbyellinae. We shall concern ourselves here chiefly with the Plect- ellinae, which we believe are very close to the Del- tatretidae in structure. The Plectambonitinae and Sowerbyellinae in their simple cardinal process show re- lationships with orthoid brachiopods rather than with the Strophomenacea where they have persistently been classified. Here again external form appears to have been the chief guide in classification. Within the Plectellinae Opik places Plectella Laman- sky, Ingria Opik, and PaltFostrofhomena Holtedahl. He believes in the uniqueness of Plectella and in this he has the support of Kozlowski, who maintains this to be a good genus. Schuchert (1929)*' unites Plectella with Plectamhonites Pander, but in this the two European authors do not concur. In our study of these genera we are wholly dependent upon the litera- ture and hence our conclusions may be in error. We regard Palceostrofhomena as a possible member of the Dinorthidae, close to Valcourea. The two remaining genera are characterized by having a chilidium and an imperforate deltidium. Fine radial lines mark the out- side of the shells and there is no spondylium. In the dorsal valve of both Plectella and Ingria the cardinal process is orthoid, not plectambonitoid. In other words, it does not form a tentlike structure as in Sozverbyella. Furthermore, the brachiophores are orthoid as in Deltatreta and are supported by lateral extensions of the notothyrial platform as in Vellamo and other Clitambonitina. It is our contention, then, that these shells are aberrant Clitambonitidae, without spondylia. Development of the Plectambonitinae and Sowerbyellinae may have come from this line. In America, Plectella and Ingria find their nearest rela- tives in Deltatreta and Pomatotrema. These two genera differ from the Plectellins in the presence of an apical foramen and incomplete chilidium only, all other structures being in agreement. *" Acta et Comment. Univ. Tartuensis, A, vol. 17, pt. 1, 1930, pp. 55-58. " Foss. Cat., Pars 42, p. 98. GENERA OF THE SUBORDER ORTHOIDEA 113 Subfamily CLITAMBONITIN,-E Schuchcrt and Cooper 1931 (^ Orthisinin.T Waagcn 1884) The typical and most prolific subfamily of the Clitambonitida?, having a spondylium simplex. Em- braces the following genera: Cl'ttamhonites Pander Vellamo Opik Clinambon, n. gen. Eitlattd'm Schuchert and Cooper Hemipronites Pander Pahlenella Schuchert and Cooper Apomatella Schuchert and Cooper Species (the first thirteen of Pander 1830) Proniles aduenden?* P.dta P. convexa P. excelsa P. humilis P. la/a P. oblonga P. flana P. fracefs P. frarufia P. rotunda P. tetragona P. transversa Orthisina schmidtiVMcn 1877 ? Orthisina concava Pahlen 1877 Genus CLITAMBONITES Pander 1830 PI. 7, figs. 17, 19-23,26 Pander, Beitr. Geogn. Russ. Reiches, 1830, p. 7U, pi. 17, fig. 6. Syn. Proniles Pander, Ibid., p. 71, pi. 17, fig. 6. Syn. Orthisina D'Orbigny, Compt. Rend., Acad. Sci. Paris, vol. 25, 1847, p. 267. Genolectotype (Dall, and Hall and Clarke). — Pronites adscendem Pander 1830. Description. Exterior. — Shell semielliptical, hinge-line less than the greatest width of the valves; lateral profile biconvex or convexo-concave; dorsal valve convex; ventral valve usually subpyramidal; anterior commissure rectimarginate, dorsal valve sul- cate in youth only. Ventral interarea the longer, strongly apsacline to procline; deltidium prominent, rarely perforate; dorsal interarea the shorter, usually anaciine; chilidium well developed. Surface multi- costellate, commonly imbricate and spinose. CostelL-E crossed by elevated concentric lines of growth. Test fibrous, impunctate. Ventral interior. — Provided with a spondylium sim- plex which is as wide as long or longer than wide, the sides usually flexed inward and the base usually flat. The median septum extends to about the middle of the valve. The musculature was evidently confined to the upper or inner side of the spondylium, the adductors probably at the base, the diductor and adjuster scars (if any) on the sides. Dorsal interior. — Notothyrial cavity small, cardinal process linear, brachiophores orthoid, supported by lateral extensions of the notothyrial platform, forming, with the median septum, an inverted "anchor." Ad- ductor field quadripartite; anterior adductor set the larger. Geologic range. — Lower and Middle Ordovi- cian of Europe. Discussion. — The features best characterizing Clitambonites are (1) the subpyramidal ventral valve, (2) the convex dorsal valve; (3) the inclination of the ventral interarea; and (4) the external ornamen- tation. The ventral exterior is usually slightly convex or nearly flat, but in one exceptional species (C. con- cava) it is concave. The ventral interarea is usually catacline or decidedly procline, the angle from the hori- zontal being not uncommonly more than 80', but it may also be very strongly apsacline. The dorsal valve is always more or less strongly convex, always less deep than the ventral one, with the greatest con- vexity usually located at the middle of the valve. The most striking external feature of Clitambonites, however, is the sculpture, which sets it apart at once from Vellamo, the shell of the former being strongly lamellose, each interval of growth making a distinct layer ("like steps or roof-tiles," Hall and Clarke, p. 236), and the costella produced forward as flanges, so that there is a costellate border about the margins of the shell when they are viewed from the inside, somewhat as in A try pa (see pi. 7, figs. 19, 20). This condition is approached in Vellamo by V. squamata, but in that form there are differences in the spondylium and external form that prevent its being placed in Clitambonites. The deltidium of Clitambonites is typical of all this family, namely, wide and highly convex and perfor- ated by a large pedicle foramen. It is interesting to note, however, that the foramen in the apex of mature shells is rarely visible, being sealed up during early maturity. It is not uncommon that the internal sur- face of the deltidium is strengthened by a longitudinal '■' Verneuil (Geology of Russia) placed all of Pander's species of Proniles except P. oblonga under the name Orlhis adscendens. From our present point of view, it seems apparent that \'crneuil never understood Pander's discrim- inating work, and due to his great influence unjustly sub- merged many of the species and generic concepts of the earlier investigators. 114 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA thickening or low septum. The spondylium is sessile at its posterior extremity but is elevated on a septum in its median portion and free at the front end. The umbonal cavities commonly have some filling but never to the degree shown by Vellamo diversa from Anticosti. It was Hall and Clarke^* who revived Pander's " Klitamhonites" as a generic term, and restricted the name, as they thought, "pretty nearly" to Pander's term Pronites. They, however, did not appreciate the features of Gonambonites, nor of the tribe of Orthisina verneuili which has been recently made the genotype of the genus Vellamo by Opik. Genus VELLAMO Opik 1930 PI. 7, figs. 5, 8-11, 14-16, 18, 24, 25, 27-32; pi. 29, figs. 8, 14 Opik, Acta et Comment. Univ. Tartuensis, A, vol. 17, pt. 1, 1930, p. 212. Genoholotype. — Orth'ts verneuili Eichwald 1841, Urwelt Russlands, II, p. 51, pi. 2, figs. 3-5. Description. Exterior. — Shell narrowly semi- elliptical, hinge-line straight, as wide as or wider than the total width of the shell; cardinal extremities usu- ally acutely angular. Lateral profile piano- to slightly concavo-convex; anterior commissure rectimarginate to broadly sulcate. Ventral palintrope long, strongly apsacline to catacline but never procline. Dorsal pal- intrope shorter, anacline. Deltidium perforate in ma- ture forms, with the apical foramen sealed in old in- dividuals. Surface coarsely costellate, the costellse being crossed by fine elevated lines of growth. Test fibrous, impunctate. Ventral interior. — Delthyrial cavity deep, teeth strong, forming a thickening along the sides of the del- thyrial margin; crural fossettes shallow. Spondylium slightly depressed so that it is usually wider than high; median septum low but long, extending nearly to the front margin. Internal lateral spaces marked by irreg- ular granules. Dorsal interior. — Notothyrial cavity small, shallow, divided into two chambers by the cardinal process. Brachiophores orthoid, supported by lateral growths anteriorly, formed by the swelling of notothyrial plat- form. Median ridge thick and broad, extending to the front of the muscle area; sockets shallow; adductor field quadripartite, the anterior set being the larger and divided from the posterior pair by low ridges at right angles to the median ridge. An oblique pallial ridge is given off from the antero-lateral extremities of the anterior adductor scars. Internal surface marked by low granules. Geologic range. — Middle and Upper Ordovician (Chazy to Richmond). «" Pal. N. Y., vol. 8, pt. 1, 1892, p. 233. American Species Orthisina diversa Shaler 1865 Clitatnbonites americanus Whitfield 1877 C diversa altissima WincheU and Schuchert 1 893 C. rogersensis Foerste 1910 C. ruedemanni Raymond 1 92 1 C. trentonensis Raymond 1 92 1 aff. C. multicosta (Hudson) European Species Orthisina adscendens Davidson (Brit. Foss. Brach., vol. 3, pt. 7, p. 278) (non Pander). O. comflectens 'Wivcid.n 1907 O. emarginata Pahlen 1877 O fyramidalisYztii&n X'iill O. squamata Pahlen 1877 O. verneuili wesenbergensis Pahlen 1877 Orthis verneuili Eichwald 1841 Clitambonites comflectens albida Reed 1917 C. humilis Fuchs Skenidium grayitiT>a.vi6i&on 1883 S. shallochiense Davidson 1883 Vellamo farva Opik 1930 V. fyramidalis arcuato Opik 1930 V . fyramidalis fahleni Opik 1930 V . fyramidalis simflex Opik 1930 K. wWwjOpik 1930 Distinguishing characters. — Vellamo is char- acterized externally by the wide hinge-line, apsacline palintrope, concavo- to plano-convex profile, and non- imbricate shell sculpture. Discussion. — From the above it is apparent at once that Vellamo differs in important details from Clitam- bonites. In the former the palintrope is usually long, flat or curved noticeably, as a rule moderately or strongly apsacline, and only very rarely catacline. Clitambonites altissimus and C. trentonensis among American species have nearly catacline pahntropes. Vellamo differs further from Clitambonites in not possessing the strongly lamellose or frilled ornamenta- tion. The surface sculpture is like that visible in the umbonal region of Clitambonites as pointed out by Opik. The type of ornamentation in Clitambonites is approached in Vellamo only by Orthisina squamata Pahlen, which is placed here because of internal similarities. Internally it is difficult to define any substantial difference between the two genera. The spondylium of Vellamo is usually wider than long and this de- pressed appearance is in contrast to the elongate or narrow one of Clitambonites. Inside the dorsal valve the fundamental structures of the two genera are essen- tially the same, the only perceptible variations being in a rather thicker notothyrial platform in Vellamo and the flange-like border around the internal margin of Clitambonites. Opik points out further that the chi- lidium of Vellamo is relatively short and wide and the muscle-scars generally wider and shorter, but there appears to be little generic value in these features. GENERA OF THE SUBORDER ORTHOIDEA 115 The deltidium of Vellamo appears to be formed in precisely the same manner as that of Clitamhonites, but at maturity the apical foramen is not sealed as in the latter genus though it is closed in old-age specimens. Clittimbomtfs appeared first geologically (Kunda), and died out with the Middle Ordovician. Vellamo, on the other hand, appeared later (Chazy) and en- dured through the rest of the Ordovician. Genus CLINAMBON. n. gen. (Gr. klino, bend, and amhon, umbo) PI. 8, figs. 24, 28 Genoholotype. — Anomites anomala Schlotheim 1822, Nachtnige, p. 65, pi. 14, fig. 2. Description. Exterior. — Subquadrate in outline, valves very unequal, the ventral one pyramidal, the dorsal flat and elongate. Anterior commissure unipli- cate. Ventral interarea strongly procline, longer than the dorsal one. Interarea of the dorsal valve unusually long; chilidium wide and strongly arched. Deltidium perforate in young shells but foramen sealed in old ones. Valves multicostellate as in Vellamo. Ventral interior. — There is a prominent spondylium, the sides of which are infolded at the front, giving a pinched-in effect. The median septum is short. Dorsal interior. — Exactly as in Vellamo except that the structures are greatly overdeveloped. The cardinal process is fused to the ventral border of the ponderous chilidium. Median ridge short. Notothyrial platform ponderous and lateral thickenings greater than in any other member of the Clitambonitidas. Brachiophores flat blades clearly visible near the sockets. Geologic range. — Middle Ordovician (Jewe and Kegel, Do) of Estonia. The genotype is the only known species. Discussion. — The unusual development of the dorsal interarea and the ponderous chilidium are fea- tures which set this shell aside from Vellamo. The ornamentation is like that of the latter genus, and the appearance in the geologic column comes long after the disappearance of Clitamhonites, indicating relation- ships with Vellamo rather than any other group of the Clitambonitinx. Genoholotype. — Orthisina marginata Pahlen 1877, Mem. Acad. Imp. Sci. St. Petersh., 7th ser., vol. 24, no. 8, p. 33, fig. 8, pi. 3, figs. 11-15, pi. 4, figs. 1-3. Description. Exterior. — Sub-strophomenoid to hebertelloid in lateral profile and outline; ventral interarea longer than dorsal, usually strongly apsa- cline. Deltidium having an apical foramen; chi- lidium complete. Surface multicostellate. Test fibrous, impunctate. Ventral interior. — Delthyrial cavity deep; teeth large; spondylium supported at the front by a promi- nent median septum and laterally by low, very short septa. Muscle attachments confined to the spondy- lium. Adductor and diductor tracks linear. Sub- peripheral rim low, but prominent. Dorsal interior. — Notothyrial cavity shallow ; noto- thyrial platform swollen, with prominent thick lateral extensions which support the brachiophores. Median ridge strong; brachiophores orthoid; cardinal process simple, cemented to the under surface of the chilidium. Subperipheral rim prominent; adductor field large, with the muscle attachments commonly elevated. Geologic range. — Middle Ordovician (Kuckers) of Europe. Species Orthisina marginafd Pahlen 1877 O.f'yro/! Eichwald 18+0 O.volborthi Pahlen 1877 Gonambrinltes marginatus asfer Opik 1930 G. marginatus magnus Opik 1930 G. fanderi'd'^W. 1930 Hemifronites carrickensis Reed 1917 H. thomsoni Reed 1917 ? G. infiexa Pander 1830 Discussion. — Estlandia differs from Gonambonites, which it resembles and with which it has usually been associated, in having an apical foramen, median septa in both valves, and a complete chilidium. The orna- mentation of Estlandia and its internal structure, how- ever, suggest relationship with Vellamo. As in the Orthidas, Dinorthid.x, and other groups, it is reason- able to expect a convexo-concave stage of development. Estlandia appears, then, to be a convexo-concave stage of Vellamo, of rather short geological range and local development. Genus ESTLANDIA Schuchert and Cooper 1931 PI. 8, figs. 6, 8, 9; t. fig. 10 Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, p. 245. Gonambonites (pars) Opik, Acta et Comm. Univ. Tartu- ensis. A, vol. 17, 1930, pp. 222-227, pi. 19, figs. 229- 236, 248; pi. 20, figs. 236-239; t. figs. 24, 25. Genus HEMIFRONITES Pander 1830 Pi. 8, figs. 10-15 Pander, Beitr. Gcogn. Russ. Reiches, 1830, pp. 71, 74, pi. 3, fig. 14, pi. 18, fig. 6, pi. 28, fig. 22. Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 1892, p. 238, figs. 9, 10. Genolectotype (Dall). — H. tumida Pander 1830, 116 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA Description. Exterior. — Shell rotund to sub- semielliptical, strongly biconvex; hinge-line straight, usually narrower than the total width of the shell; cardinal extremities obtusely angular or rounded. An- terior commissure rectimarginate; dorsal sulcus obso- lete or only faintly impressed. Ventral interarea longer than the dorsal, curved, apsacline ; delthyrium closed by a deltidium which is usually imperforate in adults. Dorsal interarea short, orthocline to faintly apsacline or anacline ; chilidium prominent. Surface finely mul- ticostellate ; test fibrous, impunctate. Ventral interior. — Delthyrial cavity deep, spondy- lium longer than wide, sessile posteriorly but supported by a low septum in front, the septum being continued forward nearly to the front margin. Adductor ( ? ) track occupying the central depressed portion of the spondylium, diductor ( ? ) tracks on each side at the base and partially on the sides of the dental plates. Interior lateral spaces covered by subreniform ovarian markings. Subparallel pallial impressions bounding the inner margins of the ovarian markings and separated by the median septum. Dorsal interior. — Notothyrial cavity and sockets shallow, brachiophores orthoid, short, blunt, and sup- ported as in Gonamhonttes. Cardinal process a linear ridge on the thick notothyrial platform. Median ridge strong, extending to the middle of the valve. Anterior adductor impressions elongate, separated from the shorter posterior pair by a narrow ridge at right angles to the median elevation. Lateral spaces occupied by pallial and ovarian markings. Geologic range. — Lower and Middle Ordovician of northwestern Europe. European Species (of Pander 1830 except where otherwise stated) H emifronit es i^jualis H. dta H. brevis H. circularis (pi. 16, B, is same figure as H. maxima) H . ex f ansa H. lata H . latissima H. maxima H. obtitsa H. orbicularis H. feralta H . ferlata H . ■prominens H. rotunda H. sfherica H. transversa H. tumida Orthis hemifronites Von Buch 1 840 O. radians Eichwald ? H. globosa (pi. 16, B, fig. 6). This is the same figure as H . elongata and both have the appearance of a hycofhoria ? H. tenuistriata WeWcr 1907 (China) Distinguishing characters. — Hall and Clarke used the name Hemifronites for shells with tumid, subequally convex valves, hinge-line narrower than the greatest width, the greatest thickness of the ventral valve not at the beak, the surface finely multicostellate, and the deltidium non-perforate. Discussion. — Hemifronites differs from its nearest relative Clitamhonites s. s. in several important char- acteristics. It is usually subequally convex, the ventral palintrope is never procline, and the deltidium is im- perforate. This contrasts strongly with Clitamhonites in which the ventral valve is usually subpyramidal, and the deltidium perforate at least in early maturity. The ornamentation of Hemifronites is also different in being much finer and in lacking the strong concen- tric lamellje or frills of Clitamhonites. Internally there is also considerable variation be- tween Hemifronites and Clitamhonites. In the ven- tral valve of the former, to judge from H. tumida, the spondylium is sessile for most of its length but at its front end it is elevated on a low septum or ridge which extends nearly to the front margin. The walls of the spondylium are slightly convex inward and converge rapidly, forming a narrow floor at the base of the structure. This type of spondylium contrasts with that of Clitamhonites in which it is sessile only at the posterior and is elevated on a rather high septum. The front end of the spondylium of Clitamhonites is free and the floor rather wide. One of the specimens in the Schuchert Collection (Cat. No. 369; see pi. 8, fig. 10) shows markings on the spondylium that can be interpreted only as the muscle impressions. Homologizing these marks with similarly situated scars in orthids, the writers believe that the central linear track represents the position of the adductor muscles. This track occupies most, but not all, of the narrow floor of the spondylium. It is bounded on each side by low ridges. The diductor impressions are thought to be narrow linear tracks located outside the ridges bounding the adductor track at the line where the dental plates unite with the septum. The impressions of these tracks are also visi- ble for a short distance on the inside surface of the central lamellae. They are marked oS by scarcely perceptible ridges along the margin of the muscle track. Adjustor muscle impressions were not definitely determined. Ovarian markings of irregular elevated lines are also well exhibited in the same specimen. These are subreniform areas such as are so common in Orthis, Dolerorthlsy Paurorthls, etc. The shallow grooves separated by the median ridge and bounded by the ovarian impressions are taken to represent the two main trunks of the pallial marks, homologous to the two sinuses extending from the anterior ends of the diductors of Orthis, Bllllngsella, etc. It is therefore evident from the above that the important internal GENERA OF THE SUBORDER ORTHOIDEA 117 shell features of H r mi promt rs differ from those of Ort/iii s. s. only in having a spondylium. The dorsal interior of Hfmipronitfs is likewise dis- tinctly orthoid. The brachiophores arc rather flat plates, divergent and oblique to the vertical. The space between the sloping outer face of the brachio- phore and the wall of the valve forms the socket. The brachiophores are supported and usually nearly en- gulfed by adventitious shell of the notothyrial plat- form, but never to the extent seen in Vrllnrrio. The median ridge is rather high and extends to the middle of the valve. The posterior diductors are smaller than the anterior pair which are elongate and elliptical. Finally, the whole interior of the valve outside the muscle field is marked by elevated, irregular radiating ridges, and there can be no doubt that the spaces between some of these represent pallial sinuses although no definite system could be discerned. Verneuil'" united nineteen of Pander's species under Von Buch's name Orthis hem'tpronites. This is an- other example of Verneuil's "lumping" attitude and his failure to understand Pander's ability in species making. There can be no doubt that many of Pander's species of Hem'tpronites are insufficiently drawn, but even so, it must be said that all of his nine- teen forms can not be put into one species. There is great need for a critical revision of the species occurring in the "Glauconite" horizons by someone thoroughly familiar with the local stratigraphy of these shells. Opik has referred Pander's species H. maximus to the genus Clitambonites. A large specimen in the Schuchert Collection (Cat. No. 365), however, agrees with Pander's figure (pi. 16, B, fig. 5) except in the arching of the ventral palintrope, and it shows all the characteristics of /fron/Ve^ (see pi. 8, figs. 12, 13). We believe, therefore, that Opik has not figured H. maxima of Pander but actually has a new species not referable to Hemipronites but with a shell surface which relates it to Vellamo squamata. Genus PAHLENELLA Schuchert and Cooper 1931 (After Pahlen, in recognition of his fine work on Orthisina) PI. 7, figs. 1-4 Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, p. 245. Genoholotype. — Orthis trigonuln {ex parte) Eichwald 1840, Sil. Schicht. Esthland, I, p. 148; Pahlen, Mem. Acad. Imp. Sci. St. Petersb., 7th ser., vol. 24, no. 8, 1877, p. 46, pi. 4, figs. 22-24. Description. Exterior. — Semiclliptical, hinge- line straight ; cardinal extremities acute; lateral profile concavo-convex; anterior commissure sulcate; ven- ' Geology of Russia, vol. 2, Paleontology, 1845, p. 205. tral valve provided with a prominent fold. Ventral palintrope the longer, moderately apsaclinc; dcltidium perforate. Dorsal intcrarea nearly obsolete, hyper- cline. Surf.ice coarsely multicostellate. Test fibrous, impunctate. Ventral interior. — Teeth prominent, crural fossettes deep; spondylium short, supported by a low septum which is not, however, produced forward beyond the anterior end of the spondylium (sec pi. 7, fig. 2). Dorsal interior. — Notothyrial cavity nearly obsolete, brachiophores as in Clitambonites and Gonamhonitrs. Cardinal process, along with the median ridge and notothyrial platform, fused with a sessile shield-shaped plate of adventitious shell underneath the muscle field. Posterior adductor scars the smaller. Geologic range. — Lower Ordovician (lower Biii^ or Kunda formation) of western Russia. Only known species the genotype. Discussion. — In this interesting little shell there are several features that set it apart from the rest of the Clitambonitidas. Externally the concavo-convex pro- file is not in itself unique, but this, when combined with a prominent ventral fold and a corresponding dorsal sulcus, makes an aggregate of characters suffi- ciently distinct for generic discrimination. Internally there are additional striking variations in each valve. In the ventral valve, the spondylium is supported on a septum so low as to give it the appearance of being sessile; the septum is furthermore very short and in this is very unlike the long ones in Clitambonites. In the dorsal valve, the development of an elevated adductor field is a convergence toward the type of structure seen in Leptelloidea Jones. The disc is shield-shaped and produced anteriorly into a short process, while the pattern of the muscle marks does not recall those of Clitambonites. Genus APOMATELLA Schuchert and Cooper 1931 (Gr. a, without; potiia, cover) PL 7, figs. 6, 7, 12, 13 Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, p. 245. Genoholotype. — Orthisina ingrica Pahlen 1877, Mem. Acad. Imp. Sci. St. Petersb., 7th ser., vol. 24, no. 8, p. 48, pi. 2, figs. 18-21. This name is suggested for the small Orthisina ingrica Pahlen which is devoid of a deltidium and chilidium. This loss of important features is a con- vergence toward the Orthida: and appeared mther early in this genus of the Clitambonitida;. The ventral valve is subpyramidal, with a catacline or procline inter- area. Internally the spondylium is short and elevated upon a high septum. The interior of the dorsal valve was not observed. The exterior is multicostellate. 118 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA The genotype is the only known species and comes from the Canadian (Walchow) in the vicinity of Leningrad. Subfamily GONAMBONITIN^ Schuchert and Cooper 1931 Aberrant Clitambonitids having a strophomenoid or substrophomenoid profile and a sessile spondylium (pseudospondylium). Dorsal and ventral median septa are lacking. In the species studied by us the del- tidium is imperforate. The subfamily embraces only the one genus, Gonamhonites Pander. Genus GONAMBONITES Pander 1830 sensu lato PI. 8, figs. 1-5, 7, 20, 21, 25, 26 Pander, Beitr. Geogn. Russ. Reiches, 1830, p. 77, pi. 3, fig. l,pl. 16a, fig. 3, pi. 28, fig. 15. Opik, Acta et Comment. Univ. Tartuensis, A, vol. 1 7, pt. 1, 1930, pp. 220-222. Genolectotype (Schuchert and LeVene). — G. lata Pander. Description. Exterior. — Shell strophomenoid or substrophomenoid in appearance, semielliptical ; hinge- line wide and straight, cardinal extremities acutely angular; lateral profile convexo-concave; lateral com- missure flexed dorsally; anterior commissure slightly sulcate, there being a faint sulcus in the dorsal valve. Ventral interarea the longer, plane, strongly apsa- cline; dorsal interarea short, apsacline to anacline. Deltidium imperforate in G. -planus; notothyrium closed by a chilidium. Ornamentation multicostellate. Shell substance fibrous, impunctate. Ventral Interior. — Delthyrial cavity deep; teeth small; spondylium (.'' pseudospondylium) sessile and strengthened on each side by a short lateral septum or several septa.®^ The muscles were attached to the upper surface of the spondylium. The adductor tracks are linear. Diductor and adjustor scars can not be differentiated but the combined marks are situated on the sides of the dental plates. Subperipheral rim low. •^ A note in regard to the spondylium (?) of G. flanus will be of interest. This is sessile and strongly suggests the pseudospondylium common to Finkelnburgia and Linofor- ella. The extra "supporting plates" are directly ventral to the dental plates and may actually be a continuation of them, and the floor of the spondylium may therefore be composed of callus. We had before us no very young shells, from which the true character of the spondylium could be ascertained. Should this ventral muscle field prove to be a pseudospondylium, the relationship of Gonamhonites flanus to the Clitambonitina: may prove to be still more remote than we had suspected. Dorsal interior. — Notothyrial cavity shallow or ob- solete; notothyrial platform swollen and extended laterally; chilidium and cardinal process protruding above the interarea. Cardinal process thin, linear; brachiophores orthoid, supported by lateral extensions of the notothyrial platform (see pi. 8, fig. 7). Sockets shallow. Adductor field subflabellate ; median ridge obsolete or rudimentary. Subperipheral rim low. Geologic range. — Lower Ordovician (Canadian) of Europe (Walchow (Bn) for G. -planus Pander). Species. — We offer here no list of species because we had available for study very few forms of Gonam- botntes. Of G. planus there was an abundance of material, but we had no specimens of G. latus. We had specimens of "G." inflexus and G. excavatus Pander but these do not agree morphologically with G. planus, from which our description was made up. We will not re-assign Pander's species from his figures or descriptions because both are rather inadequate and mistakes would be inevitable. Discussion. — The old genus Gonamhonites has been regarded by most writers as a synonym of Clitamhonites and was so placed by Hall and Clarke in 1892. Dall in 1877 listed G. lata, Pander's first species, but never selected a type. Schuchert and LeVene (1929), following Dall's lead, selected G. lata as the type of the genus. A little later Opik (1930) made a critical study and revision of the genus and selected as the type G. planus Pander. Opik's selec- tion unfortunately can not stand as Schuchert and LeVene's choice has precedence. In the description of Gonamhonites presented above, we have based our remarks on G. planus for two reasons: (1) This is the only material we had avail- able, G. latus being entirely lacking from the Schu- chert Collection; and (2) Verneuil (Geology of Russia) placed G. latus in the synonymy of G. planus. It is a known fact that Verneuil took liberties with Pander's work that would be unheard of to-day, and it is very probable that some of the shells placed by him under G. planus deserve recognition as indepen- dent species. From Pander's figures of G. latus, there is to us a strong superficial resemblance to G. inflexus, a species that clearly is not a Gonamhonites. If it is shown by subsequent workers that G. latus has affinities with G. inflexus, a wholly different interpretation of the genus will be required, and it may be found neces- sary to set off G. planus as an independent generic group on the basis of its unusual spondylium (pseudo- spondylium) and non-perforated deltidium. In the absence of material, we feel obliged to follow Verneuil's interpretation of G. latus as a probable synonym of G. planus, although we feel strongly that Verneuil as a rule took many liberties. On this basis we are justified in regarding "G. planus" as a "cotype," as it were. Therefore, as explained under Estlandia, we were forced to make a new genus for shells differ- ing very widely from Gonamhonites as described above. GENERA OF THE SUBORDER ORTHOIDEA 119 Superfamily DALMANELLACEA Schuchert and Cooper 1931 Progressive and terminal Orthoidca, probably de- rived out of the Orthidx, and most easily character- ized by their endopunctate test and a primitively bilobed cardinal process. Embraces the following families: Dalmanellida: Schuchert Wattsellidx Schuchert and Cooper Bilobitid.-e Schuchert and Cooper Mystrophoridx Schuchert and Cooper Rhipidomellidas Schuchert Heterorthid.T Schuchert and Cooper Schizophoriida- Schuchert Linoporellida: Schuchert and Cooper Tropidoleptidae, n. fam. tion. We shall not be surprised, however, if someone points out that punctation had more than one origin. Punctation once introduced, the superfamily shows shell variations similar to those of the Orthacea, a superfamily that arose in the Cambrian, dominated the Ordovician, and vanished with the Devonian. The Dalmanellacea, on the other hand, arose early in the Middle Ordovician (first ones known in the Chazy), differentiated greatly in the late Silurian and early Devonian, spread over most of the world, constantly throwing off new genera, and yet apparently died out with the Permian. The genetic relations seem to be as shown in Table 9. Table 9 MYSTROPHORIDiE DaLMANELLID/E Proschizophoria BiLOBITIDiE Bilobites Aulacella Heterorthina ORTHIDiE Discussion. — As is seen in the definition of the Dalmanellacea, we lay the superfamily characteristics in the endopunctate shell and the beginning of a bilobed cardinal process. We further are forced to assume that the punctate shell started in a single species of the Orthacea, but unfortunately do not know in which of the older families this tendency originated, since in none of this superfamily is there any endopunctate species or genus. It is this utter lack of endopuncta- tion among the Orthacea that leads us to our assump- Family DALMANELLIDA Schuchert 1929, emended (=Dalmanellin2e Schuchert 1913) Progressive and terminal Dalmanellacea in which the ventral muscle field is bilobed in front, diductor scars not enclosing the adductor impressions. Brachio- phores simple, bladelike, without fulcral plates. 120 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA Geologic range. — Ordovician to close of Devonian. The family has the following genera: Dalmanella Hall and Clarke Car'inijerella Schuchert and Cooper Aulacella Schuchert and Cooper Proschizofhoria Maillieux Levenea Schuchert and Cooper Heterorthina Bancroft Genus DALMANELLA Hall and Clarke 1892 PI. 17, figs. 2-5, 7, 10, 13, 19-27, 29-31, 33; t. fig. 8 Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 1892, pp. 205, 223, pi. SB, figs. 27-39. Sardeson, Amer. Geol., vol. 19, 1897, pp. 91-111. €yn. Onniella Bancroft. Genolectotype (Schuchert and Cooper). — Or- this testudinaria Hall and Clarke {non Dalman) 1892, pi. 5B, figs. n-?,\=Orthl5 rogata Sardeson 1892, Bull. Minnesota Acad. Nat. Sci., vol. 3, p. 331, pi. 5, figs. 1-4. Description. Exterior. — Subcircular to subquad- rate; hinge-line straight, narrower than the widest part of the shell; cardinal extremities rounded ; lateral profile piano- to very unequally biconvex; anterior commissure sulcate, the sulcus usually being pro- nounced; ventral fold faint or prominent. Ventral palintrope longer than the dorsal, apsacline, beak in- curved; delthyrium open. Dorsal interarea short, anacline, notothyrium closed by the cardinal process. Ornamentation fascicostellate, having prominent ele- vated growth-lines in the interspaces. Test fibrous, punctate. Ventral interior. — Delthyrial cavity deep, teeth stout; crural fossettes deep; dental plates strong, flaring somewhat anteriorly; muscle field bilobed in front; diductor scars elongate, usually extending a little anterior to the front end of the adductor track; adjustor scars situated at the base of, or on the sides of, the dental plates, usually narrow, elongate. Ad- ductor track linear, narrow or wide. Pallial sinuses consisting of a main trunk directed slightly antero- laterally, taking its origin at the anterior ends of the diductor impressions. It trifurcates shortly after its origin, one trunk being directed antero-medially, one antero-laterally, and a third secondary sinus extending directly laterally. These secondary trunks break up into subsidiary rami toward the margin. Dorsal interior. — Notothyrial cavity completely filled by the cardinal process; brachiophores simple, divergent, bladelike plates, subtrigonal in plan, sup- ported only by adventitious shell deposited about the cardinal process and on the inner side of the brachio- phore. Cardinal process short, myophore trilobate or multilobate; median ridge low, extending nearly to the front or terminating at the front margin of the muscle field. The latter small, suboval or subcircular in outline; anterior adductor scars the larger, separ- ated from the posterior pair by low ridges at right angles to the median elevation. Geologic range. — Middle Ordovician (Chazy) to Silurian. American Species Orthis corfulenta Sardeson 1892 O. emacerata Hall 1860 O. futilis Sirdeson 1897 O. «^«o/a Sardeson 1897 O.meeki M\]\eT 1875 O. niultisecta Meek 1873 O. porrecta Sardeson 1897 O. rogata Sardeson 1892 Dalmanella bassleri Foerste 1909 D.resufinata Raymond 1921 D. whittakeri Raymond 1921 ? Orthis tersa Sardeson 1892 European Species Dalmanella navis Opik 1930 Onniella avelinei Bincioh 1928 O. broggeri Bancroft 1928 O.reuschi Bancroft 1928 Distinguishing characters. — ^The members of this genus are to be recognized chiefly by the plano- or nearly plano-convex profile, fascicostellate ex- terior, bilobed ventral muscle area, and primitive brachiophores. Discussion. — Hall and Clarke suggested the name Dalmanella, first using it as the "Group of Orthis testudinaria" under the genus Orthis. These authors distinguished thirteen group names under the old genus Orthis, but they never designated these "groups" as genera or subgenera. In describing Dalmanella as the "Group of Orthis testudinaria Dalman" they had be- fore them American shells which they correlated with Dalman's species. The group included, however, a het- erogeneous lot from which several genera have already been split. We will show that Orthis testudinaria Dalman non Hall and Clarke actually belongs to a group which is pretty much restricted to the Upper Ordovician and early Silurian and is totally distinct from the American Ordovician Dalmanellas with which Hall and Clarke obviously dealt in their descrip- tion of the group. This situation then brings up an intricate nomenclatorial problem as to what the geno- type of Dalmanella really is. If one adheres strictly to the rules of nomenclature, he would say that Hall and Clarke chose O. testudinaria of Dalman and the European species should therefore constitute the type of the genus. This would, however, restrict one of our commonest names to an heretofore obscure divi- GENERA OF THE SUBORDER ORTHOIDEA 121 sion of dalmancllids, namely, Wattsella Bancroft, rep- resented, among others, by two species, D. edgeivood- frisis Savage and D. trstudinarM Dalman, both of early Silurian age. Obviously the confusion caused by so designating the genotype for a group of shells that Hall and Clarke never had in mind would be unjust to those two authors, and would offset any advantage obtained by classifying the biological properties. Looking at the nomenclatorial problem in another way, it can be justly contended that Hall and Clarke did not have before them typical examples of Dalman's Orthis testutiifwria, but American shells that had from the beginning been erroneously identified as this species."" Therefore O. testudhwria of American authors in general and specifically of Hall and Clarke is not the same as O. testudinana Dalman of Sweden. Accordingly, one of the several species figured by Hall and Clarke under this name can be selected as the type of the genus, and we now select as the genotype of Dalmanella the specimens illustrated by them on plate 5B, figures 27-31, which are of Sardeson's Orthis rogata. It may be objected by "legalists" that this pro- cedure is invalidated by Schuchert's having named O. testud'mnria Dalman as the genotype of Dalmanella in his work of 1897,*' a procedure in which he was followed by many others. This objection is granted willingly and would be valid if O. testudinaria Dalman were a native American shell, as pointed out before. Schuchert et al. never questioned Hall and Clarke's identification of the species; it was then the order of the day everywhere to identify similar shells as O. testu- dinaria Dalman throughout the Ordovician. It can be contended, further, that the followers of Hall and Clarke, by using their name unchallenged, did not alter the instability of the identification. In further support of the contention that Hall and Clarke did not base their group discussion on O. testu- dinaria Dalman is the excellent work of Sardeson (1897) in which he describes as new species Dal- manellas usually referred to O. testudinaria. He says (p. 106): None of the above described species are like, or similar to, Dalman's figures, and to the best of my knowledge, there is ample room for preference of Meck's view, that our forms may none of them be identical with those described as Orthis testudinaria by Dalman, or those identified with that species in England. '- We have learned from Doctors Reeds, Croneis, and Rucdcmann, curators of the Hall material in the American Museum of Natural Histor\', the Walker Museum of the University of Chicago, and the New York State Museum, respectively, that there are no specimens of O. testudinaria Dalman in these collections from Borcnshult, the type locality. »' Bull. 87, U. S. Geol. Surv., 1897, p. 199. Raymond"^ corroborates this statement in the re- mark that ". . . . no one seems to have proved that the real Dalmanella testudinaria is found in America." The present study, moreover, shows that Dalman's species actually has an internal structure totally distinct from that of D. testudinaria Hall and Clarke = D. rogata (Sardeson) and is deserving of a different generic designation. Onniella Bancroft (pi. 17, figs. 1, 6, 8, 9, 11, 12, 18) has essentially the same internal structure as Dal- vianella (emend.), but differs chiefly in being more transverse and having a somewhat different ornamen- tation. We do not consider these external details of generic value and therefore place this genus in the synonymy of Dalmnnella (emend.). Should Paleon- tology progress to the point where ribbing characters are considered of value in genus making, it would probably be found that Dalmanella meeki will also conform to the characters of Onniella. Dalmanella emacerata and D. ignota are among the few Paleozoic brachiopods in which nearly the full complement of muscle-scars has been observed (see pi. 17, figs. 19, 22). Sardeson figures (his pi. 5, fig. 6, of ignota) a specimen showing, besides the diductor, adductor, and adjuster scars, two small accessory diductor scars. The specimens that Sardeson studied are now in the Schuchert Collection and there can be little question as to the presence of these marks although they can not be seen so clearly as the figures would indicate. They are not isolated but are continuous with the diductor scars. They may actually represent the back end of the diductor impression rather than separate and special scars. The adductor marks, as in many other genera, are individually semielliptical but together form an elliptical or oval impression super- imposed over the adductor track. In the dorsal valve the brachiophores have no fulcral plates and this differentiates the true Dalmanella from Wattsella and Idiorthis of the Wattsellidas, at least so far as the dorsal valves are concerned. The simple, bladelike brachiophores may become spoonlike by the development of adventitious tissue on their inside sur- face, and then they become attached to the dorsal sur- face of the valve; the true brachiophore plate may in that event be almost completely obscured. A common mature and old-age condition is a growth of shell sub- stance over the cardinal process, both shaft and myo- phore ; and in extreme instances the adventitious shell may completely fill the notothyrial cavity and encroach on the brachiophores. Dalmanella tcrsa is diflncult to place, since its ex- terior and cardinalia are much like those of Rhipidom- ella. Furthermore, in the ventral valve the diductor scars show a marked tendency to enwrap the adductor field. The species is thus intermediate between Rhipi- domella and Dalmanella, and is believed to indicate the probable origin of Rhipidomella. "Geol. Surv. Canada, Bull. 31, 1 92 1, p. 14. 122 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA Genus CARINIFERELLA Schuchert and Cooper 1931 (Lat. carina, keel; and jerre, to bear) PI. 18, figs. 9-12, 15, 16 Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, p. 246. Genoholotype. — Orthis carinata Hall 1843, Geol. N. Y., Rept. Fourth Dist., p. 267, fig. 1, as figured by Hall and Clarke (1892). Description. Exterior. — Transversely semicir- cular, margins rounded, cardinal angles obtusely rounded; hinge-line narrower than the greatest width of the shell; lateral profile convexo-concave to une- qually biconvex; anterior commissure sulcate; fold nar- row, subcarinate; sulcus deep, narrow ; ventral inter- area short, apsacline, beak not prominent, incurved, umbo low, convex, and sulcate. Ornamentation mul- ticostellate, with elevated growth-lines covering the whole surface. Test fibrous, punctate. Ventral interior. — Delthyrial cavity deep; teeth strong; dental plates thick, nearly obsolete in adults; muscle field bilobed in front; diductor scars elongate, expanded anteriorly; adductor track linear, not en- closed in front by the diductor scars. Aggregate adductor scar elliptical. Adjuster scars narrow, diver- gent, short, placed posterior to the diductor impres- sions. Short pallial sinuses extending antero-laterally for a short distance in front of the diductor scars. Dorsal interior. — Cardinalia confined to the imme- diate vicinity of the hinge, sockets deep, oblique; brach- iophore plates widely divergent and extending verti- cally to the floor of the valve, without fulcral plates; cardinal process very small, shaft short, myophore trilobed; median ridge low, extending to the anterior margin of the muscle area where it merges into the fold produced by the ventral sulcus. Muscle area small as a whole, not extending to the middle of the shell. Adductor scars separated by horizontal ridges; anterior adductors the smaller. Geologic range. — Upper Devonian. American Species Orthis carinata Hall 1843 DalmaneUa carinata efsilon Williams 1908 D.elmiraW\\Yi3.m.% 1908 Z). /io^a Williams 1908 {non Hall 1867) D. Virginia Williams 1908 D. Virginia beta Williams 1908 European Species Carinijerella dumonti (Verneuil) Distinguishing characters. — The distinctive features of this genus are the convexo-concave profile of the shell, the aberrant sharp fold and the corre- sponding deep, narrow sulcus, the Dalmanella-WVt ven- tral musculature, the strongly divergent brachiophore plates, and the small muscle area of the dorsal valve. Discussion. — The members of this new genus have ' been referred variously to Schizophoria and Dalman- eUa. Schuchert referred them to the former in 1897, but in 1908 Williams®^ restudied the different species and concluded that their affinities placed them in clos- est association with DalmaneUa. The combination of characters displayed by this group, however, necessitates the creation of a special designation for them. The ventral musculature is similar to that of Dal- maneUa and Schizophoria in the anterior bilobation, but the adjuster scars are much shorter and more oblique, and the median ridge (euseptoid) common to the Upper Devonian Schizophorias is not well devel- oped. Instead of being strongly elevated and con- tinued in front of the diductor scars as a ridge, it is divided in Carinijerella and forms a distinct V in front of the diductors. Williams considers this forked septum an important character of the genus "Dal- manellt^' although it is by no means common to all the "species." In the dorsal valve the variations of this genus from Schizophoria are most readily perceived in the nature of the cardinalia and the musculature. The brachio- phore plates are widely divergent, bladelike, attached directly to the inner surface of the valve. The sockets are wide and elongate and are not defined by a fulcral plate as is so characteristic in Schizophoria. The mus- cle area is rather small, but the fact that the individual scars are separated by a horizontal ridge contrasts with Schizophoria in which the scars are separated by an oblique ridge. In this respect Carinijerella resembles the genus Proschizophoria most closely. The adductor scars of Schizophoria, furthermore, are usually larger and somewhat flabellate. Pallial markings are very indistinct in most of the specimens, but when visible show the same elements common in the genus Levenea. There is a lateral trunk extending slightly obliquely from the ridges separating the adductors and another pair extends forward from the antero-median ex- tremity of the anterior adductors. In Carinijerella dumonti the anterior trunks bifurcate near their point of origin so that there are four trunks originating in the same points as the pallial sinuses of Levenea. Car- inijerella is evidently an Upper Devonian branch of the Dalmanellidas, aberrant chiefly in its external form. Genus AULACELLA Schuchert and Cooper 1931 (Gr. atihx, furrow) PI. 19, figs. 7, 8, 10, 11, 13 Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, p. 246. Genoholotype. — Orthis eijelensis de Verneuil 1850, Bull. Soc. Geol. France, 2d ser., vol. 7, p. 25. ^^ Proc. U. S. Nat. Mus., vol. 34, pp. 38-41. GENERA OF THE SUBORDER ORTHOIDEA 123 Description. — This genus externally resembles Rhipuiomi-lltt and Th'u-nicUa hut has a well marked fold on the ventral valve and a sulcus on the dorsal which are not reversed in the young stages. Internally the arrangement of the ventral muscle-scars is similar to that of Rhipuiotiiflla in the imprisonment of the adductor field by the diductor scars. The diductor impressions are never broadly flabellate as in Rh'tpi- domi-lla and the adjustor marks are usually clearly visible as in CannijereUti. Furthermore, the diductor impressions are separated by a low ridge which is forked much as in Cnriniferrlta. This is a feature never shown by Th'tetnflla or Rh'tfuiomrlla in which the median ridge is always direct and unforked. In the dorsal valve the cardinalia are ponderous and Strongly resemble those of Cnriniferelln and Rhifi- domdla but are totally unlike those of Thicmella which are delicate and confined. In our classification Aulac- ella is placed in association with Dalmnnella and C artntjerella because of the close similarity of the ventral musculature and dorsal cardinalia. The only known species is the German Orthis eifelensis. Genus PROSCHIZOPHORIA Maillieux 1911 PI. 19, figs. 25, 26, 30, 32 Maillieux, Bull. Soc. Beige Geol., Pal., d'HydroL, vol. 25, 1911, p. 177, pi. B, fig. I. Genoholotvpe. — Orthis fersonata Zeiler 1857, Verb. Nat. Hist. Ver. Bonn, vol. 14, p. 48, pi. 4, figs. 9-11, emend. Kayser 1892, Jahrb. k. Preuss. Geol. Landesanst., for 1890, p. 98, pi. 11, figs. 3-5, pi. 12, figs. 1-4. Description. Exterior. — Like Schizophorin, shell large and thick. Ventral interior. — Delthyrial cavity deep; teeth large; dental plates thick and strong, continued as ridges for some distance around the margins of the diductor scars; muscle area large, cordate, wider than long; diductor scars large, divergent, not enveloping the adductor scars in front; adductor impressions forming two semielliptical impressions, together making an oval scar; the adjustor scars elongate marks on the outside of the diductor impressions. Dorsal interior. — Cardinalia ponderous; brachio- phores as in Carinijerella, divergent; sockets deep; shaft of cardinal process a stout linear ridge, rounded ventrally and continued forward nearly to the anterior ends of the adductor scars; adductor impressions sub- equal, separated from each other by low ridges at right angles to the median ridge. Geologic range. — Lower Devonian (Coblenz- ian) the only known species being the European P. personata (Zeiler). Distinguishing characters. — This genus diflFers from Schizophoria, which it most closely resembles, by the cordate ventral muscle area. The ventral adduc- tor ridge so prominent in Schizophoria is lacking here. In the dorsal valve the adductor scars are subequal in size and separated by a horizontal ridge at right angles to the median elevation. The prominent cardinal process without expanded myophorc is another signifi- cant variation from Schizophoria. Discussion. — The name Proschizophoria was not happily chosen, as this genus is neither the earliest of the Schizophorias nor is it the progenitor of the group. The structure of the dorsal valve obviously relates it to Carinijerella and the Devonian representatives of the Dalmanellidx. Neaverson*" places Orthis provulvaria Maurer in this genus, but its dorsal valve has the musculature and pallial markings of Schizophoria, Genus LEVENEA Schuchert and Cooper 1931 PL 18, figs. 19-23,25-32; t. fig. 14 Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, p. 246. Genoholotvpe. — Orthis subcarinata Hall 1857, 10th Rept. N. Y. State Cab., p. 43, figs. 1, 2. Description. Exterior. — Subquadrate to subcir- cular in outline, hinge-line narrower than the greatest shell width ; cardinal extremities rounded ; lateral pro- file unequally biconvex to plano-convex; ventral inter- area longer than dorsal, apsacline; delthyrium open. Notothyrium closed by the cardinal process. Surface multicostellate. Test fibrous, punctate. Ventral interior. — Delthyrial cavity deep; teeth strong, located at the angle of the delthyrial and hinge margins. Crural fossettes deep. Dental plates strong in the young, obsolete in old forms, thickened on the inside to form the fossette. Muscle field pentagonal in outline ; diductor tracks deeply impressed, elongate ; adductor track wide, elevated, shorter than the diduc- tor marks. Adjustor tracks located on the sides of the dental plates, strongly impressed. Pallial markings as in Isorthis and Schizophoria. Dorsal interior. — Brachiophore plates blades like those of Dalmanella ; cardinal process prominent, shaft short or obsolete, myophore lobate; adductor field sub- circular or widely elliptical; adductor scars subequal. Pallial markings similar to those of Isorthis (see t. fig. 14). Geologic range. — Silurian to Middle Devonian. American Species Orthis lenticularis Vanuxein 1 842 O. quadrans Hall 1861 O. subcarinata H.1II 1857 Dalmanella macra Dunbar 1920 D. subcarinata Dunbar 1919 (non Hall) ? Orthis concinna Hall 1859 ? O. solilaria Hall I860 "" Stratigraphical Palaeontology, 1928, p. 256. 124 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA European Species Orthis canaliculata Lindstroni 1860 Distinguishing characters. — Levenea is most like Isorthis in all except cardinalia, but differs in its unequally biconvex or plano-convex lateral profile, and pentagonal ventral muscle field. This genus is close to Isorthis in its ventral interior but differs in hav- ing less widely divergent pallial markings. In the dorsal valve it differs still more strongly from Isorthis in not having a fulcral plate, or as elongate an adductor field. Discussion. — Of special interest in Levenea is the remarkable deposition of adventitious shell, especially in the dorsal valve about the various structures of the car- dinalia. Shell matter is laid on the inside surface of the brachiophores and extends around their dorsal face to the wall of the valve. The socket is a deep excava- tion in this adventitious shell and in some specimens may simulate a socket-plate (see pi. 18, figs. 19, 21, 29). This support of the brachiophore plates is exactly the same as that seen in Dalmariella s. s. The cardinal process is usually rather small and bilobed, without a pronounced shaft. In old forms a secondary shaft is produced forward over the median ridge. In such specimens the process has the appearance of a fly's head with proboscis protruded. This genus is named for Miss Clara Mae LeVene, in recognition of her assistance in the preparation of this monograph: first, by long-continued bibliographic work on the brachiopods, begun in connection with the second edition of Zittel-Eastman in 1913, and carried on to form the basis for the Schuchert-LeVene generic catalogue of 1929; and second, by editorial revision and other work accompanying the transforma- tion of the hand-written manuscript into the present printed volume. In the dorsal valve the cardinalia are similar to those of Dalmanella. The brachiophore plates are bladelike, divergent, supported by adventitious shell, which forms a rather thick notothyrial platform. The cardinal process is small, the shaft short, the myophore lobate. The adductor field is somewhat shield-shaped with individual scars subequal. Discussion. — Heterorthtna differs from Heterorthis in lacking the widely divergent diductor scars in the ventral valve, and the thickened ridges parallel to the anterior margins. The deposition of adventitious shell about the cardinalia takes more the form of that seen in Dalmanella rather than that of Heterorthis. In the latter the brachiophore plates are thickened at their dorsal and distal extremities, the thickening being pro- longed as curved margins around the adductor field. The cardinal process of Heterorthis is also different from that of Heterorthtna. The myophore of the former is elongated ventrally and the median lobe drawn out posteriorly into a rather sharp carina. The lateral lobes are reduced to small ridges or bosses. In Heterorthina, on the other hand, the process is small, bilobed, and cleft in front, as seen commonly in Dal- manella. No trace of a heterorthoid chilidium was seen in Heterorthina. The internal structure of the genus relates it more closely to Dalmanella than to Heterorthis, and for this reason we are placing it in the family Dalmanellidae. Geologic range. — Upper Ordovician of Europe and possibly of the Ohio Valley. European Species Heterorthina frteculta Bancroft 1928 Orthis ellipoides Barrande 1 847 American Species ? Dalmanella jairmountensis Foerste 1909 Genus HETERORTHINA Bancroft 1928 PL 17, figs. 17, 28, 32; pi. 18, figs. 1-8 Bancroft, Mem. and Proc. Manchester Lit. and Philos. See., vol. 72, 1928, p. 59. Genoholotype. — H. frtpculta Bancroft 1928. Description. — Externally the shells of this genus resemble Heterorthis in outline, but the dorsal valve, instead of being flat or concave, is very gently convex and bears a shallow sulcus. The surface is rather finely multicostellate. In the ventral valve the teeth are short and flaring and their anterior ends are con- tinued forward as low ridges along the outer margins of the diductor scars. The muscle area is long, extend- ing nearly to the middle of the valve or a little beyond. It is strongly lobate in front, the diductor scars are elongate with subparallel sides, and their anterior ends extend in front of the elongate adductor track. Adjus- ter marks were not seen. Family WATTSELLID^ Schuchert and Cooper 1931 Progressive and terminal Dalmanellacea originating in the Dalmanellida, with subcircular or shield-shaped shells, cordate ventral muscle field, and fulcral plates defining the sockets. Geologic range. — Upper Ordovician to Lower Devonian. Embraces the following genera: Wattsella Bancroft Resserella Bancroft Horderleyella Bancroft M endacella Cooper Idiorthis McLearn Parmorthis Schuchert and Cooper Fascicostella Schuchert and Cooper GENERA OF THE SUBORDER ORTHOIDEA 125 The genetic relations of the Wattscllida: appear to be as shown in Table 1 0. Genus WATTSELLA Bancroft 1928 PI. 22, figs. 9, 13, 14, 17-29 Bancroft, Mem. Proc. M.inchester Lit. and Philos. Soc., vol. 72, 1928, p. 55, pi. 1, figs. 1-5. Genoholotype. — Watisella wattsi Bancroft 1 928. Description. Exterior. — Subcircular, hinge-line straight, narrower than the greatest width of the shell; cardinal extremities rounded; lateral profile unequally biconvex, the ventral valve .ilways with the greater Table 10 Fascicostella Parmorthis Idiorthis <■ Mendacella > Horderleyella Resserella Dalmanellio^ convexity. Anterior commissure rectimarginate to faintly sulcate. Dorsal sulcus shallow, deepest posteri- orly; ventral fold conspicuous only at the posterior. Ventral palintrope longer than the dorsal, curved, apsa- cline, beak incurved, umbo strongly convex. Dorsal palintrope short, faintly anacline, beak inconspicuous; umbo sulcate, delthyrium and notothyrium open. Sur- face multicostellate ; shell fibrous, punctate. Ventral interior. — Delthyrial cavity deep; teeth strong; crural fossettes deep; dental plates sharp, divergent, commonly continued forward as a ridge on the lateral periphery of the diductor scars. Muscle area subcordate, more or less gently bilobed in front. Adductor track linear, squarish in front. Diductor scars elongate, rounded in front, and narrowing behind; adjustor scars not separable. Dorsal interior. — Cardinalia prominent, brachio- phores elongated into slender points, extending nearly directly ventrally; brachiophore supports triangular in side view, convergent, and uniting with the median ridge; sockets defined by a small fulcral plate. Car- dinal process small; shaft slender, and only defined in the sp.icc between the brachiophore supports; myo- phorc lobed. Median ridge prominent, extending to the middle of the valve; muscle area oval, widest in front. Anterior adductor scars the larger. Geologic range. — Upper Ordovician into early Silurian. American Species Dalmdnella edgezvooJensis Savage 1913 Shells of this genus are common in eastern North America and are now parading as Dalmanella tesludinaria European Species Orthis tesludinaria Dalman 1828 Terebratula miguis Sowerby 1839 Watisella wattsi Bancroft 1928 ? W. multiflicata Bancroft 1928 Distinguishing characters. — This genus differs from Dalmanella in its more cordate muscle area, the subparallel brachiophore supports which converge dor- sally to meet the median ridge, the presence of fulcral plates defining the socket, and the exceedingly long tenuous brachiophores. Discussion. — In restricting Dalmanella to shells of the type of Orthis rogata Sardeson, which Hall and Clarke mistakenly identified as Orthis testudinarta Dalman, the writers have left the European dalman- ellid group without a proper generic name. Bancroft's recently proposed Wattsella has a structure identical with that of Orthis testudinarta Dalman and must therefore include that species. Bancroft laid generic value on the differences in the cardinalia of the dal- manellids and based several genera on their variations. The cardinalia of Wattsella, as well as the ventral musculature, are strikingly different from those of Dalmanella as restricted by us. In the latter genus the diverging brachiophores are typically bladelike, and are never provided with fulcral plates defining the socket. In Wattsella, on the other hand, the brachio- phore plates converge dorsally to meet the outside margins of the median ridge. It is these plates that make the parallel slots in internal molds (see pi. 22, fig. 20). The median ridge extends to the middle of the valve, is low and wide. The cardinal process is normally rather small, with a short shaft and trilobcd myophore which in its lateral lobes in adults or old forms may be extended anteriorly some distance. When such is the case, it gives this structure the appear- ance of being fissured. In some specimens the median lobe may be extended forward along the shaft as in Isorthis, so that the cardinal process simulates in form the head of a bee or fly. In mature forms of Wattsella, as in most other genera of the Dalmanellacea, adventitious shell is deposited about the brachiophore plates. In the genus 126 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA under discussion, this secondary shell is deposited mainly between the brachiophore plates in connection with the cardinal process, but in Orthis testudinarta Dalman from Borenshult, Sweden, the brachiophores are swollen on the inside at their junction with the median ridge and a very noticeable amount has also been deposited in the cavities between the brachiophore and the fulcral plates. If this deposition were carried to its extreme, the fulcral plates would be obliterated and the cardinalia would simulate those of Dahnanella, Levenea, and other genera of the Dalmanellidas. In the ventral valve the differences are not so strik- ing. However, the diductor scars are not so long and flexuous, and the muscle impressions as a whole are more heart-shaped than in Dalmanella. Wattsella is abundant in the Upper Ordovician of England and Wales and in the early Silurian of Sweden in strata previously regarded as Ordovician. In the United States, it is known for the first time in the early Silurian (Edgewood limestone of the Alexan- drian series) of Illinois and Missouri. This is a strik- ing fact which lends strength to the assertion made in the discussion of Dalmanellay namely, that shells having the structure of Orthis testudinarta Dalman are as yet unknown in the North American Ordovician. In the Silurian of Arisaig some of the specimens referred by McLearn*' to Dalmanella elegantula actually belong to Wattsella. It is not improbable that some of the Silurian dalmanellids from Anticosti referred to D. testudinarta by Twenhofel will prove to be Wattsellas when the interiors are studied. Further, many of the "Dalmanellas" collected by Williams in the Silurian of Maine prove to belong to Bancroft's genus. Genus RESSERELLA Bancroft 1928 PI. 17, figs. 14-16 Bancroft, Mem. Proc. Manchester Lit. Philos. See, vol. 72, 1928, p. 54. Genolectotype. — Orthis canalis Sowerby 1839, in Murchison, Sil. Syst., pi. 13, fig. 12a. Description. — Externally, Resserella (as here restricted) forms a very precise homoeomorph of Dal- manella as described in this memoir. The valves are unequally biconvex, the dorsal being flatly convex and the ventral rather strongly so. There is a shallow sulcus on the dorsal valve that is defined from the vicinity of the beak to the anterior margin. The beaks and the interareas have the same features exhibited by Dalmanella s. s. The external surface is multicostel- late as in the American shells. Inside the ventral valve the identity with Dal- manella is carried out to the form and lobation of the muscle area. In the dorsal valve the cardinalia of mature shells are identical with mature Dalmanella. "Geol. Surv. Canada, Mem. 137, 1924, pp. 53-54. In Resserella the adductor impressions of the dorsal valve are subequal and form a rather circular pattern. Discussion. — From the above i^ will be seen that the interior and exterior of Resserella and Dalmanella are very close if not identical in mature shells. For this reason we had regarded Resserella as a synonym of Dahnanella (restricted) in the early phases of our work. However, in correspondence with Mr. Ban- croft, he has assured us that there is a very close relationship between Resserella and Wattsella, to be seen in the presence of fulcral plates in the early growth stages of the former. If these fulcral plates exist in Resserella, we admit the validity of the genus. We have not seen such plates in the material available to us and must accept Bancroft's statement. The recognition of Resserella as a distinct genus does not invalidate our genus Parmorthis, based on Dalmanella elegantula, which Bancroft places in his Resserella. The type selected by Bancroft for his genus is Orthis canalis of Sowerby. As originally described, this species was composite, including within it Ordovician and Silurian specimens. On the basis of the latter, Davidson made Sowerby 's species a synonym of Orthis elegantula Dalman. The specimen from the Ordovician, however, according to Sowerby's fig- ures and remarks (pp. 630, 640), is different from the Silurian forms, which clearly belong to "Dalmanella" elegantula (Dalman) and were so referred by David- son. On the other hand, specimens sent us by Mr. Bancroft prove to be of the larger Ordovician form, and it is on this shell and not on the "elegantula" type that Resserella is based. Resserella as thus restricted consists of shells that are more circular as a rule than those of Parmorthis. Fur- thermore, the species of the latter genus have a rather definite ribbing system unknown in Resserella, con- sisting of a rather prominent median fascicle in both valves that is so well developed in some of the species as to produce a flattened or smooth portion in the middle of the valves at the front. On the interior, however, the diiTerences between Resserella and Parm- orthis are still more pronounced. In the ventral valve the strong arching of the beak produces a very deep delthyrial cavity and strong dental plates, much thicker than those of Resserella. Moreover, the ven- tral muscle field of Parmorthis is not decidedly lobate as is the corresponding area in Resserella. Inside the dorsal valve the differences are equally striking. In Resserella as known to us there is no prominent thick- ening produced laterally along the periphery of the adductor field, whereas this same area in Parmorthis has a very distinctive shape shared only by the closely related Fascicostella. Here the adductor area is elon- gate and the anterior impressions are triangular. An- other very important difference between the two genera is the striated teeth and sockets of Parmorthis, which are very rare in the whole brachiopod class. It is clear, therefore, that Resserella and Parmorthis GENERA OF THE SUBORDER ORTHOIDEA 127 are quite distinct. There are also notable differences between Rrsscrella and Wattsflla, particularly in the ribbing characters. We are, however, willing at pres- ent to accept Mr. Bancroft's statement regarding the relationships of these two genera in the absence of specimens of immature Resserella. Genus HORDERLEYELLA Bancroft 1928 PI. 22, figs. 30, 32-35 Bancroft, Mem. Proc. Manchester Lit. Philos. Soc., vol. 72, 1928, p. 178, pl. 1, figs. 15-18. Genoholotype. — H. fUcata Bancroft 1928. Description. Exterior. — Shells rather small, sub- semielliptical or subquadrate in outline; hinge-line straight, cardinal extremities obtuse. Lateral profile subequally biconvex. Anterior commissure sulcate, sul- cus shallow; ventral fold subcarinate. Interareas sub- equal, ventral one apsacline, dorsal anacline. Ventral umbo rather strongly convex, dorsal umbo gently convex. Surface strongly fascicostellate. Test prob- ably punctate. Ventral interior. — Delthyrial cavity rather deep; dental plates short and strong; muscle area short, sub- pentagonal; diductor impressions subtriangular. Ad- ductor impression linear, occupying a depression formed by the posterior extension of the fold. Pallial impres- sions unknown. Dorsal interior. — Notothyrial cavity shallow, brach- iophores long, brachiophore plates convergent dorsally and uniting with the median ridge. Sockets defined by fulcral plates. Geologic range. — Ordovician of England and Wales. The species are : H. flicnta Bancroft, H. n. sp. Discussion. — This genus was originally placed by its author in the Harknessellinae, but it appears to us to belong nearer Wattsella, since its brachiophores are supported by convergent plates and there is a fulcral plate bounding the socket. From Wattsella it differs in the greater convergence of the brachiophore sup- ports, which do not appear as parallel slits in internal molds. Genus MENDACELLA Cooper 1930 (Lat. mendax, lying) PI. 22, figs. 1-5, 8 Cooper, Jour. Pal., vol. 4, 1930, pp. 377, 380, pl. 36, figs. 2, 16-18 (uteris), also figs. 6, 9 {mullochiensis). Genoholotype. — Orthis uheris Billings 1866, Cat. Sil. Foss. Anticosti, p. 42. Description. Exterior. — Similar to Rhifidomella but with the valves subequally convex. The ventral valve is always the larger of the two. Ventral interior. — Delthyrial cavity moderately deep; teeth large; dental plates strong, subparallel or divergent, continued forward as ridges on the outer margins of the diductor scars. Muscle area bilobed in front; diductor scars long and narrow; adductor ridge wide or narrow, carinatc or flat, never extended to the front of the diductor scars. Adjuster scars small or large, narrow. Pallial markings not observed. Dorsal interior. — Cardinalia confined ; brachio- phores stout, bluntly pointed, slightly divergent; sock- ets deep, defined by a small concave fulcral plate. Cardinal process small, shaft slender, not extending anterior to the ends of the brachiophore plates, myo- phore expanded lobate; a low broad median ridge extending from the ends of the brachiophore plates nearly to the middle of the valve. Adductor scars subequal, separated in some species by low ridges at right angles to the median ridge. Geologic range. — Upper Ordovician and early Silurian. American Species Orthis tequivalvis Shaler 1865 ^ O. uheris Billings 1866 Rhifidomella rhynchonellijormis (Shaler) 1865 Schizofhorella arisaigensis McLearn 1924 ? Rhifidomella circulus (Hall) 1843 ? R. lenticularis Foerste 1903 ? R. tenuilineata Foerste 1913 European Species Orthis minuscula Barrande O. mullochiensis Davidson (Sil. Brach., p. 221) Distinguishing characters. — This genus is dis- tinguished especially by the Dalmanella-Wke. character of the interior and the Rhipidomella-Vke exterior. Ex- ternally, however, it differs from Rhipidoinella in hav- ing the ventral valve usually the larger. Internally the variations of the muscle-scars, dental plates, and car- dinalia from those of Rhipidomella are striking. The muscle area does not have the broadly semiflabellate diductor scars enclosing the oval adductor ones. The diductor impressions of Mendacella are quite direct and narrow, and ordinarily only slightly divergent; they have a rather strong resemblance to the same scars in Dalmanella. Discussion. — There is some variation in the ventral musculature between the various species placed by us in this genus. In M. uheris from the Ellis Bay forma- tion (late Ordovician) the diductors are frequently separated by a low wide ridge, but in the same species from the Silurian Gun River formation the ends of the diductors are separated from each other by a very narrow ridge only. The other Silurian species such as M. arisaigensis and M. mullochiensis all resemble the Silurian M. uheris in this respect. There is also a vari- able development of the adjustor scars within the same species and in different species. 128 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA From the structure of the cardinalia with its moder- ately divergent brachiophore plates, strengthened by fulcral plates, it is to be deduced that Mendacella is more closely related to Wattsella and Parmorthis than to Dalmanella and its allies. Genus IDIORTHIS McLearn 1924 PI. 21, figs. 34-38 McLearn, Geol. Surv. Canada, Mem. 137, 1924, p. 56, pi. 3, figs. 19-24, pi. 4, figs. 1-4, pl. 28, fig. 10. Genoholotype. — /. matura McLearn 1924. Description. Exterior. — Suborbicular, hinge- line narrow, cardinal extremities rounded ; lateral pro- file piano- to concavo-convex, the dorsal valve usually convex at the umbo but flat or concave in front of the middle. Anterior commissure slightly sulcate ; ventral interarea the longer, curved, apsacline ; beak incurved ; delthyrium open. Dorsal interarea short, anacline. Ornamentation multicostellate. Test punctate. Ventral interior. — Delthyrial cavity deep; teeth strong; dental plates thick, divergent, continued as ridges around the periphery of the diductor scars. Muscle field cordate, occupying nearly half the length of the valve. Diductor scars large, irregularly sub- triangular, longer than the adductor track which is elongate, rectangular; adductor scars elliptical; ad- juster scars not visible, or very narrow and situated at the base of the dental plates. Dorsal interior. — Cardinalia ponderous, brachio- phores and their supporting plates not separable, sub- parallel or slightly divergent. Cardinal process tri- lobed, its shaft obscured by the median ridge which is thick and clavate posteriorly but tapers to a point toward the front and terminates at the margin of, or just in front of, the anterior end of the muscle field. Muscle area reduced by the encroachment of the crural base supports which have forced the posterior adduc- tors "up on the truncated anterior face of the crural lamella, which, expanding laterally, curve around the postero-lateral borders of the greatly enlarged and almost circular anterior adductor scars" (McLearn). Geologic range. — Early and } Middle Silurian. American Species Idiorthis ovila McLearn 1924 /. matura McLearn 1924 European Species ? Orthis eJgelliana (Salter MS.) Davidson (Sil. Brach., p. 228) Distinguishing characters. — These are the ex- tremely large ventral muscle area, strong divergent dental plates, subparallel arrangement of the brachio- phore supports, large size and structure of the median ridge and cardinal process, and nature of the dorsal musculature. These characters are combined with a concavo-convex exterior. Discussion. — Idiorthis most closely resembles Wattsella in the internal structure of the valves. The ventral muscle field is obcordate as in the former genus, with a rather wide adductor track. In the dorsal valve the brachiophore plates, situated on each side of the median ridge, are subparallel. The sockets appear to be defined by small socket-plates. The encroachment of the adductor muscles upon the front ends of the brachiophore supports in the type specimens is due to their being gerontic individuals. This fact also accounts for the peculiar structure of the cardinal process and the median ridge. Accordingly it is our view that Idiorthis as now understood is an old-age stage of Wattsella, but in the absence of young speci- mens this can not be proved. Idiorthis has not been identified outside of the Arisaig section but McLearn has suggested a resemblance of Orthis edgelliana Salter to its genotype. Genus PARMORTHIS Schuchert and Cooper 1931 (Gr. farmey shield) PL 21, figs. 1-16, 29 Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, p. 246. Genoholotype. — Orthis elegantula Dalman 1828, K. Svensk. Vet. Akad. HandL f. 1827, p. 117, pl. 2, fig. 6. Description. Exterior. — Suborbicular, elongate elliptical or shield-shaped in outline; margins convex, front margin frequently slightly produced; hinge- line narrower than the greatest shell width; cardinal extremities rounded; lateral profile plano-convex, ven- tral valve ventricose, dorsal valve with a shallow sul- cus; ventral interarea strongly curved, slightly apsa- cline, umbo swollen, beak strongly curved; dorsal interarea short, anacline, beak nearly obsolete; del- thyrium usually open, notothyrium closed by the car- dinal process. Multicostellate, rarely fasciculate. Test punctate. Ventral interior. — Delthyrial cavity deep; teeth ponderous, crenulated on the inner surface; crural fossettes deep; accessory socket large; dental plates strong, vertical, subparallel or only slightly divergent; muscle area confined to the delthyrial cavity; adduc- tor track usually narrow, linear, slightly elevated; diductor scars triangular, not extended appreciably an- terior to the adductor track, covering the walls of the delthyrial cavity; adjustor impressions narrow; a slight median thickening extends anteriorly from the adductor track, and on each side of this ridge and GENERA OF THE SUBORDER ORTHOIDEA 129 closely adjacent to it, a pallial trunk runs forward nearly to the anterior margin. Dorsal interior. — Cardinalia confined; brachio- phores large, divergent, bearing long blunt points given off from the inside antero-ventral part; sockets very deeply defined on the inner side by the sloping crenu- lated outer face of the crural base and a small concave fulcral plate which unites the brachiophore to the floor of the valve. Denticle large; muscle area elon- gate-oval in outline; anterior adductors the smaller; peripheral ridges low; cardinal process small, bilobed or trilobed, shaft produced forward to the line of divi- sion between the diductors, where it is cleft. Geologic range. — Silurian. American Species Dalmanella concavoconi'exa Twenhofel 1928 D. ip-ingfelJemis Foerste 1917 D. wildroneniii Foerste 1917 Parmorthis crassicostata, n. sp. ? Orihis media Shaler 1865 European Species On his basalis Dalman 1828 O. eUgantulj Dalman 1828 O. visbyensis Lindstrom 1860 Distinguishing characters. — This genus is dis- tinguished externally by its plano-convex contour, strongly inflated ventral valve, and peculiar ornamen- tation. Internally, the stout brachiophores with their blunt processes, oval muscle area, unusual articulating apparatus with its crenulated teeth and sockets, and ventral muscles and pallial sinuses all combine to indi- vidualize the genus. From Dalmanella s. s., Parmor- this is differentiated by its more inflated ventral valve, different ornamentation, differently shaped ventral muscle area, divergence of the brachiophore plates at their junction with the floor of the valve, and possession of a concave fulcral plate. It differs from Isorthis in not possessing a complicated pallial system and in cer- tain details of the cardinalia. Discussion. — There is a feature of the ornamenta- tion of these shells which is characteristic. In the sulcus of the dorsal valve there is usually a concentra- tion of costellae due to the accelerated appearance of secondary and tertiary ribs. This frequently produces a nearly smooth or nearly plane area in the front of the sulcus that is ordinarily elevated as a slight fold in its center. There is a corresponding smooth area in the median portion of the ventral valve which is com- monly depressed slightly below the general level of the shell. Thb peculiarity occurs in nearly all of the species studied. The articulation of Parmorthis is unusual for the strength of its parts and their specialization. It is the only genus known among the orthids (except Trofid- oleptus, now placed in the Dalmanellacea) that possesses ci cnulated teeth and sockets. The details of the articu- lation are described on page 25. Noteworthy also in this genus is the occurrence of a small deposit of secondary shell in the apex of the del- thyrium. This has nothing whatever to do with a deltidium, being merely an apical callus such .is occurs commonly in some spiriferid genera. In the dorsal valve the brachiophores not uncommonly bear elon- gate, bluntly pointed processes on their inner dorsal and distal extremities. If these and similar processes occurring in the same position in other orthid genera could be homologized with the crura of the rhyncho- nellids, it would be possible to prove that the brachio- phores in reality are the same as the crural bases. "Dalmanella" visbyensis (Lindstrom) shows an in- teresting variation from the usual type of Parmorthis. In this form the ventral beak is more strongly arched than is usual, actually overhanging the dorsal interarea. Along with this variation goes a concave dorsal valve. Within the dorsal valve the myophore of the cardinal process is directed backward (posteriorly and dorsally) so far as to be visible from the dorsal side. Seen from the inside, one perceives that the inner margins of the shaft and the brachiophore plates are grown together. Orthis elegantula Dalman differs importantly from our Dalmanella and from Resserella. In Dalmanella the ventral muscle field is lobate and the pallial mark- ings widely divergent, whereas in Parmorthis the ven- tral pallial markings are closely appressed and sub- parallel, and the muscle area is never lobate and is confined to the delthyrial cavity. In the dorsal valve important differences also occur. The adductor field of Parmorthis is elongate, and in our Dalmanella it is subcircular. Further, the sockets of Parmorthis are defined by concave fulcral plates, which are lacking in Dalmanella. These differences we regard as of sufl^- cient importance to separate the two genera and place them in different families. Parmorthis crassicostata, n. sp. PI. 21, figs. 4, 5 Shell rather small, subcircular, of the type of P. ele- gantula but having rather coarse ribs, there being about 34 to the valve. Measurements of the holotype: length, 9 mm.; width, 9 mm. Holotype, Cat. No. 913, Schuchert Collection, Yale University. Silurian (Niagaran), Martin's Mills, western Tennessee. Genus FASCICOSTELLA Schuchert and Cooper 1931 (Lat. jasc'uy bundle; costella, a little rib) PI. 22, figs. 6, 7, 10-12, 15, 16, 31 Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, p. 246. 130 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA Genoholotype. — Orthis gervillei Defrance 1827, Diet. Sci. Nat., vol. 51, p. 152. Description. Exterior. — Dalmanelloid, subcircu- lar to subquadrate; lateral profile piano- to slightly concavo-convex; anterior commissure rectimarginate or sulcate ; sulcus on the dorsal valve shallow ; ven- tral interarea curved, beak incurved; dorsal interarea anacline; ornamentation strongly fascicostellate, the chief distinguishing feature of the genus. Ventral and dorsal interior. — Essentially the same as in Orthis elegantula Dalman (^PartTiorthis). Geologic range. — Upper Silurian ? to Lower Devonian of Europe, with the following species: Orthis dorsoflicata Beclard O. gervillei Defrance O. sedgwicki D'Archiac and Verneuil Discussion. — Fascicostella is distinguished from all other Dalmanellacea by its peculiar ornamentation, since it has the greatest development of fasciculation seen in any orthid. Internally the species most closely resemble Parmorthis elegantula of Gotland. How- ever, the hinge-line is proportionately wider and the cardinal extremities less rounded and the ventral valve never so ventricose. The ventral musculature is like that of P. elegantula, but has larger adjustor scars. The pallial sinuses are also like those of the Gotland species in being closely together, subparallel, and sepa- rated by a narrow, low ridge. The anatomy of the dorsal valve agrees closely with that of P. elegantula. The close similarity of the internal anatomy of Fasci- costella to that of Parmorthis elegantula suggests that the genus had its origin in that form. Fascicostella seems to be localized in the Lower Devonian of Europe, except for Kozlowski's report** of "Dalman- ella" gervillei in the highest Silurian of Poland (Borszczow). Family BILOBITID^E Schuchert and Cooper 1931 Aberrant, specialized Dalmanellacea originating in the Dalmanellidas, having an emarginate anterior mar- gin, narrow hinge-line, and a deep sulcus in each valve. The only genus is Bilobites Linnasus, extending from the Upper Ordovician into the early Middle Devonian of Europe and North America. Genus BILOBITES Linnaeus 1775 Linna:us, Syst. Nat., ed. Miiller, vol. 6, 177S, p. 325. Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 1892, p. 204, pi. 5B, figs. 11-14. Syn. Diccelosia King 1850. Genoholotype. — Anomia biloha Linnaeus 1767, Syst. Nat., 12th ed., p. 1154. Description. Exterior. — Small, bilobed, anterior margin emarginate, hinge-line narrower than the greatest width of the shell; lateral profile unequally biconvex; anterior commissure broadly sulcate. Ven- tral interarea the longer, curved, apsacline, umbo swollen, sulcate, beak incurved; dorsal interarea short, anacline, beak projecting slightly, incurved; surface multicostellate ; shell coarsely punctate. Ventral interior. — Umbonal cavity deep ; teeth strong; dental plates thick and rather obscure; muscle area bilobed, thickened along the margins; divided centrally by a sharp ridge corresponding to the external sulcus; diductor scars semiflabellate, divergent, not enveloping the adductor impressions; adductor scars borne on the central ridge, obscure; adjustor scars obscure. Dorsal interior. — Cardinalia thick, brachiophores ponderous, long, bladelike, widely divergent; sockets shallow; cardinal process thick; myophore bilobate, crenulated on its posterior face, its shaft extended for- ward and merging with the median ridge formed by the impression of the external sulcus on the inside of the shell. Geologic range. — Upper Ordovician (White- house group of Girvan; Richmondian of Gaspe, Quebec); widely in the Silurian; Lower Devonian (New Scotland, United States) and early Middle Devonian (Bohemia, Ggi). American Species Bilobites acutilobus (Ringueberg) 1888 B.bilobus (Linna:us) 1767 B. indentus Cooper 1930 B. varicus {Cor\r3iA) 1838 European Species Bilobites bilobtts (Linnasus) 1767 B. verneuilianus Lindstrom Orthis dimera Barrande Discussion. — External form most readily distin- guishes Bilobites from all other genera of the Dal- manellacea. Internally the ventral musculature and cardinal process are decidedly dalmanelloid. The brachiophores appear to be modified dalmanelloid, bladelike, and suggest an origin out of Dalmanella. Beecher®" has figured a deltidium perforated at the apex in young Bilobites, but such a structure has not been observed in any of the mature forms studied or in any other punctate shell. If this actually exists only in the young of Bilobites, it is one more crumb of evi- dence that a delthyrium covered by a deltidium is actually the primitive condition in early brachiopod Pal. Polonica, vol. 1, 1929, p. 70. 'Studies in Evolution, 1901, p. 402. GENERA OF THE SUBORDER ORTHOIDEA 131 growth stages, as is so forcibly indicated by the occur- rence in the early Cambrian of Nisusi/i and the later Billingsellidx with their covered delthyria. Beecher'"" has suggested the derivation of BUob'ttes "from a radicle haying, in many respects, the charac- ters of the group represented by PLitystrophia bijorata. This statement has been construed by Cumings and others as "a virtual demonstration of the derivation of BUob'ttes from Platystrophia." Beecher and Cumings showed the external resemblances between the nepionic stages of the two genera, but the former merely pointed out that the two may have come from a platystrophoid radicle. This is true from the standpoint of exteriors, but certainly the internal features of Bilobiies and Platptrofhia are diverse. On an earlier page it has Genus MYSTROPHORA Kayser 1871 PI. 16, figs. 1-5; t. figs. 20, 21 Kayser, Zeits. deut. geol. GcscU., vol. 23, 1871, pp. 612- 614, pi. 13, fig. 5. Genolectotype (Williams and Breger 1916). — Orthis areola Quenstedt 1871, Petref. Deutschl., p. 589, pi. 57, fig. 27. Description. Exterior. — Transversely subquad- rate to subsemicircular in outline; cardinal extremi- ties obtuse or angular; lateral profile plano-convex; anterior commissure sulcate, sulcus deep, ventral fold low, marked by a median costella. Ventral palintrope long, strongly apsacline, beak slightly curved; del- ^ ^ ^ Aor Op. cit., p. 404. '"Amer. Jour. Sci. (4), vol. 15, 1903, p. 40. thyrium usually open but may be closed at the apex by callus. Dorsal interarea short, plane, anacline; notothyrium partially closed by the cardinal process. Surface finely multicostellate ; shell fibrous, punctate. Ventral interior. — Delthyrial cavity deep; dental plates low, divergent. At the apex between the dental plates there is a flat callus deposit, probably for pedicle attachment. The cavity beneatli this is usually com- pletely filled by punctate shell substance. Dorsal interior. — Notothyrial cavity deep; cardinal process with a long shaft and lobate myophore. Brach- iophores elongate, pointed. Brachiophore supports con- vergent and uniting with a median septum, which is so high as to divide the shell into two chambers. Muscu- lature unknown. Geologic range. — Middle Devonian. European Species Orthis areola QncnUcil 1871 O. deshayesi Rigaux 1873 Scenidium jallax GUrich 1 896 | p^^yication not seen S. folonkum Ourich 1 896 ) Distinguishing characters. — The name My- strophora has usually been regarded as a synonym of Skenidium, despite the fact that its author showed that some of the shells to which he applied the name were 132 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA actually punctate. Mystrofhora, however, differs so strongly from Skenidium in other details that it could easily have been determined as generically distinct with- out the aid of punctation. The lobate cardinal process, together with the punctation, at once establishes Mystro-phora as a dalmanellid. It differs further from Skenidium in the ventral valve, which has no spondy- lium; instead, the divergent dental plates are attached directly to the floor of the valve. In the apex, grow- ing to the inner sides of the dental plates, is a flat plate, probably for pedicle attachment, similar to that seen occasionally in Schizofhoria and other members of the Schizophoriidas. Discussion. — In describing Mystrofhora, Kayser did not definitely designate a genotype. Underneath Fig. 21. — Mystrofhora areola (Quenstedt). Section about 2 mm. from the beak, showing relations of apical plate and dental lamellae, x ca. 12. the heading "Subgenus Mystrophora" is placed a refer- ence to Quenstedt's figures. At the end of his discus- sion, he names as typical species "M. Lewisii Davids." from the Silurian and "M. areola Quenst." in the order here given. Kozlowski^"' makes the suggestion that if future work on Skenidium and Mystrophora should prove the former to be punctate, the name Skenidium could be used for the punctate species and Mystrophora with "M. lewisii" as the type for the impunctate group. In the absence of definite informa- tion one way or the other, he includes both punctate and impunctate forms under Skenidium. It is clear from Kayser's text that he regarded Orthis areola as the type of his subgenus. This view was also held by Williams and Breger,^*"* who say, "The type of Mystrophora was Quenstedt's Orthis areola." Schuchert and LeVene'"* more recently cite Quenstedt's species as the type, and it is so regarded in the present study. Williams and Breger evidently considered Mystro- phora a subgenus of Dalmanella in describing D. (M.) elevata. Their species is clearly a dalmanellid, and probably a Wattsella if one can depend on the ventral musculature. Mystrophora is an aberrant and specialized dal- manellid paralleling Skenidioides in its internal dorsal "^Tal. Polonica, vol. 1, 1929, pp. 46-47. "' U. S. Geol. Surv., Prof. Paper 89, 1916, p. 61. "* Foss. Cat., Pars 42, 1929, p. 86. Structure, but this feature has not yet evolved into the large hinge-plate of Skenidium. Genus KAYSERELLA Hall and Clarke 1892 PI. 16, figs. 7, 8, 10 Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 1892, p. 259, figs. 15-17. Genoholotype. — Orthis lepida Schnur 1853, Pal- aeontogr., vol. 3, p. 218, pi. 45, figs. 9a, b. Description. Exterior. — Dalmanelloid, subcircu- lar in outline; hinge-line narrower than the greatest width of the shell; cardinal extremities obtuse; lateral profile unequally biconvex, the ventral valve with the greater convexity; anterior commissure rectimargin- ate; dorsal valve provided with a shallow sulcus. Dorsal interarea long, nearly plane, strongly apsacline, beak very gently curved; delthyrium with elevated margins, partially covered by a convex deltidium; dorsal interarea short, anacline, notothyrium closed by the cardinal process and narrow chilidial plates. Sur- face multicosteUate. Shell punctate. Interior. — There was only one specimen of this genus in the Schuchert Collection, and sections of it proved disappointing because the shell is greatly min- eralized. However, a few points regarding the in- ternal structure were obtained. The delthyrial cham- ber is deep and surrounded by adventitious shell, which is also connected with the deltidium. Dental plates were not observed. In the dorsal valve the brachio- phores are long, but the brachiophore plates could not be seen. The cardinal process is lobate as in other dalmanellids. Anterior to the cardinalia the median septum is extended so as nearly to touch the inner wall of the ventral valve, giving exactly the same appearance as in Mystrophora. Geologic range. — Middle Devonian of Germany. Discussion. — It has been customary for taxonomists to consider Kayserella a streptorhynchid, but, as Kozlowski'"^ has pointed out, the shell is punctate exactly as in Dalmanella. This excludes the genus at once from Streptorhynchus and its allies, since, as this author has also shown, the Strophomenacea all have impunctate shells. Furthermore, the cardinal process is clearly dalmanelloid and not strophomenoid. These two characteristics, then — endopunctate shell and or- thoid cardinal process — are sufficient to place Kayser- ella among the Dalmanellacea. The genetic relations of the genus are not yet clear, but as it is best to refer it, on the basis of our present imperfect knowledge, to the family with which it most closely agrees, we place it provisionally with the Mystrophoridae, though it probably does not belong here. We lay least family value on the sporadic reap- pearance of the deltidium and chilidium, and most on the internal features and the dalmanelloid exterior. 10" Pal. Polonica, vol. 1, 1929, p. 89. GENERA OF THE SUBORDER ORTHOIDEA 133 Family RHIPIDOMELLID^ Schuchcrt 1913, emended (=partim Rhipidomellinse Schuchert 1913) Subcircular, progressive, and terminal Dnlmanella- cea with broad flabellate diductor scars, completely enclosing the elliptical adductor field in front. Brach- iophores short, without fulcral plates. This large family begins in the early Silurian and dies out in the late Permian. It embraces the follow- ing genera: RhipiHomcUa Oehlert Perditocard'tnia Schuchert and Cooper Plijtyorthis Schuchert and Cooper Thiemella W^illiams 11. The genetic relations seem to be as shown in Table Table 11 Thiemella Platyorthis Rhipldomella- -> Perditocardinia Dalmanella (D. tersa ?) Genus RHIPIDOMELLA Oehlert 1890 PI. 19, fig. 3; pi. 20, figs. 22-24, 26, 27 Oehlert, Jour. Conch. (3), vol. 30 [38], 1890, p. 372. Horn. Rhifidomys Oehlert 1887. Genoholotype. — Terehratula mtchelim L'Eveille 1835, Mem. Soc. Geol. France, vol. 2, p. 39, pi. 2, figs. 14-17. Description. Exterior. — Subtrigonal to circular, anterior margin not uncommonly emarginate, hinge- line narrow; lateral profile unequally biconvex, the brachial valve having the greater convexity, with the ventral one concave at the front in many species; an- terior commissure faintly uniplicate or rectimarginate ; in some instances a sulcus on each valve; ventral interarea the longer, curved, apsacline, umbo swollen or gently convex, beak incurved; dorsal interarea greatly reduced, ortho- to apsacline; delthyrium open, notothyrium usually closed by the cardinal process or partially by chih'dial plates; surface multicostellate, hollow costcUx- numerous. Fibrous, punctate. Ventral intrrior. — Delthyrial cavity shallow; dental plates abbreviated, teeth strong, divergent, elongate; a broadly curved ridge extending from the bases of the low dental plates around the margin of the muscle field; muscle field large, flabellate, not confined to the delthyrial cavity, occupying from one-third to five- sixths the length of the valve and usually deeply im- pressed; diductor scars semiflabcllate, separated from each other by a sharp or low broad ridge, completely enclosing the adductor scars, which form an elliptical patch just anterior to the pedicle callist; adjustor scar commonly discernible on the outside of the diductor scar; pedicle callist occupying the delthyrial cavity. Dorsal interior. — Cardinalia confined, sockets wide, deep, without concave fulcral plates; brachiophores long, bluntly pointed, supported by adventitious sub- stance deposited beneath their anterior edge; sharp processes or points on the ends of the brachiophores have been interpreted as crura; cardinal process large, myophore commonly ponderous, lobatc; shaft short. Median ridge extending to the middle of the shell. Muscle field quadripartite, the posterior scars the larger. Ovarian and pallial impressions occupying the area of the shell not covered by the muscle marks. Geologic range. — Silurian (Clinton) to close of Permian. American Species Rhifidomella aha (Hall) 1863 R. altiroslris Mather 1915 R. arkansana Girty 1911 R.assimilis (Hall) 1859 R. burlingtonensis (HiW) 1858 ^. guadrata Bancioh 1928 Subfamily HARKNESSELLIN^ Bancroft Heterorthidae having biconvex valves, coarser orna- mentation than is usual in the Heterorthinje, a promi- nent fold on the ventral valve and an equally promi- nent sulcus in the dorsal. Embraces the following genera: Harknessella Reed Reuschella Bancroft Stneathenella Bancroft Genus HARKNESSELLA Reed 1917 PI. 20, figs. 6-10 Reed, Trans. Roy. Sec. Edinburgh, vol. 51, pt. 4, 1917, p. 862, pi. 11, figs. 3-7. Genoholotype. — Orthls vespertlllo Sowerby 1839, Sil. Syst., pi. 20, fig. 11. Description. Exterior. — Shell generally small or medium in size, usually subquadrate in outline; hinge- line straight; cardinal extremities acute or more rarely obtuse; lateral profile unequally biconvex, the dorsal valve usually having the greater convexity; anterior commissure sulcate; sulcus deep, ventral fold usually low but prominent. Ventral interarea longer than the dorsal, apsacline; umbo gently convex. Dorsal interarea prominent, moderately anacline, umbo gently convex. Surface costellate to fascicostellate. Shell punctate. "" Mem. and Proc. Manchester Lit. .ind Philos. Soc, vol. 72, 1928, p. 69. '"'Syst. Sil. Boheme, vol. 5, pt. 1, pi. 66, fig. II, 11a and I 4. Distinguishing characters. — Harknessella is distinguished by the contour and profile of the valves, the subquadrate form being rather unusual in punctate shells. In the ventral interior the muscle-scars are dalmanelloid and in the dorsal the cardinalia suggest those of Heterorthls. This structural ensemble, com- bined with a fascicostellate exterior, makes a combina- tion unique among the Dalmanellacea. The external form is similar to that of Carinlferella of the Upper Devonian. Discussion. — The greatest morphologic interest is in the dorsal valve and in the cardinalia. As stated above, the greatest similarity is with Heterorthls. In Harknessella the brachiophores are rather long, extend- ing dorso-ventrally. They are unsupported except for adventitious shell deposited on the inside and along the dorsal edge, which unites them to the valve. This is similar to the condition in Heterorthls, but in Harkness- ella there is a much greater development of the noto- thyrial platform. This is usually swollen about the inside surfaces of the brachiophores, and in some in- stances the platform is nearly flush with the ventral edge of the brachiophores. In front of the notothyrial platform are two deep indentations separated by the median ridge; it is in these pits that the posterior adductor muscles were lodged. The cardinal process is distinctly lobed as in all Dalmanellacea but the shaft may be so swollen as to hide or obscure the original lobation. In one undescribed species the cardinal process is delicate, with a slender shaft but an expanded and lobed myophore. The distinction between Harknessella and Smeath- enella and Reuschella is rather difficult to see if one deals with either the actual shell or a wax replica thereof, but in internal molds is not so troublesome. GENERA OF THE SUBORDER ORTHOIDEA 139 In HarknessflLi (see pi. 20, fig^. 8, 9) the base of the brachiophores and their supporting tissue are triangular in plan, with the antero-lateral extremity of the tri- angle somewhat drawn out. The adductor pits are represented by two posteriorly directed acute lobes on each side of a median depression. Other differences are discussed farther on. Harktu-sscUa is not uncommon in the Middle Ordo- vician (Caradocian) of the British Isles but is so far unknown in North America. Subgenus REUSCHELLA Bancroft 1928 PI. 20, figs. 11-15 Bancroft, Mem. and Proc. Manchester Lit. and Philos. Soc., vol. 72, 1928, p. 180, pi. 2, figs. 9-12, t. fig. 5. Genoholotype. — R. semiglohata Bancroft 1928. Description. Exterior. — Shells commonly rather large, subquadrate in outline, hinge-line straight, car- dinal extremities acute; lateral profile unequally bicon- vex or convexo-concave, the dorsal valve having the greater convexity. Anterior commissure more or less strongly sulcate; dorsal sulcus deep in young stages, obsolete or prominent in old stages; ventral fold sub- carinate. Ventral interarea the longer, strongly apsa- cline ; dorsal interarea anacline or orthocline. Umbos gently convex. Surface costellate to fascicostellate. Shell punctate. Ventral interior. — Delthyrial cavity shallow; teeth strong; dental plates stout, flaring, rather short, con- tinued as ridges on the lateral margins of the muscle area. Muscle field pentagonal in outline, gently bilobed in front. Adductor field linear, strongly im- pressed; diductor scars elongate, tear-shaped. Adjus- ter scars rather large. Pallial markings unknown. Dorsal interior. — Notothyrial cavity obsolete, as it is filled completely by the notothyrial platform and the cardinal process; brachiophores bladelike as in Heter- orthis, cemented to the valve in front by adventitious substance. Sockets deep; cardinal process with a lobate myophore, shaft slender or swollen. Adductor impressions occupying troughs bounding the median ridge. Geologic range. — Middle Ordovician (Caradoc- ian-post-Harnage) of the British Isles, with the follow- ing species: Reuse hella bilobata (Sowerby) 1839 R. horderleyensis Bancroft 1928 R. semiglobataBancioh 1928 Discussion. — Reuschella differs from Harknessella in its coarser ornamentation and in details of the car- dinalia. Bancroft's description of this genus was made entirely from internal molds, and the distinctions from Harknessella that he points out and figures appear very slight. From specimens kindly sent to us by Bancroft we could discern no fundamental differences in the cardinalia. We recommend therefore that Bancroft's genus be considered a subgenus of Harknessella. Genus SMEATHENELLA Bancroft 1928 PI. 20, figs. 1-5 B.incroft, Mem. and Proc. Manchester Lit. and Philos. Soc., vol. 72, 1928, p. 177, pi. 2, figs. 1-5. Genoholotype. — S. hamagensis Bancroft 1928. Description. Exterior. — Shell rather large, sub- quadrate in outline; hinge-line straight; cardinal ex- tremities usually obtuse; lateral profile unequally biconvex or convexo-concave, the dorsal valve having the greater convexity. Anterior commissure sulc.ite; ventral fold strongly carinate. Interareas subequal, ventral interarea apsacline, dorsal anacline; umbos gently convex. Surface unequally costellate as in Rafinesquina. Ventral interior. — Delthyrial cavity confined, shal- low; dental plates short and stout, muscle area rhom- boidal in outline. Dorsal interior. — Like Harknessella; notothyrial cavity closed by the cardinal process and notothyrial platform. Brachiophore plates slender, heterorthoid. Sockets deep, median elevation corresponding to ex- ternal large sulcus. Geologic range. — Middle Ordovician (Carodoc- ian) of the British Isles, with the single species S. har- nagensis Bancroft. Distinguishing characters. — Smeathenella is very closely allied to Harknessella but differs in its ornamentation, lateral profile, and in details of the cardinalia. The ornamentation has been described by Bancroft as "markedly Rafinesquinoid," that is, there are coarser costells separated by many fine costellas. This ribbing, in connection with a much less convex dorsal valve than is usual in Harknessella or Reuschella, will serve to distinguish the genus. Other differences setting apart this genus from the other two are to be found in the interior of both valves. The dental plates are usually proportionally shorter and the mus- cle field is smaller. In the dorsal valve the cardinalia are more slender and the adductor pits not as deeply excavated. Family SCHIZOPHORIID^ Schuchert 1929 Progressive and terminal Dalmanellacea, probably derived out of the Dalmanellidas,^"* having biconvex or lenticular valves in the older formations and in the younger growth stages, but in mature and old forms with the dorsal valve usually larger than the ventral. Surface ornamented by costellse, often hollow. In the '"* PionoJema, earliest of the Schizophoriidx, appears simultaneously with the DalmancUidx. It is therefore diffi- cult at present to state with assurance what stock gave rise to Schizofhoria and allies. 140 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA aberrant Enteleti'nae this ornamentation is superim- posed over broad undulations or plicae. Shell fibrous, punctate. The ventral interior is especially marked by promi- nent diductor scars, separated by a median ridge which becomes more and more elevated in the more advanced and geologically more recent genera. The dorsal cardinalia consist of tusklike brachiophores having widely divergent supporting plates; the cardinal process is typically dalmanelloid; the dorsal adductor scars are separated by an oblique ridge in the later forms. The cardinalia, being alike throughout the family, are its most significant feature. The dorsal pallial sinuses consist usually of three pairs; the two inner ones, given off at the anterior ends of the anterior adductor scars, bifurcate immediately at their inception and ex- Table 13 Enteletinae Schizophoriinae ■ -> Isorthina: tend to the front margin as four subparallel trunks. The third pair is given off from the oblique ridge sepa- rating the adductor scars and extends to the front margin nearly parallel to the other sinuses. The inner four trunks are usually the more prominent. Geologic range. — Ordovician (Chazy) to Permian. The Schizophoriidas are subdivided into the follow- ing subfamilies: Schizophoriinffi Schuchert Enteletinse Waagen Isorthinas Schuchert and Cooper The genetic relations appear to be as shown in Table 13. Discussion. — The Schizophoriidas is one of the most easily recognized and longest lived families of the Dalmanellacea. It has its inception in the early Middle Ordovician (Chazy) with Pionodema as the oldest representative of the family, starting with the biconvex or lenticular shell phase. The passage from Pionodema to Schizophoria appears to be almost im- perceptible. In Pionodema the ventral septum is low and long, being of the euseptoid type (Fredericks), and is the place of attachment of the adductor muscles. This septum increases in height during geological time and attains its final development in Orthotichia and Enteletes. In Pionodema the adductor scars are clearly visible on the median ridge, and although in the geologically higher forms no adductor scars are visible on the floor of the valve, the presumption is that they were also borne on the septum. The cardinalia are nearly uniform throughout the Schizophoriidae from Pionodetna to the last of the Enteletes in the Permian. The brachiophores are always shaped like the tusk of a boar, sharp and curved. The supporting plates are widely divergent and al- ways have a small fulcral plate defining the socket. The cardinalia are perhaps the most distinctive struc- tures in the whole group and their uniformity from genus to genus throughout the range of the family is a rather definite expression of the value of the cardinalia as a family characteristic. The modification of the dorsal musculature from Pionodema to Orthotichia has some value in chrono- logical stratigraphy. In Pionodema the adductor scars have a quadripartite arrangement, the posterior scars directly in back of the anterior pair and separated by horizontal ridges. Later, however, in Devonian and Mississippian times, the posterior adductors have migrat- ed laterally and taken a position outside of and posterior to the anterior pair. These scars, further, are rather oval in outline instead of subcircular or quadrate as in the earlier forms, and are separated by an oblique ridge. The two oval scars one above the other pro- duce a subflabellate appearance much like that of the ventral valve of Rhifidomella. The lateral migration of the posterior scars leaves a large central area occu- pied only by the much reduced median septum. The higher, more specialized forms such as Orthotichia, Enteletes, Parenteletes, etc., continue the dorsal mus- culature of the later Schizophorias. Subfamily SCHIZOPHORIIN^ Schuchert 1929 Lenticular, or convexo-concave Schizophoriidae not having numerous strong undulations or plicae developed at the front end of the shell. Geologic range. — Middle Ordovician (Chazy) to Permian. Embraces the following genera: Pionodema Foerste Schizophoria King Orthotichia Hall and Clarke Aulacophoria Schuchert and Cooper The genetic relations are thought to be as shown in Table 14. In this subfamily we place Pionodema as the origi- nating stock of the Schizophoriidae, this genus giving rise to Schizophoria out of which came the culminat- ing Orthotichia. Pionodema and Schizophoria form interesting homoeomorphs with Schizophorella and GENERA OF THE SUBORDER ORTHOIDEA 141 Hebertella of the Plectorthin.r. Schizophorclla is the lenticular type of hebertelloid evolution, just as Piotwderna represents the same phase in the develop- ment of Sch'tzofhor'ut. The convexo-concave profile of Hebertella corresponds to the similar shell form in Schizophoria. Genus PIONODEMA Foerste 1912 PI. 23, figs. 1-10, 12-14; pi. 29, fig. 1 Foerste, Bull. Denison Univ., vol. 17, 1912, p. 139. Cooper, Jour. Pal., vol. 4, 19.30, pp. 369-382. Horn. Bathyc Aulacophoria ->Enteletin« Schizophoria *. -^ ISORTHIN^E ' Pionodema Description. Exterior. — Semi-oval or subglobose, margins convex, cardin.-il extremities obtuse, hinge-line slightly narrower than the greatest width of the shell, lateral profile lenticular or globose, the anterior part of the ventral valve becoming concave at the front; anterior commissure unisulcate or uniplicate; ventral interarea longer than the dorsal, gently curved, apsa- cline, beak slightly incurved, umbo prominent; dorsal interarea curved, orthocline, umbo convex, sulcate; delthyria open or partially closed by an apical plate; multicostellate, with hollow costellx; shell fibrous, punctate. Ventral interior. — Delthyrial cavity deep; teeth small, triangular; crural fossettes oblique; dental plates sharply defined, widely divergent, continued for- ward as a slight ridge of adventitious shell for a short distance along the lateral margin of the muscle area; muscle area not confined to the delthyrial cavity, longer than wide; diductor scars divergent, subsemi- flabellate ; adductor impressions consisting of two elon- gate semielliptical scars on a small ridge between the diductors; adjustor scars elongate, situated at the base of the dental plates. Apex closed by a small apical plate, the front of which is bevelled slightly below the level of the interarea. This plate clearly served for the att.ichment of the pedicle. Ovarian impressions occupy the umbo-lateral spaces. Dorsal interior. — Cardinalia confined ; brachiophore supporting plates not separable, vertical or nearly so, divergent at their bases; brachiophores sharp, diver- gent, shaped like a boar's tusk; sockets small, excavated beneath the palintrope, defined by a small concave ful- cral plate which also serves to bind the brachial appara- tus to the wall of the valve; cardinal process very small, expanded and bilobed posteriorly, its shaft being extended forward to merge with the low median ridge that extends forward nearly to the middle of the shell. Muscle area very faintly impressed, anterior adductor scars the larger. Geologic range. — Early Middle Ordovician (Chazy) to Upper Ordovician (Maysville). American Species Dalmanella bellula (Meek) 1873 Z). «>c«/r»Vw Stauffcr 1918 S. resufinoitles (Cox) 1857 5. sedalie?tsis Wellcr 1914 5. senecta Hall and Clarke 1 892 S. j/r«a/tt/d (Schlothcim) 1813 S.striatula tnarylanJ tea Clarke and Swartz 1913 S.swallovi (Hall) 1858 5. /«//«'<;««/ (Vanuxem) 1842 Rhifidomella iubelliftica (White and Whitfield) 1 862 Foreign Species Schizofhoria beaumonti (Verneuil) 1849-1850 S.fragilis Kozlowski 1929 S. injracarbonica Janisevskij 1911 S. interstriata Janisevskij 1911 S. frovulvaria (Maurer) S. resufinaia {MiiUn) 1809 S. resupinata lata Demanet 192 1-1923 S. resupinata rotundata Demanet 1921-1923 5. striatula (Schlotheim) 1813 S. fw/fijrw (Schlotheim) 1820 Distinguishing characters. — Schizophoria is distinguished by its convexo-concave profile, the diver- gent or subparallel diductor scars in the ventral valve separated by a low median ridge (euseptoid) which bears the adductor marks, and in the dorsal valve by the widely divergent crural apparatus, characteristic muscle marks, and pallial trunks. The adductor mus- cles are separated by a curved, oblique ridge, a feature which at once separates this genus from Proschizo- phoria. The pallial marks consist of four or six sub- parallel trunks. Schizophoria closely resembles Hebert- ella externally, but the fundamental difference in shell structure and cardinalia serves to differentiate them immediately. The similarities and differences between Orthotichia and the genus under discussion are pointed out under the former genus. Discussion. — Schizophoria is a long-ranging genus and for this reason shows considerable variation in its internal anatomy. In the ventral valve the variation is seen in the musculature and the dental plates. The diductor scars vary from subparallel and extremely elongate in some species (S. vulvaria and S. beaumonti, Lower Devonian of Germany) to widely divergent and large or small (5. propinqua, S. provulvaria, and S. niultistriata). In S. iowensis the muscle area is obcordate and much like that of Hebertella. Occa- sionally the adjustor scars are considerably developed, 144 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA with the result that the muscle area is notably ex- panded in front and is a trapezium in outline. The median ridge carrying the adductor scars varies from thin to very wide and in a few forms (S. lowensls, S. striatula) is extended forward beyond the anterior margin of the muscle area. In some specimens of Schizophoria from the Pennsylvanian the median ridge has attained the profile and height seen in Orthotichia. In the ventral valves of S. resufinata, there are two palHal sinuses extending forward from the anterior ends of the diductors and bowing outward in a con- siderable curve. In the dorsal valve there are some notable variations in the musculature and pallial markings. In speci- mens of the genotype, S. resufinata, the arrangement of the adductor scars is nearly identical to that seen in Orthotichia, with the exception that the anterior scars are not in contact and that the space between the longitudinal sets is not so great as in the latter genus. In the early Schizophorias the muscle field is compact and the right and left sets of adductors are in close proximity, being separated only by a low median ridge, but in the later forms the longitudinal sets migrate laterally so that there is a notable space between them occupied only by a remnant of the formerly large median ridge. This lateral spreading of the two longi- tudinal sets of muscles reaches its maximum in Orthoti- chia. The generic difference between Orthotichia and Schizophoria, therefore, is certainly not very great. The pallial markings are usually well developed, especially in the dorsal valve ; S. macjarlani shows the common type. This species has four parallel trunks extending anteriorly from the front margin of the anterior adductors to the shell margin, scarcely branch- ing during their passage. Rarely there are six of these trunks, the two outside ones corresponding to the lateral trunks of horthis. So deep are these sinuses in S. macjarlani that they have impressed themselves deeply in the shell, so that the slightest exfoliation of the shell laminze shows these sinuses as ridges. The so-called S. profinqua (Hall) has pallial markings of the horthis type and has accordingly been placed with that genus. The earliest undoubted Schizophoria is S, senecta from the Clinton of New York and probably also from Port Daniel, Quebec. Therefore the group must have its inception in the Ordovician, in Pionodema, whose external resemblance to Schizophoria has already been remarked upon. Furthermore, in the ventral valve of Pionodema there is an incipient adductor ridge and the structure of the cardinalia in the dorsal valve is identical in every fundamental detail with that of Schizophoria. Genus ORTHOTICHIA Hall and Clarke 1892 PI. 24, figs. 12, 15,22-24,27 Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 1892, p. 213, pi. 7, figs. 11-15. Genoholotype. — Orthis ? morganiana Derby 1874, Bull. Cornell Univ., vol. 1, p. 29, pi. 3, figs. 1-9, 11, 34, pi. 4, figs. 6, 14, 15. Description. Exterior. — Externally like Schizo- phoria but usually more finely ornamented. Ventral interior. — Umbonal cavity deep ; teeth strong; dental plates strong, divergent, extended nearly to the middle of the valve as high septal ridges about the lateral margins of the muscle area; the cavity defined by these plates is centrally divided by a median septum, which takes its origin a short distance in front of the apex, rising to its maximum height a little in front of the ends of the dental plates, where it is abruptly truncated. Diductor and adjustor scars confined to the areas between the dental plates and the median septum; adductor scars attached to the sides of the median septum. Dorsal interior. — Cardinalia strong; brachiophores strong, tusklike, supported by strong plates that diverge around the lateral margins of the muscular area; all of these structures are intimately fused together; the sockets are deep and are defined by a thin concave ful- cral plate which unites the crural apparatus to the palintrope. Cardinal process small, "multifid"; muscle area quadripartite, the anterior adductors subtriangular or irregularly oval, situated so that their antero-median extremities are in contact, the posterior extremities being divergent and separated by the expanded ex- tremity of the low median ridge; the anterior adduc- tor scars on the outside of and slightly posterior to the anterior pair and separated from them by a narrow, curved, oblique ridge. Anterior scars narrow sub- parallel grooves separated by a low ridge. Pallial markings as in Schizophoria. Geologic range. — Pennsylvanian to Permian. Distribution world wide. American Species Orthotichia morganiana (Derby) 1874 O. schuchertensis Girty 1903 O.kozLowskii King 1930 (= 1931) ? O. texana Girty 1928 Foreign Species Orthotichia chekiangensis Chao 1927 O. free hi Fliegel O.indica (Waagen) 1884 O. marmorea (Waagen) 1884 O. morganiana (Derby) 1874 O. morganiana chihsiaensis Chao 1927 Distinguishing characters. — Orthotichia is dis- tinguished by the presence of three strong septal plates in the ventral valve, the outer two being the divergent dental plates; the inner or median ridge rises to a crest at the front of the muscle area as in Enteletes and is obliquely truncated. This genus most closely resembles GENERA OF THE SUBORDER ORTHOIDEA 145 certain forms of Mississippian and Pennsylvanian Schiz- ophorut but differs from them in the degree of devel- opment of these plates. In Orthotkh'ui the dental plates are always high and extend to the front of the muscle area as high ridges. Some Pennsylvanian species of Schizophorui approach Orthot'tch'ui in the accelerated development of the median septum but in them the dental plates have not attained the specialized character of those in Orthotichla. Discussion. — The dorsal valve has the general characteristics of the Pennsylvanian Schizofhoria. The posterior adductor scars are situated behind and out- side of the anterior impressions, being separated by a low oblique ridge that is antero-laterally directed. The individual scars are rather elongate, tapering posteriorly but expanded toward the front. This gives the mus- cle-scars a flabellate outline where viewed in the aggregate. It was Derby's idea'** that there were six adductor impressions in the dorsal valve. The smooth spaces on each side of the low median ridge were interpreted as muscle-scars but there is no crenu- lation or any other sign of muscle attachment in the three spaces. Hall and Clarke*" also suggested a third pair of dorsal muscles in Schizofhoria macfarlani. The specimen to which they refer is an old obese shell in the Schuchert Collection. What appear to be lateral muscles are actually pits left by the incomplete cover- ing up of the brachiophore apparatus by adventitious shell. Orthotichia structurally forms the passage between Schizofhoria and Enteletes but this does not necessarily mean that the latter actually developed out of Ortho- tichia. Enteletes probably developed directly out of the more plastic Schizofhoria, and Orthotichia may then be a terminal branch of the Schizofhoria line. Genus AULACOPHORIA Schuchert and Cooper 1931 (Gr. aulax, furrow; fhoreein, to bear) PI. 29, figs. 2, 5, 10 Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, p. 247. Genoholotype. — Orthis keyserlingiana De Ko- ninck 1843, Desc. Anim. Foss. Terr. Carb. Belgique, p. 230, pi. 13, fig. 12. Description. — Aulacofhoria is proposed for shells having an interior like that of Schizofhoria but having a deep sulcus on the dorsal valve and a strong fold on the ventral one. Besides the genotype, two other European species are placed in this genus, Enteletes injracarbonica Janisevskij from the Lower Carbonifer- ous of the eastern Urals in the vicinity of Khabarny "" Bull. Cornell Univ., vol. 1, 1874, p. 30. "'Pal. N. Y., vol. 8, pt. 1, p. 214. and E. uralica Gorsky from the Middle Carboniferous of the eastern slope of the Urals. Schizofhoria {Enteletes?) mesoloba Janisevskij may be another form belonging in this genus. In the dorsal valve the brachiophore plates are widely divergent as in Schizofhoria and Enteletes, and the sockets are defined by fulcral plates as is usual in the family. In the ventral valve the prominent dental lamellae are subparallel but no prominent median sep- tum was detected, such as occurs in Orthotichia or in Enteletes. The ornamentation is very much like that of Enteletes, much more so than like Schizofhoria. Aulacofhoria thus occupies an intermediate position structurally between Enteletes and Schizofhoria. It differs from the former in being less ventricose and inflated and in not possessing the prominent ventral median septum. The great development of the median septum in Enteletes, according to Gorsky, may be an adaptation to the great inflation of the valves to accom- modate the attachment of the adductor or closing muscles. Gorsky"* maintains that the type of structure shown by Enteletes injracarbonica Jan. represents the earliest phase of Enteletes. It is our idea, on the other hand, that Aulacofhoria is a terminal offshoot from Schizo- fhoria. We base this conclusion on the greater re- semblance of the interior to Schizofhoria than to Enteletes. Subfamily ENTELETIN^ Waagen 1884 Aberrant, globular or strongly biconvex Schizo- phoriidas that are usually strongly plicate in front, or atavistically ? smooth. Geologic range. — Pennsylvanian and Permian. Embraces the following genera: Enteletes Fischer de Waldheim Enteletina Schuchert and Cooper Parenteletes King Enteletella Likharev Enteletoides Stuckenberg The genetic relations seem to be as shown in Table 15. Discussion. — The Enteletinx in their compara- tively short life span have tried to adopt several of the features developed in other groups by more tedious routes of evolution. Parenteletes tried the cella or camera of Mcrista. Enteletella developed a spon- dylium for muscle attachment. Further, there was no stability in the development of the fold and sulcus. This trait is not unusual among the orthids but in the Enteletinx a ventral fold became persistent in two '" Bull. Cora. Geol. Leningrad, vol. 43, no. 9, 1927, pp. 1184-1186, pi. 18, fig. 9. 146 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA stocks and therefore of use in the definition of two genera along with other features. The evolution of the Enteletinae is somewhat analo- gous to that of the Derbyias and Meekellas which lived at about the same time. In the last named genus the shell also develops broad undulations at the front as in Enteletes. It would be interesting to know what conditions of ecology in the Pennsylvanian and Permian seas could have produced such parallel devel- opments in these two different stocks. Or is it an expression of old age in these terminal stocks that arose so far back in the Ordovician? Table 15 Ente etina Enteletes Parenteletes * Enteletella Enteletoides Schizophoria Genus ENTELETES Fischer de Waldheim 1825 PI. 24, figs. 1-3, 5-10, 25 Fischer de Waldheim, Notice sur la Choristite, 1825, p. 6; Oryct. Gouv. Moscou, 1830-1837, p. 144, pi. 24, figs. 10-11 {choristites ^^ lamarcki) , pi. 26, figs. 6-7 {glaber). Genoholotype. — E. chorhtltes Fischer de Wald- heim 1825 =^Sfirifer lamarcki. Description. Exterior. — More or less globular, margins convex; hinge-line narrower than the great- est width of the shell; cardinal extremities rounded; lateral profile unequally biconvex, the dorsal valve being the more convex; lateral commissure dorsally deflected near the hinge; anterior commissure unipli- cate; ventral interarea the longer, curved, apsacline, beak strongly incurved, umbo swollen; dorsal inter- area short, curved, orthocline to apsacline, beak strongly incurved, umbo tumid; delthyrium open. notothyrium partially closed by the cardinal process. Surface finely costellate, and plicate anteriorly. Shell fibrous, punctate. Ventral interior. — Umbonal cavity deep ; teeth strong ; crural fossettes oblique ; dental plates strongly developed, advancing, slightly convergent or subpar- allel; the space between these plates divided medianly by a thin bladelike septum rising to an apex at its anterior extremity, truncated sharply in front so that it terminates in a line with the anterior termination of the dental plates; muscle area compressed laterally; diductors and adjustors confined to the narrow areas between the dental plates and the septum, while the adductors were probably attached to the latter. Dorsal interior. — Cardinalia strong; brachiophores strong, curved, supported by thin plates that are ex- tended forward and around the lateral margins of the muscle area as far as the anterior margin. Sockets strong, defined by a socket-plate on the outside margin of the socket; along the hinge-line a strong denticle; cardinal process small ; muscle area like that of Ortho- tichia, the anterior adductor scars large, subcircular; the posterior adductors forming two small scars on the outside of and slightly posterior to the anterior adduc- tors and next the ridges from the brachiophore supports. Geologic range. — Middle Pennsylvanian to Per- mian in many parts of the world. American Species Enteletes andii {T>'Oih\gnY) 1842 E.dumbleiGnxy 1908 E . hemiflicata ( H all ) 1852 E.leonardensis King 1930 (= 1931) E.Uumbonus King 1930 (= 1931) E.flummeri King 1930 (= 1931) E. u'oljcamfensis King 1930 (= 1931) E.wordensis King 1930 (= 1931) Foreign Species Enteletes contractus Gemmellaro 1 898 E. elegans G&cameAixo 1898 E. haugi Gemmellaro 1 898 E. hemiflicatus naia Fredericks 1923 E. kayseri Waagen 1 884 E . Itevissimus Waagen 1884 E. lamarcki Fischer de Waldheim 182 5 E. meridionalis Gemmellaro 1 898 E. microflocus Gemmellaro 1 898 E . oehlerti GemmcWiTO 1898 E . subiiquivalvis Gemmellaro 1898 E. tschernyscheffi Diener E. waageni Gemmellaro 1898 ? E. fentamera Eichwald Distinguishing characters. — Enteletes is dis- tinguished externally by its elliptical outline, globular profile, and plicate anterior. Internally the subpar- allel dental plates and sharp, crested median septum GENER.A OF THE SUBORDER ORTHOIDEA 147 are diagnostic characters. From Orthotichia it differs in the presence of the anterior ph'cations and in the sub- parallelism of the dental plates. It differs from Entele- t'tna and Parentcletes in having the fold on the dorsal valve. EnteleteUa differs from this genus in the posses- sion of a spondylium. Discussion. — Waagen**^ proposed two groups of EnteUtes: ( 1 ) the "ventrisinuate" group having the fold on the dorsal valve; and (2) the "dorsosinuate" group which has the fold on the ventral valve. King^"" has recently separated the latter group as a new genus called Parenteletes. The new generic group is, however, not homogeneous and should be further split into two groups on the basis of internal structure. The term Enteletes, then, must be re- stricted to shells of the E. lamarcki and E. hemiflicatus type, which have the fold and sulcus in the usual position. Specimens having a small sulcus in the fold and a plication in the sulcus are not excluded from Enteletes; this condition does not alter the uniplicate character of the anterior commissure and is not con- sidered of sufficient import for the separation of a new group. VVaagen*"^ suggested that Enteletes evolved from Orthotichia by the development of plicas in the anterior part of the shell, and this idea has gained quite general acceptance. Enteletes is the culmination of a long line of orthid evolution that began in Chazy time with Pionodema. It appears in the Middle Pennsylvanian and ranges through the Permian. By loss of the plicas in late Permian time it is thought to have returned to the ancestral Orthotichia-like condition, this atavistic phase of Enteletes being termed Enteletoides by Stuckenberg. It is thought by the writers that Orthotichia, which ranges from the Pennsylvanian to the Permian, is probably a terminal stock of the Schizophoriidas and may not have given rise to Enteletes. We suggest the separate development of Enteletes out of Schizophoria in the Pennsylvanian at about the same time as the appearance of Orthotichia. Gorsky'"^ has claimed the development of Enteletes from Orthotichia by way of simply plicated stocks like Aulacofhoria. We hold, rather, that Aulacophoria is a side line out of Schizophoria, and one that probably did not survive beyond the middle Pennsylvanian. Genus PARENTELETES King 1931 PI. 24, figs. 11, 13, 14, 16, 18, 21, 26 King, Bull. 3042, Univ. Texas, 1930 (1931), p. 48, pi. 1, figs. 16-20; pi. 2, figs. 1-3. "• Mem. Geol. Surv. India, Pal. indica, ser. XIII, vol. 1, pt. IV', fasc. 3, pp. 5 5 3-563. '" Bull. 3042, Univ. Texas, 1930 (1931), p. 48. "' Op. cit., p. 564. 122 Op. cit., 1927. Genoholotype. — P. cooperi King 1931. Description. Exterior. — Externally like Ente- letes, but somewhat more transverse, dorsosinuate; lateral profile unequally biconvex, the dorsal valve the larger; anterior commissure sulcate ; ventral interarea the longer, apsacline, beak strongly incurved, umbo swollen, delthyrium large; dorsal interarea short, curved, apsacline, beak strongly incurved, umbo tumid, notothyrium wide. Surface multicostellatc and plicate, rugose, with hollow costella-, the plica broad and angular. Shell fibrous, punctate, the punctx arranged in narrow radial rows. Ventral interior. — Delthyrial cavity deep, teeth elongate, sockets pointed; dental plates strong, sub- parallel posteriorly but diverging strongly anteriorly; area between them divided by a median septum which originates very close to the apex, rounded on its dorsal extremity, rising as it progresses forward to reach its maximum height at the point where the anterior ends of the dental plates die out. Here it is abruptly trun- cated and its extremity forms the crest of a V-shaped camera or cella, formed at the point where the internal sulcus corresponding to the fold has its origin. Muscu- lature as in Enteletes. A small plate for pedicle attach- ment is in the apex. Dorsal interior. — Cardinalia and musculature like those of Enteletes. Geologic range. — Upper Pennsylvanian and Lower Permian of Europe and America. American Species Parenteletes cooperi King 1931 Foreign Species Enteletes dieneri Schellwien 1900 E. suessi Schellwien 1892 E. suessi acuticosta Schellwien 1892 Distinguishing characters. — Parenteletes dif- fers from Enteletes in having the fold on the ventral valve, and in the possession of a V-shaped camera or cella under the anterior portion of the median septum. This internal character also serves to differentiate the genus from Enteletina, which closely resembles it externally. King has shown that Parenteletes in America ap- pears in geological time before Enteletes, and this appears to indicate that the genus under discussion evolved separately from Orthotichia or Schizophoria and is not a modification of Enteletes. Girty'"^ and Waagen'"'' have suggested that Parenteletes (the dor- sosinuate group) developed out of an Enteletes having '" U. S. Gcol. Surv., Prof. Paper 5 8, 1908, p. 290. '" Mem. Geol. Surv. India, Pal. Indica, ser. XVIII, vol. 1, 1887, p. 562. 148 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA a small sinus in the dorsal fold and a plication in the sulcus, by enlargement of said plication. While this is possible, the fact remains that Parenteletes was devel- oped earliest. Furthermore, there is much more simi- larity between Orthotichia and Parenteletes in the divergence of the dental plates. Therefore the evi- dence points to an independent development of the two genera out of Orthotichia. E. suessi, figured by Schellwien,^^^ we place in asso- ciation with American species of Parenteletes, as it shows, according to his figure, the same internal char- acteristics as the American forms. ScheUwien re- marked on the external similarity between E. suessi and the Indian E. latesinuatus Waagen, but pointed out the internal differences and on this basis separated the two species; his figure shows the walls of the camera uniting with the dental plates, but in American forms these are discrete. There is a possibility that the figure has not been drawn accurately. The camera of Parenteletes is a remarkable con- vergence toward the type of muscle platform in Merista, Dayia, and Cyclosfira, although the one in Parenteletes differs in bearing a median septum on its crest. Genus ENTELETINA Schuchert and Cooper 1931 PI. 24, figs. 17, 19, 20 Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, p. 247. Genoholotype. — Enteletes latesinuatus Waagen 1884, Mem. Geol. Surv. India, Pal. Indica, ser. XIII, vol. 1, pt. IV, fasc. 3, pp. 559-560, pi. 57, figs. 4-6. Description. Exterior. — Externally Enteletina is identical with Parenteletes but internally it has all of the features of Enteletes. It therefore is essentially an Enteletes with the fold on the ventral valve and com- prises Waagen's division of "dorsosinuates." The dental plates are not strongly divergent and there is a crested median septum which does not have the peculiar V-shaped camera so characteristic of Parenteletes. The presence of this chamber, the function of which is not understood unless it be for muscle attachment, is therefore not a distinction between the Asiatic and American and European "dorsosinuate" Enteletes. The writers have selected Waagen's species E. late- sinuatus to serve as the genotype, as both internal and external characters of it have been admirably figured. The Indian forms are all Upper Permian in age and may represent a development from Enteletes by the enlargement of plica5 in the ventral sulcus, such as sug- gested by Waagen and Girty. (See under discussion of Parenteletes.) We have placed in this genus only "'Abh. k. k. geol. Reichsanst., Bd. 16, Heft 1, 1900, p. 12, pi. l,fig. 18. the Late Permian species of India, and do not include here such Pennsylvanian and Lower Permian forms as E. suessi and variety acuticosta and E. dieneri, since their internal structure is clearly that of Parenteletes. Enteletes globosus Girty, probably from the Word of late Middle Permian age, is placed here provisionally since its internal structure is not known. Geologic range. — Permian of India and Texas. Indian Species Enteletes acutiflicatus Waagen 1884 E. jerrugineaWiigtn 1884 E. latesinuatus Wiigen 1884 E. fentameroiJes Waagen 1 884 E. suMirvis WiAgen 1884 American Species ? Enteletes globosus Girty 1908 Genus ENTELETELLA Likharev 1924 PI. 24, fig. 4 Likharev, Bull. Com. Geol., Leningrad, vol. 43, no. 6, 1924, pp. 719, 721, pi. 5, figs. 1-3. Genoholotype. — E. nikschitschi Likharev 1924. Description. — Enteletella has the external form and ornamentation of a ventrisinuate or true Enteletes, closely resembling E. microflocus Gemmellaro. It differs internally from Enteletes, however, in the possession of a spondylium. The dental plates unite with the median septum in such a way as to enclose a portion of it, which projects as a ridge for the attach- ment of the adductors inside the spondylium. The dorsal edge of this ridge is somewhat club-shaped, and the lower part of the median septum remains free. The dorsal valve is, in all respects, the same as that of Enteletes. The genotype is the only species known. Geologic range. — Lower Permian of northern Caucasia. Genus ENTELETOIDES Stuckenberg 1905 Stuckenberg, Mem. Com. Geol., n. sen, livr. 23, 1905, pp. 59, 129, pi. 6, fig. 8, pi. 9, fig. 8. Gorsky, Bull. Com. Geol., vol. +3, no. 9, 1927, pp. 1184- 1186, pi. 18, fig. 7 (subrossicus) . Genoholotype. — E. rossicus Stuckenberg 1905. Distinguishing characters. — Stuckenberg pro- posed this name for shells having a convexo-concave profile and multicostellate external ornamentation, recalling Orthotichia in both of these characteristics. According to that author the shell completely lacks interareas. Internally the arrangement of the dental plates and median septum is identical with that of GENERA OF THE SUBORDER ORTHOIDEA 149 EnieUtes. In other words, this is essentially an EnteUtes showing atavistic tendencies toward Ortho- Uchia in the loss of the radial plications and globose profile. Discussion. — The absence of interareas, while not impossible, is not convincing from the figures presented by Stuckenberg, especially figure 8e of plate 6. Such an arrangement of the dental plates and their connec- tion with the lateral margins of the shell as indicated suggests a wide hinge-line, and the latter presupposes a palintrope. The shells of Orthotichia and Enteletes are usually thin and very susceptible to crushing; this may be the reason for the apparent lack of inter- areas in Stuckenberg's specimens. Parallelism of the dental plates would be an important clue to deter- mining the ancestry of this form, but the figures of the author are not specially convincing on this point (see his fig. 8b, pi. 9). It may be questioned whether an Enteletes that has atavistically lost its radial orna- mentation would likewise lose its most characteristic internal features and the globose contour of the shell as well. Schellwien has made much of the supposition that many species now referred to Orthotichia may actually represent atavistic Enteletes and for this reason he referred all smooth forms having the internal struc- ture of Enteletes to that genus, placing there even the genotype of Orthotichia. It is important, if it can be proved that atavistic forms of Enteletes actually exist, that these be separated under a new designation and not included in Orthotichia, which is essentially an incipient or potential Enteletes. It is also important to determine when these atavistic tendencies appear. They may have appeared soon after the origin of the genus, occurring several times during the Upper Penn- sylvanian and Permian, or they may have been delayed till near the end of the Permian, in which case the matter is more simple. King (1931) maintains that the Permian Enteletes, in large part, are more strongly plicated than the Pennsylvanian species, and that the forms with faint plications do not even resemble Ortho- tichia in profile. If the atavism is actually carried to the degree of the resumption of the convexo-concave form of Orthotichia, the only reliable clue to the rela- tionship of atavistic shells would be in the parallelism of the septal plates of the ventral valve. The paral- lelism of the plates in Enteletes is frequently more apparent than real, and it is often difficult to evaluate the degree of divergence that does exist. It will thus be seen that it is extremely difficult to establish the fact of atavism in these shells. It seems to the writers that unless the atavistic tendencies are inaugurated in the nepionic stage of Enteletes, Enteletoides should have the globose outline and interior of the former genus. If the return of the previous characters takes place in the nepionic stage, all objections in regard to profile in internal characters noted above lose their force. Another species referred here is Enteletoides sub- rossicus Gorsky 1927, from the Middle Carboniferous on Kamenka River, eastern Urals. Subfamily ISORTHIN^ Schuchert and Cooper 1931 Divergent biconvex Schizophoriidae, having the ven- tral musculature, cardinalia, and pallial markings of Schizophoria. The only known genus is Isorthis Kozlowski, probably derived out of early Silurian Schizofhoria. Genus ISORTHIS Kozlowski 1929 PI. 21, figs. 17-28, 30-33; pi. 23, figs. 15, 19; t. fig. 13 Kozlowski, Pal. Polonica, vol. 1, 1929, pp. 29, 75, pi. 2, figs. 24-41, t. figs. 16-18. Genoholotype. — Dalmanella {Isorthis) szajno- chai Kozlowski 1929. Description. Exterior. — Transversely subellipti- cal, hinge-line straight, narrower than the greatest width of the shell; cardinal extremities rounded. Lateral profile unequally to subequally biconvex; an- terior commissure rectimarginate to faintly sulcate. Dorsal sulcus shallow. Ventral palintrope longer than the dorsal, curved, gently apsacline. Beak curved, umbo swollen; delthyrium open. Dorsal palintrope short, faintly apsacline or faintly anacline ; notothyrium closed by the cardinal process. Ornamentation multi- costellate. Shell fibrous, punctate. Ventral interior. — Delthyrial cavity deep ; teeth large; crural fossettes deep; dental plates strong in young specimens, obsolete in old ones. Muscle field deeply impressed, bilobed in front ; diductor scars elon- gate, subparallel; adductor track elevated, usually nar- row, adductor scars semielliptical when visible ; adjus- tor scars located on the sides of the dental plates, usually not clearly visible. Pallial markings promi- nent, consisting of two main trunks extending antero- laterally from the anterior end of each diductor. Dorsal interior. — Brachiophores as in Schizophoria, i. e., bladelike plates, with fulcral plates and usually supported by adventitious shell (see below). Cardinal process small, bilobed, trilobed, or multilobed, com- monly modified by deposition of adventitious testaceous deposit. Median ridge low. Muscle area subcircular in outline, usually with thickened peripheries. Adductor scars subequal in size. Pallial sinuses consisting of three pairs of trunks as in Schizophoria; two originate at the antero-medial ends of the anterior adductor scars and extend anteriorly; a second set starts just outside the former and extends antero-laterally. The third 150 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA set appears at the line separating the anterior and pos- terior adductor sets, each trunk bifurcating into sub- sidary ones (see discussion below). Geologic range. — Late Silurian (upper Ludlow) to Middle Devonian. American Species Dalmanella arcuaria Hall and Clarke 1 892 D.ferelegans (HaU) 1857 D. fygmtta Dunbar 1920 D.rockhousensis Dunbar 1920 Schizofhoria frofinqua (HaU) 1847 European Species Dalmanella (/.) szajnochai Kozlowski 1929 Ortkis canalicula SchnuT 185 3 (1851?) O. decifiens Barrande 1 847 O. loveni Lindstrom O. neglecia Barrande 1 847 O. occlust Barrande 1847 O. tetragona Roemer 1844 O. irigeri Verneuil Distinguishing characters. — Isorthis is distin- guished from Dalmanella by its strongly biconvex lateral profile, lack of prominent fold and sulcus, pecu- liar ventral and dorsal musculature, and brachiophores with fulcral plates. Discussion. — The ventral musculature with its elongate and divergent diductor impressions most strongly resembles that of certain species of Schizo- fhoria. The diductor tracks are deep and elongate and are separated by a prominent ridge which carries the adductor muscles. A strong pallial trunk extends an- teriorly from the ends of the diductor impressions. Branching of these primary trunks has not been observed. In the dorsal valve the cardinal process is small, but, contrary to Kozlowski's statement, it has a lobate myo- phore, as shown by /. canalicula (Schnur) and other species placed in the genus. The pallial sinuses are well shown in /. tetragona^ in which four trunks appear to originate at the antero-median extremities of the anterior adductor scars. In reality there are only two main trunks but they bifurcate almost at their origin and send out two secondary branches, one pair antero- medially, the other antero-laterally. A second pair of primary pallial sinuses is given off from the horizontal elevation dividing the adductor scars. This trunk bifurcates in about half the distance to the margin, sending one branch antero-laterally, the other postero- laterally. In makeup these sinuses are much like those of Schizophoria, as seen in S. tuUiensis and S. senecta, in which all the trunks originate at the same place as in Isorthis, but since the shell ridge dividing the adductor scars is oblique they extend toward the front in a sub- parallel arrangement. Schizofhoria frofinqua (Hall) has pallial markings exactly like those of Isorthis and is therefore provisionally placed in that genus. Of interest in Isorthis is the great development of adventitious shell deposited upon the cardinalia and in the vicinity of the dorsal muscular area. In some instances primary structures are nearly completely obscured. Isorthis appears to be common in Europe, being well represented in the Lower Devonian of Bohemia and in the younger Eifelian of Germany. In America it is common in the Helderberg (New Scotland) (/. ferelegans, I. rockhousensis, I. fygmcea), and in the Silurian of Tennessee it is represented by /. arcuaria. Family LINOPORELLID^ Schuchert and Cooper 1931 Aberrant, specialized Dalmanellacea, having a dor- sal cruralium and a shell ornamentation recalling Porambonites, Geologic range. — Silurian, with the genera Linoforella Schuchert and Cooper and Orthotrofia Hall and Clarke. Discussion. — The ventral musculature of Linofor- ella is close to that of Pionodema and Schizofhoria. In the dorsal valve, however, the convergence of the brachiophore plates to meet a median septum is such a radical departure from the usual structure of the Schizophoriidae as to warrant the erection of a family to recognize this variation. The schizophorioid ventral muscles and the punctate shells indicate the probable origin of Linoforella, but the presence of a cruralium shows a distinct divergence from some stock as yet unknown, but probably to be looked for among the late Ordovician Schizophoriidae. Genus LINOFORELLA Schuchert and Cooper 1931 PI. 18, figs. 13, 14, 17, 18,24,33 Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, p. 247. Genoholotype. — Orthis functata Verneuil 1848, Bull. Soc. Geol. France (2), vol. 5, p. 343. Description. Exterior. — Semioval to subglobose, margins convex; hinge-line narrower than the greatest width of the shell; cardinal extremities rounded; lateral profile subequally biconvex; anterior commis- sure slightly sulcate; ventral interarea the longer, curved, apsacline, beak strongly incurved, delthyrium open; dorsal interarea anacline, beak incurved; sur- face multicostellate, the striae being marked by a row of coarse pits which do not perforate the interior shell layer. Test fibrous, punctate. Ventral interior. — Delthyrial cavity deep ; teeth small, fossettes lacking; dental plates strong and GENERA OF THE SUBORDER ORTHOIDEA 151 prominent, subparalkl, produced forward as prominent ridges on each side of the diductor scars; a median ridge extends from the front of the muscle area nearly to the margin of the shell and is commonly extended between the diductor impressions nearly to the apex. Muscle field longer than wide; adductor scars borne on the median ridge; diductor scars elongate, narrow; muscle area thickened in front; irregular, wavy ovarian markings occupying the lateral spaces. The palintrope overhangs the delthyrial cavity. Dorsal interior. — Cardinalia strong; notothyrial cavity deep; brachiophores forming a thickening below the notothyrial margin and terminating in short, blunt points supported by stout plates that converge to meet a median septum and thus make a cruralium; sockets shallow, excavated in the margin of the hinge and the outer face of the crural base, defined by an obscure fulcral plate; cardinal process unique, consisting of an elongate thickened myophore and a thin septum-like shaft that is continuous with the median septum. Median septum high, extended forward about half the length of the valve. Muscle area elongate-oval, divided by the median septum, commonly with an ele- vated periphery. Elevated, wavy visceral markings occupy the spaces outside of and posterior to the muscle area. Geologic range. — Middle Silurian or Gotlandian (Niagaran). American Species Orthit functostriata Hall 1852 European Species Orthis functata Verneuil 1 848 ? O. turgida McCoy 1 85 I Distinguishing characters. — Ltnoforella is unique among punctate orthoid genera in the conver- gence of its ornamentation toward the kind seen in Porambonttes. There are, however, internal and ex- ternal differences that when combined with the geolog- ical occurrences separate the two genera. Externally Linoporella has the outline and profile of Pionodema, differing thus from Porambonttes, and internally the ventral musculature, dorsal cruralium, and cardinal process further serve to differentiate the two. Discussion.— Hall and Clarke (ISgZ)'-" were aware of this group of orthids, saying of O. functata and O. punctostriata that they could not be placed in their divisions of the old genus Orthis; they did not, however, give them a generic name. The internal features of Linoporella separate the genus from all other Dalmanellacea. In the ventral valve of young shells the dental plates are sharply defined by deep umbonal cavities and extend directly '^'Pal. N. Y., vol. 8, pt. 1, p. 217. to the floor of the valve. In most specimens these plates are prolonged along the floor of the valve as low ridges. The diductor scars are semielliptical or semi- oval in plan and are separated by a depressed adductor ridge. The latter is low in the posterior of the shell but is elevated to a sharp crest at a point just in front of the anterior ends of the diductor scars, and then descends to disappearance near the front margin. Along the top of this ridge is a shallow, longitudinal groove which usually does not extend anterior to the crest. In older shells the front margins of the diduc- tor scars are elevated on a low callus deposit which is elevated forward and inward along the median ridge and rises to disappearance at the crest. Posteriorly and laterally the callus decreases in thickness to become only a film on the sides of the dental plates and floor of the valve. This callus is significant, however, because it obscures the front ends of the dental plates and produces a pseudospondyliurp. Careful observation of old specimens will commonly show low extensions of the dental plates into or beyond the callus and its upturned anterior border. The delthyrial region of Linoporella is another significant example of obsoles- cence of primary structures (bases of dental plates in this instance) by secondary deposit of shell. In this tendency it also parallels Porambonttes, although the degree to which the deposition is carried does not result in structures that are precisely alike. The dorsal interior is also unique in Linoporella, being a convergence to that seen in Skenidio'tdes. The brachiophores are elongate, toothlike, bluntly pointed processes extending into the interior of the valve. Their forward growth produces a thickening under the palintrope which lies on them and extends over the notothyrial cavity for a short distance. These processes merge into discrete convergent plates which unite near the floor of the valve with a sharp median septum. In old shells the cavities between the brachio- phore supports and the floor of the valve are filled up with adventitious shell, causing the development of a structure simulating a sessile cruralium. In reality, however, in young shells the brachiophore supports in their line of attachment with the septum curve ventrally so that their anterior ends rise to a point on the summit of the septum. The cardinal process has a simple shaft that thickens toward the front and merges with the median ridge to form a continuous septum, the thicken- ing at the front of the cruralium being the only mark of union of the two septa. The myophore in old shells is a triangular thickening of the shaft at the posterior, the compressed sides of the triangle bearing the crenu- lations or marks of muscle attachment. The median ridge usually extends to about the middle of the valve as is usually the case in the orthids. The muscle field is rather narrow, elongate oval, with the posterior adductor scars the larger. Each anterior mark is bipartite as in many other orthid genera. Elevated, 152 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA thickened margins similar to those occasionally seen in Devonian Schizophoria are not infrequent in Linoporella. Linoforella is represented by a single species in the Gotland of Sweden and by a closely related form in the Niagaran of the United States. Reed^" states that Orthis ■polygramma fentlandica Davidson has pits between the costellae as in Linoporella, but all of the figures indicated by him display neither the external outline and profile nor the internal characters of the genus under discussion. Hence we feel that this species can not be placed in association with Linoporella. Orthis turgida McCoy, on the other hand, has the external outline and profile and most of the internal features of the genus we are here proposing. David- son's figures^^* show the cruralium exactly as it is in Linoporella, and in the ventral valve (fig. 20a) the muscle area is somewhat more expanded than is typical in our genus. In the descriptions and figures, how- ever, there is no mention of the external pits so charac- teristic of L. punctata. However, we refer O. turgida to Linoporella with a query because of the close similar- ity of internal structure, which we consider of more generic importance than the external ornamentation. Discussion. — Orthotropia has been well figured but never has been adequately described. Since the genus was made known it has been erroneously classi- fied among the Pentameracea, with which it has no relationships. Orthotropia appears to be most closely related to Linoporella, having in common with it a pseudospondylium, a cruralium, and a simple orthoid cardinal process (so far as could be determined from the specimens studied). The only known specimens of Orthotropia are in the form of internal molds preserved in a hard dolo- mite. This is the type of specimen that was available to Hall and Clarke and also the kind preserved in the Teller Collection at the National Museum. The ex- ternal sculpture and the shell substance are therefore unknown. This being the case, it has been impossible to determine the exact relationships of Orthotropia to Linoporella. Should the shell substance prove to be punctate and the surface to have the characteristic linoporellid pores, our genus must become a synonym of Orthotropia. However, in the absence of this im- portant information it has seemed best to designate the group of Orthis punctata as a new genus. Genus ORTHOTROPIA Hall and Clarke 1894 Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 1894, pi. 84, figs. 3-7. Genoholotype. — O. dolomitica Hall and Clarke 1894. Description. Exterior. — Shell rather small, hinge-line straight, narrow; cardinal extremities ob- tuse. Lateral profile unequally biconvex, the ventral valve having the greater convexity. Anterior com- missure faintly uniplicate. Ventral interarea long, apsacline; beak curved. Delthyrium probably open. Dorsal interarea short, anacline; notothyrium open. Nature of shell substance and external surface unknown. Ventral interior. — Dental plates prominent, discrete. Muscle field elevated at the front end by a callosity which forms a prominent pseudospondylium. Diduc- tor tracks narrow; adductor field narrow, elevated on a low ridge in old shells. Dorsal interior. — Notothyrial cavity deep. Brachio- phore plates convergent, forming a cruralium with the median septum, which is thin. Adductor field suboval. Cardinal process a simple ridge, possibly with expanded myophore. Geologic range. — Silurian (Racine dolomite). The genotype is the only known species. '"Trans. Roy. Soc. Edinburgh, vol. 51, pt. 4, 1917, p. 857. '2« Brit. Foss. Brach., vol. 3, pt. 7, pi. 32, figs. 19a, 20a. Family TROPIDOLEPTID^, n. fam. (Tropidoleptinas Schuchert 1896) Genus TROPIDOLEPTUS Hall 1857 Hall, N. Y. State Cab., 10th Rept., 1857, p. 151, figs. 1,2; Ibid., 20th Rept., 1867, p. 280. Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 1893, p. 302. Genoholotype. — Strophomena carinata Conrad 1839, 3d Ann. Rept., N. Y. Geol. Surv., p. 64. In this family there is a single genus, Tropidoleptus, which, because of its anomalous structure, has been buffeted about between the Strophomenacea and the Terebratellidas of the Terebratulacea. It is not the purpose here to redescribe the anatomy of Tropidolep- tus; this has been done very adequately by Hall and by Hall and Clarke. It is our purpose, however, to make some suggestions regarding the taxonomic posi- tion of the genus. The anomaly in Tropidoleptus is the early appearance of what looks like a terebratelli- form loop, and this character has been taken by some workers as proof of relationship with the Terebrat- ellidae. It was early held that the Devonian genus Tropidoleptus was the forerunner of the Terebrat- ellidje, neglecting the fact that the latter division did not appear till Mesozoic time, an enormous time interval that in itself is strong evidence against any relationship between the genus under discussion and the Terebratellidse. In the 1913 edition of Zittel-Eastman, Tropidolep- tus was placed in association with the Strophomenacea, GENERA OF THE SUBORDER ORTHOIDEA 153 where its position is equally anomalous with the one described above. The presence of endopunctas at once rules the genus out of the Strophomenacea (Kozlowski having shown that the shell substance of the stropho- menoids is impunctate), and there are therefore only two possible places to put it, either in the Dalmanellacea or in the Terebratulacea. It appears to us now, after a long study of the orthoid genera, that the best — at any rate, the least anomalous — position is in the Dal- manellacea. We would place it here because of the punctate shell, the open delthyrium, and the general orthoid nature of the brachial processes, which was suggested by Hall long ago. The endopunct.-c of Trop'uioUptus are very similar to those of Dalmanella and allies, but are not so close to those of the Tere- bratulacea in pattern, the latter being much more dense. The structure of Tropidoleftus accords well with that of the dalmaneUoids in having an open delthyrium, a wide hinge-line, and an interarea on both valves. In the dorsal valve there is a small chilidium such as one finds in Heterorthis and other punctate genera. The ventral musculature is suggestive of the orthids, as Hall pointed out. In the dorsal valve the cardinal process is bilobed but is not of the type common in Strofhomena and allies, in which the lobes are usually isolated. The teeth and sockets are grooved much as in Parmorthis among the dalmaneUoids. The so called loop is of course the most difficult structure to account for. This in itself, however, does not have the appear- ance of the true terebratelloid loop that grows out from the septum and unites with the descending lamellae of the crura; rather these processes are nothing other than very long crura as in some Rhynchonellas, but instead of remaining free they appear to unite with the median septum. We hold that this type of arm sup- port could have developed many times, instead of but once. In conclusion, then, we believe that Tropi- doleptus is a highly specialized and terminal dalman- elloid of short geological life." At present it is difficult to say definitely out of what dalmaneUoids Tropidoleptus may have come, but we suggest that it may have been out of the Parmorthis stock. PART V. THE GENERA OF THE SUBORDER PENTAMEROIDEA Derived out of the Orthacea, the Pentameroidea retain their impunctate tests, but most of them tend with time to lose more and more of their transverse shells with wide interareas and to become elongate and subrostrate, smooth or costate, with well developed spondylia and characteristic cardinalia. The delthy- rium is usually open but may be modified by deltidia or marginal thickenings. The Pentameroidea begin in the Middle Cambrian and die out with the Devonian. They include the superfamilies Syntrophiacea Schuchert and Cooper and Pentameracea Schuchert. Superfamily SYNTROPHIACEA Schuchert and Cooper 1931 Specializing Protremata derived out of the Orthacea (probably the ancestral stock of the Billingsellidas ) , developing a more or less lobate exterior, interiors with either pseudospondylia or spondylia of the simplex type, and occasionally cruralia. Cardinal process absent or rudimentary. Delthyrium and notothyrium open. Test fibrous, impunctate. Geologic range. — Middle and Upper Cambrian, Ozarkian, and Lower Ordovician. Embraces the Clarkellidae, Syntrophiidx, and Huen- ellid Huenella SVNTROPHIIDiE Syntrophioides ^ Syntrophia Orthacea (probably out of an early Middle Cam- brian ancestor of the Billingsellidae) GENERA OF THE SUBORDER PENTAMEROIDEA 155 Family CLARKELLID/E Schuchert and Cooper 1931 Syntrophiacea, externally like Syntroph'ta and with a sf)ondylium simplex or a pseudospondylium. In the dorsal valve the brachiophore supports are divergent, discrete plates. Ventral pallial markings as in Billingselln. Geologic range. — Middle Cambrian to Lower Ordovician. Embraces the following genera: Syntrophioides Schuchert and Cooper Syntrofhitut Ulrich Clarkella Walcott Yangtzeella Kolarova Genus SYNTROPHIOIDES Schuchert and Cooper 1931 PI. 15, figs. 20, 23 Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, p. 247. Genoholotype. — Billingsella harlanensis Walcott 1905, Proc. U. S. Nat. Mus., vol. 28, p. 236; Camb. Brach., 1912, p. 746, pi. 87, figs. 5-5d (as Wimanella harlanensis) . Description. Exterior. — Subelliptical to subsemi- elliptical, hinge-line straight; cardinal extremities usu- ally obtusely angular; lateral profile biconvex; an- terior commissure uniplicate; dorsal fold not greatly pronounced. Ventral interarea long, apsacline, del- thyrium open ; surface marked only by fine, concentric lines of growth. Test probably fibrous, impunctate. Ventral interior. — Delthyrial cavity rather shallow, dental plates in the type specimens not sharply marked ; musculature clearly billingselloid, having a prominent central adductor track which widens toward the front and is cleft into halves by a low ridge; the front of the adductor track elevated slightly. Diductor tracks smaller than the adductor ones, rounded in front, separated from the latter by low ridges. From the anterior ends of the diductors two pairs of pallial sinuses extend in an antero-lateral direction nearly to the front border as in Billingsella. Dorsal interior. — Notothyrial cavity shallow, brach- iophore plates convergent and resting on the floor of the valve ( ? ) ; musculature prominent, posterior ad- ductors larger than the anterior pair, suboval in out- line; anterior pair about half the size of the posterior one. Two pallial sinuses have their origin at the point of contact with the posterior adductors in the mid-line of the shell and are separated by a slight median eleva- tion. They extend in this closely appressed condition as far as the front margins of the anterior adductors. where they branch abruptly toward the antero-lateral margins. Geologic range. — Middle Cambrian of Tennes- see with Billingsella (or Wimanella) harlanensis Wal- cott and ? B. (or W. ) saffordi Walcott. Discussion. — This genus is readily separated from Billingsella by its concentrically marked surface, syn- trophiid form, and dorsal musculature. B. harlanensis was placed in Wimanella by Walcott because of its smooth shell, but since that genus appears to have a costellate surface, the species under consideration can not be left there. The dorsal musculature of Syntrophioides is well defined and strongly suggests that seen in Syntrophina and Clarkella, representing a divergence probably out of some ancestor of the Billingsellidas of the early Middle Cambrian. In these circumstances we see how the slight generic differences in these early Cambrian forms may have given rise to later independent families and even superfamilies. Genus SYNTROPHINA Ulrich 1928 PI. 15, figs. 1, 2, 15-19, 30,31 Ulrich in Weller and St. Clair, Missouri Bur. Geol. Mines, 2d ser., vol. 22, 1928, p. 74 (without description). Genoholotype. — Syntrophia campbelli Walcott 1912, Camb. Brach., pp. 801-802, t. figs. 73A-F. Description. Exterior. — Subelliptical, hinge-line narrower than the greatest width of the shell ; cardinal extremities rounded. Lateral profile strongly bicon- vex; ventral valve deeply sulcata, dorsal valve with a prominent fold; anterior commissure uniplicate. Ven- tral palintrope short, apsacline; delthyrium open. Dorsal interarea shorter than the ventral, apsacline to anacline; notothyrium open. Surface marked only by fine concentric lines of growth. Test fibrous, impunctate. Ventral interior. — Delthyrial cavity deep, teeth strong, dental plates convergent, forming a true spon- dylium simplex. Muscle impressions borne on the upper surface of the spondylium; diductor marks on the sides and floor of the spondylium at the back; adductor impressions (?) in front of the diductor scars at the narrow end of the spondylium. Pedicle callist at the posterior. Pallial marks two broad trunks ex- tending antero-laterally from beneath the spondylium. Umbonal cavities and umbo-lateral spaces marked by low radial ridges. Complete or incomplete accessory septa occur in the vicinity of the spondylium in some specimens and show a convergence toward Clarkella. Dorsal interior. — Notothyrial cavity deep, brachio- phores long, brachiophore supports divergent, extend- ing to the floor of the valve. Cardinal process rudi- mentary, elevated on a narrow shelf at the posterior of the notothyrial cavity. Posterior adductor scars the 156 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA larger, situated a little outside the anterior pair. A prominent pallia! sinus extends antero-laterally from the anterior end of each adductor muscle impression. Geologic range. — Upper Cambrian to Lower Ordovician of North America. American Species Syntrofhina 3 n. spp. (Ulrich Coll.) Syntrofhia cam f belli Walcott 1908 S.isis Walcott 1924 S. nuniina Walcott 1905 S. fdmata Cleland 1900 S.ferilla Walcott 1924 S. rotundata Walcott 1905 ? Triflesia ■primordialis Whitfield 1877 Distinguishing characters. — Syntrophina may be recognized by its external resemblance to Syntro- fhia and by the presence of a ventral spondylium, but differs in having two dorsal divergent brachiophore supports and consequently no cruralium simplex. Discussion. — Interesting features of this genus are the well defined muscle marks on the dorsal side of the spondylium, the peculiar shelf at the posterior of the notothyrial cavity, and the dorsal muscle and pallial marks. Internal molds from Phillipsburg, Quebec, preserving the spondylia, show on them a median ridge toward the anterior, which widens and becomes some- what more elevated, terminating at the back end of the median septum. This anterior ridge or elevation may represent the impression of a deeply sunk adductor track. Behind the "adductor track," and surrounding it partially, are the probable impressions of the diductor muscles on the floor and sides of the spondylium (see pi. 15, fig. 15). The correct identification of these muscle marks is attended with some difficulty, but the suggested arrangement would be in accord with the mechanics of opening or closing a brachiopod shell and would homologize with the same marks seen in valves not provided with a spondylium. In the dorsal valve the brachiophores project for some distance into the valve and are supported by stout plates which show as subparallel or slightly divergent slots on internal molds (see pi. 15, figs. 2, 17, 19, 30). These plates form the lateral walls of a deep notothy- rial cavity, which has at the back end a low shelf built on the sides of the brachiophore supports and the pos- terior slope of the notothyrial depression. In the mid- line of the valve and the shelf at its back end there is in some specimens a low median elevation, interpreted as the cardinal process (pi. 15, figs. 30, 31). This shelf and the cardinal process probably mark the seat of attachment of the diductor muscles. The adductor muscle scars are thickened at their front ends and give off broad pallial sinuses that ex- tend toward the antero-lateral extremities of the valve. The musculature of Syntrophina strongly resembles that of Syntrophioides and Finkelnburgia. The genus Syntrophina has a long series of species (mostly undescribed) continuous from the Upper Cambrian into the Lower Ordovician. So far as known there are no Syntrophia s. s. in the Cambrian or Ozarkian. It is true that Walcott considered S. rotundata as the Cambrian representative of Syn- trophia, but this appears not to be correct, since the type specimens in the United States National Museum have the characters of Syntrophina. The reference of the species to Syntrophia is due to Walcott's mistaking a ventral valve for a dorsal, thereby giving the essential structure of Syntrophia. Other specimens in the same lot (Cat. No. 52493), however, have the features of Syntrophina. Ulrich's new genus Syntrophinella, as yet unde- scribed, is internally like Syntrophina but is multi- costellate externally. Its genotype is S. typica, n. sp., illustrated on our plate 15, figures 4, 5, 13. Genus CLARKELLA Walcott 1908 Pi. 15, figs. 6-11 Walcott, Smiths. Misc. Coll., vol. 53, 1908, p. 110; Camb. Brach., 1912, p. 809, pi. 104, figs. 2-2d. Genoholotype. — Polytaechia ? montanensis Wal- cott 1905, Proc. U. S. Nat. Mus., vol. 28, p. 295. Description. Exterior. — Like Syntrophia, with a prominent fold and sulcus. Test fibrous, impunctate. Ventral interior. — There is a spondylium simplex but in some species it is supported by two or more lateral accessory septa. Two strong divergent pallial sinuses extend antero-laterally from the umbonal cavities. Dorsal interior. — Essentially like that of Syntro- phina, but there are accessory lateral septa, at most two in number, which are united with the descending brachiophore supports. There are six pallial sinuses radiating from the umbonal cavities and in front of the muscle area. Geologic range. — Upper Cambrian (Ozarkian). American Species Polytaechia montanensis Walcott 1905 Syntrofhia nanus Walcott 1924 Distinguishing characters. — The character which gives Clarkella generic standing is the structure of the cardinalia with its prominent accessory plates. Discussion. — In the original definition of the genus, Walcott described spondylia in both valves, supported by accessory septa. This structure can not be ques- tioned in the ventral valve, since there is here a spon- dylium simplex supported by a stout median septum and two or more accessory septa. The latter are, how- ever, not universally present, being lacking in S. nonus GENERA OF THE SUBORDER PENTAMEROIDEA 157 (Walcott). In the dorsal valve there is no cruralium as Walcott has described and illustrated. Instead of there being a continuous spoon-shaped plate supported by lateral septa, there are two plates beneath the brachiophores which converge medially but never unite. Each plate is supported by two or more septa, the usual number being two, one inner and one outer (see pi. 15, figs. 7, 10). CLirkella is evidently very closely related to Syntro- fhina and is characteristic of the Ozarkian. It is widely distributed geographically, being known from Montana, British Columbia, and Phillipsburg, Quebec. plex which is supported for nearly its entire length by a septum which is short and thick; septum extending for half the length of the valve. There are accessory lateral septa up to four in number aiding in the support of the spondylium. Between these are other low radial ridges. Dorsal interior. — Notothyrial cavity deep; sockets deep, at the posterior the brachiophore plates form a small, deep, spoon-shaped muscle platform supported at the back by one or two pairs of lateral septa. After a short distance the platform, which is evidently for the attachment of the diductors, ends, but the two lateral «> I Fig. 22. — Yangtzedla foloi (Martelli). Serial sections through the beak of a mature shell. The internal structure allies this genus with Clarkella Walcott and proves conclusively that it has no relationships with Triflesia. The stippled areas indicate adventitious shell substance which fills the umbonal chambers of both beaks. Such a filling is common in many genera of spondylium-bearing shells. The shell sectioned was 19.3 mm. in length. All structures disappeared 8.5 mm. from the beak. Distances from beak: 1 — 1.4 mm. 2—2.5 3—3.2 4 — 4.5 5 — 5.8 mm. 6 — 6.8 7—7.3 Genus YANGTZEELLA Kolarova 1925 PI. 15, figs. 24-26; t. figs. 22, 23 Kolarova, Bull. Geol. Soc. China, vol. 4, 1925, p. 219, pi. l,t. fig. 1. Genoholotype. — Triflecia foloi Martelli 1901, Bull. Soc. Geol. Ital., vol. 20, fasc. 1, pp. 302-304, pi. 4, figs. 13-22. Description. Exterior. — Outline subquadrate, hinge-line straight, narrower than the total width of the shell ; cardinal extremities rounded ; lateral profile biconvex, the dorsal valve having the greater con- vexity. Anterior commissure uniplicate ; ventral sul- cus very deep, defined only on the front half of the shell; dorsal fold pronounced only in the anterior area. Ventral interarea longer than the dorsal, apsa- cline; beak curved slightly and may or may not be resorbed by pedicle pressure; delthyrium open. Dorsal interarea curved, apsacline, beak strongly curved, umbo inflated, notothyrium open. Surface nearly smooth, marked by concentric growth-lines and distant lam- ellae, and by faint radiating ridges. Test fibrous, impunctate. Ventral interior. — Delthyrial cavity deep; teeth strong ; dental plates thick, forming a spondylium sim- septa persist, each bearing a laterally directed shelf and a rather long brachial process. Geologic range. — The only known species is y. foloi (Martelli) of the ? Lower Ordovician of China. Distinguishing characters. — The chief diag- nostic characters of Yangtzeella are its nearly smooth exterior, Triflesia-hke or syntrophioid outline and pro- file, ventral spondylium simplex which may be sup- ported by two or four accessory lateral septa, and in the dorsal valve a spoon-shaped plate supported by two primary lateral and two accessory lateral septa. It is impossible to say whether or not this plate bore muscles. Discussion. — Yangtzeella shows several interesting features. One of these is the great amount of adven- titious shell substance deposited in the umbonal cavities of the valves, chiefly in the ventral one. In Kolarova's sections (his pi. 1, figs. 2-6) the ventral shell for about 3 mm. from the beak is wholly test, and the same is true of the Yale specimens for 2.8 mm. from the beak. However, the septa show clearly in the .idventi- tious substance so that none of the shell anatomy is obscured in the sections. The supporting plate beneath the ventral spondylium is commonly split or fractured so that it appears to be a duplex septum, and yet it is not precisely like the 158 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA duplex septum of Pentamerus, but is single as in Ck- tambonites or Vellamo, which occasionally also have the septum fractured medially. The spondylium is supported for practically its whole length and there are generally two lateral septa as in some species of Clark- ella. At its front end the spondylium hangs free of the septum, which is prolonged forward as a ridge on the floor of the valve. Between the main septa on the floor of the valve are numerous accessory radiating ridges, such as are common in Syntrofhina and Clark- ella. These radiate from each of the umbonal cavi- ties, the larger ones forming the septa. They are interpreted by Kolarova as ridges or septa between the diductor muscles. According to our view, they are Fig. 23. — Yangtzeella foloi (Martelli). Section cut through an adult specimen, showing septa of both valves and notothyrial chamber of the dorsal valve, x about 4. for attachment of the ovarian bodies, and so far as our knowledge goes, the ventral muscle marks in spon- dylium-bearing shells are always located on the upper or dorsal surface of the spondylium, and such shells never have additional muscle-scars on the valve floor. Low ridges upon the spondylium of Yangtxeella sug- gest that the muscles here were also attached to the upper surface of the spondylium. The dorsal valve is especially interesting and quite unlike any other brachiopod except Clarkella. There is a narrow, spoonlike platform supported by two lateral septa, and two or more accessory ones. After about 3.5 mm. in a specimen 20 mm. long, the spoon terminates, but the lateral septa persist and are overlain by horizontal plates or shelves that may or may not be supported by septa. The brachiophores consist of rather long processes extending into the valve from the extremity of the brachiophore plates, as in Syntrofhina. This shell was originally referred to Triflesia and in the 1929 classification of Schuchert was placed in asso- ciation with this and allied genera. Yangtxeella, how- ever, has no forked cardinal process, and besides, the presence of a spondylium and cardinalia like those of Clarkella shows that it belongs with the Syntrophiidae, where Miss Muir-Wood placed it.^ It is closest to Clarkella in a broad sense, having many septa in each valve. However, Clarkella does not have a spoon- shaped dorsal plate or lateral processes such as occur in Yangtzeella. Family SYNTROPHIID^ Schuchert 1896 Smooth biconvex Syntrophiacea having well devel- oped interareas, a spondylium simplex, and a cruralium simplex. Geologic range. — Lower Ordovician. Embraces but one genus, Syntrofhia Hall and Clarke. Walcott has referred to this family his Swantonia, a genus whose taxonomic position is wholly unknown, but we leave it here as we are not able to place it. Genus SYNTROPHIA Hall and Clarke 1891 PI. 15, figs. 3, 27-29, 32 Hall and Clarke, Pal. N. Y., vol. 8, pt. I, 1891, p. 270; pt. 2, 1893, p. 216, pi. 62, figs. 1-10. Genoholotype. — Triflesia lateralis Whitfield 1886, Bull. Amer. Mus. Nat. Hist., vol. 1, no. 8, p. 303, pi. 24, figs. 9-11. Description. Exterior. — Transversely oval, hinge- line narrower than the greatest width, cardinal ex- tremities obtuse; lateral profile biconvex; anterior commissure unipHcate; fold and sulcus usually shal- low; areas on both valves; ventral interarea apsa- cline, delthyrium open; dorsal interarea anacline, notothyrium open. Surface marked only by fine con- centric lines of growth. Test fibrous, impunctate. Ventral interior. — "Teeth small" ; dental plates uniting with a median septum to form a spondylium simplex that is free at its anterior end. Muscle con- fined to the spondylium. Dorsal interior. — Much like the ventral, the brach- iophore supports converging and uniting with a median septum to form a cruralium. Pallial trunks radiating from the muscle area as in Syntrofhina. Geologic range. — The only known species is the genotype, which comes from the Lower Ordovician (upper Beekmantown) of Vermont. Distinguishing characters. — The presence of a spondylium and a cruralium is the chief distinguishing feature. Discussion. — When Hall and Clarke based Syn- trofhia on Triflesia lateralis, they were struck most by the external form and the presence of a spondylium. >Zool. Record, 1926. GENERA OF THE SUBORDER PENTAMEROIDEA 159 For these reasons they included in the genus many similar species, making, as we now see, a rather hetef- ogeneous lot of shells. Since Triplesui lateralis is the genotype, the name must be based upon it, and as so restricted, all of the other species formerly placed here must be taken out of the genus. Specimens of Syntrophui are exceedingly rare and none were available to us for sectioning. It is therefore not known with certainty whether the genus possessed a spondylium simplex or one of the duplex type. Polished specimens in the National Museum at Wash- ington do not show a suture line in the septum of the spondylium, hence the presumption is that the latter was of the simplex type, ? Genus SWANTONIA Walcott 1905 Walcott, Proc. U. Camb. Brach. S. Nat. Mus., vol. 28, 1905, p. 296; 1912, p. 796, pi. 104, figs. 5-5b. Genoholotype. — Camerella antiquata Billings 1861, Pal. Foss., vol. 1, pp. 10-11, fig. 13. Description. Exterior. — The species and genus are based on a single ventral valve. Shell ovate, mod- erately convex, subrostrate, with a shallow sulcus toward the front; multicostate (8-12 costs). Inter- area narrow and long; delthyrium open; teeth rudi- mentary, muscle-scars not preserved. Geologic range. — Lower Cambrian of Swanton, Vermont (S. antiquata (Billings) 1861) and Nevada {S. weeksi Wz\coK 1905). Discussion. — There is some superficial resemblance here to Camerella, but the internal structure is very different. According to Walcott, S. antiquata has a "narrow, strong, concave shelf or area; the area or shelf is free from contact with the bottom of the valve, a recess or chamber existing beneath it." This feature he has taken to indicate a free spondylium, but the presence of such a structure may be questioned. The specimen is preserved as a mold of the interior. In the vicinity of the beak the specimen is indented on each side, the indentation representing the position of the lateral interareas. Immediately under the beak, however, there is no excavation such as would be ex- pected if there were a free spondylium, and the internal mold is united to the matrix rock just as if there were an open delthyrium now filled up. Were it not for the strong costje on the outside of the shell, the posi- tion of Swantonia among articulate brachiopods might be questioned. Swantonia therefore appears to be a narrow-hinged form without a spondylium, but its family position can not at present be determined, especially in the absence of any knowledge of the dorsal valve. S. weeksi is also based on a single ventral valve, and shows external characters only. However, as Walcott referred the genus to the Syntrophiida:, we will leave it here provisionally. Family HUENELLID^ Schuchert and Cooper 1931 Syntrophiacea externally like Syntrofhia but costate or costcllatc. Ventral valve with a pseudospondylium ; dorsal brachiophore plates discrete, subparallel. Geologic range. — Upper Cambrian and Ozar- kian. Embraces but two genera, Huenella Walcott and Huenellina Schuchert and Cooper. Genus HUENELLA Walcott 1908 T. fig. 24 Walcott, Smiths. Misc. Coll., vol. 53, 1908, p. 109; Camb. Brach., 1912, p. 804, pi. 103, figs. 1-li. Genoholotype. — Syntrofhia iexana Walcott 1905, Proc. U. S. Nat. Mus., vol. 28, p. 294. Description. Exterior. — Subelliptical in outline; hinge-line straight, always narrower than the total width of the valves; cardinal extremities rounded. Lateral profile biconvex. Anterior commissure unipli- cate; dorsal fold broad in front, prominent; sulcus deep. Ventral interarea longer than the dorsal, apsa- cline, delthyrium open ; dorsal interarea anacline, noto- thyrium open. Surface costate or costellate, especially toward the front. Test fibrous (?), impunctate. Ventral interior. — Delthyrial cavity deep ; teeth strong and somewhat elongate; dental plates well developed, converging to the floor of the valve to form a pseudospondylium which bears the muscle-scars. Two broad paUial trunks diverge antero-laterally from either side of the pseudospondylium and follow the shell sulci nearly to the front margin. Dorsal interior. — Notothyrial cavity deep; brachio- phore supports short, convergent, but cemented directly to the floor of the valve. At the anterior they are extended forward and laterally as low ridges which separate the anterior from the posterior adductor scars. Cardinal process simple, very faintly visible or absent. Adductor impressions elongate, tear-shaped or oval, directed antero-laterally. A pallial sinus is given off from the anterior extremity of each adductor impres- sion. The inner trunks extend along the margins of the sulcus, while the outside trunks pass .ilong the anterior portion of the lateral lobes. Geologic range. — Upper Cambrian and Ozar- kian of North America. Species Costate section Huenella ainormis {W ikon) 1905 H.iillingii (Walcott) 1905 //. A^rj Walcott 1924 H. Iexana (Wilcon) 1905 H.texanalttviusculus iyizXcoxx) 1905 160 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA Costellate section H.ic etas 'Wilcott 1924 H.juba Wilcott 192+ H. lesleyi Wilcoti 1908 ? H.simonW Aeon 1924 ? H. vermontana Walcott 1912 f H.weedi^i\co\X 1924 ? Hebertella battis (Billings) 1 865 Distinguishing characters. — Huenella is recog- nized most readily by its syntrophinoid outline and profile, costate or costellate exterior, pseudospon- dylium, convergent brachiophore supports, and dorsal musculature. Fig. 24. — a-c, Huenella abnormis (Walcott). a, ventral internal mold, showing muscular impressions on the imprint of the spondylium. b, c, dorsal internal mold and interior, showing musculature and brachiophore plates {brf). ov, ovarian impressions (mistaken by Walcott for muscle marks); di, diductor impressions; ad, adductor impres- sions; Ps, pallial sinus. d, e, H. texana (Walcott), ventral and dorsal exteriors, showing coarse ribbing and strong fold and sulcus. After Walcott 1912, pi. 103. Discussion. — Huenella differs from Syntropkia and Syntrofkina not only in the external costation but also in internal characters. Internally there is a pseudo- spondylium bearing the muscle impressions; at the front it is elevated slightly on a prominent thickening of secondary shell. From the musculature of H. ab- normis the myology of this genus would appear to be essentially the same as that of Syntrofhina. However, the diductor and adductor scars are not flabellate im- pressions outside the pallial trunks as Walcott describes and figures in that species (1912, p. 806, pi. 103, fig. 2b). As he figures these impressions, the adductor scars are posterior to the diductors; their position and arrangement are accordingly unlike the ventral mus- culature of any other known brachiopod. Walcott has evidently mistaken impressions that are in the position of, and are usually considered to be, ovarian impres- sions. Furthermore, in the same figure the muscula- ture is shown clearly on the floor of the pseudospon- dylium and is essentially the same as that which is seen commonly in Syntrofhina. This genus as now constituted may be divided into two groups on the basis of the external ornamentation, as shown above. H. Simon and H. weedi, according to their external form and sculpture, suggest affinities with Billingsella rather than with Huenella. Genus HUENELLINA Schuchert and Cooper 1931 PL 15, figs. 14, 21, 22 Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, p. 247. Genoholotype. — Huenella triflicata Walcott 1924, Smiths. Misc. Coll., vol. 67, no. 9, p. 526, pi. 125, figs. 1-15. Description. Exterior. — Externally much like Platystrofhia, being strongly sulcate and costate. Hinge-line shorter than the greatest width of the shell, and interareas well developed on both valves. Del- thyrium and notothyrium open. Ventral interior. — Delthyrial cavity deep, teeth strong and long, with well defined crural fossettes; dental plates strong; pseudospondylium scarcely ele- vated in front. Adductor impression subcircular, di- ductor scars indefinite. Umbo-lateral spaces marked by elevated subradial ridges. Dorsal interior. — Notothyrial cavity deep, brachio- phores long, curved slightly; brachiophore supports prominent, curved, and extending directly to the floor of the valve; cardinal process rudimentary or absent. Attached to the outside of the brachiophores and the lower (anterior) side of the palintrope is a lateral septum which extends obliquely toward the lateral margins. Geologic range. — Upper Cambrian (Ozarkian) of Novaya Zemlya, Arctic Russia. The only known form is H. triflicata (Walcott). Discussion. — The main distinction between this genus and Huenella is to be found in the lateral septa developed under the anterior part of the dorsal palin- trope. There is nothing among the Cambrian brach- iopods exactly like this feature and it is diflBcult to understand what purpose it could have served other GENERA OF THE SUBORDER PENTAMEROIDEA 161 than additional strengthening of the palintropc. The brachiophore supporting plates are well developed, dis- crete, and convergent toward the floor of the valve, defining a rather deep notothyrial cavity. The brach- iophores, judging by internal molds, were long, slender processes extending from the point of union of the accessory septal plate and the palintrope. In the ventral valve a feature of particular interest is the shallow pit at the front end of the pseudospon- dylium. Such a pit also occurs in Syntrophina, and is in exactly the same position as the anterior adductor scar in Billingsella. Behind this pit can be seen the central adductor track and on each side of this, impressions which are regarded as diductor scars. This antero- central pit may represent the final place of the adductor scar in its forward migration during growth. Superfamily PENTAMERACEA Schuchert 1896 Specializing Protremata probably originating in the Syntrophiacea (family Syntrophiidse ) , characterized by spondylia that in the primitive families are nearly always supported by single septa (^^ spondylium sim- plex) and in the derived families by double septa (coalesced septal plates = spondylium duplex), but in rare instances in any of the families may hang free TabU 17 Pentameridx -^Stricklandidae Camerellidae Syntrophiidx or posteriorly supported by a remnant of the septum. The more important family characters occur in the dorsal cardinalia, which may be discrete or medially coalesced into a cruralium that bears the adductor mus- cles. Cardinal process usually absent, or rudimentary as a linear ridge or slight boss. Rudimentary or modi- fied remnants of the deltidium occur rarely, and chilidia never. Shells narrow-hinged, with small interareas, or decidedly rostrate with plane areas; exterior smooth, costate or multicostate. Test fibrous, and, so far as known, without endopunctje. Geologic range. — Middle Ordovician to close of Devonian. Embraces the following families: Camerellidae Hall and Clarke Pentamerida; McCoy ? Stricklandidjc Hall and Clarke The genetic relationships appear to be as shown in Table 17. After completing their survey of the great group of orthids, the writers turned naturally and logically to a study of the Pentameracea. They had embarked on this course and carried on their research for about six weeks when it was interrupted by the removal of the junior author to Washington. This had at once dis- advantages and advantages. At Washington it was possible to add observations on the strnnge genera Brooksina and Harpidium and on the even more aberrant Cymbidium. In the National Museum col- lections also are some fine specimens of Orthotropia which show the necessity for removal of this genus to the Dalmanellacea near Linoporella. As the presentation of the pentamerids now stands, it represents a survey of two of the finest collections of these shells in this country. However, the work can not be considered as final. In any study such as this, in which the elucidation of internal details depends on the destruction of materials, it would have been desirable to have still more specimens. As an example of this need it might be stated that, in all of the speci- mens of Pentamerus studied, we did not find a single example which retained a deltidium, yet Hall and Clarke reported such a structure. Furthermore, where there was abundance of common species, the rarer forms were represented in each collection by one or two specimens which could not be sacrificed to serial sectioning. Therefore much remains still to be done. We have, however, seen, and in the majority of cases sectioned, specimens representing the genotype of nearly all the pentamerid genera. If nothing else, the study strives to make clear exactly what each genus is, and, as far as possible, to eliminate from each the non-typical species. It has therefore been necessary to create a few new genera. Besides Orthotropia, the probability is that Str'icklandla will eventually have to be removed from its honored place among the pentam- erids. On the other hand, as we have shown on earlier pages, the Porambonitid.'c belong with the Orthacea. The old family Pentamerid.-c is, however, now divided into three, the Camerellida;, Stricklandidas, and the very varied Pentameridae, on the basis of certain in- ternal characteristics. Recent studies by Kozlowski have shown that the Camerophoriidae are spondylia- bearing Rhynchonellacea. 162 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA TECHNIQUE During the progress of this work, the junior author has improved the old Zugmayer process of serial sec- tioning so as to leave a remainder of the specimen, and this is highly advantageous. Serial sectioning has heretofore resulted in total destruction of the speci- men, or in the destruction of so much of it that the remainder was worthless. In our work we have fol- lowed the course described below: Before sectioning, the specimen is measured by micrometer. Sectioning proceeds slowly, the structures being sketched at inter- vals with the aid of a camera lucida, thus showing the significant changes in spondylia, septa, and cardinalia. This procedure is continued until that plane is reached where the ventral teeth can be seen inserted in the sockets of the dorsal valve. The specimen is then cemented by balsam to a glass slide and the whole measured. Sectioning is now resumed from the an- terior end of the shell and continued until the internal lamellae begin to appear. The specimen is measured, the shell structures sketched, and this procedure fol- lowed until the section is too thin to grind further, when it is covered by a glass slip. In this way at least a thin section of the specimen remains to form a record. In sectioning from the anterior end, the distance from the plane where internal structures first appear must be added to the length from the beak to the plane of articulation; addition of the thickness of the thin slice representing the distance from the last plane of the anterior sections to the plane of the articulation must also be made. In work demanding the sectioning of brachiopod material it is desirable that a replica of the specimen first be made, then the original should be photographed for dorsal, ventral, lateral, anterior, and posterior views, and these photographs should be reproduced along with those of the other type material. MORPHOLOGY OF THE PENTAMERACEA The pentamerid is a rather distinctive shell, easily recognized by its usually strongly biconvex valves and its spondylium duplex and elongate median septa in both valves. It was formerly the practice of most writers to place nearly all of the spondylium-bearing shells among the Pentameracea, but recent advances in brachiopod studies, and especially those of Kozlowski, have shown that the spondylium is a rather common structure developed independently and in different ways in many stocks. For example, such an unmis- takable orthid as Enteletes has been shown by Likharev" to have developed a spondyhum in its altera- tion to Enteletella. Among the punctate spiriferids, - BuU. Com. Geol., vol. 45, no. 6, 1924, pp. 720-721. Cyrt'ina has a spondylium, and among the terebratulids Amfhigenia may be cited. No one would now consent to the union of these genera to the Pentameracea. Although the spondylium is of itself not a distinctive feature of this group of shells, it is, in combination with certain dorsal structures, a rather important additional character. Considering the Pentameridas as a whole, there is a distinctive tripartite division of the dorsal septa which is characteristic of the family and appears to be its most important taxonomic feature. The family Camerellidas does not accord with the charac- teristic pentameroid dorsal structure and it is for this reason that we have given it family rank. It has singular resemblances to the Syntrophiidas and when material is at hand showing in detail the pallial mark- ings of the Camerellidas, it may be necessary to make still other family arrangements than those offered in this book. Such material, in the form of internal molds, is singularly lacking in both the collections studied. We again find among the Pentameracea, as we did among the Orthacea and Dalmanellacea, that dorsal structures are most diagnostic of family and mutual relationships. The following discussion is designed to summarize the significant points of pentamerid anatomy. Exterior. — There are several features of the ex- terior of the pentamerid shell that are of some interest: 1. V entricos'tty . — Among the Camerellidas none of the shells are flattish but all are rather globular and the valves strongly gibbous. The Pentamerida;, on the other hand, have a number of stocks that are rather flattish but may have developed gibbous stocks as offshoots. Such, for example, is the development of the subcylindrical Pentamerus from the rather flat- tish P. oblongus. C onchidium is evidently a strongly gibbous stock from the start and it is difficult to ascer- tain its ancestry; the best suggestion appears to be that it came out of Clorinda or Barrandella. In the Gypidulinas nearly all the genera have at least one valve strongly arched. 2. Ornamentation. — It is at present difficult to say whether the first of the pentamerids was smooth, or ribbed as is known to be the case among the orthids. In the Camerellidas the species of the primitive genus Camerella are partially smooth and partially ribbed. It would appear safe to say that this stock started from a smooth one like the syntrophiids and subsequently became anteriorly ribbed, since the culminating genus of the family, Anastrofhia, is the most strongly costate member. In the Pentamerinje, on the other hand, the problem is not so easy, since the ribbed and the smooth stocks appear nearly simultaneously, or, as seen from present knowledge, the ribbed stocks have a httle the better of it from the point of view of time. Among the Gypidulinas — Barrandella, Clorinda, Gyfidula, and GENER.\ OF THE SUBORDER PENTAMEROIDEA 163 Siehrrella — smooth and costate stocks are essentially contemporaneous, although in America the Harrand- ellas and Clorindas evidently appear first, in the Brass- field formation. With the Pcntamcrina; the first to appear is Plotymerella (Brassfield-Alexandrian). It may be the forerunner of Pentamerus, since the latter nearly always shows broad radial costs, but whether these represent secondary dying out of stronger costa- tion or incipient development of the same is not known. Conch'uiium was evidently always costate but may have given rise to two secondarily smooth genera. 3. Interareas. — It has been the common belief among paleontologists that the pentamerids have lost their interareas. This is certainly in part true. The reduction of the interareas, and their subsequent loss in a few stocks, are consequent upon the narrowing of the hinge-line and the development of rostration. It is very possible that this rostration has gone hand-in-hand Fig. 25. — Conchidium biloculare (Linnseus), from Klin- tehamn, Gotland. Section showing unusual deltidial cover of ventral valve. See pi. 29, fig. 4. The whole ventral interior, except for the two roughly oval areas on either side of the spondylium, is filled with adventitious shell, represented by the stippled portions. with the elongation and narrowing of the spondylium in the Pentamerinae in order to produce more surface for attachment of the diductor muscles. In the Gypidulinae, where the spondylium has not been so pronouncedly narrowed, there are usually preserved well marked remnants of the interareas. This is par- ticularly true of Sieberella. Another feature of pentamerid shells in the vicinity of the beaks is the production of prominent flattened areas which greatly simulate an interarea. This fea- ture is perhaps most strongly developed in Brooksina which has practically no interarea but has a broad flat region that extends from the beaks of both valves to the point of contact of the ribbed portions. These false cardinal areas are best developed in that genus and in Capellinirlla, both groups having the convexity of the valves reversed from the normal, and there may be some mechanical connection between the two phenomena. Ventral interior. — Important features of the ventral valve are the delthyrium and its accessories, the deltidial cover and deltitidal plates, and the spondylium and its supporting septum. /. Delthyrium. — So far as our observations have extended, the delthyrium of the Camerellida: is open except for marginal thickenings along the dclthyrial border, alluded to below. No deltidial cover such as occurs in Conchidium has been seen. 2. Deltidial cover. — In Conchidium there is a truly remarkable cover to the delthyrium which we hesitate to call a deltidium, preferring the more non-committal term deltidial cover. As commonly described, this has been termed a "concave deltidium." Our investiga- tions show that this structure is concave toward the anterior portion of the delthyrium, but when followed to the vicinity of the beak it rises above the margins of the delthyrium and projects dorsally in the form of a sheath with rather rectangular sides and depressed center. A similar structure exists in one specimen of Harpdium, but was erroneously described as convex deltidial plates. These two occurrences are the only ones noted by us in which there is a cover plate over the delthyrium. A concave "deltidium," presumably like the above, has been reported in Pentamerus but we have not seen such. The structure in Conchidium appears to be a pedicle sheath ; it is not known definitely whether it had an open foramen at the apex, but it appears to have had one. In the specimen of Harpid- ium preserving the deltidial cover the beak is crushed down on the cover so that its precise structure can not be determined. Parenthetically it may be added that among the orthids and strophomenids no such cover as this has been observed. 3. Deltidial plates. — Hall and Clarke announced the presence of deltidial plates in several genera studied by them and even went so far as to use these struc- tures as one of the generic distinctions between Gypidula and Sieberella. The present writers have not observed in any of the pentamerids that have come under their observation any clearly developed, typical deltidial plates. We have seen thickenings along the delthyrial margins of many of the genera, and such structures as these were termed "pscudo-deltidial plates" by Booker."' These thickenings, in our ex- perience, never restrict the delthyrium in any notable degree, certainly not nearly so noticeably as do similar thickenings in Glossorthis, Hesperorthis, and other orthid genera. Similar structures were observed also in specimens of Conchidium that have the deltidial cover as well. It therefore appears to us that they are of little taxonomic significance, certainly not sufficiently important to warrant the removal of the Pentameracea (restricted) to the Telotremata, as Kozlowski has done. ' Booker, Jour. Proc. Roy. Soc. N. S. Wales, vol. 60, 1927, p. 134. 164 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 4. Sfondylium. — The spondylium is an important feature of the pentamerids. It is always of the duplex type as described by Kozlowski, in which the septum is composed of two closely apposed pieces. One tendency among the pentamerids, achieved in only two instances, is the reduction of the septum and the elongation of the spondyhum. In extremely ven- tricose forms the rostral chamber is elongated and narrowed, so much so in some species that the sides are only a couple of millimeters apart. This elongation and narrowing of the spondylium is evidently for greater surficial spreading of the diductor muscles, per- mitting a strong pull without a breaking strain on the sides of the muscle platform. 4. Supporting plates consist of two subparallel septa extending from the middle of the valve and unit- ing with the inner, convex surfaces of the alar processes. It is clear that the function of these plates is to support and make rigid the rest of the cardinalia. In Camer- ella the supporting plates converge inward and unite with a septum. Musculature. — Of great interest among the Camer- eUidae is the position of the dorsal adductor scars in front of the cruralium in Camerella and forward of the parallel plates in Par astro fhinella and Anastrophia. This is very different from the situation of the same muscles in the Pentameridas, in which they are located 26 27 28 Figs. 26-28. — Longitudinal sections of pentamerids. 26, Pentamerus; 27, Pmtameroides; 28, ConchUium s. s. BP, brachial process; 5fiP, base of same; /^, inner plate; Mj, median septum; O/, outer plate; O//, outer plate and septum; Sf, spondylium. Dorsal interior. Sefta. — The dorsal interior of the Camerellida is fundamentally different from that of the Pentameridas and will be treated first. Here the dorsal septa can be divided into four more or less distinct parts: (1) the brachial supports, (2) the alar processes, (3) the fulcral plates, and (4) the support- ing lamellae. 1. The brachial supports appear to be extensions from the postero-ventral extremity of the alar processes. They are blunt and short, and are defined by ridges along the posterior margin of the alar plates. 2. The alar processes appear as concave plates, con- vex inward, which define the walls of the notothyrial cavity at the posterior. These processes in their for- ward extension curve outward. The alar extensions may represent the complete supports of the lophophore, taking the place of the elongate processes in other forms. 3. Fulcral plates are small plates, concave dorsally, which define the sockets much as in the Orthoidea (Plectorthidae and Schizophoriidae). within the confines of the subparallel or divergent septa or within the subrostral vault of the crurahum. In the Camerellidas the situation of the adductors is much like that of the adductors in the Syntrophiid.-e and Huenellidas, and it is very hkely that future study will show the origin of the Camerellida; out of these groups. This supposition, however, must await the dis- covery of the pallial marks in the Camerellidje and will be decided by the course of these structures on the inner surface of the dorsal valve. As stated above, the muscle-scars of the Pentam- erids are borne within the septal plates of the dorsal valve, and in the spondylium of the opposing one. This enclosure of the dorsal muscles by the septa is held by us to be a markedly characteristic feature of the Pentameridx, differentiating them from the CamereUidae. Cardinalia. — The cardinalia of the Pentameridx are the most characteristic feature of the genera and probably of the family as well. These lamellae are GENERA OF THE SUBORDER PENTAMEROIDEA 165 divisible into four distinct units termed by Leidhold:* (1) The inner crural plate, (2) the outer crural plate, (3) the crural band or border, (4) the septal plate. We prefer to term the first two of these parts simply the outer and inner plates, since we do not feel that it is at present certain that they are the homologues of the crura such as occur in the Rhynchonellacea and Terebratulacea. 1. The inner plates are curved, convex ventrally, and unite with the outer plates and the wall of the valve to form a prominent umbonal chamber. The sockets into which the ventral teeth fit are notches in the inner plates where they unite with the wall of the valve. The designation "inner plates" is not entirely apt, since, when the valves are seen in ventral view, the plates are actually the outermost part of the cardinalia. Leidhold evidently coined his expressions from a specimen that had been split longitudinally (this at least is the way he figures the structures). Thus seen from the side, the plates are innermost, being next to the spondylium. 2. The outer plates are rather thin and low, and unite with the dorsal edge of the inner plates and are separated from the septal or supporting plates by (3) a band or longitudinal thickening, the "Cruralleiste" of Leidhold. 4. The septal or supporting plates unite with the floor of the valve or with each other if a cruralium is present, and they support the other structures. They are commonly rather long and may be the best devel- of)ed of the tripartite cardinalia. Further discussion is necessary regarding the Crural- leiste or base of the brachial support as we prefer to term it. This band is prolonged into a free process that commonly extends to the front end of the ventral spondylium. It is this elongate, free process that undoubtedly was the support of the lophophore. The Cruralleiste represents the growth path of this brachial support or rather the remnant of the brachial support which has been enclosed and encroached upon during the forward growth of the inner and outer plates. These brachial processes are rather stout in the Gypidu- linx but in some members of the Pentamerina are long and slender. In Brooksina, with its strongly arched valve, the processes are exceedingly long. The cardinalia of the Pentamerida; as herein de- scribed are absolutely distinctive, nothing like them being known elsewhere. There is a slight difference between the septa of the Gypidulina: and the Pentam- erinje. That of the former is commonly bowed or convex outward, especially where the inner and outer plates unite with the base of the brachial support. This is notably true of Gypidula, Sieberella, and Pentamer- ella. In the Pentamerinae, on the other hand, the plates are generally higher and consequently not notably bowed. * Abhandl. preuss. geol. Landesanst., n. ser., Heft 109, 1928, pp. 51-53. GENERIC AND EVOLUTIONARY TRENDS The following discussion points out the more im- portant generic and evolutionary trends among the pentamerids observed by us, and indicates as well what characters we regard as of most import in defining the genera. Loss OF INTERAREAS AND DEVELOPMENT OF A PLANAREA. — In all of the pentamerids the inherited interareas are reduced to mere remnants and in many of the genera they have totally disappeared. But along with their vanishing comes the development in a few stocks of planareas or so-called false areas, plane (or nearly so) surfaces developed on either side of the delthyrium and largest in Brooksina. This develop- ment of planareas is of little taxonomic significance so far as present knowledge goes. The great reduc- tion of the interareas is of course characteristic of the whole superfamily. RosTRATiON. — Elongation of the beaks is best developed in Conchtdium, Harfidium, and Lissoccelina, and along with this extreme rostration has come a great arching of the ventral beaks over the dorsal ones. Reversion of normal convexity. — This is a common tendency among many brachiopod stocks and the cause of it is not fully understood. In Strofhomena of the Strophomenacea it has been correlated by Sarde- son" with a pendant growth habit, but this gravity causation does not appear to be responsible for the reversion in Brooksina and Cafelliniella. Reversion of fold and sulcus is another common feature, and one that is not uncommonly attended by rather profound alterations of the interior, and yet Pentamerella and Sieberella are structurally alike in- ternally though the fold and sulcus are reversed. In Virgiana, Twenhofel' says that the reversion of fold and sulcus takes place during growth, hence he regards it as a generic character, but to us this is certainly not so important as the internal features. Trilobation. — The flattened pentamerids such as Pentamerus and Rhifidium developed pronounced tri- lobation, which may be tied up with their incurrent and excurrent canals as explained in the Orthidx. This trilobation is also exhibited in the galeate pentam- erids and the Camerellida by the development of a fold and sulcus. Ornamentation. — It appears clear that the first of the known Camercllid<-c were practically smooth during early growth and that later the stock became more and more costate. Among the Gypidulina; the first members to appear, Clorinda and Barrandella, were smooth, and were followed by costate genera. Pan-Amer. Geol., vol. 51, 1929, pp. 37-38. 'Geol. Surv. Canada, Mus. Bull. 3, 1914, pp. 27-28. 166 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA Gyfiduln becomes secondarily smooth or nearly so in the late Devonian. On the other hand, smooth Gyfidula fseudogaleata may represent a totally differ- ent stock generically from any of the others. The first of the known American Pentamerinae are the costate Platymerella and Virgiana. They were followed by smooth forms (Pentamerus, Pentamer- oides, etc.), and these were later succeeded by Con- chidium and allies. The strongly costate forms are certainly characteristic of the high Middle and Upper Silurian, especially of central United States and Alaska. Loss OF THE VENTRAL SEPTUM. — Many pentam- erids eliminate more or less of the ventral septum. Complete loss was nearly achieved by Pentamerella and some species of Gyfidula, and total loss of the septum is the case in Cymbidium and Holorhynchus. On the other hand, Platymerella and Virgiana, earliest of the Pentamerinas, had exceedingly short dorsal septa, and some later stocks had longer ones. Development of a cruralium. — Among the galeate Pentamerinae Sieberella and Barrandella alone have cruralia. In the Pentameridas the genus Pentam- eroides is the only one known to have this structure. Conchidium with a cruralium is theoretically possible, but so far none is known. PARALLEL TRENDS The subfamily, on the basis of external characters, is at once separable into two divisions, Laeves and Costatae, with the following arrangement in equivalent states of development: Smooth Pentamerus Pentameroides Lissocaelina C apelliniella Holorhynchus Har-pidium, Costate Platymerella, Rhifidium None as yet known Conchidium, Brooksina Cymbidium None known, but to be looked for in Conchid- ium with abbreviated septum We do not mean to say that these two groups repre- sent two genetic lines. This is possible, but as yet we do not know pentamerids well enough to state it as a fact. There are too many possibilities in the way of smooth stocks becoming costate and costate stocks going back into the smooth state. However, our scheme is interesting, even if not established. It will be noticed that out of the six possibilities, four of the smooth stocks have a costate genus in a corresponding stage of development. A number of interesting parallel developments were seen during this study in various groups of the pentam- erids. One of these is in the development of a cruralium. Camerella has a cruralium in the dorsal valve but its homoeomorph, Parastrophinella, has the dorsal septa separate. In Barrandella there is a cru- ralium but in Clorinda, its homceomorph, the dorsal plates are widely separate. The same is true of Pen- tamerus and Pentameroides, and of Sieberella and Gypidula. Hall and Clarke^ regarded this develop- ment of a cruralium as of not more than specific value and placed more reliance on the external form of the shell. The trend of more recent times is, how- ever, to take the opposite view, regarding the internal variations as of greater importance than the exterior. Accordingly we recognize the presence of a cruralium as of generic value. SUMMARY In summarizing the above remarks, we can not sur- vey the evolution of the pentamerids in any clearer way than by showing our conceptions of the interrela- tions of the various members of the Pentamerinae. ' Pal. N. Y., vol. 8, pt. 2, p. 246. Family CAMERELLID^ Hall and Clarke 1894 (Syn. Parastrophinina; Schuchert 1929) Primitive, small, usually multicostate Pentameracea with the dorsal shell the deeper and with small, nar- row ventral interareas; probably derived out of the Syntrophiidse. All have a spondylium duplex. Dorsal cardinalia tripartite, with the septal plates either dis- crete or united into a cruralium supported by a median septum; arm supports short. Adductor muscles at- tached to the floor of the dorsal valve in front of the septal plates. Delthyrium open and but rarely modi- fied by narrow delthyrial marginal growths. Embraces the following genera: Camerella Billings ? Branconia Gagel Parastrophinella Schuchert and Cooper Anastrophia Hall Metacamerella Reed The genetic relations appear to be as shown in Table 18. GENERA OF THE SUBORDER PENTAMEROIDEA 167 Discussion. — Hall and Clarke say:" Whatever may be the oscillation in form and the varia- tion in secondary characters presented by Camarella, Para- strofhu and their allies, present evidence indicates that they must be regarded as the genetic precursors, as they are the secular precedents of the great group of true pentameroids. And on page 355 they erect the family Camerellidre for CamereUa, Parastrophui, Anastrophia, and ? Bran- coma; all the other genera referred here by them are now excluded from this family. Table 18 Camerellid^ Anastrophia Parastrophinella Metacamerella CamereUa 2, ^ Branconia Syntrophiid^ Beecher and Clarke' have shown that in early age the shells of Anastrophia are normally biconvex, and the brachial valve scarcely deeper than the opposite. ... In this condition the form of the shell resembles that of normal individuals of Camarella volborthi, and in this series of forms, beginning in Camarella where senile shells evince a gibbosity of the brachial valve and a tendency toward reversion of convexity, and ending with the Lower Helderberg Anastrophia verneuili, we have a consecutive and gradational development in internal struc- ture, which is accompanied by more abrupt variations in exterior.*" As conceived by the present writers, the Camcrellidx consist of shells in which the dorsal valve is usually more convex than the ventral. The shells are com- monly multicostate. The ventral interior has a promi- nent spondylium duplex, but the most important family *Pal. N. Y., vol. 8, pt. 2, p. 341. » N. Y. State Mus., Mem. I, 1889, p. 32. " Hall and Clarke, op. cit., p. 225. characters arc in the dorsal valve in connection with the cardinalia. The brachial processes are supported by alate plates, concave outward, and these arc, in turn, supported by septal plates which are cither discrete and fastened directly to the floor of the valve as in Para- strophinella and Anastrophia, or converge and unite with a median septum to form a cruralium as in CamereUa. Another feature of significance is the attachment of the adductor muscles to the floor of the dorsal valve entirely outside the septal plates. This is a more primitive feature than in the derived Pentam- eridas, in which the adductor muscles are confined either within or on the dorsal septal plates or cruralium when such a structure is present. The family as now constituted contains one genus, Branconia, which is of doubtful value, and another, Metacamerella, which has not been adequately de- scribed. The former, which occurs in the Middle Ordovician of Europe, is probably identical with Cam- ereUa. Metacamerella, on the other hand, suggests an Anastrophia, having a ventral fold, but the alate proc- esses of the dorsal valve were not reported by the nomenclator of the genus. In the Camerellidas the delthyrium appears to be open or partially encroached upon by marginal growths which have been called dcltidial plates, but they are not like those in the Telotremata and it appears proba- ble that true deltidial plates do not occur in this family. Genus CAMERELLA Billings 1859 PI. 25, figs. 3-6, 8-13, 16, 20-22, 24, 27-30, 43 Billings, Canadian Nat., vol. 4, 1859, p. 301. HaU and Clarke, Pal. N. Y., vol. 8, pt. 2, 1893, p. 219, pi. 62, figs. 11-18 (Camarella). Horn. Parastrofhia Hall and Clarke, Ibid., p. 221, pi. 63, figs. 1-3. Syn. Parastrophina Schuchert and LeVcne 1929. Genolectotype (Hall and Clarke). — C. volhor- thi Billings 1859. Description. Exterior. — Subglobular to subpen- tagonal in outline; hinge-line narrow, cardinal ex- tremities broadly rounded; profile unequally biconvex, the dorsal valve having the greater convexity in mature individuals; anterior commissure uniplicate. Ventral interarea narrow, nearly obsolete; delthyrium open, so far as known. Dorsal interarea obsolete. Surface costate on the anterior half, smooth posteriorly. Shell substance fibrous, impunctate. Ventral interior. — Delthyrial cavity deep; teeth strong; dental and septal plates convergent to form a spondylium duplex. Septal plates in conjunction and prolonged in front of the spondylium for some distance. Dorsal interior. — Notothyrial cavity deep and elon- gate; brachial supports rather short and blunt; septa! plates, which buttress the brachial support, elongate 168 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA and convergent, forming a cruralium duplex; short alx present just anterior to the brachial supports. Geologic range. — Middle Ordovician (Chazy) to Middle Silurian of North America. Species Camerella n. sp. C. volborthi Billings 1859 Parastrofhia greenei (Hall and Clarke) 1 895 (placed here after a study of Hall and Clarke's figure) P . hemiflicata {HnW) 1847 P. hemifUcata rotunda (Winchell and Schuchert) 1893 P. rotundijormis WiUard 1928 P. ? scojieldi (Winchell and Schuchert) 1893 Distinguishing characters. — Camerella as here restricted is characterized by the presence of a spon- dylium duplex, a cruralium duplex, and dorsal septa having short alate extensions on the outside. Discussion. — ^The genotype selected by Hall and Clarke is C. volborthi Billings, which proves to be structurally identical with Parastrofhia hemiflicata Hall, the type species of that genus. Both forms have the same kind of spondylium and cruralium and possess the ala which are so much better developed in Anastro- fhia. It is evident, then, that Parastrofhia Hall and Clarke 1893 (a homonym replaced by Parastrofhina Schuchert and LeVene 1929) and Camerella Billings 1859 are synonyms, and the latter name has priority. Regarding Cafnerella, Hall and Clarke say that these shells have "in effect, a rhynchonelloid exterior," with a well defined spondylium supported by a short median septum, and an open delthyrium without del- tidial plates. The cardinalia consist of "crural plates converging and forming a short, very small median cavity, which is supported by a long septum," and this hinge structure "is similar to that of Camarotoe- chia . . . The crura are short, and the lateral divi- sions of the hinge-plate small. No cardinal process exists. The internal structure of Camerella is, thus, not unlike that of Syntrofhia, notwithstanding the wide difference of exterior." It is evident that there is needed a revision of the species commonly referred to Camerella under the current conception of the genus." The present writers exclude from this genus C. hella Fenton, C. ambigua (Hall), C. tnornata Weller, and the following species of Billings: C. varians, C. longirostris, C. fanderi, C. costata, C. folita, and C. farva. Some of these " For the early Middle Ordovician (Chazy) shells which have usually been called Camerella, but which have a differ- ent internal structure, we proposed the name Rhyncho- camara in our "Synopsis" of 193 1, and left the genus under the family Camerellids. We are now convinced that it is in reality a rhynchonellid. For the convenience of workers on that group its description is amplified in the Appendix to this work. species must be removed to entirely different groups. C. tnornata Weller does not have the external form of either Camerella or Rhynchocamara and should probably be assigned to ? Cyclosfira. "Camerella" fanderi appears from etched specimens to possess spiralia and is without a spondylium, as is also C. longirostris. "Camerella" ottawaensis from Paquette Rapids (pi. 16, figs. 6, 9, 13) apparently belongs to Orthorhynchula. In Camerella the ventral muscles are confined to the spondylium as is normal for spondylium-bearing shells, but in the dorsal valve internal molds show clearly four adductor impressions bisected by the septum of the cruralium. This must be regarded as a primi- tive character and is in contrast to the Pentameridas in which the muscles are enclosed by the septal plates or are confined to the cruralium. In the Camerellidae the cruralium is the base of attachment of the diductor muscles. Genus BRANCONIA Gagel 1890 (Compare with C amerella) Gagel, Beitr. z. Naturkunde Preuss. hrgb. von d. Physik.- Oekonom. Gesell. z. Konigsberg, vol. 6, 1890, p. 62, pi. 4, fig. 12. Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 1893, p. 223. GENOHOLOTyPE. — B. borussica Gagel 1890. Description (translation from Gagel). — Outline transversely lengthened, ventral valve (? dorsal) strongly arched, provided with a thick, keel-like ele- vated fold, which is divided in the middle by a low, but definite groove. On the anterior margin the ventral (dorsal ?) valve is provided with a deep indentation; the beak is small but so strongly incurved that it touches the dorsal valve and no foramen can be perceived. Hinge margin somewhat narrower than the greatest shell width, slightly curved. Dorsal valve (? ventral) flattened, with a small beak, from each side of which there runs a small flat surface that joins with the shell proper at an angle. Close under the beak begins a very deep, steep sinus; at the anterior margin the dor- sal valve is drawn out into a rather noticeable tongue which is bent gradually toward the remaining valve at a right angle ; the tongue fills up the indentation of the anterior margin of the ventral (? dorsal) valve. Un- fortunately this part of the anterior margin is quite damaged, so that the exact outline can no longer be ascertained. The outer shell layer is preserved in a few places only, and shows a very fine, concentric ornamentation. Both valves are distinguished by the possession of a very large, strong, median septum which extends from the beak to half the length of the shell. Aside from these septa, which are clearly visible on the outer surface of the shell, nothing is known of the interior. Discussion. — Gagel thought Branconia was a rhyn- chonellid but found that it agreed with no known GENEILA OF THE SUBORDER PENTAMEROIDEA 169 genus. It does not belong to Rhynchonella or Cam- erophoria, since the sinus is situated on the dorsal (? ventral) valve; moreover, from Rhynchonella as well as from Enton'ut and Pentamerus it is clearly dif- ferentiated in having a very large strong septum in each valve. The only existing example was found in a "Lower Silurian" (Ordovician) erratic boulder thought to belong to the Jewe horizon of the Estonian Ordovician. Hall and Clarke remark on the fact that the author may have reversed the position of the ventral and dorsal valves. The presence of a septum in each valve and the occurrence of the shell in rocks of probable Trenton age suggests a relationship of Branconia to Camerella. However, in the absence of definite information re- garding the nature of the interior and in the absence of comparative material, the genus will stand for the present. Genus PARASTROPHINELLA Schuchert and Cooper 1931 PI. 25, figs. 23, 25, 26; pi. 29, fig. 7 Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, p. 248. Genoholotype. — Pentamerus reversus Billings 1857, Geol. Surv. Canada, Rept. Prog, for 1856, p. 295 ; Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 63, fig. 11. Description. Exterior. — Shell subglobular in out- line, hinge-line narrow, lateral profile biconvex, the dorsal valve always the more convex and usually arched somewhat over the ventral interarea. Anterior commissure broadly uniplicate. Ventral interarea nar- row, apsacline, beak curved slightly, delthyrium open, so far as known. Dorsal interarea obsolete, beak in- curved, umbo strongly arched. Surface costate, the costje being faint or obsolete at the umbones as in Camerella. Shell structure fibrous, impunctate. Ventral interior. — Like that of Camerella, with a prominent spondylium duplex, the septal plates of which are prolonged for some distance in front of the spondylium. In one species and in some individuals of other species the spondylium is sessile at the posterior of the shell. Dorsal interior. — Like that of Anastrophta, with the septal plates discrete or convergent only at their front ends. Alse somewhat better developed than in Camer- ella. Adductor scars anterior to the cruralium. Geologic range. — Upper Ordovician to Middle Silurian of North America. Species Parastrophia divergens Hall and Clarke 1895 P. latiflicala Hall and Clarke 1895 P. multiflicala Hall and Clarke 1895 P. ops (Billings) 1862 /». rw/ff/d (Billings) 1857 Distinguishing characters. — This new genus differs from Camerella srnsu stricto chiefly in the divergence of the septa of the dorsal valve, which do not form a cruralium duplex. It thus forms, with the other Camerellidx, a parallel scries with the Gypidulina:. Discussion. — Hall and Clarke proposed the name Parastrophia and typified the genus by Atrypa hemi- plicata Hall. Little did they realize that they had selected a genotype whose internal structure is identical with that of Camerella volbortht Billings. However, several of the later species usually placed under the old name of Parastrophia prove to have features in the dor- sal valve differing from those of Camerella and are here placed under the new name Parastrophinella, chosen to preserve a semblance of the old term. It is known from internal molds that the adductor muscles of the dorsal valve of Parastrophinella were situated entirely outside of the cruralium as in Camer- ella. In this respect, Parastrophinella agrees with the other members of the family. Genus ANASTROPHIA Hall 1867 PL 25, figs. 14, 15, 19, 33-36, 38-42 HaU, N. Y. State Cab., 20th Rept., 1867, p. 163. Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 1893, p. 224, pi. 63, figs. 31-38. Horn. Brachymerus Shaler 1865 Genolectotype (Hall and Clarke). — Pentamerus verneuili Hall 1857, N. Y. State Cab., 10th Rept., p. 104, figs. 1, 2. Description. Exterior. — Globular to subellipti- cal; hinge-line straight, narrow; cardinal extremities broadly rounded; lateral profile unequally biconvex, anterior commissure uniplicate, the dorsal fold usually being defined only in the anterior half of the valve. Ventral interarea small and narrow, cleft by an open delthyrium; dorsal interarea obsolete; beak curved, commonly resorbed. Surface multicostate ; shell sub- stance fibrous, impunctate. Ventral interior. — Teeth narrow, sharp, with deep crural fossettes, the teeth forming a strengthening ridge along the delthyrial margin; dental plates con- vergent, making with the septal plates a spondylium duplex which is sessile or nearly so at the posterior of the shell and is supported in front by the median septum. Scar of pedicle attachment located just be- neath the dental ridges. Dorsal interior. — Notothyrial cavity deep; brachial supports stout curved processes, carinate on their pos- terior face, sloping laterally into a concave fulcral plate which is attached to the inner shell wall. Sockets deep. Septal plates nearly vertical lamell.-c, subparallel or con- verging slightly toward the anterior. On the outside of these plates and beneath the brachial supports are winglike plates which are concave outward. In the 170 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA middle of the space between the septal plates is a low ridge with diductor scars on each side which probably served as a cardinal process. The adductor impres- sions are four in number, a posterior pair located on the outside of the anterior ends of the septal plates, and a larger anterior pair that is elongate or sub- trigonal in outline. Geologic range. — Middle Silurian to Lower Devonian. American Species Anastrofhia brevirostris (Hall) 1852 A. internascens Hall 1879 A.interflicata CHiW) 1852 A.verneuili (Hall) 1857 European Species Anastrofhia deflexa {Siov/erhy) 1839 A.magnifica Kozlowski 1927 Distinguishing characters. — Anastrofhia is dis- tinguished from Parastrophinella chiefly by the more completely costate exterior. Discussion. — ^There is some variation in the spon- dylium of Anastrofhia. In some specimens it is sup- ported for its whole length by the ventral septal plates, but in others the posterior end rests on the floor of the valve. In one unique individual (pi. 25, fig. 42) the walls of the spondylium have bent inward and united, forming a tubular chamber open at the front and back. Such a structure must have impaired the activity of the muscles and is regarded as a pathologic case. In many of the specimens the scar of the pedicle attachment is to be seen in a depression on each side of the spon- dylium just beneath the tooth ridges. The ventral beak and the dorsal umbo as well are commonly more or less abraded by the pedicle. The cardinalia of the dorsal valve are typical of the family. There are long brachial supports and outside of these are broad, alate expansions. The posterior ends of the septal plates unite with the inner surfaces of the alae, which are carinate, thickened in the back, and slope off on the outside to unite with a thick, concave plate that serves as a socket. Adductor scars are visible at the front ends of the septal plates, not having become enclosed as in the Pentamerinae. several longitudinal plications chiefly developed towards front end; beak high, incurved, with open delthyrium; small false area on each side; interior with small subum- bonal spondylium and short median septum. Brachial valve with lower beak than opposite valve; low median fold near anterior end, composed of several longitudinal plications; interior with pair of long recurved crura, pair of long parallel median septa, and muscle-scars as in Parmtrofhia. Shell thick, fibrous, punctate externally. Discussion. — In the absence of material. Reed's genus is difficult to place in our taxonomic scheme. From the morphology as described by him, it is evident that Metacamerella is very close to Parastrophinella and to Anastrofhia. It differs from the former extern- ally in possessing a fold on both valves. Unfortunately Reed's illustrations do not adequately bear out this statement, as figure 18, of the dorsal valve, shows a decided sulcus and appears to be a totally different shell from the one under consideration. Figures 15 and 17, however, clearly have a fold. According to Reed, the muscle marks of the dorsal valve are like those of Camerella (Parastrofhia), but the figures again, except for figure 18, do not show them. Should figures 14 and 18 represent opposite valves, the genus would not be difficult to place and we could say at once that it is structurally a Parastrophinella with fold and sulcus reversed, and clearly belongs in association with Ana- strofhia and Camerella. The question of punctation among pentamerids is a very important one. Reed states that the punctation is external, and we have shown in our discussion of orthids that exopunctse have no taxonomic significance beyond genera. This is best seen in Porambonites and Linoforella, genera that have developed the same kind of exterior, but the former has an impunctate shell and the latter is endopunctate as in the Dalmanellacea. From the internal characters of Metacamerella it would appear to be impunctate as are all other pentamerids. From the above discussion it seems best to place Metacamerella close to Anastrofhia, the chief known generic difference of the former being a fold on the ventral valve. Were it not for Reed's assertion that Metacamerella possessed a dorsal fold, which seems unlikely, to judge by the ventral valve he figures, we would exclude from his genus his figures 15 and 17. Genus METACAMERELLA Reed 1917 Reed, Trans. Roy. Soc. Edinburgh, vol. 51, art. 4, 191 7, p. 934, pi. 23, figs. 14-18. Genoholotype. — Stricklandia ? balcletchiensis {balclatchiensis) Davidson 1883, Brit. Foss. Brach., vol. 5, Sil. Suppl., p. 166, pi. 9, figs. 27-29. Original description. — Shell oval, biconvex. Pedicle- valve with low median fold near anterior end composed of Family PENTAMERIDS McCoy 1844, emend. Terminal, usually rostrate Pentameracea, probably derived out of the Camerellids. Shells smooth, costate or multicostate. Ventral interareas short and narrow, but often obsolete, or plane areas are developed. In nearly all there is a more or less large spondylium GENERA OF THE SUBORDER PENTAMEROIDEA 171 duplex, supported by cither very long or short double septa; but in two genera the spondylium hangs free. Dorsal cardinalia — the most characteristic feature of the family — tripartite; consisting of discrete plates, or united into a cruralium that bears the adductor muscles. When the plates are discrete, the adductors are on the floor of the shell between them. Brachial processes very long, and in some forms terminally divergent. A cardinal process is usually absent or, when present, is a rudimentary septum or a low callosity. A concave deltidium and incipient deltidial plates are sporadically developed, but in general the delthyrium is open. Shell substance fibrous, impunctate. Geologic range. — Silurian and Devonian. Embraces two subfamilies: Gypidulins Schuchert and Pentamerinae Waagen. Discussion. — The Camerophoriina; of the Devo- nian and Carboniferous, long included in the Penta- meracea, are now considered to be spondylia-bearers of the Rhynchonellacea, and the Porambonitidas are in the present work shown to be much modified Orthacea. Subfamily GYPIDULIN^ Schuchert 1929 Pentamerids more or less galeatiform, with small ventral interareas. Ventral valve always the more con- vex and deeper. Cardinal process simple, but usually absent. Shells smooth, or with some low or many rounded costae. Geologic range. — Silurian and Devonian. Includes the following genera:'^ Clorinda Barrande (syn. Barrandina Booker 1926) Barrandella Hall and Clarke Gyfidula Hall Sieberella Oehlert Pentamerella Hall ? Zdimir Barrande The genetic relations are about as shown in Table 19. Discussion. — We include in the subfamily Gypidu- linae all of the galeatiform Pentameridae having the internal structure of the family. This subfamily was rather prominent in the Silurian and Devonian but evidently did not survive the latter period. As pre- viously constituted, it included Virgiana, but the struc- ture of the dorsal septal plates in the latter genus is so close to that in Pentamerus that it must be referred to the Pentamerinae. Its previous constant association with the Gypidulinje is undoubtedly due to the errone- ous figure of the interior published by Hall and Clarke. " Our genus Lioccetia, which was included in this sub- family in 1931, is now seen to be a rhynchonellid; it is further described in the Appendix to this work. Clor'ttida and Barrandella show some internal varia- tions from the rest of the subfamily. At the junction of the inner and outer septal plates of the dorsal valve there is a small process extending in a ventral direction and into the notothyrial chamber. This feature has not been observed in Gyp'tdtda, Sirherella, or Penta- ?nerclla, but was noticed in one specimen of Clorinda. In the Gypidulinae occur some interesting paral- lelisms. Barrandella and Clorinda are externally alike but in the former the plates of the dorsal valve form a Table 19 Gypidulin^ Pentamerella Pentamerin* Barrandella CAMERELLIDiC cruralium whereas in the latter they are discrete. The same relations also hold for Sieberella and Gypidula. These striking features are not only of great interest but are of considerable importance to students of strati- graphy and paleogeography. Genus CLORINDA Barrande 1879 PI. 26, figs. 8-12, 21; t. fig. 29 Barrande, Syst. Sil. Boheme, vol. 5, 1879, p. 109, pi. 1 19, figs. 1-4. Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 1893, p. 244. Syn. Barrandina Booker 1926 (genotype, B. wilkinsoni Booker 1926 ^ Pentamerus linguifera var. wilkinsoni Etheridge 1892 (pars)), Jour. Proc. Roy. Soc. N. S. Wales, vol. 60, p. 131. Genoholotype. — C. armata Barrande 1879. Description. Exterior. — Subgaleatiform ; hinge- line straight, cardinal extremities broadly rounded; lateral profile biconvex, the ventral valve having the greater convexity. Anterior commissure uniplicate; ventral sulcus usually ill defined; anterior of ventral 172 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA valve produced into a long tongue; dorsal fold low, best defined at the front. Ventral interarea narrow and short, curved apsacline to orthocline, beak strongly curved, umbo inflated; dorsal interarea obsolete, beak curved, umbo swollen. Valves unornamented except by concentric lines of growth. Shell structure fibrous, impunctate. Ventral interior. — Delthyrial cavity shallow; teeth small, dental and septal plates forming a small spon- dylium duplex, which is free at its anterior end. Dorsal interior. — The septa of the dorsal valve are subparallel or divergent as in Gyfidula. At the junc- tion of the inner plates with the base of the brachial process there is a carina pointing ventrally. The brachial process is wide, moderately long. Geologic range. — Silurian of North America, Europe, and Australia. and accordingly we feel that this carina has no signifi- cance in generic taxonomy. A plate of this kind was seen also in one specimen of C onchidium, and Booker shows them as well in Gyfidula galeata from Gotland. We therefore hold that Clorinda is the smooth equiva- lent of Gyfidula, having its fold on the dorsal valve. In this latter respect it differs from certain smooth Gypidulas (G. fseudo galeata^ which may ultimately be referred to a new subdivision. Booker has recently proposed a new genus Bar- randina for shells having the external form of Clorinda, but thought to differ from the latter in the dorsal interior. He defines his genus in part as follows: At the junction of the crural plates and septa a pair of curved, outwardly convex plates are developed. These are attached throughout their entire length to the cruralium, at the junction of the crural plates [inner plates] and septa 1^. ^'^ Fig. 29. — Serial sections of Clorinda fseudoUnguijera Kozlowski. After Kozlowski. ca, carina at the limit of crural plates and crural septa; cr, crural plates (inner plates); dt, teeth; scr, crural septa (outer plates); sm, spondylium; s, septum. American Species Athyris {?) tumidula Billings 1866 European Species Clorinda ancillans Barrande 1879 C. armata Barrande 1879 C. bubo Barrande C. fseudoUnguijera Kozlowski 1 929 Distinguishing characters. — Clorinda is a smooth shell, internally resembling Gyfidula, and hav- ing the fold on the dorsal valve. Discussion. — Inside the ventral valve the septum is very short and the spondylium hangs free for most of its length. The dorsal cardinalia are like those of Gyfidula and have rather long brachial processes as is typical of the family. The most notable distinction to be seen from Gyfidula is the carina which runs along the base of the brachial process at its junction with the inner plate. In pentamerids of this type from Australia a carina may be seen also at the junction of the base of the brachial process with the outer plate. [outer plates], either along the median line of the convex side of the plate, or at the edges, being then intercalated between the septa and crural plates. These plates extend beyond the anterior termination of the crural plates and septa for fuUy one-third of their length and terminate at a point slightly anterior to the end of the spondylium. The sub-genus Barrandina has been erected for the recep- tion of certain Australian Pentamerids, with the fold on the brachial valve and sinus on the pedicle valve, in which the cruralium is modified by the development of an extra plate at the junction of the septa and crural plates. The two species comprising the sub-genus were first described by Etheridge as Pentamerus linguifera var. zvilkinsoni. Subsequent work on the pentameroids of the Yass district has revealed a series of forms paralleling in their structures the Barrandella and Sieberella series of Europe and America, but all characterized by the development of an extra plate in the cruralium. It is our belief that the "extra plate" of Booker is actually the same as the brachial process of all other pentamerids but has more pronounced ridges than is usual along the junction of the inner and outer plates with the base of the brachial process. In our opinion the internal structure of Barrandina does not differ GENERA OF THE SUBORDER PENTAMEROIDEA 173 generically from that of Clorinda. In B. minor Booker the septal plates are more widely divergent than in B. wilkinsont, in which they are subparallel. The latter species is approaching Barrandella, but the former is quite a typical Clorinda. We therefore regard Barranditia as a synonym of Clorinda. Genus BARRANDELLA Hall and Clarke 1893 PI. 26, figs. 1-3, 5-7; t. fig. 30 Hall and Clarke. Pal. N. Y., vol. 8, pt. 2, 189.% pp. 241, 245, pi. 71, figs. 1-3, t. fig. 173; H.indbook, pt. 2, 1895, p. 844, figs. 457-458. Genoholotype. — Atrypa linguifera Sowerby 1839, in Murchison, Sil. Syst., p. 629, pi. 13, fig. 8. Distinguishing characters. — Barrandella is to the genotype. Later Schuchert (1897) regarded it as a synonym of the older Clorinda. Recently the name has been revived by Booker (1926) and Koz- lowski (1929), the latter recognizing Barrandella as a genus and Clorinda as a subgenus, but we prefer to regard both groups as of generic rank. Geologic range. — Silurian of North America and Europe. American Species Clorinda ventricosa (Hall) 1860 ? C.jornicata (Hall) 1852 European Species Barrandella linguifera (Sowerby) 1839 ^ .. Oh c\^n\^T 185 3 P. felagica Barrande 1879 (in part) Distinguishing characters. — Gyfidula is most readily recognized externally by its galeate form and the position of the fold on the ventral valve. Intern- ally the differential characters are in the dorsal valve, in the discreteness of the brachial supports, which form a double track on the dorsal surface when the shell is eroded or seen in section. Discussion. — ^The ventral morphology of Gyfidula presents no departures from that in Clorinda, Pentam- erella, and other genera of this subfamily. The septum is exceedingly variable in its length and this is true like- wise of the spondylium. The latter in some species (G. romingeri and G. coeymanensis) is free for more than three-fourths its length. The internal surface is marked with elongate pustules and low ridges which are interpreted usually as evidences of ovarian struc- tures. The teeth are sharp and slender. The dorsal interiors of G. coeymanensis and G. romingeri, of which excellent examples are at hand, are especially instructive in illustrating the characters of the genus. In these the dorsal septal plates are divisible clearly into three parts, termed by Leidhold'* (1) inner crural plates, (2) outer crural plates, and (3) septal plates (see pp. 164-165). A linear cardinal process was observed by Hall and Clarke in the genotype, but the occurrence of such a structure is not general throughout the various mem- bers of the genus. The usual condition is for the diductors to attach to the floor of a deep pit under the beak. It has been noted in some instances that the pit is divided by a low ridge which may be inter- preted as a cardinal process, although it was appar- ently never functional as such in the sense of bearing the muscles. Hall founded the genus Gyfidula on Petitamerus occidentalis and G. Iwvtusculus and stated distinctly that the lamella of the dorsal valve were separate and diverging. This therefore excludes from Gyfidula shells like Sieberella sieberi in which the septal lamellae unite with a median septum. Hall and Clarke, reason- ing with no knowledge of homceomorphy and the confusion it may cause by obscuring the true relation- ships of genera, maintained that the union or diver- gence of the crural lamellas was of little import, being only of specific value. In so holding, they could find no distinction between the two genera except that Sieberella was lacking in "deltidial plates" and a "car- dinal area." This distinction between the genera has been perpetuated by most recent authors. Schuchert in 1897 placed Sieberella in the synonymy of Gyfidula >' Abh. preuss. geol. Landesanst., N. F., Heft 109, 1928, pp. 49-53. GENERA OF THE SUBORDER PENTAMEROIDEA 175 and this was also Leidhold's solution (1928).''' In 1913,'^ however, the former author indicated S'leber- ella as a subgenus of Gyfidula and in the Zittel- Eastman Text-book of Paleontology both were given independent rank. Recently Belanski (1928), in a study of the Iowa Upper Devonian pentamerids, notes the internal differences mentioned by Hall and Clarke and also the fact that Sicherella sieberi actually has a cardinal area. The latter fact has been corroborated by the present writers. This heretofore superficial dis- tinction therefore fails, with the result that the chief difference between the genera lies in the dorsal interior. It is just as correct to assume a line of reasoning diametrically opposed to that of Hall and Clarke: commissure sulcate; ventral fold usually defined only on the front half of the shell. Ventral interarea rather wide for the subfamily, curved, apsacline, delthyrium open, beak incurved strongly, umbo inflated. Dorsal interarea obsolete, beak incurved, umbo swollen. Sur- face multicostate ; shell substance fibrous, impunctate. Ventral interior. — Like Gyp'ulula. Dorsal interior. — The tripartite character of the car- dinalia is essentially the same as that of Gypidula, but the septal plates, instead of uniting directly with the floor of the valve, unite with a low median septum. Geologic range. — Silurian and Devonian of North America and Europe. (D (t^ QX) \J \j Fig. 31. — Sieberella sieberi Barr. Sections showing internal structure, dorsal septa to form a single plate. Distance of sections from beak: 1 — 1.6 mm. 2—2.8 3—3.5 4 — +.0 Of greatest significance is the union of the 5 — 4.8 mm. 6—7.9 7—9.2 the internal structures can be seized upon as diagnostic, the cruralium of Sieberella distinguishing that genus from Gypidula, which has divergent lamellae. The striking external resemblance in form we therefore attribute to homoeomorphy. Genus SIEBERELLA Oehlert 1887 PI. 26, figs. 25, 30, 33; t. fig. 31 Oehlert in Fischer, Man. Conch., 1887, p. 1311. HaU and Clarke, Pal. N. Y., vol. 8, pt. 2, 1893, pp. 241, 246, pi. 72, figs. 4, 5, t. fig. 176. Genoholotype. — Pentamerus sieberi Von Buch in Barrande 1847, Brach. Sil. Schichten Boehmen, p. 103, pi. 21, figs. 1, 2. Description. Exterior. — Outline galeatiform as in Gypidula, hinge-line faintly arcuate; cardinal ex- tremities rounded. Lateral profile biconvex, the ven- tral valve usually with the greater convexity. Anterior "Op. cit., p. 50. *' Marj'land Geol. Surv., Lower Devonian, p. 342. American Species Sieberella emarginata Belanski 1928 S.insolita Belanski 1928 S. roemeri Hall and Clarke 1893 (in part) European Species Sieberella multistriata Roemer S. sieberi (V. Buch) 1847 S. sieberi rectifrons (Barrande) 1879 Discussion. — Hall and Clarke restricted the name Sieberella to shells not having a cardinal area on the ventral valve, this being their sole distinction. A re- study of the genotype, however, shows that it possesses a well defined area. Nevertheless, there is a funda- mental internal difference, the convergence of the septal plates to meet a median septum. This type of structure is not common and is restricted seemingly to rather transverse shells which are costate over nearly the whole surface. Sieberella has the fold restricted to the ventral valve and is thus a homoeomorph of Gypidula. The two genera form a striking example of internal variation combined with a stable external expression. 176 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA Genus PENTAMERELLA Hall 1867 PI. 26, figs. 4, 13-17, 19,20; t. fig. 32 Hall, 20th Rept., N. Y. State Cab., 1867, p. 163. Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 1893, pp. 242, 245, pi. 71, figs. 21-29. Genolectotype (Hall and Clarke). — Atrypa arata Conrad 1841, 5th Rept. N. Y. Geol. Surv., p. 55. Description. Exterior. — Outline subtriangular to subpentagonal ; hinge-line narrow; cardinal extremi- ties rounded ; lateral profile biconvex, the ventral valve having the greater convexity. Anterior commissure uniplicate; dorsal fold usually low, in some species nearly obsolete. Ventral interarea narrow, curved, apsacline, beak incurved strongly, umbo swollen; del- P. intrdineata (Winchell) 1 866 P. missouriensis Branson 1922 P. favilionensis (HaW) 1860 Distinguishing characters. — Externally Pen- tamerella most closely resembles Gypidula, but differs in having the fold on the dorsal valve. Internally, it is similar to SieberelLa in having the septal plates unite with a low septum to form a cruralium. Discussion. — Of interest in Pentamerella is the ex- tremely short septum of some of the species. In shells referred to P. favilionensis , from Moreland, Kentucky, the septum may be confined to the very posterior of the shell as a short rib. In the brevity of its sep- tum Pentamerella represents an advanced generic development. o o 6 6 <5 O I 8 Fig. 32. — Pentamerella aff. P. ■favilionensis (Hall), Alpena, Michigan. Of prime interest in these serial sections is the extreme brevity of the median septum in the ventral valve. It was apparently a ridge on the posterior wall of the shell. X 1 . Distance from beak: 1 — 1.9 mm. 2—3.9 3—4.2 4—4.9 5 — 5.5 mm. 6— 6.9 7— 7.6 8—11.25 thyrium with incipient deltidial flates. Dorsal inter- area obsolete, beak curved under that of the ven- tral valve. Surface usually multicostate, occasionally smooth. Shell substance fibrous, impunctate. Ventral interior. — Teeth narrow and sharp; spon- dylium duplex shallow; floor of spondylium longi- tudinally striated, supporting septum short; intern.il umbo-lateral ovarian spaces pustulose. Dorsal interior. — Notothyrial cavity deep; plates supporting brachial processes broad and flat, supported by thin curved plates which unite in the mid-line of the shell to form a cruralium that in some specimens is supported by a low duplex septum. In the apex is a shallow pit divided by a low ridge ; the two pits are the areas of the diductor scars. Geologic range. — Devonian (Middle and Up- per) of North America. Species Pentamerella arata (Conrad) 1841 P.dubia (Hall) 1860 P. juhoneiisis Branson 1922 Booker^* has recently published figures of "Pentam- erella" molongensis Mitchell, which are obviously not of this genus. In this form the septal plates are widely divergent as in Clorinda or Gyfidula. The same is true also of "Pentamerella" sublinguifer Maurer, figured by Leidhold.'^ In this shell the dorsal septa are wide-spread and the form is certainly not a Pen- tamerella. The figures suggest Clorinda, as the shell is quite smooth. Hall and Clarke say that in Pentamerella "there are occasionally evidences of lateral, erect or convex growths upon the margins of the delthyrium, which may be interpreted either as remnants of a resorbed convex deltidium, or as highly accelerated secondary deltaria" (pp. 341, 342). We have also seen these plates, and interpret them as incipient deltidial plates. "Jour. Proc. Roy. Soc. N. S. Wales, vol. 60, 1926, pp. 140-142. " Abh. preuss. geol. Landcsanst., N. F., Heft 109, 1928, p. 57, pi. 4, fig. 14, t. fig. 25. GENERA OF THE SUBORDER PENTAMEROIDEA 177 Genus ZDIMIR Barrande 1881 B.irrande, Syst. Sil. Centre Bohcmc, vol. 6, 1881, p. 171, pi. 292, figs. 1 7-20. Genoholotype. — Z. solus Barrande. Discussion. — A single isolated valve from the De- vonian of Bohemia, of "unusual and enigmatic" ap- pearance, was named Z/iimir and provisionally referred by Barrande to the Pelecypoda. The specimen, ac- cording to the author, resembles the arched valve of certain elongate pentamerids, but is slightly inequi- lateral. He states further that the specimen has a moderately developed, curved beak, almost as in the pentamerids, without trace of an area, and the surface of the beak is broadly open. The exterior is orna- mented by cost.T diverging from the beak. No muscle or pallial marks were observed. Aside from the allu- sions to its possible affinities with the pentamerids, Barrande compares the specimen with Isocardia and places it in the Pelecypoda. It was later referred to as scarcely separable from Uncites gryfhus by Freeh.** Novak,'* however, studied and prepared a specimen of Z. solus in the Bohemian museum at Prague, showing that fragments of the shell substance adhering to it had the structure of brachiopod shells and that it had a spondylium sup- ported by a septum. Ztlimir is therefore quite clearly a pentamerid. In 1913 Schuchert"" considered the genus as a synonym of C onchidium . The latter genus is, however, not certainly known in the Devonian. Barrande has described many species of Gyp'tdula from Bohemia but Novak claims that Zdimir is not referable to any of them. Two courses can be taken with this doubtfully valid genus: (1) the name can be restricted to the existing specimens; or (2) it can be placed provisionally as a synonym of Conchidium which is known doubtfully from the Devonian. C apeUinirllit Strand Holorliyncluts Kiaer Harpidium Kirk Costa tse Rhifid'tunt Schuchcrt and Cooper Conchidium Linna;us Brooksina Kirk Platymerella Foerste Cymbidiuni Kirk Virgiana Twenhofel The genetic relationships are thought to be as shown in Table 20. Discussion. — The Pentamerinx are a large and interesting group of shells intimately tied together by their rather elongate and non-galeate form as com- pared with the Gypidulina. Internally their spon- dylium is like that of the other subfamily. In the dorsal interior the only difference of note lies in the septa, which are less bowed than in the Gypidulinas. The subfamily is provisionally divided into two groups on the basis of its external sculpture. The significance of this division is discussed in the pre- liminary remarks. Suffice it to say here, that nearly every smooth stock is represented by an equivalent costate one. For example, Pentamerus represents a flattish smooth stage, and has its costate equivalent in Rhifidium. The writers found that Orthotropia, which has long been classified with the Pentamerina;, is actually an orthid. In the ventral valve it has a pseudospon- dylium and in the dorsal there is a short cruralium occupied by a prominent, simple cardinal process. The structure of Orthotropia allies the genus with Lino- porella, but its exterior is at present unknown. Subfamily PENTAMERINX Waagen 1883 Large, rostrate, elongate, non-galeate Pentam- eridas, usually without distinct interareas. It is in this subfamily that the internal characters of the family are t)'pically developed. Shells smooth, costate or multicostate. Geologic range. — Silurian and early Devonian. Has the following genera: Laeves Pentamerus Sowerby Pentameroides Schuchert and Cooper Lissocceitna Schuchert and Cooper " Zeits. deut. geol. Gesell., vol. 38, 1886, p. 920. '» Ibid., vol. 40, pp. 588-590, 1888. ^^ Zittel-Eastman Text-book of Paleontology, 2d ed., vol. l,p. 394. Laves or Smooth Pentamerinse Genus PENTAMERUS Sowerby 1812 PL 27, figs. 4, 8, 9, 10, 12, 15, 17-19; pi. 29, fig. 11; t. fig. 26 Sowerby, J., Min. Conch., vol. 1, 1812, p. 73, pi. 28. Davidson, Brit. Foss. Brach., vol. 1, Introd., 1851-1855, p. 97; vol. 3, pt. 7, 1866-1871, pi. 18, figs. 1-12, pi. 19, figs. 1, 2 {oblongus). HaU and Clarke, Pal. N. Y., vol. 8, pt. 2, 1895, p. 236, pi. 67, figs. 11-13, 20, pi. 68, figs. 1-8, pi. 69, figs. 1-14, pi. 70, figs. 1-5, t. figs. 169-171 (all of oblongus) . Syn. Pentaiicre Blainville 1824, Diet. Nat. Sci., vol. 32, p. 301. Genolectotvpe (Hall and Clarke). — P. Itpvis Sowerby 1812. Description. Exterior. — Shell large, pentagonal- hexagonal in outline, commonly trilobate in front; 178 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA hinge-line gently arcuate; cardinal extremities round- ed; lateral profile subequally biconvex. Anterior com- missure usually rectimarginate. Fold present or ab- sent on both valves. Interareas obsolete; ventral beak curved over the dorsal; umbo prominent. Surface P. cylindrktis Hall and Whitfield 1872 P. diver gens Foerste 1909 P. oblongus Sowerby 1812 P.ovalis HaU 1852 P. farvulus Savage 1913 />. ^ffwrw Whitfield 1882 Table 20 Lissoccelina Pentameroides CapeUiniella Pentamerus Holorhynchus Platymerella «■ Rhipidium (costate) Brooksina Cymbidium / Conchidium Harpidium (smooth) .Virgiana Clorinda (GvPiDULiNiE) smooth or with faint undulations at the front. Shell substance fibrous, impunctate. Ventral interior. — The dental plates are convergent and with their septal plates unite to form a spondylium duplex. Dorsal interior. — A tripartite division of the crural plates is recognizable. Thin inner plates unite with the walls of the valve to form lateral chambers. The sockets are notches in the partitions of these chambers next to the wall of the valve. The brachial processes are long. The septal plates are discrete, divergent or parallel, continued to the internal surface of the valve. Between the septal plates a low median septum divides the septal chamber into two parts. The cardinal process is a callosity under the beak. Geologic range. — Throughout the Silurian of Europe and North America and probably widely elsewhere. American Species Pentamerus bisinuatus McChesney 1861 P. comfressui Kindle and Breger 190+ P. corrugatus Waller and Davidson 1 896 European Species Pentamerus borealis Eichwald 1842 P. esthonus Eichwald 1860? P. gothlandicus Lebedeif 1 892 P. Iwvis Sowerby 1812 P. oblongus Sowerby 1812 P. samojedicus Keyserling 1 846 P. schmidti Lebedeff 1 892 Distinguishing characters. — The genus Pen- tamerus is characterized by its comparatively smooth exterior, long spondylium duplex, discrete septal plates, and tripartite cardinalia. Discussion. — Hall and Clarke state that Pentam- erus "is an exceedingly plastic type," and limit the name to smooth shells typified by P. Icevis Sowerby. The latter is considered by European paleontologists as the young of P. oblongus. The specific name P. oblongus has in America been applied to a wide variety of forms, among them P. oblongus subrectus Hall and Clarke. In this form the septal plates of the cardinalia are united to form a duplex septum which supports a shallow cruralium. GENERA OF THE SUBORDER PENTAMEROIDEA 179 This development is so different from Pfntainerus that we are distinguishing it as a new genus under the name Pe'titamerouifs. The most interesting internal feature of Pintam- erus is the long brachial process which reaches to the anterior end of the spondylium. The two processes appear to remain separate, judging by serial sections prepared by us. Genus PENTAMEROIDES Schuchert and Cooper 1931 PI. 27, figs. 13, 14; pi. 28, figs. 19, 22; t. fig. 27 Schuchert and Gx)per, Amer. Jour. Sci. (5), vol. 22, 1931, p. 248. Genoholotype. — Pentamerus suhrectus Hall and Clarke, 1893, Pal. N. Y., vol. 8, pt. 2, p. 238, pi. 69, figs. 2, 3, 8-10. Distinguishing characters. — Externally the members of this genus are precisely like Pentamerus, but internally there is a cruralium in the dorsal valve that in some specimens is sessile for a short distance at the posterior but is elevated at the front. In some species, however, the cruralium is supported by a promi- nent median septum for its whole length. In all of the species the dorsal septum is rather long and is not uncommonly longer than the ventral one. A careful study of internal molds, a common mode of occurrence of the genus, shows that the adductor muscles were attached to the upper surface of the cruralium, for no trace of muscle scars has been observed on the floor of the valves. We have selected Pentamerus suhrectus as the type of the genus. This is a well defined species charac- terized by its shouldered dorso-lateral extremities. There are a number of unnamed species occurring in the Wisconsin dolomites, especially at Bailey's Harbor. Here occurs a flat form much like the Clinton P. ob- longus, but another type externally nearly identical with Pentamerus cylindricus is also known. Discussion. — Pcntameroides has a wide distribution in Silurian strata, being fairly common in Iowa and Wisconsin. It was also found associated with a small Stricklandia in the Silurian beds on the shore of the Bay of Chaleur at Black Point postoffice. New Bruns- wick. Some of the shells called Pentamerus borealis by European writers are of this type. It is thus clear that there must be a drastic revision of the species of Pentamerus s»nsu lata. Geologic range. — Widely distributed in the Silu- rian of North America and Europe, but the species are not at all worked out, being referred to other genera and mainly to Pentamerus. American Species P.tubrectus Hall and Clarke 1893 Genus LISSOCOELINA Schuchert and Cooper 1931 (Gr. lissosy smooth ; ko'ilia, belly) PI. 28, figs. 7, 14, ? 13, ? 15, .? 16, ? 21 Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, p. 248. Genoholotype. — Pentamerus fergibbosus Hall and Whitfield 1875, Pal. Ohio, vol. 2, p. 139, pi. 7, figs. 10, 11. Distinguishing characters. — This name is pro- posed for smooth shells having the external form of Conc/iuiium in profile and outline and the internal structure of Pentamerus. It represents an offshoot from the Pentamerus line in the Middle Silurian. The genotype is perhaps the best known example. The ventral valve is elongated and the ventral beak is arched over that of the dorsal valve as in C onchid'tum. In Pentamerus sensu stricto there is not the great develop- ment of the beak and the strong arching of the valves. Even in the young of "fergibbosus" the same shape of the adult is faithfully preserved and forbids any relationship with Pentamerus s. s. In the ventral inte- rior the median septum is extremely long and in the dorsal valve the septa are rather close together and subparallel as in Pentamerus. Externally the shell preserves no trace of radii. We are placing in this association, but somewhat doubtfully, P. maquoketa Hall and Clarke. In this species the ventral septum is short and in this respect suggests Harfidium but otherwise agrees fairly well with that of Lissocoelitia. Subgenus CAPELLINIELLA Strand 1928 Strand, Arch. Naturgeschicht., Berlin, vol. 92, A8, 1928, p. 38. H.ill and Clarke, Pal. N. Y., vol. 8, pt. 2, 1893, p. 248, pi. 70, figs. 6-14. Horn. CafelliHia Hall and Clarke 1893. Genoholotype. — C. mh-a (Hall and Clarke) 1893. Description. Exterior. — Subpentagonal in out- line; hinge-line exceedingly narrow; cardinal ex- tremities obsolete. Anterior commissure rectimargin- ate. Lateral profile unequally biconvex, the dorsal valve always with the greater convexity. Ventral interarea exceedingly narrow, almost obsolete. Del- thyrium open. Dorsal interarea obsolete, beak strongly curved, umbo swollen. Surface marked by obscure radiating costa; and a broad low fold on each valve as in Pentamerus. Shell substance fibrous, impunctate. Interior. — Essentially as in Pentamerus. Geologic range. — Silurian of Wisconsin. Discussion. — Specimens of this subgenus appear to be rather rare and restricted geographically to Wis- 180 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA consin. It forms a remarkable example of reversion of relative convexity of the valves and from its com- paratively smooth shell is clearly an offshoot from Pentamerus. The ventral median septum is rather long, reaching about to the middle of the valve. It extends practically to the beak, showing the spondylium to have been supported for its whole length. The spondyhum as shown on interiors is, however, rather short, shorter than usual in Pentamerus. In the dorsal valve the septal plates are rather closely placed and nearly parallel. The portion bounded by them is divided by a low median ridge as is usual in the pentamerids. Genus HOLORHYNCHUS Kiaer 1902 PI. 27, fig. 20 Kiaer, Norg. geol. unders. aarbog for 1902, 1902, p. 68, figs. 1-7, pp. 103-110. Genoholotype. — H. giganteus Kiaer 1902. This name was proposed by Kiaer for shells ex- ternally like Pentamerus ohlongus Sowerby except that they are transverse and have a free spondylium in the ventral valve. The structure of the dorsal valve is like that of Pentam,erus. Geologic range. — The only known form, the genotype, is from the Silurian of Norway. Genus HARPIDIUM Kirk 1925 Kirk, Proc. U. S. Nat. Mus., vol. 66, 1925, pp. 1-5, pi. 1, figs. 1-6, pi. 2, fig. 7. Genoholotype. — K. insignis Kirk 1925. Description. Exterior. — Shells large, longitudi- nally subtriangular ; hinge-line narrow; cardinal ex- tremities obsolete. Lateral profile unequally biconvex, the ventral valve usually having the greater convexity. Anterior commissure emarginate, sulci of both valves rather shallow. Interareas obsolete or exceedingly narrow. Beaks of both valves strongly arched, the ventral one bent over that of the dorsal. Beaks acutely pointed; umbo swollen; delthyrium covered by a prominent deltidium. Shell thick, fibrous, impunctate. Ventral interior. — Spondylium duplex long and nar- row, septum short. Dorsal interior. — As in C onchidium and Pentam,- erus, there are three sets of plates. The inner ones form chambers at the back of the valve ; the outer ones bear long brachial processes and are supported by low septa. The septal plates are subparallel and always discrete. Species H arfidium insignis Kirk 1 92 5 H.latus Kirk 1925 H. rotunius Kirk 1925 Distinguishing characters. — Harpidium may be recognized easily by its smooth exterior, strongly incurved beaks, unusually long spondylium and very abbreviated septum. Discussion. — This very interesting genus has ob- vious similarities to Conchidium. These are seen in the elongate beaks, the disproportionate size of the valves, and the whole physiognomy of the shell. Fur- ther, Harfidium and Conchidium have the peculiar concave type of deltidium, the only instances of its occurrence in the Pentameridas. Despite these obvious relationships there are two points of difference of considerable significance. One of these is the smooth surface which, together with the external configuration, makes this shell unique. The other difference is found in the abbreviated septum. In typical Conchidiufn the septum extends for nearly the full length of the valve and supports the spondylium for nearly its entire length. In Harpidium, on the other hand, the septum is unusually short and the spondylium hangs free for nearly its whole length. In one specimen the length of the septum is 17 mm., that of the spondylium about 54 mm., with a greatest width of 1.5 mm., while the length of the shell meas- ures 75 mm. One wonders how the spondylium, hanging free as it does, can be strong enough to serve as the seat of attachment of the opening and closing muscles of the shell. Strength is evidently obtained by the narrowness of the spondyhum and the broad surface of muscle attachment. Of most interest in H arfidium is the deltidium, which Kirk described as "convex, elevated, deltidial plates." These are, however, not deltidial plates, but are the protruding sides of the deltidium, which is a continuous plate convex postero-dorsally. This struc- ture is like that of Conchidium (see pi. 29, fig. 4; t. fig. 25). Harpidium is evidently restricted to the Silurian of the Pacific province in Alaska. At Louisville, Ken- tucky, occurs a species, usually called Pentamerus per- gibbosus Hall and Whitfield, which is externally like Harpidium in being smooth, but is like Conchidium internally, and for this shell, which represents a stage of pentameroid evolution, we have proposed the new name Lissocaelina. Costatje or Costate Pentamerina: Genus RHIPIDIUM Schuchert and Cooper 1931 Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, p. 249. Genoholotype. — Pentamerus knappi Hall and Whitfield 1872, 24th Rept. N. Y. State Cab. Nat. Hist., p. 184. This name is proposed for rather flattish shells that have commonly but erroneously been classed with GENERA OF THE SUBORDER PENTAMEROIDEA 181 Conchid'tum. They are the costatc equivalent of Pen- tamcrus and agree in many other external details with that genus. For example, they are distinctly trilobate, the trilohation being produced by an elongate tongue extending forward from both valves. This trilobation is carried to its extreme in this genus in the species R. trilohata (Kindle and Breger). Rhip'uiiurn is a contemporary of Pentamerus, and probably lived before Conch'uiium arrived in American seas. Internally the genus is like Pentamerus in the ar- rangement of its septa. Externally the ribs are as a rule implanted or split at the front. Other species besides the genotype assigned to this genus are: ConchiJium multicostatum (Hall) 1860 C. riysius (Hall and Whitfield) 1872 (see Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, p. 235) C. tenukoita (Hall and Whitfield) 1 872 C. triloiala Kindle and Breger 1904 Genus CONCHIDIUM Linnaeus (1753) 1760 PI. 28, figs. 1-6, 8-12, 17, 18, 20, 23, 26; pi. 29, fig. 4; t. figs. 25, 28, 33 Linnxus, Syst. Nat., ed. 12, vol. 2, 1760, p. 163. Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 1893, p. 231, pi. 66, figs. 11-14. Syn. Gyfidia Dalman 1828, K. Vet. Akad. Handl. for 1827, pp. 93, 100, pi. 4, figs. la-g. Syn. Antirhynchonella Quenstedt 1868-1870, Petref. Deutsch., vol. 2, pp. 231, 727. Genotype. — C. b'doculare Linnasus 1753, Mus. Tess., pi. 5, fig. 8. Description. Exterior. — Subtriangular to sub- trapezoidal in outline, hinge-line very narrow; lateral profile biconvex, the ventral valve usually having the greater convexity. Anterior commissure rectimargin- ate, sulcate or uniplicate, fold and sulcus usually nar- rowly pointed and uncurved; delthyrium covered by a specialized "deltidium" ; dorsal beak obtusely pointed, umbo more or less swollen. Shell thick in the posterior region, costate or multicostate ; shell substance fibrous, impunctate. Ventral interior. — Spondylium duplex elongate, sup- ported by a duplex septum usually for more than one- half its length. Septum exceedingly long. Dorsal interior. — Septal plates divergent, consisting of three parts: (1) The inner plates which form two chambers on each side of the interior, uniting laterally with the walls of the valve. The sockets are excavated in the antero-lateral extremities of these plates adjacent to their junction with the wall of the valve. (2) Outer plates. The structures noted above are sup- ported by two diverging lamella; which unite with the valve directly. The line of union is marked by a thickening. (3) Brachial supports. Two small plates. expanded at the free ends and united to the thickening at the juncture of the outer plates and the inner plates. The brachial processes extend into the cavity of the valve, in some specimens, for a distance greater than the length of the septal plates. At their distal extremi- ties they curve laterally. Between the septal plates is a low median septum, dividing the adductor impressions. Geologic range. — Late Silurian. American Species Conchidium arcticum Holtedahl (Rept. 2d Norw. Arctic Exped., "Fram," 1898-1902, No. 32, 1914, p. 5) C.colletti (Miller) 1891 C. crassoradius (McChesney) 1861 C.cumberlandicum Prouty 1923 C. exfonens Hall and Clarke 1895 C. greenii Hall and Clarke 1893 C. lajueatum (Coniid) 185 5 C. lindenense Foerste 1903 C.littoni (Hall) 18 59 C. nettelrothi Hall and Clarke 1 893 C. obsoletum Hall and Clarke 1895 C.occidentde {WiSS) 1852 C. scofarium Hall and Clarke 1895 C . unguiforme (Ulrkh) 1886 ? C. georgits Hall and Clarke 1895 (may be Gyfidula) European Species Conchidium biloculare Linnaeus 1 760 C. miinsteri Kiaer 1901 ? C. tenuistriatum Walmstedt Distinguishing characters. — The features of Conchidium that distinguish it from the other Pentam- erinas are the strongly multicostate exterior, usually strongly biconvex valves, and extended beak, below which there is a narrow interarea, a wide delthyrium, and in some species a modified deltidium. Internally Conchidium is like Pentamerus. Discussion. — It will be profitable to discuss several morphologic features of Conchidium, i. e., the del- tidium, the spondylium duplex, and the cardinalia. Deltidial plates have been reported in many of the pentamerids, such as Gyfidula. These plates have also been termed pseudodeltidial plates by Booker.^' In Conchidium, many specimens show these thickenings along the margins of the delthyrium, but in addition they have a concave or, in some instances, an elevated plate covering the delthyrium. The two types — plates and "deltidium" — may be observed on the same speci- men. Obviously, then, the thickenings along the del- thyrial margin are not true deltidial plates. This statement will become more certain after a detailed =' lourn. Proc. Roy. Soc. N. S. Wales, vol. 60, 1927, p. 134. 182 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA account is given of the occurrence and structure of the "concave deltidium" of C onch'tdtum . This plate in most specimens is truly concave and bears a median groove giving it the appearance of having been formed by the symphysis of two plates. One specimen of C. biloculare, however, shows toward the end of the beak that the plate is elevated above the margins of the delthyrium, forming a cover with nearly rectangular sides. In another specimen of the same species the plate is elevated fully 3 mm. dorsally from the del- specimens is more than half the length of the ventral shell and in some it extends for nearly the full length. The cardinalia, are, as is usual in the Pentameridae, the most interesting and significant structures of the genus. Here, as in Gyfidula, Pentamerella, etc., three distinct "plates" can be determined. The inner crural plates are convex and unite with a narrow flat shelf forming the inner postero-lateral margins of the valve. The union is marked by a low ridge and groove which forms the suture line between the valve and plate. ^ CD ci) (2) ^^ Fig. 33. — Conchidium biloculare Linn. Sections 1-3 show the delthyrial plate in place at a considerable distance (4.2-6.1 mm.) from the beak. It is here rather concave, having been depressed from an elevated structure like that in fig. 25. In section 4 lateral plates are visible in the dorsal valve. Similar plates were seen in Strkklandia (see pi. 29, fig. 6) but their function and meaning are unknown. Distance from beak: 1 — 4.2 mm. 2—5.0 3—6.1 4 — 7.4 5—8.1 6—8.7 7 — 9.9 mm. 8—11.0 9—12.1 10—14.4 11—15.0 thyrial margin, having nearly rectangular sides, and bears a shallow median groove. There is no visible evidence that it has developed by the symphysis of two plates. Further, the "deltidial plates" are visible along the delthyrial border in specimens carrying the cover plate. This modified deltidium must therefore have been a sort of pedicle sheath housing the proximal portion of the pedicle. Conchidium is usually defined as lacking a "cardinal area" (interarea), but in all of the specimens exam- ined a narrow but very definite interarea could be seen. The spondylium duplex in Conchidium is remark- able for the great depth of the delthyrial cavity and the great length of the septum. The latter in most The inner plates thus form narrow lateral chambers. Posteriorly the terminations of the chambers are thick- ened, and together with a boss or callus beneath the beak form the "cardinal process" or seat of diductor attachment. The outer plates appear to be represented by narrow longitudinal ridges, the base of the brachial process. On the inner sides of some specimens, each of these ridges bears a plate directed dorso-medially. These are probably posterior extensions of the brachial process which extend anteriorly with a slight curve for a considerable distance ; they are curved very slightly laterally but none were observed to unite into a "loop." The two inner plates and bases of the brachial processes are supported by thin divergent septal plates. GENERA OF THE SUBORDER PENTAMEROIDEA 183 Of the species heretofore referred to Conchidium. several must be removed to other genera. C. ktuippi, for example, does not have the elongated and ex- tremely incurved beak of this genus; it is essentially a rather strongly costatc Pcntamrrus and belongs to our Rhip'uiium. Likewise C. Icgoensis Foerste and C. cras- iipititi Hall and Clarke do not belong in Con- chidium, the non-bifurcation of the ribs being of little significance. Savage" has correctly removed C. de- cussatum to the genus Virgiana. Genus BROOKSINA Kirk 1922 PI. 28, fig. 24 Kirk, Proc. U. S. Nat. Mus., vol. 60, 1922, pp. 2-5. Genoholotype. — B. alaskensis Kirk 1922. Description. Exterior. — Transversely subellipti- cal in outline; hinge-line narrow; cardinal extremi- ties obsolete. Lateral profile unequally biconvex to convexo-concave, the dorsal valve strongly convex, the ventral gently so, flat or strongly concave. Ventral interarea narrow, almost obsolete. Ventral cardinal slopes flat and defined by prominent ridges extending from the beak to the point of juncture of the costellate portion of both valves. Delthyrium probably always open. Dorsal interarea obsolete ; umbo swollen. Sur- face of the valves multicostellate ; shell substance fibrous, impunctate. Ventral interior. — Spondylium duplex supported by a long duplex septum. Spondylium narrow but exceedingly deep. Dorsal interior. — Septa and lamellae as in Con- chidium and other genera of the Pentamerina:. Geologic range. — Upper Silurian of Alaska. Possibly Lower Devonian of the Urals. American Species Brooksina alaskensis Kirk 1922 Asiatic Species Pentamerus of talus Tschernyschcw 1885? (wow Barrande) Distinguishing characters. — Brooksina may be differentiated from all other pentamerids by its con- vexo-concave profile and costellate exterior. Discussion. — As would be expected, the interior of this genus differs somewhat from other members of the family because of the reversed convexity of its valve. The ventral septum is long and extends to the anterior margin of the spondylium. Toward the front "Jour. Geol., vol. 26, 1918, p. 335. it is strongly corrugated. The spondylium is exceed- ingly narrow and deep and the anterior margin is rather straight. Postero-latcrally its sides flare outward noticeably where they meet the delthyrial margins. In the dorsal valve all parts of the cardinalia can be distinguished as in Conchidiutn and there is a long brachial process which terminates near the anterior end of the spondylium. In a specimen 30 mm. long this process is 9 mm. in length. The young of Brooksina are almost equally biconvex and this gives a clue to its ancestry. It certainly must have come out of some Conchidium stock, perhaps out of the " biloculare" line or some form resembling C. nysius (Hall and Whitfield). It certainly did not come out of the "knighti" stock, with which it is contemporaneous. Brooksina is in the same stage of development as Cafelliniella and is thus to Conchidium what Cafel- liniella is to Pentamerus. These two forms and Anastrophia are the only representatives of this stage of evolution so far observed in the pentamerids, but other "reversed" forms as aberrant developments are to be expected. Genus CYMBIDIUM Kirk 1926 T. fig. 34 Kirk, Proc. U. S. Nat. Mus., vol. 69, art. 23, 1926, p. 2, pi. 1, figs. 5-13; t. fig. 34. Genoholotype. — C. acutum Kirk 1926. Description. Exterior. — Subelliptical in outline; lateral profile subequally convex, the dorsal valve with the greater convexity. Interareas obsolete; delthyrium open so far as known. Each side of the delthyrium bounded by a broad flattened area. Dorsal beak strongly arched, umbo swollen. Surface multicostate, shell substance fibrous, impunctate. Ventral interior. — Delthyrial cavity deep; bounded by a free spondylium which extends for a long distance forward. Dorsal interior. — Septal plates widely spaced, rather short. Inner plates exceedingly short. A low short median ridge exists in the middle of the notothyrial cavity. Discussion. — Cymbidium is a most unusual brach- iopod and its careful description by Kirk leaves little to be added. Of special interest is the free spondylium, which is a very rare structure in this class of organisms. Careful sectioning by Kirk failed to reveal any trace of a septum or any evidence that the spondylium had previously been sessile. Among the pentamerids Hol- orhynchus is the only other genus having a free spondylium. In the dorsal valve the septa are clearly pentameroid although not absolutely typical. Division into three 184 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA sets of plates is not entirely clear. The inner plates appear to be exceedingly short, making small chambers at the rear as in Conchtd'tum. In front of the inner plates the septal ones are low and bear elongate processes as is usual in pentamerids. It is difficult to trace the lineage of Cymhidium. From its external form it would appear to be related to Conchidium {"biloculare") but the spondylium is much wider than is usual in that genus. The loss of the ventral septum appears to be rare among pentam- erids, and in the orthids with spondylia is seen only in the Middle Cambrian Protorthis and Loferia. Genus PLATYMERELLA Foerste 1909 PI. 27, figs. 2,3,5, 11 Foerste, Bull. Sci. Lab. Denison Univ., vol. 14, 1909, p. 70, pi. 1, figs. lA-D; vol. 19, 1920, p. 223, pi. 23, fig. 5. American Species Platymerella man?:ie?!sis Foerste 1909 Conchidium crassiflica Hall and Clarke 1 895 Distinguishing characters. — Platymerella was distinguished by its nomenclator as unique for the fol- lowing reasons: (1) absence of a straight hinge mar- gin, (2) non-galeatiform exterior, (3) ventral and dorsal beaks approximate, (4) median septum short. Discussion. — Some time after the appearance of the original description of Platymerella, Dr. Foerste redescribed the genus on the basis of new material showing perfectly the character of the internal struc- ture, which had hitherto been unknown. As shown by these specimens, the genus has essentially the same structure as Pcntamerus, but the plates in both valves are unusually short, actually making it difficult to distinguish the ventral from the dorsal, as stated by 12 5 4. ^ F,c. Z\.—Cymbidium acutum Kirk. Sections cut through the beak of a dorsal valve, showing pentameroid character of the septal plates. Distance from beak: 1 — 1 mm. + — 5 mm. 2—3 5—6 3—4 Genoholotype. — p. manniensis Foerste 1909. Description. Exterior. — Longitudinally suboval, hinge-line narrow; cardinal extremities rounded. Lateral profile subequally biconvex; anterior commis- sure rectimarginate. Fold and sulcus indistinct, or ventral sulcus shallow and dorsal fold low. Beaks subequal, incurved. Surface multicostate ; shell sub- stance fibrous, impunctate. Ventral interior. — Spondylium duplex short, septum short. Spondylium directed antero-dorsally. Ovarian areas marked by elongated pustules. Dorsal interior. — Cardinalia confined to the poste- rior, exceedingly short. Septal plates short, discrete or united by extra shell deposit to form a small chamber which is U-shaped in cross-section, making a pseudo- cruralium. Brachial processes as in Pentamerus. In front of the pseudocruralium is a long, slender, but low median ridge. The adductors are elongate impres- sions on each side of the median ridge. Geologic range.— Early (and ? Middle) Silurian. Foerste. The published description of the genus leaves little to be added now that the interiors are known. However, Foerste figures and describes two types of shells in Platymerella. In one of these, the septal plates of the cardinalia are united at their junction with the valve by extra testaceous substance which forms a pseudocruralium, as may be seen in Foerste's figures 5E and 5F. The other type appears in figures 5G and 5H; it shows two elongate septal plates ex- tended for a considerable distance along the inner surface of the valve, and strongly resembles Pentam- erus. In the type which forms a pseudocruralium there is no trace of the extensions of the septal plates forward, and, on the other hand, in the type with the elongate, parallel plates there is no evidence of a median septum. There are two possible explanations: (1) we are here dealing with homceomorphs, one type representing Platytnerella and the other being refer- able to Pentamerus; (2) we may look at the problem from another angle and maintain that both types belong to the genus Platymerella, the form with the pseudo- GENERA OF THE SUBORDER PENTAMEROIDEA 185 cruralium being developed from the type with the parallel plates by deposition of adventitious shell mat- ter between the plates. Specimens from Mannie, Tennessee, examined by us appear to conform to the first type, that with the pseudocruralium, and it is to this form that the name must be restricted unless further acquisition of material shows that Platymerella s. s. develops from a type with parallel plates and no adventitious material. One specimen from Illinois sec- tioned by us is of the Pentamerus type, and probably the Alexandrian species from Illinois now referred to P. mantuensis are actually not the same species and may not be of the same genus. Foerste has suggested that Platymerella is closely related to Pentamerella. The cardinalia are sugges- tive of this relationship but the shape of the valves, the nature of the beaks, and the structure of the spon- dylium are quite different. Furthermore, there is an enormous time discrepancy between the two genera which makes the relationship rather remote. Platy- merella is of great interest because it is the earliest known pentamerid (together with the Pentam.erus-\\\it form referred to above). Here again a plicated form introduces a large group of shells. Genus VIRGIANA Twenhofel 1914 PI. 27, figs. 1,6, 7, 16; t. fig. 35 Twenhofel, Geol. Surv. Canada, Mus. Bull. 3, 1914, p. 27. Genoholotype. — Pentamerus barrandei Billings 1857, Geol. Surv. Canada, Rept. Prog, for 1856, p. 296. Description. Exterior. — Elongate-oval, hinge- line short, lateral profile biconvex, the ventral valve having the greater convexity. Anterior commissure sulcate, uniplicate, or nearly rectimarginate. Ventral valve usually with a low fold, which in young stages forms a median plica in a low sulcus; the dorsal valve in young shells usually has a noticeable fold, which may reverse at maturity into a sulcus or may remain as a low fold. Interareas obsolete, ventral beak in- curved strongly, umbo swollen; dorsal beak curved under the ventral. Surface unevenly multicostate ; shell substance fibrous, impunctate. Ventral interior. — Delthyrial cavity deep ; spon- dylium duplex long, free anteriorly; duplex septum short. Dorsal interior. — Dorsal valve provided with two short, stout septal plates which extend nearly vertically and attach directly to the internal surface of the valve. Geologic range. — Early Silurian of North America. Species ClorinJa barrandei Billings 18 57 Pentamerus decussatus Whiteaves 1891 Virgiana anticostiensis Twenhofel 1928 V. major Savage 1916 V. mayvillensis Savage 1916 Distinguishing characters. — Virgiana is most readily identified by its strongly convex ventral valve and relatively gently convex dorsal valve, elongate spondylium, and short, discrete septal plates. Discussion. — In the ventral valve of young indi- viduals of V. barrandei, for 2 or 3 millimeters there is a shallow sulcus, but after this distance a low median plica develops in the depression. As growth continues, the sulcus becomes shallower and finally disappears, but the median plica enlarges and continues to the front margin as a low fold. In some varieties the median plica consists of a bundle of low costs. The fold at the front margin is very gentle and is usually narrow. On the dorsal valve of young individuals of V . bar- randei the young are provided with a low plica, in- dented medially by a shallow stria. With continued growth and bifurcation of the median plica, the fold is lost in the ornamentation of the valve. In some speci- mens the antero-median portion of the valve is de- pressed into a very shallow sulcus. It is this phenom- enon of the "reversion" of the fold and sulcus that Twenhofel considered the most important generic character of his genus. However, in the specimens from Anticosti in the Peabody Museum there are many in which the dorsal fold maintains its identity through- out the length of the valve. This is especially true of the small V. anticostiensis. Furthermore, some specimens of V. mayvillensis preserve the fold from near the beak to the front margin. More important in the diagnosis of the genus than reversion of the fold and sulcus is the internal struc- ture of the valves. The ventral shell has an exceed- ingly long spondylium which is free for most of its length. In the dorsal valve the cardinalia are remark- able for their simplicity and shortness; in one indi- vidual having a length of 36.5 mm. the septal plates extend for only 5 mm. These plates arc pentameroid, uniting directly with the valve. This structure differs from that of Clorinda in having the plates nearly parallel in horizontal section and in not possessing the inner carin.t which project into the cavity bounded by the plates. The structure therefore is not greatly different from that of Conchidium and we are remov- ing the genus to the subfamily Pentamerina: where it properly belongs. Hall and Clarke""^ figure an interior of Pentamerus barrandei showing a spondylium in both valves, that "Pal. N. Y., vol. 8, pt. 2, 1893, p. 243, t. fig. 174. 186 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA of the dorsal one being shallow and supported by a very low median septum. This figure is utterly incorrect and gives an entirely wrong impression of the interior of Virg'iana. The dorsal structure of this genus is precisely like that of Pentamerus in composition and most nearly resembles that of Platymerella in the brevity of its septa. discrete, with more or less long brachial supports. The only genus is Strlcklandia Billings of the Silurian. The origin of this family is not yet known, but it may have come out of camerellids. Instead of retain- ing the shell form of the camerelHds, however, the Stricklandida; developed into wide-hinged types. It is possible also that the stricklandids came out of a much O (D O O C) s o 6 10 II Fig. 35. — Serial sections of Virgiana barrandei (Billings), from Anticosti. Notice the general similarity of the structure to that of Pentamerus s. s., pi. 29, fig. 11. The difference between the two genera internally is the brevity of the dorsal septa in Virgiana, in this specimen a little more than 2 mm. Distance from beak: 1 — 1.0 mm. 7 — 6.4 mm. 2—2.2 8— 7.7 3 — 2.9 9 — 8.9 4 — 4.4 10—10.5 5—5.2 11—11.5 6—5.9 The shell sectioned was 41.8 mm. in length. All trace of the spondylium disappeared at 24 mm. from the beak; the ventral septum disappeared at about 5 mm. These facts indicate an exceedingly long spondylium and very abbreviated septum. ? Family STRICKLANDIDA Hall and Clarke 1894 Divergent large Pentameracea, probably developed out of the Camerellidas. Spondylium short, supporting a short septum. Cardinalia simple but specialized and older stock, the Syntrophiids. In neither supposition are the annectant genera known, but it is more natural to look for them in the later Ordovician, in which event the origin would be out of the camerellids. The Stricklandias are first met with in the early Silurian of North America in faunas of northwestern GENER/\ OF THE SUBORDER PENTAMEROIDEA 187 European origin, and the ancestral stock is to be looked for in the later Ordovician of Europe. Interareas are characteristic of the Orthacea and Strophomcnacea of the Protremata, and in the Tclo- tremata are of common occurrence only among the Spirifcracea. They manifest themselves in some of the Pcntameracea only and their appearance here, say Hall and Clarke,"^ "may be regarded as the resumption of a primitive or original character which was normal" for the Protremata. Regarding interareas among the rostrate pentam- erids, Hall and Clarke say:"^ Every now .ind then specimens will show a clearly devel- oped cardinal area; always in Stricklandinia, frequently and normally in Gyfidula, rarely and of exceptional occurrence in Pentimterdla. Stricklandinia possesses so straight and long a hinge, so sharply defined an area and so short a spon- dylium, that it is more natural to regard this genus as the accompaniment, rather than the close organic kin of the other pentameroids, deriving its differentials directly from those long-hinged and straight-hinged shells of the early Silurian [=: Ordovician], which constitute the genus Syntrofhia. Genus STRICKLANDIA Billings 1859 PI. 28, figs. 25, 27, 28; pi. 29, fig. 6 Billings, Canadian Nat. and Geol., vol. 4, 1859, p. 132, figs. 8-9 {lens). HalJ and Clarke, Pal. N. Y., vol. 8, pt. 2, 1893, p. 249, pi. 73, fig. II. Syn. Stricklandinui Billings 1863.=^ Genolectotype (Hall and Clarke). — 5. gasfeen- w Billings 1859. Description. Exterior. — Shells variable in size but tending to be large, elongate-oval, transversely oval, or subcircular in outline ; hinge-line straight, cardinal ex- tremities rounded. Lateral profile biconvex, the dorsal valve usually having the greater convexity. Anterior commissure rectimarginate or gently uniplicate; ven- tral sulcus shallow or obsolete or raised into a low fold. Dorsal fold usually present, low. Ventral inter- area wide, curved, apsacline; delthyrium open. Dor- sal interarea reduced, apsacline. Surface smooth or multicostate. Shell substance fibrous, impunctate. Ventral interior. — ^Teeth small, dental plates short and stout, uniting to form a small spondylium duplex. Supporting septa short. Dorsal interior. — Internally there are two slightly divergent plates which slope toward each other but do ■* Pal. N. Y., vol. 8, pt. 2, 1894, p. 3 36. " Op. cit., p. 342. ^^ Stricklandia, a plant, does not invalidate Stricklandia Billings 1859, as that author supposed. not unite. The inner margins of the plates are thick- ened and give off long processes which form the arm attachments. Adductor impressions are elongate pits on the floor of the valve, the anterior pair slightly divergent distally, the posterior pair narrower and subparallel. Geologic range. — Lower and Middle Silurian. American Species Stricklandia aniicostiensis Billings 1863 S. billingsiana {Dzwion) 1880 S.brevis Billings 1859 S. breviuscula (Savage) 1916 S. canadensis Billings 1859 S.castellana (White) 1876 S. chafmani (Hall and Clarke) 1895 5. circularis (Savage) 1916 S.davidsoni {'&\\\\ng%) 1868 S. dejormis (Meek and Worthen) 1870 S. gasfeensis Billings 1859 S. manitouensis (M. Y. Williams) 1919 S.melissa (Billings) 1874 S.multilirata {'WKxtfidd) 1877 S.norzcoodi (Yoersii) 1906 S . fyriformis (Sivigc) 1916 S. fyrijormis elongata (Savage) 1916 S. fyrijormis varicosa (Savage) 1916 S.salteri (Billings) 1868 5. j/ria/j (Twenhofel) 1928 S. triplesiana (Foersle) 188 5 Clorinda becsciensis (Twenhofel) 1928 European Species A try fa lens Sowerby 1839 Sfirifer liratus Sowerhy 1839 Pentamerus microcamerus McCoy 1859 Distinguishing characters. — Stricklandia is best identified externally by its lenticular outline, wide hinge-line, and well developed interarea. The spon- dylium and its supporting septum are very short. The cardinalia consist of two concave plates giving off long brachial processes from their posterior inner margins. Discussion. — Billings' genus differs widely from all members of the Pentameracea in several features, both internal and external. The most striking external variation is the width of the hinge-line, which is narrow in all other known members of the super- family. Along with the wide hinge-line goes a short but wide interarea, seen best in the genotype. All of the pentamerids retain remnants of the interarea but they are confined to the margins of the delthyrium and in some species are nearly obsolete. The spondylium is a typical duplex one, and is re- markable for the abbreviation of both the septum and the spoon. These features in themselves are of no GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA great taxonomic importance, but in combination with the peculiarities of the dorsal valve form a unique ensemble. The structure of the cardinalia is compar- able to that of Orthorhynchula among the early Rhyn- chonellacea, is much simpler than in the Pentameridae, but not more so than in the Camerellidas. In Ortho- rhynchula the crural bases are concave plates hanging free in the valve and bear long, curved crural processes. Regarding Stricklandia, Hall and Clarke have this to say: These pentameroids are principally remarkable for the unusual development of the cardinal areas of both valves in the larger and more typical species, and the straight orthoid hinge in the earlier and smaller members of the group. The combination of such features with an internal chambered structure is not of frequent occurrence among these genera. [The interareas] are sharply defined on both valves, and so persistent are they that we look for the origin of this combination, not among the various pentameroids. . . but to the small, transverse shells of the early faunas to which the term Syntrofhia has been applied, as Syntrofhia ? arachne and S. ? arethusa of the Lower Ordovician; these species, however, are not of the genus Syntrofhia but may belong to Huenella. This origin may not be the correct one, but in that event why do we not find any Ordovician stricklandid ? We are therefore inclined to look for the origin of the family in the Camerellidae, with the idea that the small inherited interareas are redeveloped into the much larger ones of Stricklandia. In the dorsal valve of Stricklandia, according to Hall and Clarke, "the short dental plates, at their inner angles, bear long crural processes," and are analogous with those of Amfhigenia, but do not unite to form a hinge-plate or cruralium as in that genus. On page 355 these authors erect the family Strick- landidas for the genera Syntrofhia and Stricklandia. The present authors agree that Stricklandia can not be incorporated in the family Pentameridae, and we there- fore accept the family Stricklandids but exclude from it Syntrofhia.^^ ^' The junior author would exclude the Stricklandid^ from the Pentameroidea entirely. The cardinalia are totally unlike anything known in that order (Syntrophiacea and Pentameracea). In his opinion the best place for Stricklandia, so far as present knowledge goes, is among the early RhynchoneUacea. APPENDIX ? Superfamily RHYNCHONELLACEA Schuchert 1896 Genus RHYNCHOCAMARA Schuchert and Cooper 1931 (Gr. rhynchos, beak; knmnra, chamber) PI. 25, figs. 1,2, 7, 17, 18 Schuchert and Cooper, Anier. Jour. Sci. (5), vol. 22, 1931, p. 248. Genoholotype. — R. flicata Schuchert and Cooper 1931. (D Cb internal structure of Rhynchocamara is essentially rhynchonelloid, not pentameroid, since the notothyrial or subrostral chamber has much the same shape as that seen in later rhynchonellids. Geologic range. — Ordovician (Chazy), so far as known. American Species Camerella bella Fenton 1928 C. variam Billings 1859 Rhynchocamara flicata Schuchert .ind Cooper 1931 CD 5 e Fig. 36. — Liocoelia froxima (Barrande). The beak and spondylium of Liocoelia are rather strongly curved, which accounts for the peculiar oval seen in section 1. Sections 4 and 5 show the rhynchonelloid chamber in the dorsal valve. Distance of each section from the beak: 1 — 1.4 mm. 2—2.6 3—3.3 4 — 3.5 .5 mm. 6-5.2 7—6.2 In a shell 18.6 mm. in length the dorsal septum persisted for 8.5 mm. and the ventral septum was present 9 mr anterior to the beak. The chief distinguishing features of Rhynchocamara are the camereUoid exterior, short hinge-line, ventral spondylium duplex, and rhynchonelloid cruralium. From Camerella it differs in outline, the former being more globular, and in the structure of the dorsal valve. The cruralium duplex of Camerella is long and nar- row, extending for perhaps one-fourth the length of the valve, whereas in Rhynchocamara it is confined to a small region beneath the beak. The dorsal Genus LIOCCELIA Schuchert and Cooper 1931 (Gr. leios, smooth; koilia, belly) PI. 25, figs. 31,32, 37; pi. 29, fig. 9; t. fig. 36 Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, p. 248. Genohoi.OTVPE. — Pentamerus froxima Barrande, see Syst. Sil. BohSme, 1879, vol. 5, pi. 9, fig. 5, pi. 81, fig. 5. 190 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA Description. Exterior. — Subpentameral in out- line; hinge-line narrow; lateral profile subequallv biconvex; anterior commissure uniplicate, the fold and sulcus being defined at the front half. Ventral beak curved over the dorsal. Surface smooth ; test fibrous, impunctate. Ventral interior. — In the ventral valve there is a long spondylium supported for nearly its full length by a high median duplex septum which extends for nearly half the length of the valve. Dorsal interior. — There is a small and short cru- ralium supported by a high septum. This cruralium is partially covered by lateral plates which overhang the notothyrial margins much as deltidial plates, repre- senting a divided hinge-plate. In a valve about 20 mm. long the subrostral chamber is only 2 mm. in length. Geologic range. — Silurian of Bohemia. Discussion. — At first it appears difficult to assign Lioccelia to its proper place among either the pentam- erids or the rhynchonellids. The external form is essentially pentameroid, and the ventral valve with its spondylium duplex is a normal pentameroid feature. But in the dorsal valve the cardinalia are rather those of the rhynchonellids, such as Camerofhoria, especially in the small subrostral vault. It would thus appear that Liocaelia is essentially a smooth Camerofhoria having an external form like that of Clorinda, Merista, or Meristella. PLATES PLATE A Brachiopod Morphology A CP, accessory cardinal process AcS, accessory dental socket Adti, adductor scars AtiJt, adductor track Adj, adjustor scars Adt, adventitious testaceous substance Br, brachiophore Brf, brachiophore plate BrP, brachiophore process CF, crural fossette CA, chilidlum CP, cardinal process CPF, crural pit filling D, deltidium DA, duplex adductor scar DC, delthyrial cavity DeP, dental plate Di, diductor scar F, foramen FP, fulcral plate las, impression of adductor scar LP, lateral plate Mbrf, mold of brachiophore plate Mcf, mold of cardinal process Mms, mold of median septum MR, median ridge MS, median septum and euseptum Ov, ovarian impression PC, pedicle callist PFF, pedicle foramen filling PPS, primary pallial sinus PS, pallial sinus PsS-p, pseudospondylium SF, filling of socket Sf, spondylium SPS, secondary pallial sinus T, tooth TPS, tertiary pallial sinus 194 PLATE A SF CPF Z^J CP-^ x\ DA-^ DeP CP /Ch Adt Ch M ms ?^^ Xi. MS W'/'J44n 10 Addt PPS 5P5 TP5 Ov MR- Adj p 5 ■'"*'/'?-f ^'"TifflnwA^"''' ■ 11 12 14 15 195 PLATE 1 YiGs. Bohemiella romingeri (Barrande) 1^ 18. — Wax replicas of dorsal interior, showing cardinalia which are orthoid in aspect. Cf. pi. 4, fig. 17. Note differences from cardinalia of BllUngsella and Eoor- ihis, figs. 13 and 28. 3. — Internal mold of small ventral valve, showing diver- gent pallia! trunks. 22. — Wax replica of dorsal exterior. Cf. fig. 6. Mid.Cambrian,Skrej, Bohemia. Cat.No. S12. x 2. 5. — Dorsal exterior, showing also ventral valve in place. Deltidium lacking and profile piano- or concavo- convex. 1 1 . — Dorsal interior, showing remarkable growth of adven- titious shell over brachiophores. Cf. pi. 5, figs. 1 7, 24. Wax impressions of molds. Mid. Cambrian, Skrej, Bohemia. Originals U. S. Nat. Mus. Cat. No. 52267. x2. Orusia atava (Matthew) 2. — Internal mold, showing slotlike molds of brachiophore plates. Erroneously referred to Eoorthis by Walcott. After Walcott, Camb. Brach., pi. 95, fig. 7b. x 0.5. Orusia lenticularis (Wahlenberg) 7. — Ventral internal mold, showing dental plates. 9. — Dorsal internal mold, showing slotlike molds of brach- iophore supporting plates, probably very similar to those of Finkelnburgia. After Walcott, Camb. Brach., pi. 98, figs. 2d, 2f'. Reduced. Oligomys exporrectus (Linnarsson) 4, 15. — Ventral internal mold, showing impression of small delthyrial cavity and muscle tracks. Mid. Cambrian (Paradoxides zone), Westrogothia, Lovened, Sweden. Cat. No. S 16. Fig. 4 x 2.65 ; fig. 15x6. 8. — Dorsal interior, showing cardinalia. Cf. Billingsella, fig. 13. The brachiophores of Oligomys, when com- plete, are similar to those of OrtAis s. s. Replica of specimen figured by Walcott, Camb. Brach., pi. 88, fig. 1 1. 17. — Dorsal internal mold, showing pallial marks. After Walcott, Camb. Brach., pi. 88, fig. Ik. Reduced. Mid. Cambrian (Agnoi/us Itevigatus zone), Gud- heim, 12.5 miles SSE of Skara, Sweden. Cat. No. 52249 U. S. Nat. Mus. Billingsella cf. pepina (Hall) 6^ 19_ — Dors.il and ventr.il exteriors, showing ornamenta- tion. 1 3. — Dorsal interior, showing cardinalia. Note short brach- iophores. Cf. Eoorthis, fig. 28. 21^25. — Ventral interiors, showing perforate deltidium. Fig. 25 shows thickening of median portion of mus- cular area. Note the large teeth. The deltidium is an arch as in other orthoids. Cf. pi. 7, fig. 16. Up. Cambrian, Grand Teton, S. of Muskrat Lake, Teton Creek, Idaho-Wyoming state line. Cat. No. S2220. x2. Figs. Billingsella lindstroemi (Linnarsson) 10. — Ventral internal mold, showing divergent pallial trunks and reniform ovarian impressions. Cf. Orthis, pi. 2, fig. 18. After Walcott, Camb. Brach., pi. 87, fig. 6e. Reduced. Mid. Cambrian {Paradoxides jorchhammeri zone), Alunbruk, Oeland I., Sweden. 27. — Ventral internal mold, showing pallial markings and adherent fragments of shell. A thin section of the shell substance (see pi. 29, fig. 12) shows it to be fibrous, and fibres are visible with a hand lens also. Mid. Cambrian (Paradoxides zone), Westrogothia, Lovened, Sweden. Cat. No. S 20. x 2. Protorthis billingsi (Hartt) 12. — Ventral internal mold, showing free spondylium. Cambrian (Acadian), Seeley St., St. John, N. B. Cat. No. S94. x2.65. 14. — Ventral valve, showing free spondylium. After Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 1892, pi. 7A, fig. 16. Reduced. Nisusia festinata (Billings) 16. — Wax replica of exterior of a small ventral valve. Low. Cambrian, near Emigsville, York Co., Penn. Cat. No. S 9. X 2. 20. — Dorsal internal mold, showing musculature. Cardi- nalia similar to those of Billingsella but more primi- tive. After Walcott, Camb. Brach., pi. 100, fig. Ig. Reduced. Low. Cambrian, near Emigsville, Penn. Eoorthis remnicha (Winchell) 23. — Dorsal interior, showing cardinalia. A low cardinal process or ridge has risen between the seats of muscle attachment on the notothyrial platform. Up. Cambrian, S. side Gallatin Valley, Mont. U. S. Nat. Mus. X 2. Eoorthis cf. remnicha (Winchell) 26. — Ventral interior, showing thickening on floor of del- thyrial cavity. This type of structure is called a pseudospondylium, but the resemblance to a spon- dylium is very remote in Billingsella and Eoorthis. 28. — Dorsal interior, showing flat, oblique brachiophores. This specimen is deprived of a cardinal process, hence the muscles were attached on the floor of the notothy- rial cavity. Up. Cambrian, Flat River, Mo. U. S. Nat. Mus. x 2. Jamesella cf. perpasta (Pompeckj) 24. — Ventral internal mold of a large specimen, showing muscular marks. Up. Cambrian, Skrej, Bohemia. Cat. No. S 1944. x2. 196 PLATE 1 197 PLATE 2 Pigs. Nicolella cf. actoniae (Sowerby) 1, 3. — Dorsal and ventral views of a well preserved indi- vidual. Cleft chilidium, or chilidial plates, visible in fig. 1. Compare angular costs and predominantly concentric finer ornamentation with those of Orthis (figs. 9, 11, 12, 15). Ordovician (Lyckholm, Fj), Piersal, Estonia. Cat. No. S151. x2. Nicolella actoniae (Sowerby) 4, 6. — Dorsal and ventral views of a large specimen, show- ing anterior bifurcation of costs. 5. — Ventral internal mold, showing muscle impressions. Ordovician (Chasmops Is.), Ostragothia, Sodra Fre- berga, Sweden. Cat. No. S 1 52. x 2. Nicolella, n. sp. 2. — Dorsal view of a specimen related to A^. moneta (Eichw.), showing coarse angular costs. Ordovician (Wierland group, Echinosphsrites Is.), Popovka near Leningrad, Russia. Cat. No. S 147. x2.65. Orthis cf. calligramma Dalman 7, 9, 1 1, 1 3. — Posterior, dorsal, ventral, and lateral views of the same specimen. Note biconvexity of shells, rounded costs, and parvicostells in the stris. Middle Ordovician, Pulkova, near Leningrad, Russia. Cat. No. 8 167. x 2. Orthis rotunda Pander 10. — Dorsal interior, showing simple brachiophores, septum- like cardinal process, small adductor impressions, and pallial sinuses radiating from them. The sockets for the teeth are the space between the brachiophores and the wall of the valve. Ordovician ( Walchow) , opposite Iswos, Russia. Mus, Comp. Zool., Harvard Coll. x 2. Figs. Orthis rotunda Pander — Cont. 16. — Dorsal interior, showing median ridge and pallial markings. 18. — Ventral interior, showing two prominent pallial trunks, closely apposed and separated by a small, low ridge. These trunks bound, on the inside, subreniform ovarian markings. It is this type of interior that char- acterizes the ventral valve of the Orthids as here conceived. Ordovician (Walchow) , opposite Iswos, Russia. Mus. Comp. Zool., Harvard Coll. x 2. Orthis callactis Dalman 8, 12, 15. — Posterior, ventral, and dorsal views of the same individual. Coarse, simple, rounded ribs covered by fine radii and finer concentric growth markings clearly visible. O. callactis is the type of the genus Orthis, and the kind of shell to which this generic name should be restricted. Compare the outline and con- tour with those of O. calligramma (figs. 7, 9, 11, 13). A critical study of large collections of European Orthis may prove the advisability of separating the O. calli- gramma type as a subgenus. Ordovician (Walchow, Glauconite Is.), Iswos, Wal- chow River, Russia. Cat. No. S 144. x 2. 1 7. — Dorsal interior, showing small, simple brachiophores. Ordovician, opposite Iswos, Russia. Shaler Mem. Exped. Colls., Mus. Comp. Zool., Harvard Coll. x2. Cyrtonotella aflf. C. frechi (Wysogorsky) 14. — A kind of biconvex Orthis with prominent concentric lines covering the costs. Ordovician (Echinosphsrites Is., C), Popovka, near Leningrad, Russia. Cat. No. S 2099. x 2. 198 PLATE 2 199 PLATE 3 p,Q5 Panderina abscissa (Pander) 1 , 4. — Dorsal and ventral views of a specimen belonging to the genotype, showing characteristic ornamentation, especially concentration of growth toward the front. Note reduced (shortened) ventral interarea well shown in fig. 1. Ordovician (Glauconite ss.), Popovka, near Lenin- grad, Russia. Cat. No. S 157. x2.25. Panderina tetragonum (Pander) 2, 3. — Dorsal and ventral interiors. After Lamansky, Mem. Com. Geol., 1905, pi. 2, figs. 11,12. Ordovician (Walchow, Bi^), Popovka, near Lenin- grad, Russia. Slightly enlarged. Paurorthis parva (Pander) 5. — Ventral interior, showing teeth, crural fossettes, median ridge, and lateral ovarian areas. The muscle area shows a broad adductor-diductor impression. Cf. Dalmanella rogata, pi. 17, fig. +. 7. — Dorsal interior, showing subradial pallial sinuses, indis- tinct muscle field, and prominent elevated median ridge. Brachiophores oblique, divergent plates sup- ported by a swelling of adventitious tissue, to be seen at posterior portion of median ridge. Cardinal process a faintly discernible ridge just behind the swelling of adventitious substance. Cf. Ddmanella rogata, pi. 17, fig- 31. 8, 10. — Ventral and dorsal exteriors of a large mdividual. Note reduced (shortened) ventral interarea and sub- fasciculate costellx. The general external resemblance of P. farva to Dalmanella is a striking instance of homoeomorphy. Paurorthis, however, differs intern- ally as illustrated above (figs. 5,7). It differs also in being impunctate (no endopunctas) but possesses nu- merous exopunctae which have led to its misidentifica- tion as Dalmanella. Ordovician (Glauconite Is.), Gornaja Scheldicha, Lake Ladoga, Russia. Cat. No. S 1 36. x 3. 6. — Ventral interior, showing broad pallial trunks sepa- rated by a low median ridge. Subreniform ovarian areas occupy lateral portions of valve. Wide adductor- diductor track clearly visible and adjuster (?) im- pressions outside these may be seen. The interior of Figs. Paurorthis parva (Pander) — Cant. Paurorthis is thus close to that of Orthis s. s., and the genus forms a remarkable homoeomorph of Dalman- ella s. s. Ordovician, opposite Iswos, Russia. Shaler Mem. Exped. Colls., Mus. Comp. Zool., Harvard Coll. x 3. Productorthis parallela (Pander) 9. — Ventral interior, showing reduced (shortened) inter- area, prominent dental plates, and peculiar muscle field. The small, elliptical impressions at the base of the dental plates have usually been interpreted as diductor scars; they are, however, smaller than is common for the scars of the diductors and probably represent adjustor muscle impressions. The diductor- adductor impressions occupy the large central elevated area. 13, 16. — Ventral and dorsal exteriors, showing imbricating lamellx or "ruffles," remarkable "productoid" form, and nearly obsolete interareas. Ordovician (Chazy, Glauconite Is.), Gornaja Schel- dicha, Lake Ladoga, Russia. Cat. No. S 126. x 3. 1 1 . — Dorsal interior, showing elongated cardinal process with its compressed myophore. The brachiophores have developed into cuplike structures which receive large teeth. Circular chilidium visible on dorsal sur- face of free or posterior end of cardinal process. Bipartite character of anterior adductor impressions clearly visible. The ribbed elevated border around the periphery of the shell is an aid in articulation. 12. — Reverse, or dorsal, surface of fig. 11, showing imbri- cated exterior and small, umbrella-like chilidium cov- ering free end of cardinal process. Ordovician (Chazy, Glauconite Is., BJ, Popovka, near Leningrad, Russia. Cat. No. S 127. x 3. Productorthis cf. eminens (Pander) 14, 15. — Ventral and dorsal exteriors of a species having finer ribbing. The "ruffled" exterior, however, is well exhibited. In fig. 15, ventral interarea nearly obsolete, and a rounded foramen formed by resorp- tion of the beak by the pedicle. Ordovician (Glauconite Is., BJ, Popovka, near Len- ingrad, Russia. Cat. No. S 122. x 3. 200 PLATE 3 201 PLATE 4 Figs. Cyrtonotella semicircularis (Eichwald) 1,4,5,11. — Ventral, dorsal, posterior, and lateral views, showing multicostate exterior, short interarea, and concave dorsal valve. Ordovician (Echinosphaerites Is.), Popovka, near Leningrad, Russia. Cat. No. S 169. x 5. Glossorthis tacens Opik 2^ 8. — External and internal views of the same dorsal valve. The latter illustrates the cardinal process and the thickening or swelling just in front of it that supports the brachiophores. The latter structures are broken off near the sockets so that their full length is not revealed. Anterior adductor scars bipartite as in Productorthis and many other orthoid genera. 7, 9. — External and internal views of an exceptionally well preserved ventral valve. Fig. 9 illustrates vvell the pseudospondylium and also the various muscle impres- sions. This pseudospondylium is entirely a secondary feature, being best developed in mature shells, and is formed as a deposit under the muscle attachments. In G. extensa, the pseudospondylium is not developed to such a marked degree. For details of the ventral interarea, see fig. 28. 12. — ^Ventral valve, tilted upward to afford a better view of the pseudospondylium. The short septum visible in front, and the thick rim, are adventitious deposits simulating a spondylium. 28. — Enlarged view of interarea of ventral valve of the same specimen illustrated in fig. 9, x 3, showing pon- derous teeth and heavy later-il (deltidial? ) plates along delthyrial margin. On the right side of the delthyrium, one of these is well shown, the suture being emphasized by slight fracturing. Mid. Ordovician (Cj), Kohtla, Estonia. Cat. No. S2100. All except fig. 28 x 1.5. Glossorthis extensa (Verneuil non Pander) 3^5^ 10. — Dors.-il, posterior, and lateral views of the same individual. Ordovician (Glauconite Is., B,), Popovka, near Len- ingrad, Russia. Cat. No. S 161. x 1.5. Productorthis parallela (Pander) 15^ 16. — Internal and external views of posterior of the same individu.il as in pi. 3, figs. 1 1, 12._ Note unusual cardinal process and cuplike adventitious shell sup- porting brachiophore in fig. 15, and umbrella-like chilidium in fig. 16. Ordovician (Glauconite Is., B,), Popovka, near Len- ingrad, Russia. Cat. No. S 127. x 3. Figs. Hesperorthis tricenaria (Conrad) 13, 14, 25. — Ventral, dorsal, and posterior views of a com- plete individual. Contrast broad interareas of figs. 14 and 25 with shortened ones of Orthis, figs. 7 and 8 of pi. 2. 26. — Smaller specimen preserving remnantal chilidium and deltidium. Ordovician (Black River), Cannon Falls, Minn. Cat. No. S199. x 1.5. 17. — Internal view of dorsal valve, showing chilidium, brachiophores, strong median elevation, and internal marginal ribbing. This type of marginal ribbing, with a cleft internal rib corresponding to the stria (groove) of the exterior, is common to many genera of the Orthida;. x 2. 18. — Same shell, tipped forward to give a better view of the convex chilidium. x 2. 29. — Enlargement (x 4) of cardinalia of fig. 1 8, to show chilidium and brachiophores in greater detail. These brachiophores are typical of the ''Orthis" type, long and pointed, grooved on the inside, triangular in sec- tion, the outer sloping face forming resting places for the teeth of the ventral valve, the crural fossette of the ventral tooth resting on the carinate, postero-ventral ridge of the brachiophores. Ordovician (Trenton), St. Paul, Minn. Cat. No. S210. 19, 27. — Ventral interiors, showing long interarea and short deltidium. Notice distinct suture-lines between mar- gins of delthyrium and deltidium. In fig. 27 the slender ridge dividing the main pallial trunks of the ventral valve is visible, and a few of the elevated lines of the ovarian areas can be seen near the hinge margins. 2 1 . — Dorsal interior, showing cardinal process and brachio- phores. Anterior portion of the cardinal process bear- ing a shallow groove. Ordovician (Trenton), ScuUsburg, Wis. Cat. No. S176. Fig. 19 X 1.5; figs. 21 and 27 X 2. 20. — Ventral valve, showing details of interarea. This specimen illustrates a feature of unusual interest: the delthyrium is extremely narrow, having been dimin- ished by the growth of margin.il plates inside the teeth. (See fig. 28.) Deltidial plate an arch under delthyrial margins. Ordovician (Black River), Minneapolis, Minn. Cat. No. S 2101. X 1.5. Hesperorthis davidsoni (Verneuil) 22, 23. — Dorsal and ventral exteriors of a well preserved individual. In fig. 22 perforations are visible on the surface of the costa:, but these extern.il apertures are not to be confused with puncts (endopunctae). 24. — Dorsal interior, showing muscle-scars and cardinalia. Silurian (Gotlandian), Gotland, Sweden. Cat. Nos. S203, S204 (fig. 24). x 1.5. 202 PLATE 4 203 PLATE 5 Figs. Eridorthis rogersensis Foerste 1, 5. — Dorsal exteriors, showing fold. In early stages the dorsal valve bears a sulcus which later reverses and becomes a fold. Ordovician (Trenton, Cynthiana), Rogers Gap, Ky. Cat. No. 78712, U. S. Nat. Mus. x 1.5. Eridorthis aff. E. nicklesi Foerste 2. — Ventral internal mold, showing Orthis-Wke muscula- ture, pallial trunks, and low median rib. Ordovician (Eden, Fulton), New Richmond, Ohio. Cat. No. S 2102. x 2. Eridorthis nicklesi Foerste 9. — Ventral valve, showing depression of fold to form a sulcus at front of valve. Ordovician (Trenton, Cynthiana), Rogers Gap, Ky. Cat. No. 7871 1 U. S. Nat. Mus. x 1.5. Glyptorthis fausta (Foerste) 4. — Dorsal interior, showing H esferorthis-\\k& cardinalia. 7. — Ventral interior, illustrating musculature of the genus. Silurian (Brassfield), near Dayton, Ohio. Cat. No. S236. xl.5. 8. — Dorsal exterior, showing incipient sulcus at posterior. Silurian (Brassfield), Centerville, Ohio. Cat. No. S238. xl.5. Figs. Schizoramma fasciata (Hall) — Cont. This genus has been described as a subgenus of Orthostrofhia, but from its internal structure is dis- tinct enough to merit generic ranic. Schizoramma does not have the small, confined ventral muscle area and the elevated adductor impressions of the other genus. Dolerorthis rustica osiliensis (Schrenk) 10, 12, 19,21. — Lateral, posterior, dorsal, and ventral views of exterior, fig. 1 0 showing faint concavity of ventral valve toward the front. 15. — Dorsal interior, showing orthoid brachiophores and cardinal process, and notothyrial platform. Silurian (Gotlandian), Gotland, Sweden. Cat. No. S282. xl.5. 20. — Ventral interior, showing closely apposed pallial trunks separated by a low septum that forks at the front. Subreniform ovarian impressions bounded by these sinuses. Compare with Orthis s. s., Hesferorthis, and Glyftorthis (pi. 2, fig. 18; pi. 4, fig. 27; pi. 6, fig. 26). 23. — Dorsal interior, showing median ridge, orthoid cardi- nalia, large posterior adductor scars, and smaller bipar- tite anterior adductor impressions. Cf. fig. 24. Silurian (Wenlock), England. Mus. Comp. Zool., Harvard Coll. x 1.5. Schizoramma ? gotlandica Schuchert and Cooper, n. sp. 3, 6. — Dorsal and ventral exteriors. Silurian (Gotlandian), Gotland, Sweden. Cotypes. Cat. No. S 228. xl.5. Schizoramma rjisis (Hall and Whitfield) 11, 16. — Ventral and dorsal exteriors of a silicified speci- men, showing ornamentation. Silurian (Niagaran, Meniscus) , W. Tennessee. Cat. No. S221. Fig. 11 X 1.5; fig. 16 x2. Schizoramma fasciata (Hall) 13. — Ventral interior, somewhat crushed, showing orthoid musculature, and, more indistinctly, the two closely apposed palli-il trunks. 14. — Dorsal interior, to show orthoid brachiophores; low, broad, median ridge; simple cardinal process; and accessory ridges on each side of it. Small adductor impressions more indistinctly visible. Silurian (Clinton), Osgood, Ind. Cat. No. S 222. X 1.5. Dolerorthis jnterplicata (Foerste) 18. — Dorsal interior, showing characteristic cardinalia. Silurian (Niagaran, Osgood), Osgood, Ind. No. 8 279. X 1.5. Cat. Dolerorthis flabellites (Foerste) 1 7, 24. — Obese dorsal interiors, illustrating interesting old- age modifications of cardinalia and notothyrial plat- form, and brachiophores heavily overgrown with ad- ventitious shell substance, which hides them almost completely. Note "orthoid" ribbing of internal mar- gin. In fig. 24, note especially adductor impressions and compare with fig. 23. Compare fig. 24 with fig. 8 of pi. 1. 22. — Ventral interior, showing orthoid musculature, and pallial sinuses bounding subreniform ovarian impres- sions. Cf. fig. 20, and ventral interiors of Orthis, Hesferorthis, and Ghftorthis. Silurian (Niagaran, Osgood), Osgood, Ind. Cat. No. S272. xl.5. 204 PLATE 5 205 PLATE 6 Figs. Ptychopleurella bouchardi (Davidson) 1, 33. — Ventral interiors, showing musculature, pedicle cal- list, and median ridge. 3, 4. — Ventral and dorsal exteriors, showing prominent costje and lamellose exterior. 6, 32. — Dorsal interiors, showing cardinal process, brachio- phores, and adductor impressions. Silurian (Gotlandian), Gotland, Sweden. Cat. No. S285. Figs. 1, 3, 4, 6 x2; figs. 32, 33 x 3. Ptychopleurella matapedia Schuchert and Cooper, n. sp. 2, 5. — Dorsal and ventral exteriors of a silicified specimen, showing strongly lamellose surface. High Silurian or basal Devonian, Upsalquitch road, Matapedia, Quebec. Holotype. Cat. No. S287. x 2. Ptychopleurella lamellosa (Twenhofel) 9. — Posterior view of holotype, showing slitlike delthyrium produced by growth of lateral plates. Ordovician (Ellis Bay), Ellis Bay, Anticosti. Cat. No. 10411 Y. P. M.' X 5. Archseorthis electra (Billings) 7. — Ventral exterior, x 2. 8. — Ventral interior, showing callus extending forward from muscular area. 16. — Dorsal interior, showing low median ridge, nearly ob- solete cardinal process, and brachiophores. x 4. Ordovician (Canadian), Levis, Quebec. Cat. No. 740, Nat. Mus. Canada. All silicified. Deltatreta typica Ulrich MS. 10, 19. — Dorsal and ventral exteriors, x 1.5, showing costel- late exterior and hollow costellas. 14. — Portion of a silicified ventral valve, x 3, showing del- tidium and large, subapical foramen. Faint radial lines also visible on interarea. 30. — Silicified dorsal interior, x 2.25, with a ventral valve lying in it. Primitive nature of brachiophores and their lateral supports like those of Vellamo plainly shown, also simple cardinal process and chilidial plates projecting posteriorly from notothyrial margin. The ventral interior shows well the pedicle callist. Ordovician (up. Canadian, ca. 1000 feet beneath top of Arbuckle Is. (formation name not yet given) ) , near Berwyn, Okla. (U. S. G. S. loc. 191K). U. S. G. S. colls., U. S. Nat. Mus. Deltatreta sp. 15. — Ventral interior, showing prominent dental plates and pseudospondylium, the latter produced by a deposit of adventitious shell beneath the muscles. Cf. ventral interior in fig. 30; cf. also fig. 13. Ordovician (2d Ceratopea zone), "inlier," sect. 25, T 6 N, R 14 W, N. E. of Wichita Mts., Okla. U. S. Nat. Mus. X 2. Figs. Deltatreta dice (Walcott) II. — Dorsal interior, showing primitive cardinalia. 13. — Ventral interior, showing pseudospondylium. Cf. fig. 15. Drift (probably Ordovician (Canadian)), St. Albans, Vt. Impressions from type specimens, U. S. Nat. Mus. Cat. No. 52248. x 1.5. Glyptorthis cf. bellarugosa (Conrad) 1 2. — Dorsal exterior, showing lamellose surface. Ordovician ("Simpson"), Criner Hills, Okla. Cat. No. S 251. x2. Glyptorthis insculpta (Hall) 17, 20, 21. — Ventral, dorsal, and lateral views of exterior. Lateral profile biconvex. Cf. same view of Hebert- ella, pi. 11, fig. 19. 1 8, 26. — Ventral interiors, showing "Orihis" pattern to perfection. Note muscle area; central adductor track straight, diductor tracks elongate tear-shaped; and adjustor scars visible at base of dental plates. Ex- tending forward from the adductor track is a low sep- tum which divides the closely apposed pallial sinuses originating at the front ends of the diductor impres- sions. Lateral spaces occupied by subreniform ovarian impressions. Elevated lines in these impressions con- sidered to be muscle attachments for ovarian bodies. 29. — Dorsal interior, showing an old shell heavily ridged by pallial markings. Note adductor scars. The car- dinalia of Glyftorthis are of the "Orthii" type, dif- fering markedly from those of Hebertella. See pi. 11, figs. 24, 26. Glyftorthis has been usually considered to be a sub- genus of Hebertella, but its lenticular profile and inter- nal structure relate it rather XoHesferorthis and Orthis. Ordovician (Richmond), Oxford, Ohio. Cat. No. S242. X 1.5. Orthostrophia strophomenoides (Hall) 22. — Dorsal internal mold, showing muscle area and pallial markings. 25. — Impression from the above, showing elevated muscle area. Devonian (New Scotland), near Clarksville, N. Y. Cat. No. S 320. X 2. 27, 28. — Ventral and dorsal exteriors, showing characteris- tic ornamentation. Devonian (Birdsong), Henry Co., Tenn. Cat. No. 9702 Y. P. M. xl. Orthostrophia aff. O. strophomenoides (Hall) 24. — Ventral internal mold, showing small muscle area and pallial markings. Devonian (Hunton), near Crusher, Okla. Cat. No. S322. xl.5. Orthostrophia dartae Schuchert and Cooper, n. sp. 23. — Ventral internal mold of paratype, showing small mus- cle area. 3 1 . — Dorsal exterior of holotype, showing ornamentation. Silurian (Bouleaux), Port Daniel, Quebec. Cat. No. S 1985. X 1.5. 206 PLATE 6 207 PLATE 7 Figs. Pahlenella trigonula (Eichwald) 1, 3. — Ventral and dorsal exteriors. Dorsal valve concave. 2. — Ventral interior, showing deltidium and sessile spon- dylium. 4. — Dorsal interior, showing short brachiophores, chi- lidium, and elevated muscle area. Ordovician (Canadian, Walchow, B2),Popovka, near Leningrad, Russia. Cat. No. S 370. x 2. Aff. Vellamo multicosta (Hudson) 5, 11. — Views of the same ventral interior, showing rem- nant of imperforate deltidium. 8. — Ventral interior, showing septum dividing two closely apposed pallial trunks, and, outside these, ovarian areas as in Glyftorthis et al. 9. — Dorsal interior, showing cardinalia and musculature. 10. — Ventral interior, showing muscle impressions on spon- dylium. These shells are doubtfully referred to Vellamo, differing from that genus in having what appears to be an imperforate deltidium, and in details of the dorsal valve. Ordovician (Chazy), Sloop Bay, Valcour Island, N. Y. Cat. No. S3 59. x 2. Vellamo trentonensis (Raymond) H. — Ventral interior, showing spondylium and its support- ing septum, and deltidium. 28. — View of interarea and deltidium. Note position of teeth. Ordovician (Galena, bed 6), Kenyon, Minn. Cat. No. S342. X 1.5. 18. — Dorsal interior, showing anchor-shaped cardin.ilia and musculature. Ordovician (Galena), Cannon Falls, Minn. Cat. No. 8 348. X 1.5. Cf. Vellamo squamata (Pahlen) 1 5. — Interarea. 31. — Interior, showing spondylium. Ordovician (Cj) , Kohtla, Estonia. Cat. No. S 2 1 0 5 . xl.S. Figs. Vellamo cf. emarginata (Pahlen) 16, 29, 30, 32. — Posterior, ventral, lateral, and dorsal views of the same individual, showing details of ornamenta- tion and contour and outline of valve. Ordovician (Mohawkian, Wesenberg), Wesenberg quarries, Estonia. Cat. No. S 339. x 1.5. Vellamo verneuili (Eichwald) 24, 27. — Dorsal and ventral exteriors of type species of genus. Ordovician (Cincinnatian, Lyckholm, F^), Kertel, Estonia. Cat. No. S 347. x 1.5. Vellamo diversa (Shaler) 25. — Dorsal interior, showing cardinalia and musculature. Ordovician (Ellis Bay), Junction Cliff, Anticosti. Cat. No. 10335 Y. P. M. x 1.5. Apomatella ingrica (Pahlen) 6, 7.- — Views showing spondylium. 12. — Ventral exterior, showing also dorsal valve in con- junction. 13. — Ventral exterior, showing procline position of inter- area. These shells may be somewhat remote from the true Vellamo line. Ordovician (Canadian, Walchow, B„), Popovka, near Leningrad, Russia. Cat. No. S2104. x 2. Clitambonites adscendens (Pander) 17, 19, 22, 23. — Posterior, dorsal, ventral, and lateral views of exterior. Note particularly ornamentation and procline interarea. Deltidium commonly imperforate in adult shells, x 1.5. 20, 21. — Ventral and dorsal interiors, the former showing the spondylium. Note peripheral border or frill. x 1.5. 26. — Enlargement of exterior of specimen in fig. 19, X ca. 4.5. Ordovician (Canadian, Kunda), Popovka, near Len- ingrad, Russia. Cat. No. S 372. 208 PLATE 7 209 PLATE 8 Figs. Gonambonites planus Pander emend. Pahlen 1. — Ventral interior, showing spondylium and its acces- sory septa. Marginal thiclcening visible also. 2, 3, 20. — Dorsal, ventral, and posterior views of a small individual, showing prominent process extending from deltidium. Fig. 20 shows deltidium somewhat crushed in. 4, 7. — Dorsal interiors, showing cardinalia and chilidial plates. Brachiophores to be seen protruding from lateral adventitious thickening just antero-laterally of notothyrium. 5,21,26. — Posterior, ventral, and dorsal views of a large shell, showing characteristic ornamentation. Ordovician (Canadian, Walchow, Bj), Wassilkowo, near Lake Ladoga, Russia. Cat. No. S 333. x 1.5. 25. — Ventral interior of a large specimen. Same locality as above. Shaler Mem. Exped. Colls., Mus. Comp. Zool., Harvard Coll. x 1.5. Estlandia marginata (Pahlen) 6. — Dorsal interior, showing cardinalia and median ridge. Brachiophores may be seen in postero-lateral portions of shell just outside adventitious thickenings anterior and laterally of notothyrium. 8. — Ventral interior, showing spondylium. Foramen in deltidium closed by a plug of shell substance. Ordovician (Kuckers, Brandschiefer), Baron Toll's estate, near Jewe, Estonia. Cat. No. S 327. x 1.5. 9. — A fine dorsal interior, showing cardinalia. Notice brachiophores outside anchor-shaped adventitious deposits. Middle Ordovician (Cgo), Kohtla, Estonia. Cat. No. S 2106. X 1.5. Presented by A. Opik. Hemipronites tumidus Pander 10. — Ventral interior with deltidium missing, showing median septum dividing two pallial trunks which in turn surround inside margins of subreniform ovarian impressions. Cf. Glyftorthis insculfta, pi. 6, figs. 18, 26, and Orthis, pi. 2, fig. 18. It will be noted that the spondylium is the homologue of the dental plates and floor of the deltidial cavity. The septum is the homologue of the small ridge dividing the pallial sinuses in Orthis s.s.; the sinuses are situated in precisely the same place and bound similar ovarian impressions. Ordovician (Walchow, Bj), Popovka, near Lenin- grad, Russia. Cat. No. S3 69. x 1.5. Figs. Hemipronites tumidus Pander — Cont. 11. — Dorsal interior, showing cardinalia. Cardinal process simple as in Orthis s. s., and brachiophores rodlike plates supported by adventitious tissue which, in this instance, spreads laterally to form an anchor-shaped mass with the median ridge. 14, 15. — Posterior and ventral exteriors, showing ornamen- tation. Ordovician (Walchow, Bj), Popovka, near Lenin- grad, Russia. Cat. No. S 369. x 1.5. Hemipronites cf. maximus Pander 12,13. — Ventral and dorsal exteriors, showing fine costellae. Ordovician (Canadian, Walchow), Popovka, near Leningrad, Russia. Cat. No. S 365. x 1. Marionella typa Bancroft 16. — Dorsal internal mold, showing dinorthid character of cardinalia. 17. — Muscle-scars and pallial markings of ventral valve. 18. — Impression showing external sculpture. Ordovician (upper Longvillian), Horderley District, East Shropshire, England. Cat. No. S2107 (fig. 18), and S2108. x 1.5. Multicostella platys (Billings) 19, 23. — Ventral and dorsal exteriors, showing ornamenta- tion of this biconvex dinorthid. Ordovician (Chazy), Speer's Ferry, Virginia. Cat. No. S92. X 1.5. Multicostella saffordi (Hall and Clarke) 22. — Ventral interior, showing musculature, which is simi- lar to that of Valcourea. 27. — Dorsal interior, with crenulated dinorthid cardinal process plainly visible. Brachiophores also similar to those of Valcourea. Multicostella was common in Chazy time, but evi- dently did not survive this stage. A deltidium or chilidium is unknown in this dinorthid stock. Ordovician (Chazy), Washburn, Tenn. Cat. No. S786. X 1.5. Clinambon anomalus (Schlotheim) 24, 28. — Posterior and lateral views, showing unusual de- velopment of chilidium, and posterior portion of dorsal valve. Ordovician (Mohawkian, Kegel, D,), Redder, Es- tonia. Cat. No. S 337. x 1.5. 210 PLATE H 211 PLATE 9 FiGS. Dinorthis sweeneyi (Winchell) 1. — Ventral interior, showing form of muscle impressions and remnantal deltidium. x 1.5. 10, 14. — Lateral and dorsal views of exterior, showing con- vexo-concave profile and costate exterior, x 1.5. 1 1 . — Dorsal interior, showing orthoid brachiophores and crenulated myophore of cardinal process. In its later stages of growth, the cardinal process may become distinctly lobate, see figs. 3 and 20. x 2. Ordovician (Decorah), St. Paul, Minn. Cat. Nos. S686 and S 693. Dinorthis pectinella (Emmons) 2. — Ventral interior of a silicified specimen, showing in- terior. X 1. 5. — Exterior, x 0.75, with its costae. Ordovician (Trenton), Curdsville, Ky. Cat. No. S685. Dinorthis (Plaesiomys) subquadrata (Hall) 3. — Dorsal interior, showing cardinal process of an old shell and lobation developed in late stages, x 2. 20. — Same, x 3, showing myophore and shaft, also orthoid nature of brachiophores and swollen notothyrial plat- form. Cf. pi. 10, fig. 25. Ordovician (Richmond), Richmond, Ind. Cat. No. S 728. Dinorthis (Pionorthis) sola (Billings) 4^ 6^ 7^ 8. — Ventral, lateral, posterior, and dorsal views of a large specimen, showing ornamentation and lentic- ular outline of the species. Ordovician (Richmond), Pt. Carleton, Anticosti. Cat. No. S760. x 1.5. 9._Ventral internal mold, showing characteristic dinor- thid muscle-scars. Ordovician, cliff, W. side Vaureal Bay, Anticosti. Cat. No. S 758. x 2. Aff. Dinorthis (Pionorthis) carletona Twenhofel 13. — Ventral internal mold, showing muscle impressions. Ordovician, Raven Nest, Anticosti. Cat. No. S 754. X 1.5. FiGs. Dinorthis (Retrorsirostra) carleyi (Hall) 21,23. — Ventral and dorsal exteriors, showing procline ventral interarea. Ordovician (Arnheim), Jefferson Co., Ind. Cat. No. S 747. xl.5. Dinorthis (Retrorsirostra) carleyi insolens Foerste 22. — Ventral interior, showing teeth, crural fossetts, curv- ing dental plates, and muscle impressions. Note absence of pallial marks. Ordovician (Waynesville) ? , Hanover, Ohio. Cat. No. S 750. xl.5. Austinella sp. 12. — Ventral interior, showing large teeth and outline of muscle area. Ordovician (Maquoketa), Spring Valley, Minn. Cat. No. S 375. xl.5. Austinella whitfieldi (N. H. Winchell) 15. — Ventral internal mold, showing muscle-scars. These and the pallial trunks diverging from the anterior ends of the diductor impressions appear to relate the genus most closely to Dinorthis. 16, 19. — Ventral and dorsal exteriors. 18. — Reverse side of fig. 15, dorsal internal mold, showing adductor impressions. This genus has been referred to as a subgenus of Plectorthis, but structurally it has nothing to do with PUctorthis or the Plectorthidse. Ordovician (Maquoketa), Spring Valley, Minn. Cat. Nos. S 374 and S 376 (figs. 16, 19). x 1.5. Austinella scovillei (Miller) 22. — Ventral interior, showing teeth, crural fossettes, curv- tinctly dinorthid in structure. Ordovician (Richmond), near Lebanon, Ohio. Cat. No. S 377. xl.5. 212 PLATE 9 213 PLATE 10 Pigs. Cyclocoelia sordida (Hall) 1, 5. — Ventral and dorsal exteriors. 2, 4.- — Ventral and dorsal internal molds, the former show- ing short dental plates and the latter the mold of the median septum. Ordovician (Maysville, Fairmount), Cincinnati, Ohio. Cat. No. S 383. x 2. Cycloccelia sectistriata (Ulrich) 3, 7, 9. — Lateral, dorsal, and ventral views of exterior. Ordovician (Maysville, Fairmount), Cincinnati, Ohio. Cat. No. S 387 (one of Ulrich's type lot). x2. Skenidioides billingsi Schuchert and Cooper 6, 11, 13, 14. — Lateral, ventral, posterior, and dorsal views of holotype. 8, 10, 12. — Dorsal interiors of paratypes, illustrating well high dorsal septum, cardinal process, and cruralium. Fulcral plates visible forming sockets. Brachiophores broken off, but when preserved, very long. Ordovician (Black River), Paquette Rapids, Ottawa River, Quebec. Cat. No. S 20 13 (holotype, 2013a). x4. Dinorthis (Plaesiomys) subquadrata (Hall) 15. — Ventral internal mold, showing characteristic muscle- scars. The adjustor scars are prominent in the whole dinorthid stock. Ordovician (Richmond), Spring Valley, Minn. Cat. No. S 732. X 1.5. 17, 18. — Dorsal and ventral exteriors, x 1. 24. — Ventral interior, showing musculature and course of pallial markings, x 1.5. Ordovician (Richmond), Richmond, Ind. Cat. No. S728. Figs. Dinorthis (Plaesiomys) subquadrata (Hall) — Cont. 25. — Dorsal interior, showing cardinalia. x 1.5. 26. — Interior of a gerontic ventral valve, showing extreme thickening of shell, x 1.5. Ordovician (Richmond), Richmond, Ind. Cat. No. S728. Valcourea magna Schuchert and Cooper, n. sp. 16, 20, 27, 29. — Posterior, lateral, dorsal, and ventral views of exterior. 28. — Ventral interior, showing musculature and pallial markings. This specimen is not provided with a del- tidium but the pedicle callist is visible. Ordovician (Simpson). Criner Hills, Okla., sect. 35, 6 S, 1 E. Cotypes. Cat. No. S 779. x 1.5. Valcourea deflecta (Conrad) 19. — ^Ventral interior, showing muscle-scars and deltidium in place. 21. — Dorsal interior, showing cardinalia. Notothyrial platform greatly thickened. 23. — Interarea of specimen shown in fig. 19, showing del- tidium, and teeth with accessory sockets. Ordovician (Black River), Allen Hunter quarry. Fountain, Minn. Cat. No. S 771. x 1.5. Valcourea loricula (Hall and Clarke) 22. — Ventral interior, showing musculature and lateral ovarian impressions. This specimen is not provided with a deltidium but in its place is a decided callist. This suggests that the deltidium, when present in Valcourea at least, is the seat of pedicle attachment. Ordovician (Black River), Allen Hunter quarry, Fountain, Minn. Cat. No. S 771a. x 1.5. 214 PLATE 10 215 PLATE 11 Plectorthis fissicosta (Hall) 1, 3, 5, 15.- — Dorsal, lateral, ventral, and posterior views of exterior. Ordovician (Maysville), Cincinnati, Ohio. Cat. No. S306. X 1.5. Plectorthis jamesi (Hall) 2. — Ventral interior, showing dental plates and muscula- ture. Cf. fig. 14. Ordovician (Corryville), Cincinnati, Ohio. Cat. No. S 316. xl.5. Plectorthis plicatella (Hall) 4. — Dorsal interior. The incurving brachiophore plates which unite with the valve under the cardinal process are clearly visible. Cardinal process like that of Hebertella, see fig. 23. 9. — Ventral interior, showing a delicate dental plate. Ordovician (Maysville), Cincinnati, Ohio. Cat. No. S298. X 1.5. Doleroides gibbosus (Billings) 6. — Ventral interior, to show characteristic musculature. Cf. fig. 24. Ordovician (Decorah sh.), Minn. Cat. No. S 537. X 1.5. 16, 18. — Dorsal and ventral views of an old individual, showing normal wide sulcus. Ordovician (Black River), Fountain, Minn. Cat. No. S 508. xl.5. Doleroides cf. gibbosus (Billings) 7, 8, 11. — Dorsal, ventral, and posterior views of exterior of a well preserved individual, showing pionodemoid outline. This particular specimen has a narrower fold and sulcus than is usual in the species. Ordovician (Black River), Chatfield, Minn. Cat. No. S 507. xl.5. Doleroides pervetus (Conrad) 10, 13. — Dorsal and ventral views of exterior. Ordovician (Black River), Lanesboro, Minn. Cat. No. S 498. xl.5. Figs. Doleroides pervetus (Conrad) — Cont. 12. — Dorsal interior, showing cardinalia. Brachiophores supported by convergent plates as in Plectorthis and Hebertella. Ordovician (Black River), Fountain, Minn. Cat. No. S 499. X 2. Hebertella occidentalis sinuata (Hall) 14. — Young individual. Note resemblance to Plectorthis, figs. 2 and 9. x 2. 17,23. — Enlarged views, x 1.75, of cardinalia of an old shell, showing shaft and compressed myophore of car- dinal process, brachiophores and their supporting plates. 19,20,22,25. — Lateral, ventral, dorsal, and posterior views of a large individual. Note convexo-concave profile. X 1.5. 24. — Ventral interior, showing muscle area with its double- track adductor impression and subcrescentic diductor scars. Adjuster impressions borne, when visible, on base of dental plates. Notice callus deposit at back end near pedicle callist; in some specimens this almost completely obliterates the posterior portion of the muscular field, x 2. 26. — Adult dorsal interior, showing cardinalia. Shaft of cardinal process simple, with compressed myophore. Brachiophores supported by converging plates which unite under the cardinal process; sockets defined by fulcral plates. This is different from the usual type in the Orthidx, in which the brachiophores are simple rods or blades supported only by a swelling of adventi- tious shell. X 2. Ordovician (Maysville), Cincinnati, Ohio. Cat. Nos. S388 (figs. 19, 20, 22, 25), S 397 (figs. 14, 24), and S451 (figs. 17, 23, 26). Hebertella frankfortensis Foerste 21. — -Ventral interior, showing bipartite adductor track. Cf. fig. 24. Ordovician (Trenton), Frankfort, Ky. Cat. No. S440. x2. 216 PLATE II 26 217 PLATE 12 Pics. Schizophorella fallax (Salter) 1 , 9. — Ventral interiors, the former an internal mold, x 2, and the latter an impression taken from it, x 1.5. Cat. No. B 52133, Brit. Mus. Nat. Hist. 2. — Dorsal exterior, x 1.5, the external mold correspond- ing to fig. 4. Cat. No. B 52123, Brit. Mus. Nat. Hist. 3. —Ventral exterior, x 1.5. Cat. No. B 52121, Brit. Mus. Nat. Hist. 4. — Dorsal internal mold, x 2. Cat. No. B 44610, Brit. Mus. Nat. Hist. 7. — Impression from the above, x 1.5, showing support- ing plates of brachiophores. Ordovician (upper Bala, Drummock group. Starfish bed), Thraive Glen, Girvan, Ayrshire, Scotland. Cat. Nos. S 486 and S 487. Impressions of British specimens. Mimella globosa (Willard) 5, 10, 14, 15. — Posterior, dorsal, lateral, and ventral views of exterior. This species was previously referred to Pionodema, with which it forms a heterochronous homoeomorph. Ordovician (Chazy), Luttrell, Tenn. Cat. No. S483. X 1.5. 6. — ^Dorsal interior, showing septum, cardinal process, and sessile cruralium. 8. — Ventral interior, showing trilobed muscle area. Ordovician (Chazy), Washburn, Tenn. Cat. No. S482. xl.5. Mimella melonica (Willard) 20. — Ventral interior, showing trilobed muscle field and pallial markings. Ordovician (Chazy), Luttrell, Tenn. Cat. No. S474. xl.5. Mcewanella lineolata (Savage) 11, 12. — Ventral valve, the latter showing the interarea. 1 8, 22. — Dorsal exterior and interior. Brachiophores of dorsal interior like those of Platystrofhia, and the plates supporting them unite beneath the cardinal process. 2 1 . — Ventral interior. Muscle-scars not defined and mus- cle field with thickened peripheral rim. Ordovician (Fernvale), old quarry S. of Regen- hardt's quarry, N. W. edge of Cape Girardeau, Missouri. Cat. No. 65872, U. S. Nat. Mus. x 1. Figs. Mcewanella raymondi Foerste 17. — Ventral exterior, to show characteristic ornamentation. Ordovician (top of Kimmswick), Cape Girardeau, Missouri. U. S. Nat. Mus. x 1.5. Platystrophia crassa (James) 13. — Dorsal interior, showing gerontic condition of car- dinalia. Ordovician (Maysville), Spring Valley, Minn. Cat. No. S583. x 1.5. Platystrophia laticosta (Meek) 1 6. — Dorsal interior, showing incurving brachiophore plates which are clearly visible only in young Platystrofhia. Cf. Plectorthis, pi. 11, fig. 4. 23. — Ventral interior of young shell, showing dental plates and muscle area. Dental plates not visible in gerontic shells, see fig. 26. 27. — Dorsal interior, showing muscle impressions and car- dinalia. The cardinal process of Platystrofhia is usually much aborted. This figure shows the brachio- phores and their supporting plates much thickened by callus deposit. Ordovician (Maysville), Cincinnati, Ohio. Cat. No. S 575. xl.5. Platystrophia cf. laticosta (Meek) 19. — Very young dorsal valve, showing strongly incurving brachiophore plates and fulcral plates. Cf. similar views of Plectorthis and Hebertella. Ordovician (Maysville), Roh's Hill, Cincinnati, Ohio. Cat. No. S 566. x 3. Platystrophia acutilirata (Conrad) 24. — Ventral interior of gerontic individual. Dental plates obsolete and interior pitted by ovarian? impressions. Ordovician (Whitewater), Oxford, Ohio. Cat. No. S545. xl.5. Platystrophia ponderosa Foerste 25. — Dorsal interior, showing a gerontic individual. Car- dinalia so thickened as to obliterate fulcral plates and exaggerate brachiophore supports, x 1.5. 26. — Gerontic ventral valve. Dental plates obsolete, owing to filling in of umbonal cavities, x 1. Ordovician (Maysville), Cincinnati, Ohio. Cat. No. S5 56. 218 PLATE 12 219 PLATE 13 PiGs. Skenidium insigne (Hall) 1,2,5,8. — Posterior, anterior, dorsal, and postero-dorsal views of three individuals, showing characteristic ex- ternal form. Figs. 1 and 8 show uncovered del- thyrium, and spondylium. 3. — Dorsal interior, showing wide hinge-plate. 4. — Ventral interior, showing spondylium. Devonian (New Scotland), Indian Ladder, Albany Co., N. Y. C.it. No. S2025. x 6. Finkelnburgia sp. 6, 1 1 . — ^Ventral exterior, and interior showing pseudospon- dylium. Ozarkian (Eminence, near top), S. slope of hill 4% to 5 miles S. of Potosi, Missouri. Wax impres- sions from originals in U. S. Nat. Mus. Cat. No. S60. x3. Finkelnburgia sp. 7^ Ig. — Dorsal interiors, showing incurving brachiophore plates and subradial muscle impressions and strongly elevated ridges about each impression. Ozarkian (Huzzah Creek section, bed 7), 10 miles E. of Slideville, Missouri. Wax impressions from originals in U. S. Nat. Mus. Cat. No. S 58. x 3. Finkelnburgia armanda (Billings) 9, 10. — Dorsal internal mold, and wax impression taken from it. 12. — ^Wax impression of ventral exterior, showing sculpture. 15. — ^Wax impression of dorsal interior. This and fig. 10 show the simple cardinal process and the incurving brachiophore plates, and the fulcral plates. Structure much like that of Hebertella or Plectorthis. Ozarkian (near base of Logan's B3) near Phillips- burg, Quebec. Cat. No. S 2109. x 2.20. Fics. Finkelnburgia sp. 13,1 7. — Dorsal and ventral exteriors of a silicified specimen. 16. — Dorsal interior, reverse of fig. 13, showing converg- ing brachiophore plates and elevated seats of muscle attachment. 19. — Ventral interior, reverse of fig. 17, showing pseudo- spondylium. Ordovician (Canadian), 3 miles S. E. of Rainy Mt., Kiowa Co., Okla. Ulrich Coll., U. S. Nat. Mus. x3. Finkelnburgia aflf. osceola (Walcott) 14. — Ventral internal mold, showing filling of delthyrial cavity. Finkelnburgia appears to be the earliest genus in the plectorthid line. Ozarkian, Crawford Co., Missouri. Cat. No. S 62. x3. Angusticardinia recta (Pander) 20, 22. — Ventral and dorsal views of this rhynchonelloid orthid. 2 1 . — Dorsal interior, showing brachiophores and their sup- porting plates which unite at the floor of the valve with the median ridge, forming a narrow notothyrial orthid. Ordovician ( Walchow, Glauconite ss., Bip) , Popovka, near Leningrad, Russia. Cat. No. S 171. x 3. Angusticardinia striata (Pander) 23. — View showing profile. Ordovician (Walchow, Glauconite ss.,B,3), Popovka, near Leningrad, Russia. Cat. No. SI 70. x 3. 220 PLATE 13 PLATE 14 Figs. Porambonites teretior (Eichwald) I, 2, 4, 5, 10. — Dorsal, anterior, posterior, ventral, and lat- eral views of a rather smooth, attenuate type. Ordovician (Mohawkian, Kuckers), near Jewe, Es- tonia. Cat. No. S 1520. x 1. Porambonites deformatus (Eichwald) 6, 7, 9. — Posterior, ventral, and lateral views of a coarsely pitted, attenuate type. Ordovician (Mohawkian, Echinosphserites Is.). Popovka, near Leningrad, Russia. Cat. No. S 1528. X 1.5. Porambonites baueri Noetling 8. — Dorsal interior, showing great internal thickening of brachiophore (spondyloid) plates. 24. — Dorsal interior of a gerontic individual, showing greatly thickened brachiophore plates. Ordovician (Mohawkian, Jewe, DJ, near Jewe, Estonia. Cat. No. S 1529. x 1. Porambonites gigas Schmidt 14. — Posterior view, showing resorbed beaks. This condi- tion of the beaks indicates that the pedicle persisted throughout the life of the animal. Ordovician (Richmond, Lyckholm, Fi), Kurkul, Estonia. Cat. No. S 1518. xO.5. Porambonites reticulatus Pander 3, 11-13. — Posterior, dorsal, ventral, and lateral views of the same individual, showing suborbicular outline and characteristic ornamentation. 15. — Anterior view of another individual, showing deep ventral sulcus and long tongue. Ordovician (Chazy, Walchow), Iswos on Walchow River, Russia. Cat. No. S 1 532. x 1. Figs. Porambonites reticulatus Pander — Cont. 29. — Enlargement of a portion of fig. 11, to show ornamen- tation in greater detail. Ordovician (Chazy, Walchow), Iswos on Walchow River, Russia. Cat. No. S 1532. Porambonites schmidti Noetling 20, 27. — Views of ventral interior, the former tipped for- ward to show better the dental plates. These plates are discrete but in old forms may unite by secondary deposition on their sides and the floor of the valve. 21,28. — Two views of a dorsal valve belonging to the ventral one above. In fig. 28, the reader is looking directly into the shell and the short brachiophore plates are not visible but the sockets are clearly dis- cernible. In fig. 21, the front of the shell is tipped forward to permit a view of the brachiophore plates, which are also discrete. There are no structures in either of these two shells or in the genus linking them to the Pentameridx. Ordovician (Mohawkian, Jewe, DJ, Spitham, Es- tonia. Cat. No. S 1530. x 1. Lycophoria nucella (Dalnian) 16, 18. — Ventral interiors, showing discrete dental plates. 22, 23. — Dorsal interiors, showing cardinal process. 30. — Same as fig. 22, enlarged three times to show greater detail. Cardinal process complete and uninjured in any way, not forked as Hall and Clarke assert. Com- pressed brachiophore plates visible, and also short brachiophores. Ordovician (Chazy, Kunda). Popovka, near Len- ingrad, Russia. Cat. No. S1515. x 1.5 (except fig. 30). 17, 19, 25, 26. — Lateral, ventral, dorsal, and posterior views of a complete individual, showing contour, profile, and ornamentation. Ordovician (Chazy, Kunda, B^), Popovka, near Leningrad, Russia. Cat. No. S 1516. x 1.5. 222 PLATE 14 PLATE 15 Figs. Syntrophina campbelli (Walcott) 1. — Ventral internal mold, showing impression of spon- dylium. 2, 19. — Dorsal interiors, showing subparallel or divergent brachiophore plates. Fig. 19 also shows impression of small callosity at back of notothyrial cavity. Ordovician (Canadian, lower Roubidoux), hill op- posite McCabe, Missouri. Cat. No. S 1553. x2.65. Syntrophina breviseptata Ulrich MS. 15, 16. — Ventral interiors, 15 being an internal mold and 16 an impression taken from it. Spondylium well shown. 17, 18. — Dorsal interiors, the latter an internal mold and the former an impression taken from it, showing divergent brachiophore plates. Ozarkian? , Phillipsburg, Quebec. Cat. No. S 1 5 5 1 . x2. Syntrophina palmata (Cleland) 30, 31. — Dorsal internal mold and impression taken from it. The latter shows the adductor impressions and the posterior callosity. Ordovician (Canadian, Tribes Hill), Fort Hunter, N. Y. Cat. No. S 1544. x2.65. Syntrophia cf. lateralis (Whitfield) 3. — Replica of dorsal interior, from original in Ulrich Coll., U. S. Nat. Mus. Shows narrow, short cru- ralium, which resembles that of some rhynchonelloids. Ozarkian ? (Bg), Phillipsburg, Quebec. Cat. No. S1574. x2. Syntrophia lateralis (Whitfield) 27, 28. — Lateral and front views of ventral interior, the latter showing musculature. 29, 32. — Posterior and dorsal views of complete individual. From Walcott, Camb. Brach., pi. 102, figs. 6e-g, after Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 62, fig. 9. Syntrophinella typica Ulrich MS. 4, 5. — Ventral and dorsal exteriors, showing faint radial sculpture. 13. — Dorsal interior. Ozarkian ? (upper Chepultepec), ^ mile S. W. of Elliottsville church, Bess quad., Alabama. Wax re- plicas of originals in Ulrich Coll., U. S. Nat. Mus. Cat. No. S 1582. X 2.65. Figs. Syntrophioides harlanensis (Walcott) 20. — Ventral interior, with pseudospondylium and diver- gent pallial markings. 23. — Dorsal interior, with adductor impressions and an- terior pallial markings. Up. Cambrian, 4 miles N. E. of Rogersville, Tenn. Replicas of Walcott's types (Cat. No. 52252, U. S. Nat. Mus.). Cat. No. S 1583. x 2. Clarkella sp. 6, 11. — Ventral interiors, showing spondylium. 7, 1 0. — Dorsal interiors, showing numerous septa support- ing brachiophore plates. These are not continuous across the middle of the valve but separate as in Syntrophina. Ozarkian? , Phillipsburg, Quebec. Cat. No. S 1 567. x3. Clarkella afif. C. montanensis (Walcott) 8, 9. — Ventral and dorsal exteriors, showing smooth surface. Ozarkian? (Logan's Bj), near Phillipsburg, Quebec. Geol. Surv. Canada, x 2. Multicostella platys (Billings) 12. — Lenticular profile. Ordovician (Chazy), Speer's Ferry, Va. Cat. No. S792. x2. HuenelHna triplicata (Walcott) 14, 21. — Ventral and dorsal internal molds. Ventral inte- rior shows an impression of the pseudospondylium and radiating ridges diverging from it and a strong ante- rior adductor (?) impression. Fig. 21 shows molds of peculiar lateral plates upon which the genus is based. Original in U. S. Nat. Mus. 22. — Replica of fig. 21, to show lateral plates. Ozarkian?, Novaya Zemlya, Russia. Cat. No. S1580. x2.65. Yangtzeella poloi (Martelli) 24, 25. — Ventral and posterior views of exterior. Up. Ordovician, Foope, China. Cat. No. S 1585. X 1.33. 26. — Dorsal exterior. Ordovician (Neichiashan), Neichiashan, Yangtze gorge, Hupeh, China. Cat. No. S 1586. x 1.30. 224 PLATE 15 -7 -A #^ ^ * 225 PLATE 16 Figs. Mystrophora areola (Quenstedt) 1, 5. — Dorsal and ventral exteriors of a large specimen. Compare the ornamentation and contour with those of Skenidium, pi. 13. 2, 3, 4. — Dorsal, lateral, and ventral views of a smaller specimen. Devonian (Givetian, crinoid bed), Gerolstein, Ger- many. Cat. No. S 2 1 1 0. x2.7. Orthorhynchula ottawaensis (Billings) 6. — ^Ventral interior of a silicified specimen, showing long dental plates. 9, 13. — Crural bases with ends of crural processes, which are extremely long when present. Certain rhynchonelliform shells from Kentucky have been referred to this species, but they should be placed with Camerella. The Ottawa forms (topotypes) have commonly been referred to Camerella but belong with Orthorhynchula. ^'■Camerella" fanderi from Paquette Rapids, an associate of C. ottawaensis, is neither a Camerella nor an Orthorhynchula, but allies itself with the spire-bearing genera. Ordovician (Black River), Paquette Rapids, Ottawa, Canada. Cat. No. S 809. x 2.7. Orthorhynchula linnejri (James) 12. — Silicified specimen from which the ventral valve has been broken, revealing the crural bases. 28, 29. — Dorsal and lateral exteriors. Ordovician (Maysville), Frankfort, Ky. Cat. No. 7539 Y. P.M. x2. 1 7. — Ventral view of exterior. 30.- — Ventral interior, showing teeth, interarea, and outline of muscular field. Ordovician (Maysville), Colby, Clarke Co., Ky. Cat. Nos. S 795 and S 796 (fig. 30). x 2. Kayserella lepida (Schnur) 7, 8, 10. — Posterior, lateral, and ventral views of exterior. Posterior view shows deltidium indistinctly. Devonian (Eifelian), Gerolstein, Germany. Cat. No. S2111. x2. p,G3. Taffia planoconvexa Butts 11. — Dorsal interior, showing orthoid character of cardi- nal ia. Low. Ordovician (Odenville), E. of Odenville, Alabama. U. S. Nat. Mus. Pomatotrema grandaeva (Billings) 1 4, 1 9. — Dorsal and ventral exteriors. 16, 23, 26, 31. — Ventral interiors, showing deltidium, den- tal plates, internal lateral ovarian ridges, and a ridge developed from the adductor impression. 18, 21. — Dorsal interiors, showing lateral ridges and simple cardinal process. The specimen in fig. 21 was actually in conjunction with that in fig. 16. Pomatotrema shows many resemblances to the Cli- tambonacea and, together with the members of the Clitambonitida:, has been placed in that superfamily. Ordovician (Beekmantown), Newfoundland. Cat. Nos. S 81 (fig. 18) and S 82. x 2.7. Pomatotrema muralis Ulrich and Cooper, n. sp. 15. — Ventral interior. 20. — Dorsal internal mold. Compare these figures with those of P. grandava. Ordovician (upper Canadian), Ardmore quad., Okla. Wax impressions of specimens in U. S. Nat. Mus., Cat. Nos. S 77 (fig. 20) and S 79. x 2.7. Planidorsa crassicostella Schuchert and Cooper, n. sp. 22, 27. — Posterior and ventral views of the holotype, the former showing chilidial plates. Ordovician (Chazy), Speer's Ferry, Virginia. Cat. No. S 763. Fig. 22x1.35. Planidorsa bella Schuchert and Cooper 24. — Dorsal exterior, x 1.35. 25. — Ventral internal mold, showing musculature, which is essentially dinorthoid but with unusually large ad- juster scars. X 2. Ordovician (Chazy), Washburn, Tenn. Holotype. Cat. No. S. 764. 226 PLATE 16 227 PLATE 17 Figs. Onniella broggeri Bancroft I, 6. — Dorsal and ventral views, showing exterior. Ordovician, Horderley District, E. Shropshire, Eng- land. Cat. No. S 21 12. x2. Onniella sp. 8, 12. — Dorsal internal mold and impression taken from it. 9, 1 1 . — Ventral internal mold and replica of it. 18. — Enlargement, x 4, of fig. 12, showing internal struc- ture in greater detail. Cf. fig. 23. Up. Ordovician, Horderley District, E. Shropshire, England. Cat. No. S 2 1 1 3. All except fig. 1 8 x 2. Dalmanella rogata (Sardeson) 2,31. — Dorsal interior, x 2.65, and enlargement of same, X 4, showing simple, bladelike brachiophores and car- dinal process. The myophore is lobed and from its center there has grown forward along the shaft a peculiar process giving the whole the appearance of the head of a bee. Ordovician (Black River), St. Paul, Minn. Cat. No. S 845. 3, 5. — ^Ventral and dorsal exteriors, showing suborbicular outline and ornamentation. 4. — ^Ventral interior, showing characteristic bilobed muscle field. Ordovician (Trenton), 3 miles S. E. of Cannon Falls, Minn. Cat. No. S 811. x 2.67. 7, 13. — Dorsal and ventral views of the exterior of Sarde- son's holotype. Ordovician (fucoid bed), Ellsworth, Wis. Cat. No. S 935. x2. Dalmanella meeki (Miller) 10,20,21. — Posterior, ventral, and dorsal exterior views, X 1.5. This transverse type of shell strongly resembles Bancroft's Onniella. 23, 24. — Dorsal interiors, showing age variations. Fig. 23 portrays the younger shell, with its more slender car- dinal process having a lobed myophore, indistinct ad- ductor impressions, and bladelike brachiophores with little adventitious thickening. The other shell shows a prominent thickening of the cardinal process, with some development of adventitious testaceous matter on the inside surface of the brachiophores, and the adductor impressions are more distinct, x 2. 33. — Ventral interior, showing well the dental plates and deep crural fossettes. Notice that the musculature varies from that usual in Dalmanella rogata and allies in not being lobate in front and in having the adduc- tor track approximately the same length as the diduc- tor impressions. The "meeki" type of Dalmanella may be referable to Onniella when that genus has become better established and its lineage put on a substantial basis, x 2. Ordovician (Richmond), Hanover, Ohio. Cat. No. S855. Figs. Dalmanella ignota (Sardeson) 19. — Internal mold, showing musculature. Adjustor scars and elongate diductor impressions visible enclosing ad- ductor impressions. Such a muscle pattern could have produced that of Rhifidomella by expansion of the diductor impressions to a semiflabellate outline. Ordovician (Wykoff), Spring Valley, Minn. Cat. No. S 947. X 2. Dalmanella emacerata (Hall) 22. — Ventral internal mold showing bilobate muscle area and pallial markings. Cf. interior of Parmorthis, pi. 21. Ordovician (Maquoketa), Granger, Minn. Cat. No. S 954. X 2. Dalmanella corpulenta (Sardeson) 25. — Ventral internal mold, showing lobate muscle-scars. X 1.5. 27, 29. — Dorsal interior and exterior, x 2. Ordovician (Richmond), Spring Valley, Minn. Cat. No. S 867. Dalmanella tersa (Sardeson) 26, 30. — Dorsal interior, showing brachiophores, and dorsal exterior. This shell is plano-convex in profile and the ventral diductor scars wrap around the adductor impressions. D. tersa has all the characters necessary to produce Rhifidomella. Ordovician (Richmond), Wilmington, 111. Cat. No. S 871. x2. Resserella canalis (Sowerby) 14. — Ventral internal mold, showing divergent pallial marks. 15,1 6. — Dorsal internal mold, and impression taken from it. Note, in the latter, structure identical with that of D. rogata (figs. 2, 31). Ordovician (low. Longvillian, Kjxrina zone). Long Lane quarries S. of Horderley, E. Shropshire, Eng- land. Cat. Nos.S 21 14 (fig. 14) and S 21 15. x 2. Heterorthina praeculta Bancroft 17. — Ventral internal mold, x 1.5, showing p.iHial markings. 28. — Ventral internal mold, showing musculature, x 1.5. 32.- — Cardinalia of dorsal valve, x 4. Bilobed myophore and cleft shaft clearly visible, also simple bladelike brachiophore plates. Ordovician (Marshbrookian, Wattsella zvattsi zone), Horderley District, E. Shropshire, England. Cat. No. S2116. 228 PLATE 17 ' -' • "' .jfft'r,-^ 23 E*-^/ '> 27 25 28 29 \ '"nr 31 32 229 26 30 PLATE 18 Figs. Heterorthina fairmountensis (Foerste) 1. — Ventral interior, showing widely divergent diductor impressions and delicate pallial markings, x 2. Cf. pi. 17, fig. 17. 2, 3. — Dorsal and ventral exteriors, x 1.5. 4. — Dorsal interior, to show cardinalia and musculature. Notice lobed myophore and cleft shaft of cardinal process, and simple divergent brachiophores. Cf. fig. 7. x2. Ordovician (Bellevue), Hamilton, Ohio. Cat. No. S852. Heterorthina praeculta Bancroft 5. — Ventral exterior, showing form of valve. Ordovician (Marshbrookian), Horderley District, E. Shropshire, England. Cat. No. S 21 17. x 1.5. 6, 7. — Internal mold of dorsal valve and wax replica made from it. Compare this replica with fig. 4; the former shows the same cleft shaft and lobate myo- phore, similar brachiophores, muscle impressions, and pallial marks. 8. — Replica of a dorsal external mold. Dorsal valve flat and shell multicosteUate. Ordovician (Marshbrookian, Wattsella uiattsi zone), Horderley District, E. Shropshire, England. Cat. No. S2116. X 1.5. Cariniferella carinata (Hall) 9. — Ventral internal mold, showing muscle field and form of muscle impressions. 15. — Dorsal internal mold, showing impressions left by brachiophores and cardinal process. 16. — ^Wax impression taken from above. Devonian (Chemung), Bath, N. Y. Cat. No. S1087. X 1.5. 10, 12. — Ventral and dorsal exteriors, showing prominent dorsal sulcus and ventral fold. Devonian (Chemung), Arkport, N. Y. Cat. No. 8 1088. X 1.5. Cariniferella dumonti (Verneuil) 1 1 . — ^Ventral interior, showing musculature. Up. Devonian, ? Belgium. Mas. Comp. Zool., Har- vard Coll. xl.5. Linoporella punctata (Verneuil) 13, 14, 18. — Ventral interiors, showing respective stages in development of pseudospondylium. Fig. 1 8 shows discrete dental plates with little or no adventitious thickening on floor of delthyrial cavity. Fig. 14 shows an advanced stage in which the deposit is ele- vated at the front and unites the dental plates, x 2. Silurian (Gotlandian), Visby, Gotland, Sweden. Riks. Pal. Zool. Avd., Stockholm. Figs. Linoporella punctata (Verneuil) — Cont. 24. — Reverse side of exterior in fig. 3 3, showing well inter- area, teeth, dental plates, and median ridge, x 1.5. 33. — Exterior of a large ventral valve, showing characteris- tic ornamentation, x 1.5. Silurian (Gotlandian), Visby, Gotland, Sweden. Riks. Pal. Zool. Avd., Stockholm. 17. — Posterior portion of large dorsal valve, showing car- dinalia. Note extremely slender shaft of cardinal process, with its thickened unlobed myophore. Brach- iophore plates uniting below with median septum to form a cruralium. Silurian (Gotlandian), Foro, Lausa, Sweden, x 2. Levenea subcarinata (Hall) 19,32. — Dorsal interiors, the former, x 1.5, showing ad- ventitious growths on inside of brachiophores, and the latter, x 2, showing well musculature and pallial marks. 31. — Ventral interior, showing characteristic musculature. X 1.5. Devonian (Birdsong), Birdsong Creek, west Ten- nessee. Cat. No. S982. 20. — Enlargement of fig. 28, x 3. 28. — Dorsal interior, turned slightly to the side, showing unmodified brachiophores in the form of flat, ob- liquely placed blades, x 1.5. Devonian (Helderberg). Albany Co., N. Y. Cat. No. S 986. 21. — Dorsal interior, showing greatly modified interior. Shaft of cardinal process modified by a process grow- ing forward from median lobe of myophore, brachio- phores coated on inside by adventitious material which cements them to the valve. Lateral pallial sinuses given off between adductor impressions clearly visible. Devonian (Birdsong), near Holladay, Tenn. Cat. No. S 980. xl.5. 23. — Dorsal interior of young individual, showing brachio- phores as simple, unmodified, oblique blades, x 1.5. 25, 29. — View, x 1.5, and its enlargement, x 3.5, to show modified brachiophores, a line of demarcation be- tween actual plates and adventitious substance being clearly visible. Devonian (Birdsong) Conrad Place, near Jeanette, Tenn. Cat. No. S 988. 30. — Ventral internal mold, showing pentagonal form of muscle field and pallial markings. Devonian (Helderberg), near Clarksville, N. Y. Cat. No. S 1014. X 1.5. Levenea cf. subcarinata (Hall) 22, 26, 27. — Posterior, ventral, and dorsal exterior views. Devonian (Birdsong), Benton Co., Tennessee. Cat. No. 9719 Y. P. M. X 1.5. 230 PLATE 18 •n ■ "3JJ..iH~l' f^^ i V 9 231 PLATE 19 Figs. Thiemella villenovia Williams 1, 5. — Dorsal and ventral exterior views, showing well the reversion of fold and sulcus which is one of the generic characters. The specimens figured are squeezes of external molds in the U. S. Nat. Mus. X 1.5. 2, 6. — Ventral internal molds, showing musculature and long septum dividing anterior ends of diductor scars and extending some distance in front of them. Fig. 2 X 1.5, fig. 6 X 2. 4. — Replica of dorsal internal mold, showing cardinalia. X 1.5. Note delicacy of structures and compare with the rather ponderous cardinalia of Aulacella, fig. 13. 9. — Dorsal internal mold, the reverse side of that in fig. 4. xl.5. Devonian (Chemung), Dyes quarry, Villenova, Chautauqua Co., N. Y. Originals in U. S. Nat. Mus. Rhipidomella vanuxemi (Hall) 3. — ^External view of ventral valve, showing swollen costellje. Devonian (Hamilton), Canandaigua Lake, N. Y. Cat. No. S 1286. X 1. Aulacella eifelensis (Verneuil) 7, 8. — Dorsal and ventral views of exterior, showing fold and sulcus, which are not reversible, x 1.5. 11. — Ventral internal mold, showing musculature. Ad jus- tor impressions clearer in this specimen than usual. Cf. the same view of Thiemella, fig. 6, and note that whereas in Aulacella the septum forks at the ends of the diductor scars, in Thiemella there is no forking and the septum extends a considerable distance in front of the muscle field, x 2. Mid. Devonian, Eifel, Germany. Cat. No. S 1096. 10. — Ventral interior, showing elongate muscle field with diductor impressions separated by a low ridge which forks in front, x 2. 13. — Dorsal interior, showing ponderous cardinalia which have a strong resemblance to those of Rhifidomella. Cf. fig. 4. xl.5. Mid. Devonian, Eifel, Germany. Schultze Colls., Mus. Comp. Zool., Harvard CoU. Perditocardinia dubia (Hall) 12,20,22. — Lateral, dorsal, and ventral views of a large specimen, showing terebratuloid form of valves. 21. — Ventral interior, showing musculature. Mississippian (St. Louis), Spergen Hill, Ind. Cat. No. S 1300. x2. 16, 17. — Ventral and posterior views of exterior. Mississippian (Carthage), Webb City, Jasper Co., Missouri. Cat. No. S 1293. x 2. Figs. Perditocardinia aflf. P. dubia (Hall) 14,15. — Ventral and dorsal interiors. Ventral interior shows clearly the lack of an interarea and consequently of a hinge-line. Dorsal interior shows ponderous cardinalia similar to those of Rhifidomella but with modifications induced by the compression of the inter- area to disappearance. Mississippian (Salem), near Valley Park, St. Louis Co., Missouri. Cat. No. S 1297. x 2. Platyorthis planoconvexa (Hall) 18,19,29. — Dorsal, ventral, and lateral exterior views. Notice particularly planoconvex profile, x 1. 23. — Ventral interior, showing flaring dental plates con- tinued as ridges along margins of muscle field, x 1.5. 24. — Dorsal interior of silicified specimen, showing pon- derous cardinal process and oblique, bladelike brach- iophores. xl.5. Devonian (Oriskany), Cumberland, Md. Cat. No. S114S. Platyorthis circularis (Sowerby) 27. — Dorsal internal mold. The pit marking the cardinal process is short, showing a short cardinal process but elevated a considerable distance in a ventral direction. 28. — Ventral internal mold, showing elongate flabellate ad- justor-diductor fields separated by a median ridge which carried the adductor muscles. Devonian (Coblenzian), Siegen, Germany. Cat. No. SI 139. xl.5. Platyorthis opercularis (Verneuil) 31. — Dorsal view of exterior, showing flatness of dorsal valve. Devonian (Eifelian), Gerolstein, Germany. Cat. No. S 1141. xl.5. Proschizophoria personata (Zeiler) (emend. Kayser) 25. — Dorsal internal mold. Cardinal process and brachio- phore plates ponderous. Anterior adductor impres- sions separated from posterior impressions by hori- zontal ridges (in the mold, depressions). 30. — Ventral internal mold, showing subtriangular muscle field. Diductor impressions long and adductor im- pressions imprisoned within them. Up. Devonian, Unkel, Germany. Schultze Colls., Mus. Corap. Zool., Harvard Coll. x 1. 26, 32. — Ventral and dorsal internal molds. Low. Devonian (Siegenerschichten), Seifen, Ger- many. Cat. Nos. S 1090 and S 1089. xl.5. 232 PLATE 19 233 PLATE 20 Figs. Smeathenella hamagensis Bancroft 1, 5. — Dorsal internal mold, and a replica of it which shows brachiophores cemented to valve to produce thornlike structures. Cf. fig. 19, Heterorthis clytie. 2, 4. — Internal mold of crushed ventral valve, and a replica taken from it, to show short dental plates. 3_ — Replica taken from dorsal exterior mold, showing fascicosteUate surface. Ordovician (basal Harnagian), Smeathen Wood, Horderley, E. Shropshire, England. Cat. Nos. S2119, S2120. xl. Muscle Harknessella vespertilio (Sowerby) 6^ 10. — ^Ventral internal mold (10), and replica. area lobate. 7,9. — Dorsal interna! mold (9), and replica. This wax impression shows clearly the nature of the brachio- phores, although the lobate character of the cardinal process is not altogether clear because of the imper- fection of the impression. The cementing of the brachiophores to the valve is clear, however. Cf. fig. 19. Ordovician (basal Caradocian, Heterorthis fatera grits), Coston District, E. Shropshire, England. Cat. Nos. S2122 (figs. 6, 10) and S 2123. x 1. Harknessella sp. 8. — Dorsal internal mold. Ordovician (basal Caradocian, Harknessella subquad- rata zone), BuUhill, Cressage District, E. Shrop- shire, England. Cat. No. S 2124. x 1. 11, 13, Reuschella bilobata (Sowerby) 14. — ^Ventral internal mold (14) and wax impression taken therefrom, showing character of muscle field. ^ 15. — Dorsal internal mold, and replica. Cardinalia close to those of Harknessella. Ordovician (Acton Scott beds). Castle Hill, E. Shropshire, England. Cat. Nos. S2125 (figs. 13, 15) and S2126. x 1. Reuschella cf. R. bilobata (Sowerby) 12. — Dorsal exterior, showing coarse costells. Ordovician (Caradocian), Cardington, Salop, Eng- land. Cat. No. S 1065. x 1. Heterorthis clytie (Hall) 16, 17. — ^Ventral and dorsal views of exterior. Ordovician (Trenton), Bourbon Co., Ky. Cat. No. S1068. X 1.5. Figs. Heterorthis clytie (Hall) — Coni. 18. — Dorsal interior, showing cardinalia. x 1.5. 19. — Posterior view of specimen in fig. 20, showing small, convex chilidium covering carinate median lobe of cardinal process, x 2. 20. — Dorsal interior, showing cardinalia, small adductor field, and pallial marks; also cementation of brachio- phores to valve. In Heterorthis and its allies this cementation takes place along the dorsal surface of the brachiophores, but in Dalmanella, Levenea, etc., it is along their flat, inner face. Shown also in this figure is the submarginal elevation in the dorsal valve, and the corresponding depression in the ventral valve can be seen in fig. 21. x 1.5. 2 1 . — Ventral interior, showing semiflabeUate pattern formed by combined diductor and adjustor scars, x 1.5. Ordovician (Hermitage), Frankfort, Ky. Cat. Nos. S 1067 (figs. 19-21) and S 1074. Heterorthis cf. H. altemata (Sowerby) 25. — Ventral internal mold, showing elongate diductor and adjustor muscles. Ordovician (Caradocian), W. Corwan, England. Cat. No. S 1073. x 1.5. Rhipidomella sp. 22. — Dorsal interior turned slightly to the side showing cardinalia. Right brachiophore shows an elongate blade-like process extending from a median swelling. Such a process is missing in fig. 23. Locality unknown. Cat. No. S 1220. x 3. Rhipidomella vanuxemi (Hall) 23. — Dorsal interior, showing swollen cardinal process and adductor field. 27. — Ventral interior, showing postero-median position of adductor field, and large, semiflabeUate diductor-ad- justor area. Devonian (Hamilton), Canandaigua, N. Y. Cat. No. S 1286. X 1.5. Rhipidomella musculosa (Hall) 24. — Ventral interior, showing unusually expansive devel- opment of diductor-adjustor fields. Devonian (Oriskany), Cumberland, Md. Cat. No. S1193. xl. Rhipidomella penelope (Hall) 26. — Ventral interior of an old shell, showing puckered margins of muscle field and swollen, irregular inner surface. Devonian (Hamilton), Canandaigua Lake, N. Y. Cat. No. SI 229. x 1. 234 PLATE 20 235 PLATE 21 Figs. Aff. Parmorthis visbyensis (Lindstrom) 1, 6, 12. — Dorsal, ventral, and posterior views of a medium- sized individual. 8. — Dorsal interior, showing adductor field and trilobed cardinal process. The concavity of the dorsal valve has caused a bending postero-dorsally of the cardinal process in order to maintain the necessary leverage in opening the valves. This makes the myophore visi- ble on the exterior of the dorsal valves (see fig. 1 ) . 1 1 . — Dorsal view of a larger shell. 15. — Ventral internal mold, showing pallial trunks swing- ing into apposition as they extend forward. This is the usual condition in Parmorthis and is a significant difference between these shells and Ddmanella s. s. Silurian (Gotlandian), Gotland, Sweden. Cat. No. S895. x2. Parmorthis elegantula (Dalnian) 2. — Dorsal interior seen from rear, showing (on left brachiophore) an elongate process or crus (?). 3. — Posterior view of a dorsal interior, showing fulcral plates and brachiophores. Parmorthis is the only dal- manellid with corrugated dental sockets, teeth, and fossettes. 13. — Dorsal interior, showing elongate-ovate adductor field with its smaller anterior adductor scars. Fulcral plates visible, and inside surface of brachiophores strength- ened by adventitious material. Silurian (Gotlandian), Gotland, Sweden. Cat. Nos. S 886 (fig. 2) and S 892. x 2. 9, 14, 16. — Posterior, dorsal, and ventral views of exterior. In fig. 14, a concentration of smaller costella; is noticeable in the antero-median portion of the valve. Such a concentration of ribs is a feature of these shells. 1 0, 29. — Two views of a ventral interior, showing ponder- ous teeth and cavernous crural fossettes. Silurian (Gotlandian), Klintehamn, Gotland, Sweden. Cat. Nos. S 890 (figs. 9, 14, 16), and S918. x2. Parmorthis crassicostata Schuchert and Cooper, n. sp. 4, 5. — A rather heavily costellate species. Silurian (Niagaran), Martin's Mills, W. Tenn. Holotype. Cat. No. S 91 3. x 2. Parmorthis waldronensis (Foerste) 7. — Ventral exterior of the common American shell that has so long masqueraded under the name of Dal- manella elegantula (Dalman). Silurian (Niagaran), Jeflferson Co., Ind. Cat. No. S 899. X 2. Isorthis perelegans (Hall) 17, 18. — Dorsal and lateral views of exterior, the latter showing biconvexity of valves. 26. — Ventral interior, showing Schizofhoria-Y\ks muscle- scars. 28. — Dorsal interior, showing cardinalia. Devonian (Helderberg), ClarksviUe, N. Y. Cat No. S 1013. X 1. Figs. Isorthis rockhousensis (Dunbar) 19. — Dorsal exterior. Low. Devonian (Rockhouse), Rockhouse, Tenn. Cat. No. 9750 Y. P. M. x 2. Isorthis szajnochai Kozlowski 20. — Posterior portion of a dorsal valve, showing divergent diductor tracks and median adductor fold. This is very similar to the condition in Schizofhoria, see pi. 23, fig. 20. x2. 25, 30, 31, 33.— Ventral, dorsal, posterior, and lateral views of a specimen representing the type species of the genus. X 1.5. 27. — Dorsal interior, showing cardinalia and adductor field. Fulcral plates visible just outside brachiophore plates. Cf. corresponding view of Schizofhoria, pi. 23, fig. 21. X 1.5. Up. Silurian (Borszczow), Wierzchniakowce, Po- land. Cat. No. S2127. Isorthis arcuaria (Hall and Clarke) 21. — Dorsal interior, showing elliptical adductor field and prominent ridges of adventitious shell extended around it. 23. — Ventral interior, showing prominent divergent diduc- tor tracks and pallial sinuses extending from them. Diductor tracks separated by a fold of shell which bears adductor impressions. Silurian (Niagaran), Decatur Co., W. Tenn. Cat. No. S 1030. x2. 32. — Lateral view, showing strongly lenticular profile of sheU. Silurian (Niagaran), Clifton, W. Tenn. Cat. No. S1022. x2. Isorthis tetragonum (Roemer) 22. — Dorsal internal mold, to show pallial markings. Devonian (Eifelian), Gerolstein, Germany. Cat. No. S994. X 1.5. Isorthis canalicula (Schnur) 24. — Ventral interior, with musculature and pallial mark- ings characteristic of the genus. Devonian, Eifel, Germany. Mus. Comp. Zool., Harvard Coll. x 2. Idiorthis matura McLearn 34. — Ventral internal mold, showing musculature. Cf. Wattsella, pi. 22. x 3. 3 5, 38. — Dorsal and ventral views of exterior, x 1.5. 36, 37. — Dorsal internal mold (37), x 1.5, and replica of it showing strong median ridge and adductor field. Cardinal process trilobed. Silurian (McAdam), Arisaig, Nova Scotia. Cat. Nos. 426 (figs. 34, 36, 37) and 426c Y. P. M. 236 PLATE 21 PLATE 22 Mendacella uberis (Billings) 1. — Dorsal interior, showing cardinalia. 2. — Ventral interior, showing musculature. 3. — Dorsal internal mold, showing elongate Dalmanella- like muscle field. Ordovician (Richmond, Ellis Bay), E. of Junction Cliff, Anticosti. Cat. No. 10415 Y. P. M. x 2. 8. — ^Ventral exterior. Silurian (Anticostian), Anticosti. Cat. No. S 1049. x2. Figs. Wattsella edgewoodensis (Savage) — Cont. 13. — Ventral interior, showing flaring dental plates and heart-shaped muscle field. 22. — Dorsal interior of a silicified shell, showing remark- ably long brachiophores (crura?). 27, 28. — Dorsal and ventral exteriors, to show ornamenta- tion. Silurian (Edgewood), Edgewood, Pike Co., Mis- souri. Cat. Nos. S1075 (figs. 13, 27, 28), and S 1083 (fig. 22). x2. Mendacella muUochiensis (Davidson) 4, 5. — Ventral internal mold (fig. 5), x 1.5, showing mus- culature, and wax impression taken from it, x 2. Silurian (lower Llandovery, Mulloch Hill), Mulloch Hill, Girvan, Ayrshire, Scotland. Brit. Mus. Nat. Hist. (fig. 5); Cat. No. S657 (fig. 4). Fascicostella sedgwicki (D'Archiac and Verneuil) 6. — Dorsal internal mold, showing adductor field like that of Parmortkis, see pi. 21, fig. 13. 7. — ^Ventral exterior, showing bundled costellae. 10. — Replica of interior. 11. — Dorsal interior, showing cardinalia. 16. — Reverse side of fig. 6, showing ventral interior mold. Of significance is the septum extending forward from the adductor track separating two pallial trunks. Devonian, Vise, Belgium. Mus. Comp. Zool., Har- vard Coll. X 2. Fascicostella gervillei (Def ranee) 12, 15. — Ventral and dorsal exteriors, showing fascicostel- late exterior. Devonian (Ffj), Koniepruss, Bohemia. Cat. No. S925. xl.5. Fascicostella sp. 3 1 .• — Large specimen, showing dorsal exterior. Devonian, Vise, Belgium. Mus. Comp. Zool., Har- vard Coll. X 2. Wattsella edgewoodensis (Savage) 9, 23, 25, 29. — Several views to show dorsal interior. Fig. 23 shows left brachiophore plate broken away to ob- tain a clearer view of the right plate. Fig. 29 is an enlargement of fig. 23, x 4. Fig. 2 5 shows socket and fulcral plates. Silurian (Edgewood), Edgewood, Pike Co., Mis- souri. Cat. Nos. S 1078 (figs. 23, 25, 29), S 1079 (fig. 9). All except fig. 29 x 2. Wattsella testudinaria (Dalman) 14, 19, 24. — Posterior, ventral, and dorsal exteriors, x 1.5. 21. — Dorsal interior, showing subparallel brachiophore plates, meeting the median ridge, x 2. Silurian, Borenshult, Sweden. Riks. Pal. Zool. Avd. Stockholm. 17. Wattsella wattsi Bancroft -Ventral exterior, a replica of the internal mold. 18, 20. — Dorsal internal mold, and wax replica of it, show- ing subparallel brachiophore plates meeting median ridge. Cf. figs. 21, 9, and 23. 26. — Ventral internal mold, showing subcordate muscle area. Ordovician (Marshbrookian, W. wattsi zone), Woolston, Horderley District, E. Shropshire, Eng- land. Cat. Nos. S2128 (fig. 17), S2129 (figs. 18, 20), and S2130 (fig. 26). x 1.5. Horderleyella sp. 30. — Dorsal interior, showing fulcral plates outside brach- iophore plates. Ordovician (Marshbrookian, Wattsella zvattsi zone), Marshbrook, N. of Horderley, E. Shropshire, Eng- land, x 1.5. Horderleyella plicata Bancroft 32. — Ventral internal mold. 33. — Wax replica of dorsal external mold. 34,35. — Dorsal internal mold (fig. 35) and wax replica of it showing cardinalia. Brachiophore plates con- vergent and supported by low lateral septa. Ordovician (basal Caradocian, H. flicata zone), Horderley, E. Shropshire, England. Cat. Nos. S 21 32-21 34. x2. 238 PLATE 22 239 PLATE 23 Figs. Pionodema subsequata (Conrad) 1, 5. — Ventral and dorsal views of the holotype. Ordovician (Black River), Mineral Point, Wis. Cat. No. 694/3 Amer. Mus. Nat. Hist, x 2. 3. — Posterior view of exterior, x 1.5. 7. — Dorsal interior, showing flaring brachiophore plates and thickenings extending forward from them. Cf. figs. 2, 4. X 3. Ordovician (Black River), near Lanesboro, Minn. Cat. No. S 1 364. Pionodema redux (Barrande) 2, 4. — Dorsal internal mold (fig. 4) and wax impression of it. Cf. fig. 7. Brachiophore plates separate, but low ridges extend from them and converge toward the median ridge. Ordovician (Dd), Mt. Drabow, Bohemia. Mus. Comp. Zool., Harvard Coll. x 1.5. Pionodema minnesotensis Cooper 6, 10, 14. — Dorsal, ventral, and posterior views of the holotype. Notice swollen ribs. Ordovician (Black River), Minneapolis, Minn. Cat. No. S 1356. x 1.5. Pionodema cf. circularis (Winchell) 8. — Dorsal interior, showing well the flaring brachiophore plates. Cf. fig. 21. Ordovician (Glade Is. of Safford (?))> Lebanon, Tenn. Cat. No. S 1360. x 2. Pionodema cf. conradi (Winchell) 9. — Ventral interior, showing musculature and apical plate. Ordovician (Black River), St. Paul, Minn. Cat. No. 1359 S. X 1.5. Pionodema uniplicata Cooper 12. — External view of ventral exterior, showing deep sulcus. Ordovician (Black River), Minneapolis, Minn. Cat. No. S 1361. x 1.5. Figs. Pionodema circularis (Winchell) 13. — Incomplete dorsal valve turned to the side in order to show the fulcral plate which defined the socket. Cf. fig. 18. Ordovician (Black River), 2.5 miles S. of High Bridge, Ky. Cat. No. S 1348. x 3. Isorthis ? tuUiensis (Vanuxem) 15, 19. — Ventral internal mold, and reverse side showing dorsal interior. Pallial marks distinctly spread apart, showing pattern clearly. Devonian (Tully), Skaneateles, N. Y. Cat. No. S1412. X 1.5. Schizophoria provulvaria Maurer 1 1. — Dorsal internal mold, showing pallial trunks. Devonian, Seifen, Germany. Cat. No. S 1401. X 1. Schizophoria vulvaria (Schlotheim) 16.— Ventral internal mold, showing exceedingly long di- ductor impressions separated by a long septum. 1 7. — Dorsal interior, showing pallial markings. Low. Devonian (Cobienzian), Laubach, Germany. Mus. Comp. Zool., Harvard Coll. x 1. Schizophoria iowensis (Hall) 18. — Same specimen as in fig. 21, turned to side to show brachiophore supports and fulcral plates. 20. — Ventral interior, showing musculature and median fold which carries adductor impressions. 21. — Dorsal interior, showing flaring brachiophore plates, fulcral plates, and musculature. Up. Devonian, Rockford, Iowa. Cat. No. S 1441. x2. Schizophoria aff. striatula (Schlotheim) 22-25. — Posterior, dorsal, lateral, and ventral views of ex- terior. Compare fig. 24 with fig. 19 of pi. 11. Devonian (Craghead Creek), E. of Toledo, Calla- way Co., Missouri. Cat. No. S 1447. x 1. 240 PLATE 23 PLATE 24 FiQS. Enteletes lamarcki Fischer 1, 5, 8, 9. — Dorsal, ventral, posterior, and lateral views of exterior, showing fine Schizofhoria ornamentation superimposed over plications. Up. Carboniferous, Mjatschkowa, Russia. Cat. No. S1495. xl. Enteletes dumblei Girty 2, 6, 10. — Ventral interior, showing three prominent septa, X 1.5. Figs. 2 and 6 seen from the side. 3. — Dorsal interior, showing flaring brachiophore supports and curved brachiophores, x 2. 7. — Small interarea of ventral valve, x 1.5. Pennsylvanian (Word) , Glass Mts., W. Texas. Fig. 2 is Cat. No. S 2224, the others are specimens in the Univ. of Texas collections. Enteletes cf. dumblei Girty 25. — Dorsal valve seen from the side, showing tusklike brachiophores and a fulcral plate in place. Permian (Wolfcamp), N. E. of Wolfcamp, Glass Mts., W. Texas. Cat. No. S 1 501. x 3. Parenteletes cooperi King 11, 13, 18. — Anterior, lateral, and posterior views of exte- rior, showing strong ventral fold and ornamentation of an adult. 14, 16. — Ventral and dorsal views of exterior of a young specimen. Pennsylvanian (Gaptank), near Gaptank, Glass Mts., W. Texas. Univ. Texas colls. Figs. Parenteletes cooperi King — Cont. 2 1 , 26. — Ventral interior of cotypes, showing septa and camera under anterior end of median septum. Permian (Uddenites zone), Hill 4752, Glass Mts., W. Texas. Cat. No. S 1494. x 2. Enteletella nikschitschi Likharev 4. — Lateral view of a complete individual, after Likharev. Low. Permian, northern Caucasia. Orthotichia afJ. morganiana (Derby) 12. — Extreme posterior portion of dorsal valve, showing cardinal process and a complete brachiophore. x 1.5. 15. — Posterior portion of a ventral valve, showing elevated median septum, x 2. 22. — Ventral view, with shell worn off, showing three septa, X 1. 23,24,27. — Posterior, ventral, and dorsal exterior views. X 1. Pennsylvanian, near Saddle Mt., S. E. of Winkel- man, Ariz., near Gila River. Cat. No. S 1493. Enteletina latesinuata (Waagen) 17. — Ventral interior, showing subparallel dental plates and simple median septum. Cf. fig. 10. 19. — Ventral exterior, showing prominent fold. 20. — Dorsal interior. Shape of brachiophore process normal, but lateral curvature undoubtedly incorrect. Middle Productus Is., Musakheyl, India. All after Waagen 1887. 242 PLATE 24 PLATE 25 Figs. Rhynchocamara plicata Schuchert and Cooper 1,2,7. — Ventral, posterior, and dorsal views of exterior, showing subrhynchonelloid form. 17. — Posterior portion of ventral interior, showing spon- dylium. 18. — Dorsal interior, showing small chamber beneath beak of dorsal valve, formed by union of crural plates with strong median septum. This is a rhynchonelloid fea- ture and places this form and its congeners with the Rhynchonellacea. Ordovician (Stones River, Central Is.), near Mur- freesboro, Tenn. Cotypes. Cat. No. S 2035. x 1.5. Camerella hemiplicata (Hall) 3, 4, 6, 10. — Posterior, ventral, lateral, and dorsal views of a somewhat crushed specimen, showing strongly con- vex dorsal valve and relatively smaller ventral valve. X 1. 43. — ^Thin slice of interior, showing spondylium, cruralium, and crural als. x ca. 4. Ordovician (Trenton), Lowville, N. Y. Cat. Nos. S1593 and SI 597 (fig. 43). 9, 1 1 . — Ventral and anterior views, showing sulcus, x 1 . 16. — Ventral view of a young shell, showing median sep- tum. X 1. 24. — Section of interior of a rather young form, showing spondylium and cruralium. A faint crural alar exten- sion can be seen, x 4. Ordovician (Trenton), Watertown, N. Y. Cat. Nos. S1588 (figs. 9, 16), S 1600 (fig. 11), and S1603 (fig. 24). Camerella volborthi Billings 5. — Ventral interior, with prominent spondylium. Ordovician (Black River), Paquette Rapids, Ottawa River, Quebec. Cat. No. 1594S, x 1.5. 20, 21. — ^Ventral and dorsal views of a young specimen. 27, 30. — Lateral and dorsal views of a larger individual. These specimens were among Billings' types. Same horizon and locality as above. Cat. Nos. 1 148b, 1 148a, Nat. Mus. Canada, x 2. 22, 28, 29. — Lateral, dorsal, and posterior views of the same individual, showing exterior. Cf. C. hemifli- cata, especially fig. 9. Same horizon and locality as above. Nat. Mus. Canada, x 2. Figs. Camerella scofieldi (Winchell and Schuchert) 8, 12, 13. — Ventral, posterior, and dorsal views of exterior. Ordovician (Galena), near Cannon Falls, Minn. Cat. No. 8 1 592. x 1. Anastrophia vernemli (Hall) 14, 15, 19. — Lateral, posterior, and dorsal views of exterior, showing implantation of ribs in fold, x 1. Devonian (Helderberg), near Clarksville, N. Y. Cat. No. S 1619. 33, 36. — Dorsal interior, showing partial side and dorsad views. Fig. 36 shows musculature well, x 1.5. 34, 35. — Spondylium in partial side and direct or dorsad view. Septum supports spondylium for nearly its whole length, x 1.5. 38, 39. — Different views of the same specimen, showing to perfection crural ala:, supporting plates, and muscula- ture. X 1.5. Devonian (Helderberg), Clarksville, N. Y. Cat. No. S 1640. 41. — Interior of both valves in conjunction, x 1.5. 42. — Same view of another specimen, in which the sides of the spondylium have grown over, restricting the area of muscular attachment to a narrow opening. xl.5. Devonian (Helderberg), near Clarksville, N. Y. Cat. Nos. S 1632 (fig. 41) and S 1631. Anastrophia deflexa (Sowerby) 40. — Thin section, showing arrangement of internal plates. There are lateral alas on the spondylium as well as in the dorsal valve. Silurian (Wenlock), Dudley, England. Cat. No. S1623. xca. 4. Parastrophinella ops (Billings) 23,25,26. — Ventral, posterior, and dorsal views of ex- terior. Silurian (Chicotte),Anticosti. Cat. No. S 1608. x 1. Liocoelia proxima (Barrande) 31, 32, 37. — Ventral, dorsal, and lateral views of exterior of this smooth rhynchonellid. See t. fig. 36. Silurian (Etage Eej), Kolednik, Bohemia. Cat. No. S 1696. X 1. 244 PLATE 25 245 PLATE 26 Figs. Barrandella linguifera (Sowerby) 1,3,5,6. — Dorsal, ventral, posterior, and lateral views of a complete individual. Silurian (Gotlandian), Gotland, Sweden. Cat. No. 8 1680. xl. Figs. Gypidula multicostata Dunbar 18. — Ventral interior, showing spondylium greatly thick- ened by growth of adventitious shell. Devonian (Birdsong), Birdsong Creek, Camden Road, Tenn. Cat. No. S 1 764. x 1. Barrandella sp. 2, 7. — Dorsal and posterior views of an internal mold, showing septa. Silurian (Brassfield), Eaton, Ohio. Cat. No. S1674. xl. Gypidula acutilobata procerula (Barrande) 22, 23, 27. — Lateral, ventral, and dorsal views, showing exterior. Devonian (Ff,), Koniepruss, Bohemia. Cat. No. S1803. xl." Pentamerella cf. arata (Conrad) 4. — Dorsal interior, showing long plates of dorsal valve which carry brachial processes. 17. — Ventral interior, showing spondylium. Septum very short. Devonian (Onondaga), Falls of the Ohio, Louis- ville, Ky. Cat. No. S 1731. x 1.5. 13. — Ventral interior, showing short spondylium and two pallial trunks. Devonian, Moreland, Ky. Cat. No. S 1726. x 1. Pentamerella arata (Conrad) 20. — Dorsal view of internal mold, showing septa. Devonian (Schoharie grit), Schoharie, N. Y. Cat. No. S 1735. xl. Pentamerella fultonensis Branson 14-16. — Ventral, dorsal, and lateral views of exterior. Devonian (Callaway Is.), Auxvasse church, Calla- way Co., Missouri. Cat. No. S 1724. x 1. Gypidula dudleyensis Schuchert 24, 28, 34, 37. — Dorsal, ventral, lateral, and posterior views, showing external form and ornamentation. Silurian (Wenlock), Dudley, England. Cat. No. S1789. xl. Gypidula coeymanensis Schuchert 26, 29. — Posterior and dorsal views of exterior, x 1 . 31,35. — Interior, showing spondylium, median septum, and brachial processes in place, x 1.5. 32. — Dorsal interior, showing plates, x 1.5. 40. — Both valves in conjunction. The spondylium and its relation to the dorsal plates are clear, and the nature of the septum is visible, x 1.5. Devonian (Helderberg, Coeymans), Clarksville, Albany Co., N. Y. Cat. Nos. S 1697 (figs. 31,35), S 1792 (figs. 26, 29), and S 1797 (figs. 32, 40). 38. — Posterior portion of ventral valve, showing spondylium and median septum. Devonian (Coeymans), Indian Ladder, N. Y. Cat. No. S 1782. X 1. Pentamerella cf. pavilionensis (Hall) 19. — Dorsal interior, showing important lamella:. Devonian, Alpine, Mich. Cat. No. S 1736. x 1. Clorinda tumidula (Billings) 8, 1 2. — Dorsal and posterior views, showing characteristic exterior. 9, 10, 11,21 . — Ventral, anterior, dorsal, and lateral views of exterior. Silurian (Gun River), Anticosti. Cat. Nos. S 1685, S 1693 (figs. 8, 12). X 1. Gypidula romingeri Hall and Clarke 36, 39. — ^Ventral and dorsal interiors, showing internal plates. Brachial processes broken from dorsal valve. Devonian, Grand Lake, Presque Isle Co., Mich. Cat. No. S 1698. x 1.5. Sieberella sieberi (V. Buch) 25, 30, 33. — Dorsal, lateral, and ventral views of two speci- mens, showing contour and profile of valves. Devonian (Ffj), Koniepruss, Bohemia. Cat. No. S1721. X 1. 246 PLATE 26 ^ ^^ A ^^ 9 1 _ >5 16 17 PLATE 27 Figs. Virgiana barrandei (Billings) 1,6,7, 1 6. — Lateral, ventral, dorsal, and posterior views of a large specimen. Silurian (Becscie River), Becscie River, Anticosti. Cat. No. S 1668. x 1. Platymerella aff. manniensis Foerste 2, 3, S, 11. — Ventral, dorsal, posterior, and lateral views of exterior. Silurian (Alexandrian, Sexton Creek), Belleview, Calhoun Co., m. Cat. No. SI 84 5. i 1. Pentamerus cf. oblongus Sowerby 4. — Internal septa in a specimen broken through the middle. 8, 12, 18. — Lateral, dorsal, and ventral views of exterior. 10. — Dorsal exterior of a young specimen. Silurian (Clinton), Rochester, N. Y. Cat. Nos. S1805,S 1833 (fig. 10), and 8 1851 (fig. 4). x 1. Figs. Pentamerus aff. oblongus Sowerby 17, 19. — Posterior and dorsal views of an internal mold, showing septa. Silurian (Niagaran), near Richmond, Ind. Cat. No. S 1808. xO.5. Pentamerus cf. cylindricus Hall and Whitfield 9, 1 5. — Ventral and dorsal views of an internal mold, show- ing impressions of septa. Silurian (Niagaran), Yellow Springs, Ohio. Cat. No. SI 826. xO.5. Pentameroides subrectus (Hall and Clarke) 13, 14. — Internal molds of dorsal and ventral interiors of the same specimen. Note single septum in dorsal valve, Silurian (Maquoketa), Jones Co., Iowa. Cat. No. S1848. x 1. Holorhynchus giganteus Kiaer 20. — Dorsal view of exterior. This genus is interesting for its possession of a free spondylium. Silurian (zone 5b), Sandviken, Boerum, Norway. Cat. No. S 2135. x 2. 248 PLATE 27 249 PLATE 28 Figs. Conchidium biloculare Linnaeus 1,5, 6. — Dorsal, ventral, and lateral exteriors of a small individual. 3. — Posterior view. 8, 11, 12, 18. — Dorsal, ventral, lateral, and posterior views of a large specimen. Posterior view shows a remnant of the deltidium in place. Ventral view shows a long crack, marking position of median septum. 9. — Dorsal tiew, showing nearly complete deltidium in place. Silurian (Gotlandian), Gotland, Sweden. Cat. No. 51859. X 1. 2. — Internal mold seen from dorsal side. 23. — Specimen broken in plane of median septum. Long brachial process visible on the right. Silurian (Gotlandian), Hejde, Gotland, Sweden. Cat. No. S 1867. x 1. 4. — Small individual in dorsal view, to show uncovered delthyrium. 26. — ^Apical portion of dorsal valve with internal plates. Silurian, Klintehamn, Gotland, Sweden. Cat. No. S1872. xl. 1 0. — Interior of apical portion of dorsal valve. 20. — Spondylium of ventral valve. Silurian, Gotland, Sweden. Cat. No. 7839 Y. P. M. xl.5. Conchidium tenuistriatum Walmstedt 17. — Exterior of large specimen, in dorsal view. Silurian (Gotlandian), Gotland, Sweden. Cat. No. 51860. xO.S. Lissocoelina pergibbosa (Hall and Whitfield) 7, 14. — Dorsal and lateral views of an internal mold, the former showing dorsal plates and the latter the unusual gibbosity. Silurian (Niagaran), Cedarville, Ohio. Cat. No. S1834. X 1. Lissocoelina ? maquoketa (Hall and Clarke) 13, IS, 16. — Ventral, dorsal, and posterior views of this gibbous pentamerid, showing internal septa. This form must be excluded from Harfidium because of the short median septum. Silurian (Niagaran), Maquoketa, Iowa. Cat. No. S1840. Figs. Lissocoelina ? maquoketa (Hall and Clarke) — Cont. 2 1 . — Lateral view of an unusually gibbous form. Silurian (Niagaran), Maquoketa, Iowa. Cat. No. S 1836. Pentameroides subrectus (Hall and Clarke) 19. — Posterior view of an internal mold, showing septa. Note single septum in dorsal valve. Silurian (Clinton), Stony Creek, Ontario. Cat. No. S 1842. X 1. Pentameroides, n. sp. 22. — View of dorsal exterior, showing single median septum. Silurian ("Stricklandia-Pentamerus" zone of the Clemville), Black Point, New Brunswick. Cat. No. S1841. X 1. Brooksina alaskensis Kirk 24. — Internal septa of a specimen broken through the middle. Silurian, S. E. shore Kosciusko Island, Davidson Inlet, S. E. Alaska. Cat. No. S 2223. x 1. Stricklandia gaspeensis Billings 25. — Posterior of dorsal valve, showing brachial processes. This structure is more nearly rhynchonelloid than pentameroid. Davidson figured the brachial process affixed to the extremities of the lateral plates bound- ing the notothyrium. Silurian (La Vieille), La Vieille Cove, E. of Gascons, Quebec. Cat. No. S 1899. x 2. 27. — Posterior of ventral valve, showing cardinal area and fractured spondylium. Silurian (Clinton), S. W. corner of Barachois at Port Daniel, Quebec. Cat. No. S 1887. x 1. Stricklandia lirata (Sowerby) 28. — Dorsal exterior. Silurian, Gotland, Sweden. Cat. No. S 1883. x 1. 250 PLATE 28 251 PLATE 29 Figs. Pionodema cf. conradi (N. H. Winchell) 1 . — ^Thin section, showing endopunctas. It is to this type of internal shell perforation that we restrict the term endopunctje. The section figured is a portion of the shell shown on pi. 23, fig. 9. Ordovician (Black River), St. Paul, Minn. Cat. No. S 1367. xca. 12. Aulacophoria keyserlingiana (De Koninck) 2,5,10. — Ventral, dorsal, and lateral views of complete individual. Anterior like that of Schizofhoria, but prominent fold and sulcus a divergence toward Enteletes. Carboniferous, Welton, England. Cat. No. S 1924. xl. Hesperorthis laurentina (Billings) 3. — ^Thin section, showing deltidium in place. Cf. Silurian, Anticosti. Cat. No. S 2221. x 4. fig. 8. Conchidium biloculare Linnaeus 4.- -Cross section of shell, showing deltidium in place. In an anterior direction from the position of this sec- tion (which is near the beaks) the deltidium becomes a flatly concave plate. In this figure the sides of the deltidium have been broken from the walls of the del- thyrial cavity. In Conchidium the deltidium was evidently a pedicle sheath open at the posterior. A similar structure appears in H arfidium. See t. fig. 25. Silurian (Gotlandian), KJintehamn, Gotland, Sweden. Cat. No. S 1872. x 3. Stricklandia davidsoni Billings —Section showing duplex spondylium and cardinalia. Notice also lateral elevated plates on cardinal region of dorsal valve. Silurian (Jupiter River), East Jupiter Cliff, Anti- costi. Cat. No. S 1898. x5. Parastrophinella reversa (Billings) —Section showing spondylium and cardinalia. Note prominent dorsal plates and folded or duplex charac- ter of alae in dorsal valve. Silurian, base of White Cliff, Cape Eagle, Anti- costi. Cat. No. S 1607. x4.5. Fics. Vellamo diversa (Shaler) 8. — Section showing spondylium. Note thickening in umbo-lateral chambers (cf. fig. 14). Note deltidium built against delthyrial edge and strengthened by shell deposit on sides of delthyrial cavity (T = trace of tooth showing as an irregularly oval spot). x4.5. 14. — Section showing umbonal chambers completely filled by adventitious shell. Note thickened deltidium and callus spread on inside of delthyrial cavity, x 3. Ordovician (Ellis Bay), W. side Ellis Bay, Anticosti. Cat. Nos. S 2222a (fig. 8) and S 2222b. Liocoelia proxima (Barrande) -Section showing spondylium and cardinalia which strongly suggest Camerofhoria and the rhynchonellids. See t. fig. 36. Silurian (Etage Eej), Kolednik, Bohemia. Cat. No. SI 696a. x4.5. Pentamerus aflf. oblongus Sowerby 1 1. — Section showing duplex spondylium and characteristic dorsal lamella. Silurian (Jupiter River), Belle River, Anticosti. Cat. No. S 1819. x ca. 4. Billingsella lindstromi (Linnarsson) 12. — ^Tangential section showing fibrous structure. Frag- ment of shell taken from specimen figured on pi. 1, fig. 27. Mid. Cambrian (Paradoxides zone), Westrogothia, Lovened, Sweden. Cat. No. S 16a. Billingsella coloradoensis (Shumard) 1 3. — ^This tangential section was figured by Walcott (Camb. Brach., p. 299, fig. 5) but the greater magnification used by him did not reveal the true structure. There are no punctse in Billingsella and the section shows the fibers. See fig. 12. Up. Cambrian, Morgan Creek, Burnet Co., Texas. U. S. Nat. Mus. 252 PLATE 29 253 INDEX The majority of the species listed in this index are not discussed in the memoir, but merely cited in the species lists to which the page number refers. Pages on which species or genera are discussed are in bold-face type. All species are listed under the specific name ; in addition, species whose generic reference has been changed are listed under the genus to which they were formerly referred. Synonyms are in italics. Asterisks signify illustrations. abnorinis (Hucnella), 159, 160* abscissa (Panderina), 81 actonia: (Nicolella), 77, 78 aculeatus (Productorthis), 82 acuminata (Platystrophia), 65 acutilirata (Platystrophia), 65, 66 prolongata (Platjstrophia), 65 acutilobus (Bilobites), 130 acutiplicata Reed (Porambonites), 102 acutiplicata Waagcn (Enteletina), 148 acutolobatus (Gypidula), 174 procerula (Gypidula), 174 acutum (Cymbidium), 183, 184* Adductor muscles, 6, 30, 31, 33,* 39, 40* Adjuster muscles, 6, 30, 31, 33* adscendens Davidson (V'ellamo), 114 adscendens Pander (Clitambonites), 26, 113 iqualis (Hemipronites), 116 xqualis (Orthis), 76 «equirostris (Porambonites), 102, 104 aKjuivalvis Hall (Plectorthis), 58 latior (Plectorthis), 58 pcrvagata (Plectorthis), 58 aquivdvis Shaler, see Mendacella uberis agilera (Nicolella), 77 alaskensis (Brooksina), 183 alberta (Nisusia), 45 alsa (Rhipidomella), 133 altaica (Rhipidomella), 134 alternata (Heterorthis), 137 altirostris (Rhipidomella), 133 altissimus (Clitambonites), 114 altus (Clitambonites), 113 altus (Hemipronites), 116 altus (Porambonites), 102 alveata (Hebertella), 60 richmondensis (Hebertella), 60 ambigua ("Camerella"), 168 americana (Vcllamo), 114 amii {'. Nisusia), 45 amocna (Plat}strophia), 65 longicardinalis (Platj'strophia), 65 Anastrophia, 162, 164, 167, 169 ancillans (Clorinda), 172 andii (Enteletes), 146 angulata (Gypidula), 174 Ancusticardima, 69, 84 Angusticardiniina:, 74, 84 annamitica (Dinorlhis), 94 annieana (Platystrophia), 65 anomala Schlotheim (Clinambon), 115 anomala Walcott (Wimanella), 50 anthonensis (Skcnidioides), 72, 73 anticosticnsis Billings (Stricklandia), 187 anticosticnsis Shaler (Plaesiomys), 94 anticosticnsis Twenhofel (Virgiana), 185 antiquata (Swantonia), 159 aperturata (Platystrophia), 66 Apical plates, 6, 22 apicalis Billings (Hesperorthis), 86 apicalis Whitfield (Pohtoechia), 110 Apomatella, 24, 107, 117 appalachia (Billingsella), 49 arachne (? Syntrophia), 188 arata (Pentamerella), 176 Arch.€;orthis, 21, 26, 74, 80 arcticum (Conchidium), 181 arcuaria (Isorthis), 150 areola (Mystrophora), 22, 131, 132* arethusa (? Syntrophia), 188 arisaigensis (Mendacella), 127 arkansana (Rhipidomella), 133 armanda (Finkelnburgia), 5 5 armata (Clorinda), 171 Articulation, 6, 24 assimilis (Rhipidomella), 133 asteroidca (Nicolella), 77 atava (Orusia), 54 atavoides (Dinorthis), 94 Athyris (?) tumidula, see Clorinda Atr}-pa dorsata, see Platystrophia galeata, see Gypidula lens, see Stricklandia nucella, see Lycophoria attenuata (Platystrophia), 65 AULACELLA, 19, 119, 122, 136 AULACOPHORIA, 141, 145, 147 aurora (Eostrophoniena), 8 5 AUSTINELLA, 94, 99 avelinei (Dalmanella), 120 avita (Idiorthis), 128 Baker, F. C, 1 1 balclatchiensis (Glyptorthis), 90 balclatchiensis (Mctacamerella), 170 Bancroft, B. B., 96, 138 Barrande, J., 177 barrandei Billings (Virgiana), 185, 186* barrandei Schuchcrt and Cooper (Heterorthis), 138 Barrandei-la, 162, 163, 165, 166, 171, 173 Barrandina, see Clorinda, 172 basalis (Parmorthis), 129 258 INDEX bassleri (D.ilmanella), 120 Bather, F. A., 1 3 Bathycoelia, see Pionodema battis (? Huenella), 160 baueri (Porambonites), 102, 103 bavarica (? Archasorthis) , 81 beaumonti (Schizophoria), 143 becsciensis (Stricklandia), 187 Beecher, C. E., 16, 130, 131, 167 Belanski, C. H., 175 bella Fenton (Rhynchocamara), 189 bella Schuchert and Cooper (Planidorsa), 101 bellarugosa (Glyptorthis), 90 bellula (Pionodema), 14-1 biforata (Platystrophia), 64 fissicostata (Platystrophia), 66 lynx reversata (Platystrophia), 65 sardoa (? Platystrophia), 66 BiLLiNGSELLA, 22, 27, 28, 32, 37, 40, 41, 48, 76 alberta, see Nisusia billingsi, see Protorthis bivia, see Nisusia dice, see Deltatreta exporrecta, see Oligomys festinata, see Nisusia harlanensis, see Syntrophioides hlcksi, see Oligomys latourensis, see Protorthis orientalis, see Nisusia quacoensis, see Protorthis romingeri, see Bohemiella rugicostata, see Oligomys saffordi, see Syntrophioides transversa, see Nisusia Billingsellids, 22, 27, 32, 34, 35, 37, 39, 41, 44, 48, 107 billingsi Hartt (Protorthis), 46 billingsi Schuchert and Cooper (Skenidioides), 72 billingsi Walcott (Huenella), 159 billingsiana (Stricklandia), 187 bilobata (ReuscheUa), 139 BiLOBlTES, 119, 130 Bilobitids, 119, 130 bilobus (Bilobites), 130 biloculare (Conchidium), 163,* 181, 182 bivia (? Nisusia), 45 Bohemiella, 50, 52 Booker, F. W., 163, 172, 173, 176, 181 borealis Eichwald (Pentamerus), 178 borealis Walcott (Wimanella), 50, 51 borussica (Branconia), 168 bouchardi (Ptychopleurella), 92 Brachidia, 6 Brachiophore processes, 6, 37 supports, 6, 37 Brachiophores, 6, 37 brachynota (Platystrophia), 65 Branconia, 167, 168 brevirostris (Anastrophia), 170 brevis Billings (Stricklandia), 187 brevis Pander (Hemipronites), 116 brevis Pander (Productorthis), 82 brevis Willard (Multicostella), 98 breviuscula (Stricklandia), 187 broggeri Bancroft (Dalmanella), 120 broggeri Lamansky (Porambonites), 102 Brooksina, 163, 165, 166, 178, 183 bubo (Clorinda), 172 Buckman, S. S., 14, 16, 17,20 burgessensis (Nisusia), 45 burlingtonensis (Rhipidomella), 133 Butts, C, 109 buttsi (Orthis), 75,86 caducus (Gypidula), 174 callactis (Orthis), 7, 37, 75, 76 calligramma (Orthis), 37, 76 subplicata (Schizoramma), 88 Caiman, W.T., 12 Camarium, 6 camerata (Platystrophia), 65 Camerella, 162, 164, 166, 167 bella, see Rhynchocamara ottawaensis, see Orthorhynchula varians, see Rhynchocamara Camerellids, 161, 162, 163, 164, 165, 166, 167, 171 campbelli (Syntrophina), 155 canadensis Billings (Stricklandia), 187 canadensis Clarke (Orthostrophia), 71 canalicula (Isorthis), 150 canaliculata (Levenea), 124 canalis (Resserella), 126 Cafellinia, see Capelliniella Capelliniella, 163, 165, 166, 178, 179 carbonaria (Rhipidomella), 133 Cardinal angles, 6 area, 6 process, 6, 34 Cardinalia, 34, 164 carinata (Cariniferella), 122 epsilon (Cariniferella), 122 carinatus (Tropidoleptus), 152 Cariniferella, 30, 39, 119, 122 carletona (Pionorthis), 95 carleyi (Retrorsirostra), 95 insolens (Retrorsirostra), 95 carrickensis (Estlandia), 115 castellana (Stricklandia), 187 Cella, 6, 30 celsa (Schizoramma), 87 chama (Platystrophia), 66 chapmani (Stricklandia), 187 chekiangensis (Orthotichia), 144 Chilidial plates, 6, 24 Chilidium, 7, 23 choristites {Enteletes), see Enteletes lamarcki chouteauensis (Schizophoria), 143 christianize (Archaeorthis), 81 cimex (Platyorthis), 135 circularis Pander (Hemipronites), 116 circularis Savage (Stricklandia), 187 circularis Sowerby (Platyorthis), 135 circularis Winchell (Pionodema), 141 circulus (? Mendacella), 127 Clarkella, 30, 154, 156 Clarkellidae, 154, 155 clarkensis (Rhipidomella), 133 clarksvillensis (Platystrophia), 65 Clements, F. E., 15, 16 INDEX 259 clcobis (Rhipidomella), 133 clermontensis (? Hebertella), 60 Clinambon, 107, 115 Clitambonacc.1, 22, 28, 43, 44, 48, 107 Clitambonites, 21, 22, 27, 29, 38, 107, 113 amcricanus, see V'ellamo complectens, see \'ellamo diversa altissima, sec \'cllamo multistriata, sec \'ellamo planus rctroflcxus, see Billlngsella reflexa rogerscnsis, see Vellamo ruedemanni, see Vellamo semiconvexus, see Pomatotrema trentonensis, see Vellamo Clitambonitidae, 17, 22, 27, 33, 35, 37, 42, 48, 110 Clitambonitin.-e, 107, 113 Clorinda, 162, 163, 165, 166, 171, 178 becsciensis, see Stricklandia fornicata, see Barrandella ventricosa, see Barrandella cljnie (Heterorthis), 40,* 137 coejTnanensis (Gypidula), 174 prognostica (Gypidula), 174 colbiensis (Platystrophia), 65 mutata (Platystrophia), 65 coUctti (Conchidium), 181 coloradocnsis (Billingsella), 41, 48, 49 Columbia (Pla^siomys), 94 comis (Gypidula), 17+ Commissure, 7, 19 complectens (Vellamo), 114 albida (Vellamo), 114 compressus (Pentamerus), 178 compta ("Nisusia"), 46 concava Holtedahl (Palxostrophomena), 100 concava Schmidt (Cyrtonotella), 77 concavoconvexa (Parmorthis), 129 concavus Pahlen (? Clitambonites), 113 Conchidium, 162, 163, 164,* 165, 166, 172, 178, 181 concinna (? Levenea), 123 conradi (Pionodema), 141 contractus (Entelctes), 146 Convexity, 7, 19, 165 convexus (Clitambonites), 113 cooperi (Parenteletes), 147 cora (Schizophoria), 143 cornuta (Gypidula), 174 parva (Gypidula), 174 corpulenta (Dalmanella), 120 corrugatus (Pentamerus), 178 corrjTillensis (Platj-strophia), 65 Costaa, 7 costalis (Hesperorthis), 86 costata (Platystrophia), 66 costata (Productorthis), 82 Costell^e, 7 craigensis (Skenidioides), 72 crassa (Platj-strophia), 66 crassicosta (Orthis), 76 crassicostata (Parmorthis), 129 crassicostella (Planidorsa), 101 crassiplica (Plat>-merella), 183, 184 crassiplicata (Cyclocoelia), 64 crassoradius (Conchfdium), 181 crispa (Glyptorthis) , 90 crispata (Glyptorthis), 90 Crura, 7, 37 Crural fossettes, 7, 25 plates, 7, 165 Cruralium, 7, 39, 166 cumberlandix (Rhipidomella), 133 cumberlandicum (Conchidium), 181 Cumings, E. R., 55, 67, 68, 131 cumingsi (Platystrophia), 65 Cycloccei.ia, 58, 64 Cyclococliinas, 56, 58, 64 Cyclospira, 6, 27 cylindricus (Pentamerus), 178 CvMBiDiUM, 29, 166, 178, 183 cypha (Platystrophia), 66 arcta (Platystrophia), 66 bellatula (Platystrophia), 66 tumida (Platystrophia), 66 Cyrtonotella, 74, 77 Dalmanella, 33,* 36, 58, 39, 119, 120, 133, 135 bellula, see Pionodema carinata epsilon, see Cariniferella cimex, see Platyorthis circularis, see Pionodema concavoconvexa, see Parmorthis conradi, see Pionodema crispata, see Glyptorthis danbyi, see Thiemella deshayesi, see Mystrophora edgewoodensis, see Wattsella electra, see Archjeorthis Ixvis, see Archararthis major, see Archasorthis elmira, see Cariniferella fairmountensis, see Heterorthina lucia, see Platyorthis macra, see Levenea perelegans, see Isorthis planoconvexa, see Platyorthis pygmaea, see Isorthis rockhousensis, see Isorthis springfieldensis, see Parmorthis sub^equata, see Pionodema gibbosa, see Doleroides perveta, see Doleroides Virginia, see Cariniferella waldronensis, see Parmorthis wemplci, see Finkelnburgia Dalmancllacea, 22, 43, 119 Dalmanellid.-c, 34, 42, 119, 125, 137 dalyana (Rhipidomella), 133 danbyi (? Thiemella), 136 dartae (Orthostrophia), 71 daunus (Archxorthis), 81 Davidson, T., 66, 92, 103, 104 davidsoni Billings (Stricklandia), 187 davidsoni Verneuil (Hesperorthis), 86 Davies, A. Morley, 14 Dayia, 6, 27 daytonensis (Platystrophia), 65 laurelensis (Platystrophia), 65 daytonensis (Glyptorthis), 90 260 INDEX decipiens (Isorthis), 150 decussata (Virgiana), 185 deflecta (Valcourea), 7, 97 deflexa (Anastrophia), 170 deformatus (Porambonites), 102, 103 deformis (Stricklandia), 187 Deltarium, 7 Deltatreta, 17, 21, 22, 23, 24, 38, 107, 108 Deltatretida:, 48, 107 Delthyrial cavity, 7 Delthyrium, 7, 21, 163 Deltidial cover, 163 plates, 7, 163 Deltidium, 7, 21 demissa (Nicolella), 77 Dental plates, 7, 2 5, 42 sockets, 8, 34, 35* dentata (Platystrophia), 66, 68 Denticles, 8, 34, 35* Derby, O. A., 39, 145 deshayesi (Mystrophora), 131 desmopleura (? Eoorthis), 51 desmopleura nympha, see nympha diablo (? Eoorthis), 51 dice (Deltatreta), 108, 109 DiCTYONELLA, 102 Diductor muscles, 8, 30, 31, 32, 33,* 40 dieneri (Parenteletes), 147, 148 Dietrich, W. O., 67 dimera (Bilobites), 130 diminutiva (Rhipidomella), 133 Dinorthida, 35, 37,44,93 DiNORTHis, 10, 18, 21, 31, 35, 36, 93, 94 carletona, see Pionorthis carleyi insolens, see Retrorsirostra Columbia, see PL-esiomys deflecta, see Valcourea iphigenia, see Plaesiomys loricula, see Valcourea meedsi, see Plaesiomys arctica, see Plaesiomys germana, see Plssiomys platys, see Multicostella recta, see Valcourea retrorsa, see Retrorsirostra retrostriata, see Heterorthis rockymontana, see Plaesiomys subquadrata, see Plaesiomys tioga, see Cariniferella transversa, see Plaesiomys ulrichi, see Plaesiomys discus (Rhipidomella), 133 disparilis (Hesperorthis), 86 divergens Foerste (Pentamerus), 178 divergens Hall and Clarke (Parastrophinella), 169 diversa (Vellamo), 114 dixoni (Orthostrophia), 71 DoLERoiDEs, 17, 36, 58, 63 DOLERORTHIS, 40, 74, 88 dolomitica (Orthotropia), 152 dorsata (Platystrophia), 66 dorsoplicata (Fascicostella), 130 dougaldensis (Loperia), 47 dubiaHall 1860 (Pentamerella), 176 dubia Hall 1858 (Perditocardina), 83, 13S dudleyensis (Gypidula), 174 dumblei (Enteletes), 146 dumonti (Cariniferella), 122 edgelliana (Idiorthis), 128 edgewoodensis (Wattsella), 125 Eichwald, E., 103 eifelensis (Aulacella), 122 electra (Archseorthis) , 80, 81, 109 Ixvis (Archseorthis), 81 major (Archsorthis), 81 elegans (Enteletes), 146 elegantula Butts (Paurorthis), 80, 109 elegantula Dalman (Parmorthis), 128, 130 elegantula McEwan (Platystrophia), 65 amplisulcata (Platystrophia), 65 triplicata (Platystrophia), 65 elegantula Walcott (Eostrophomena), 85 elkhornensis (Platystrophia), 65 EUes, Gertrude, 1 3 ellipsoides (Heterorthina), 124 ellsworthi (Rhipidomella), 133 elmira (Cariniferella), 122 elongata Willard (Multicostella), 98 emacerata (Dalmanella), 32, 120, 121 emarginata Belanski (Sieberella), 175 emarginata Hall (Rhipidomella), 133, 134 emarginata Pahlen (Vellamo) ,114 eminens (Orthis), 76 eminens (Productorthis), 83 Encuclodema, see Cycloccelia Endopunctse, 10, 42 Enteletella, 145, 146, 148, 162 Enteletes, 19, 30, 38, 140, 146, 149, 162 acutiplicata, see Enteletina choristites, see lamarcki dieneri, see Parenteletes ferruginea, see Enteletina globosa, see Enteletina infracarbonica, see Aulacophoria latesinuata, see Enteletina pentameroides, see Enteletina sublxvis, see Enteletina uralica, see Aulacophoria Enteletina, 146, 148 Enteletina, 31, 140, 141, 145 Enteletoides, 146, 147, 148 Eoorthids, 44, 50, 74 Eoorthis, 27, 37, 50, 51 bavarica, see Archxorthis christianiae, see Archsorthis daunus, see Archaeorthis desmopleura nympha, see nympha hastingsensis, see Protorthis johannensis, see Orusia newtonensis, see Finkelnburgia putillus, see Archaeorthis remnicha sulcata, see sulcata texana, see texana winfieldensis, see winfieldensis tuUbergi, see Archsorthis wimani, see Archseorthis Eostrophomena, 85 Epidermis, 41 erecta (? Nisusia), 45 INDEX 261 Eridortius, 19, 74, 91 esthonus (Pentamcrus), 178 ESTLANDIA, 38, 38,* 107, 115 eun-one (Archiorthis), 81 Euseptum, 8 excavatus (? Gonambonites), 118 excelsus (Clitambonites), 113 exfoliata (Plectorthis), 58 Exopuncta:, 1 0, 42 expansus (Hemipronites), 116 exponens (Conchidium), 181 exporrectus (Oligomys), 53, 54 extensa McEwan (Platystrophia), 65, 68 extensa Pander (Panderina), 78, 82 extensa \*crncuil (Glossorthis), 78 fairmountensis (? Heterorthina), 124 fallax Gurich (Mystrophora), 131 fallax Salter (Schizophorella), 62 fasciata (Schizorarama), 87 Fascicostella, 125, 129 fascigera (? Eoorthis), 51 fausta (Glyptorthis), 90 jernvalensis {Platystrofhia), see Mcewanella lineolata ferruginea (Entcletina), 148 festinata (Nisusia), 44 FUx, 8 fiUistriata (Deltatreta), 108, 109 filosa (Porambonites), 102 finkelnburgi (Finkelburgia), 55, 56 FiNKELNBURciA, 10, 17, 27, 28, 38, 40, 55, 71 Finkelnburgiidx, 44, 48, 54, 56, 58 fissicosta (Plectorthis), 58 fissiplica (Schizoramma), 87 fissistriata (Schizoramma), 87 flabellites (Dolerorthis), 89 dinorthis (Dolerorthis), 89 euorthis (Dolerorthis), 89 fissiplicata (Dolerorthis), 89 militaris (Dolerorthis), 89 flabellulum (Dinorthis), 94 carrickensis (PIxsiomys), 95 floydcnsis (Schizophoria), 143 Focrste, A. F., 88, 90, 99, 184, 185 foerstei (Platystrophia), 65 ampla (PlaR'strophia), 65 Fold, 8 fornicata (Barrandella), 173 fragilis (Schizophoria), 143 frankfurtensis (Hebertella), 60 Freeh, F., 177 frechi Fliegel (Orthotichia), 143 frechi Wysogorsky (Orthis), 76 freija (Orthis), 76 frenum (Lycophoria), 105 Fulcral plates, 8, 34, 38, 164 fultonensis (Pentamcrella), 176 futilis (Dalmanella), 120 galeata (Gypidula), 172, 174 gaspeensis (Stricklandia), 187 Genital markings, 8, 33 georgix (? Conchidium), 181 gervillei (Fascicostella), 130 gibbosa (Doleroides), 63 giganteus (Holorhynchus), 180 gigas (Porambonites), 102, 103 Girty, G. H., 147, 148 globata (Platystrophia), 65, 66 globosa Eichwald (Lycophoria), 105 globosa Girty (? Enteletina), 148 globosa McEwan (Platystrophia), 65 globosa Willard (Mimella), 61 globosus Pander (? Hemipronites), 116 globulosa (Gypidula), 174 globus (Gypidula), 174 Glossorthis, 10, 23, 28, 74, 78 Glyptorthinx, 74, 89 Glyptorthis, 37, 74, 89 sublamellosa, see Ptychopleurclla Gonambonites, 17, 30, 38, 107, 118 inflexa, see Estlandia marginata aspera, see Estlandia magna, see Estlandia panderi, see Estlandia Gonambonitinx, 107, 118 goodwini (Rhipidomella), 133 Gorsky, I. I., 145, 147 gothlandicus Lebedeff (Pentamerus), 178 gotlandica Schuchert and Cooper (Schizoramma), 88 grandxva (Pomatotrema), 110 grandis Portlock (Valcourea), 97 grandis Tolmachew (Rhipidomella), 134 grayix (Vellamo), 114 greenei Hall and Clarke 1895 (Camerella), 168 greenei Hall and Clarke 1893 (Conchidium), 181 greenoughi (Skcnidioides), 72 Gypidula, 162, 165, 166, 171, 173 Gypidulinx, 162, 163, 165, 171, 173, 178 Hall, H. M., 1 5 Hall, J., 174 Hall and Clarke, 18, 19, 21, 26, 27, 30, 31, 34, 35, 42, 47,48, 58, 59, 60, 76, 86, 87, 95, 104, 106, 113, 114, 116, 118, 120, 121, 142, 145, 151, 161, 163, 166, 167, 168, 169, 173, 174, 176, 178, 185, 187, 188 halli (Skcnidioides), 72, 73 hamatus (Nicolella), 77 Harknessella, 39, 137, 138 Harknessellinx, 3 5, 137, 138 harlanensis (Syntrophioides), 155 harnngcnsis (Smeathenella), 139 Harpidium, 163, 165, 166, 178, 180 hartti (Rhipidomella), 134 hastingscnsis (? Protorthis), 47 haugi (Entcletcs), 146 Hebertella, 6, 10, 19, 36, 38, 58, 59, 90, 91 celsa, see Schizoramma daytoncnsis, see Eridorthis fausta, sec Eridorthis impcrator, sec Mimella insculpta, sec Glyptorthis lineolata, sec Mcewanella mclonica, sec Mimella nicklesi, sec Eridorthis rogersensis, see Eridorthis scovillei, see Austinella vulgaris, see Mimella helena ("Protorthis"), 47 hemiplicata Hall 1847 (Camerella), 168 rotunda (Camerella), 168 262 INDEX hemiplicata Hall 1852 (Enteletes), 146, 147 naias (Enteletes), 146 Hemipronites, 17, 38, 107, 115 apicalis, see Polytoechia carrickensis, see Estlandia thomsoni, see Estlandia hemipronites (Hemipronites), 116 hera (HueneUa), 159 hermitagensis (Platystrophia), 6S Hesperorthinas, 37, 85 Hesperorthis, 7, 17, 21, 22, 23, 25, 26, 37, 39, 40, 74, 76,85 hessensis (Rhipidomella), 133 Heterorthida:, 136 Heterorthina, 36, 119, 124 Heterorthins, 35, 137 Heterorthis, 24, 36, 39, 40,* 137 hicksi (Oligomys), 53 highlandensis (Wimanella), 49, 50, 51 Hinge-line, 8 hippolyte (Archasorthis) , 81 holdeni (Plectorthis), 58 Holorhynchus, 29, 166, 178, 180 holtedaUi (BiUingsella), 48 Homoeodeltidium, 22 Homoeomorphy, 16, 57, 61, 62, 63, 89, 100, 140, 175 HORDERLEYELLA, 125, 127 horderleyensis (Reuschella), 139 HuENELLA, 27, 28, 154, 159 triplicata, see Huenellina Huenellidae, 154, 159 Huenellina, 154, 160 humilis Fuchs (Vellamo), 114 humilis Pander (Clitambonites), 113 hunnebergensis ("Protorthis"), 47 hybrida (Rhipidomella), 133 icetas (Huenella), 160 Idiorthis, 125, 128 idonea (Rhipidomella), 133 ignota (Dalmanella), 33,* 120, 121 imitatrix (Rhipidomella), 134 imperator (Mimella), 61, 62 inca (Rhipidomella), 134 inclyta (Heterorthis), 137 incurvata (? Productorthis) , 83, 84 indentus (Bilobites), 130 indianola (Eoorthis), 51 indica (Orthotichia), 144 inflata (Plasiomys), 95 inflexa (? Estlandia), 115, 118 infracarbonica (Aulacophoria), 145 infracarbonica (Schizophoria), 143 Incria, 38, 107, 112 ingrica (Apomatella) , 117 inornata (? Cyclospira), 168 inostrantzefi (Hesperorthis), 86, 87 ubjaensis (Hesperorthis), 86 viruana (Hesperorthis), 86 insculpta (Glyptorthis), 89 manitoulinensis (Glyptorthis), 90 insigne Hall (Skenidium), 73 insigne Kirk (Harpidium), 180 insolita (Sieberella), 175 Interareas, 8, 20, 163, 165, 187 intercedens (Porambonites), 102 intermedia Pander (? Porambonites), 104 intermedia Pander (Productorthis), 83 intermedia StaufFer (Rhipidomella), 133 internascens (Anastrophia), 170 interplicata Foerste (Dolerorthis), 88 interplicata Hall (Anastrophia), 170 interstriata Janischevsky (Schizophoria), 143 interstriata Willard (Dinorthis), 94, 96 intralineata (Pentamerella), 176 inyoensis ("Wimanella"), 51 iones (Taffia), 47, 85 iowensis (Schizophoria), 143, 144 magna (Schizophoria), 143 iphigenia (Plaesiomys), 94, 96 isis (Syntrophina), 156 ISORHYNCHUS, 101, 102 Isorthina:, 140, 141, 149 IsoRTHis, 36, 149 Jamesella, 34, 44, 46 jamesi (Plectorthis), 58 jerseyensis (Rhipidomella), 133 johannensis (Orusia), 54 jonesi (Harknessella), 138 juba (Huenella), 160 juvensis (Platystrophia), 65 kankakensis (Austinella), 99, 100 Kayser, E., 132 Kayserella, 24, 1 19, 132 kayseri Kozlowski (Productorthis), 83 kayseri Waagen (Enteletes), 146 keisleyensis (Ptychopleurella), 92 keyserlingianus (Aulacophoria), 145 King, R. E., 147, 149 King, W., 66, 83 Kirk, E., 183 knappi (Rhipidium), 180 knotti (Gypidula), 174 Kolarova, F. N., 158 Kozlowski, R., 10, 28, 41, 49, 80, 83, 84, 104, 106, 112, 132, 150, 161, 163, 164, 173 kozlowskii (Orthotichia), 144 kuckersensis (Porambonites), 102 kuckersiana (Orthis), 76 kuthani (Jamesella), 46 KUTORGINA, 34 laevis Sowerby (Pentamerus) , 177 Ijevis Wakott ("Protorthis"), 47 la;vissimus (Enteletes), 146 lasviusculus (Gypidula), 174 Lahusen, I., 106 Lamansky, W., 78 lamarcki (Enteletes), 146, 147 lamellosa (Ptychopleurella), 92 lapworthi (Ptychopleurella), 92 laqueatum (Conchidium), 181 Laqueus, 32 lata Lamansky (Panderina), 82 lata Pander (Productorthis), 83 latasulcata (Hebertella), 60 INDEX 263 Lateral areas, 8 plates, 9 lateralis (Syntrophia), 158 latesinuatus (Entcletina), 148 laticaudatus (Porambonitcs), 102 laticosta (Platystrophia), 66 latiplicata (Parastrophinella), 169 latissima (Paurorthis), 79 latissimus (Hcmipronitcs), 116 latourcnsis (Protorth'is), 47 latus Kirk (Harpidium), 180 latus Pander (Clitambonitcs), 1 13 latus Pander (Gonarabonites), 118 latus Pander (Hemipronites), 116 laurentina (Hesperorthis), 86, 87 legoensis ("Conchidium"), 183 lehuequetiana (Rhipidomella), 133 Leidhold, C, 165, 175, 176 lens (Stricklandia), 187 lenticularis Foerste (? Mendacella), 127 lenticularis Vanuxem (Levenea), 123 lenticularis Wahlenberg (Orusia), 54, 55, 68 atrj'poides (Orusia), 54 lyncioides (Orusia), 54 leonardensis (Enteletes), 146 leonardensis (Rhipidomella), 133 leonensis (r Thiemella), 136 lepida (Kayserella), 132 lesleyi (Huenella), 160 leucosia (Rhipidomella), 133 Levenea, 36, 39, 40, 119, 123 lewisii (Skenidioides), 72 Likharev, B., 162 lima (Porambonitcs), 102 linda (Glossorthis), 78 virgata (Glossorthis), 78 lindenense (Conchidium), 181 lindstromi (Billingsella), 41, 49 lineolata (Mcewanclla), 69 linguifera (Barrandclla), 173 wilkinsoni (Clorinda), 171, 172 linnevi (Orthorhynchula), 42 LiNo'poREi.LA, 10, 28, 38, 103, 150 Linoporellidi, 39, 150 LioccELiA, 171, 189 liratus (Stricklandia), 187 LissoccELiNA, 165, 166, 178, 179 littoni (Conchidium), 181 liumbonus (Enteletes), 146 livia (Rhipidomella), 133 logani (Rhipidomella), 133 lonensis (? Hebertella), 60 longirostris ("Camerella"), 168 LoPERiA, 44, 47 loricula (Valcourea), 97 lotis (Gypidula), 1 74 loveni (Isorthis), 1 50 lowi (? Nisusia), 45 lucia Billings (Rhipidomella), 133 lucia Clarke (Platyorthis), 135 Lycophoria, 39 Lycophoriidse, 44, 105 lynx (? Platystrophia), 66, 68 moritura (Platystrophia), 65 McEwan, Eula D., 66-69, 70 Mcewanella, 31, 69 macfarlani (Schizophoria), 144, 145 macra (Levenea), 123 magna (Valcourea), 98 magnifica (Anastrophia), 170 magnicardinalis (Rhipidomella), 133 major Savage (Virgiana), 185 major Walcott (Billingsella), 49 manitouensis (Stricklandia), 187 manniensis (Platymerella), 184, 185 maquoketa (? Lissocalina), 179 marginata (Estlandia), 22, 38,* 115 asper (Estlandia), IIS magna (Estlandia), 115 maria (Hebertella), 60 parkensis (Hebertella), 60 Marionella, 94, 96 marmorea (Orthotichia), 144 matapedia (Ptychopleurella), 93 matura (Idiorthis), 128 maximus (Hemipronites), 116 mayvillensis (Virgiana), 185 media (? Parmorthis), 129 Median septa, 9 meedsi (Plaesiomys), 94 arctica (Plasiomys), 94 germana (Pljesiomys), 94 meeki (Dalmanella), 36, 120, 121 melissa (Stricklandia), 187 melonica (Mimella), 33,* 61 melvillei (Rhipidomella), 134 Mendacella, 39, 125, 127 meridionalis (Enteletes), 146 Merista, 6, 27 merope (?? Skenidioides), 72, 73 mesoloba (? Aulacophoria), 145 mesoplatys (Rhipidomella), 134 baylorensis (Rhipidomella), 134 Metacamerella, 167, 170 Metcalf, Z. P., 12 michelini (Rhipidomella), 133 microcamerus (Stricklandia), 187 microplocus (Enteletes), 146 Microstructure, 41 Mimella, 17, 33, 58, 61 mineolaensis (Gypidula), 174 minima Pander (Paurorthis), 79 minima Savage (Rhipidomella), 134 minnesotensis (Pionodema), 141 minor (Barrandina), 173 minuscula Barrande (Mendacella), 127 minuscula Willard (Mimella), 61 minuta Pander (Paurorthis), 79 minuta Raymond (Platystrophia), 65 mira (Capelliniella), 179 missouriensis Branson (Pentamerella), 176 missouriensis Swallow (Rhipidomella), 133 molongensis ("Pentamerella"), 176 moneta (Nicolella), 77, 78 montanensis (Clarkella), 156 monticula (Ptychopleurella), 92 morganiana (Orthotichia), 40, 144 chihsiaensis (Orthotichia), 144 264 INDEX morrowensis (Platystrophia), 65 munsteri (Conchidium), 181 Muir-Wood, H. M., 158 mullochiensis (Mendacella), 127 multicosta (Vellamo), 114 multicostata Dunbar (Gypidula), 174 multicostatum Hall (Rhipidium), 181 MULTICOSTELLA, 94, 98 multilirata (Stricklandia), 187 multiplicata Bancroft (? Wattsella), 125 multiplicata Hall and Clarke (Parastrophinella), 169 multisecta Meek (Dalraanella), 120 multistriata Hall (Schizophoria), 143 multistriata Roemer (Sieberella), 175 muralis (Pomatotrema), 109 Muscle impressions, 9, 30, 39 musculosa (Rhipidomella), 134 Myophore, 9, 35 Mystrophora, 17, 22, 39, 119, 131 Mystrophorids, 39, 119, 131 nautes (? Nisusia), 45 navis (Dalmanella), 120 neglecta Barrande (Isorthis), 150 neglecta James (Plectorthis), 58 nettelrothi Foerste (Dolerorthis), 89 nettelrothi Hall and Clarke (Conchidium), 181 nevadensis (Rhipidomella), 134 newsomensis (Orthostrophia), 71 newsomensis (Rhipidomella), 134 newtonensis (Finkelnburgia), 55 nicklesi (Eridorthis), 91 NiCOLELLA, 24, 74, 77 nikschitschi (Enteletella), 148 nisis (Schizoramma), 87 Nisusia, 26, 27, 34, 44 Nisusiida:, 37, 43, 44, 50 nitida (Platystrophia), 65 nodocostata (Ptychopleurella), 92 Noetling, F., 103 NoETLINGIA, 101, 105 nonus (Clarkella), 156 norwoodi (Stricklandia), 187 Notothyrial platform, 9, 34, 39 Notothyrium, 9, 23 Novak, O., 177 nucella (Lycophoria), 105, 106 nucleolata (Gypidula), 174 nucleus (Gypidula), 174 nundina (Syntrophina), 156 nympha (? Eoorthis), 51 nysius (Rhipidium), 181 oakensis (Polytoechia), 110 oblata (Rhipidomella), 134 oblongus Pander (Clitambonites), 1 13 oblongus Sowerby (Pentamerus), 162, 178 subrectus, see subrectus obsoletum (Conchidium), 181 obtusa (Productorthis), 82 obtusus (Hemipronites), 116 occasus (Rhipidomella), 134 occidentale Hall 18 52 (Conchidium), 181 occidentalis Hall 1858 (Gypidula), 173, 174 occidentalis Hall 1847 (Hebertella), 60 sinuata (Hebertella), 60 occlusa (Isorthis), 150 ochus (? Eoorthis), 51 oehlerti (Enteletes), 146 Old-age characters, 42 Oligomys, 50, 53 Onniella, see Dalmanella opercularis (Platyorthis), 135 Opik, A., 69, 79, 100, 112, 114, 117, 118 ops (Parastrophinella), 169 optatus (Brooksina), 183 orbicularis (Hemipronites), 116 oriens ("Nisusia"), 46 orientalis (? Nisusia), 45 Orientation, 18 oriskania (Schizophoria), 143 Ornamentation, 162, 165 Orthacea, 43, 154 orthambonites (Orthis), 76 Orthambonites crassicosta, see Orthis parvus, see Paurorthis rotundata, see Orthis trigona, see Paurorthis Orthidje, 33, 34, 35, 37, 39, 42, 44, 48, 73, 94, 101, 119 Orthinx, 37, 74, 75 Orthis, 18, 21, 26, 32,* 35, 37, 40,* 74, 75, 86 acton iae, see Nicolella {squivalvis, see Mendacella uberis annamitica, see Dinorthis anticostiensis, see Plasiomys apicalis, see Hesperorthis areola, see Mystrophora attenuata, see Heterorthis balclatchiensis, see Glyptorthis basalis, see Parmorthis bellarugosa, see Glyptorthis biforata, see Platystrophia lynx, see Platystrophia bouchardi, see Ptychopleurella calligramma subplicata, see Schizoramma canalicula, see Isorthis canaliculata, see Levenea canalis, see Dalmanella carinata, see Cariniferella carleyi, see Retrorsirostra circularis, see Platyorthis concava, see Cyrtonotella concinna, see Levenea corpulenta, see Dalmanella costalis, see Hesperorthis crispa, see Glyptorthis davidsoni, see Hesperorthis decipiens, see Isorthis dimera, see Bilobites disparilis, see Hesperorthis dorsoplicata, see Fascicostella edgelliana, see Idiorthis eifelensis, see Aulacella elegantula, see Parmorthis ellipsoides, see Heterorthina emacerata, see Dalmanella euryone, see Archxorthis - INDEX 265 Orthis — Cont. cxtcnsa, sec Glossurthis f.isci.ita, see Schizoramma tissiplica, see Schizoramma flabellitcs, see Dolcrorthis dinorthis, see Dolcrorthis cuorthis, sec Dolcrorthis fissiplicata, see Dolcrorthis militaris, sec Dolcrorthis flabcllulum, sec Dinorthis carrickensis, see Plxsiomys frcchi, see Cyrtonotella frcija, see Cyrtonotella futilis, see Dalmanclla gervillei, sec Fascicostella grand is, see Valcourea hcmipronitcs, sec Hcmipronites highlandensis, see VVimanella hippolyte, see Arch.*orthis ignota, see Dalmanclla inclyta, see Heterorthis incurvata, see Productorthis inflata, see Pl.-esiomys inostrantzefi, see Hcspcrorthis ubjaensis, see Hcspcrorthis viruana, sec Hcspcrorthis interplicata, sec Dolcrorthis kankakensis, see AustincUa keisleyensis, see Ptychopleurclla keyserlingiana, see Aulacophoria kuckersiana, see Cyrtonotella lamellosa, see Ptychopleurclla lapworthi, see Ptychopleurclla lenticularis Vanuxem, see Levenea lenticularis Wahlenberg, see Orusia atrypoides, see Orusia lyncioides, see Orusia leonensis, see Thiemella lepida, see Kayscrella lewisii, see Skenidioides loveni, see Isorthis media, see Parmorthis meeki, see Dalmanclla minuscula, see Mendacella moneta, see Nicolella monticula, see Ptychopleurclla morrowensis, see Platystrophia mullochiensis, see Mendacella multisecta, see DalmancUa neglccta, see Isorthis nettelrothi, see Dolcrorthis nisis, see Schizoramma occlusa, sec Isorthis opercularis, sec Platyorthis parvula, see Paurorthis pectincUa sweencyi, sec Dinorthis sweeneyi pepina, sec Billingsclla personata, see Proschizophoria plicata, see Hcspcrorthis porcata, sec Plxsiomys porrecta, see Dalmanclla punctata, see Linoporclla punctostriata, see Linoporella pyramidalis, sec Hcspcrorthis Orthis — Cont. quadrans, sec Levenea radians, see Hcmipronitcs rcdux, sec Pionodcnia rigida, see Schizoramma rogata, see Dalmanclla rugiplicata, sec Ptychopleurclla rustica, sec Dolcrorthis saffordi, see Multicostclla sedgwicki, see Fascicostella scmicircularis, see Cyrtonotella scmiovalis, see Dinorthis stracheyi, sec AustincUa striata, see Nicolella striatocostatus, see Plassiomys subcarinata Dunbar, see Levenea subcarinata Hall, see Levenea subdivisus, see Plssiomys subfissicosta, see Schizoramma sweencyi, see Dinorthis tersa, see Dalmanclla tcstudinaria (Dalman), sec WattscUa testuditiaria (Hall and Clarke), see Orthis rogata tetragonum Roemer, see Isorthis lata, see Isorthis thakil, see Dinorthis convexa, see Dinorthis trifida, see Dinorthis tricenaria, see Hcspcrorthis trigeri, see Isorthis trigonula, see Pahlenella turgida, see Linoporella uberis, see Mendacella unguis, see Nicolella vespertilio, see Harknessella wisbyensis, see Parmorthis Orthisina, see Clitambonites adscendens, see Vellamo complectens, see Vellamo concavus, see Clitambonites diversa, see Vellamo emarginata, see Vellamo grandseva, see Pomatotrema ingrica, see Apomatella marginata, see Estlandia planus altus, see Gonambonites pyron, see Estlandia schmidti, sec Clitambonites squamata, see Vellamo verneuili, see Vellamo wescnbergensis, see Vellamo volborthi, see Estlandia Orthoidea, 43 Orthorhynchula, 26, 42 Orthostrophia, 33, 70 fissistriata, see Schizoramma Orthostrophiina:, 56, 58, 70 Orthotichia, 30, 31, 40, 140, 141, 144 Orthotropia, 152 Orusia, 38, 54, 56, 58 Osborn, H. F., 1 3 osceola (Finkelnburgia), 5 5,* 56 corrugata (Finkelnburgia), 5 5 ottawaensis (Orthorhynchula), 42 266 INDEX Otusia, 50, 52 ovalis (Pentamerus), 178 Ovarian markings, 33 ovata (Orthis), 76 ovata (Productorthis), 83 oweni (Rhipidomella), 134 Pahlenella, 10, 107, 117 Pal^ostrophomena, 94, 100 Paleoecology, 16 Palintrope, 9 Pallial markings and sinuses, 9, 32, 40 palmata (Syntrophina), 156 Pander, C. H., 113, 117 panderi Billings ("Camerella"), 168 panderi Opik (Estlandia), 1 15 panderiana (Orthis), 75, 86 Panderina, 74, 81 papyracea (Gypidula), 174 parallela (Productorthis), 83 parallelus (Porambonites), 102 Parastro-phia, see Camerella divergens, see Parastrophinella greenei, see Camerella hemiplicata, see Camerella rotunda, see Camerella latiplicata, see Parastrophinella multiplicata, see Parastrophinella ops, see Parastrophinella reversa, see Parastrophinella rotundiformis, see Camerella scofieldi, see Camerella Parastrofhina, see Camerella Parastrophinella, 164, 166, 167, 169 Parenteletes, 6, 19, 30, 145, 146, 147 Parmorthis, 22, 25, 36, 39, 125, 126, 128 parva Billings ("CamereUa"), 168 parva Opik (Vellamo), 114 parva Pander (Paurorthis), 79, 80 Parvicostells, 7 parvula Lamansky (Paurorthis), 79 parvulus Savage (Pentamerus), 178 patera ("Heterorthis"), 138 pauciplicata (Platystrophia), 65 Paurorthis, 10, 74, 79, 81 pavilionensis (Pentamerella), 176* pecosi (Rhipidomella), 134 pectinella (Dinorthis), 93, 95, 96 sweeneyi (Dinorthis), see sweeneyi Pedicle callist, 9, 33* foramen, 9 muscles, 9, 30, 31 pelagica (Gypidula), 174 peloris (Rhipidomella), 134 penelope (Rhipidomella), 134 penniana (Rhipidomella), 134 pennsylvanica (Rhipidomella), 134 pentamera (? Enteletes), 146 Pentameracea, 28, 43, 161 Pentamerella, 165, 166, 171, 176 Pentameridae, 27, 161, 162, 164, 165, 170 Pentamerinx, 162, 163, 165, 166, 171, 177 Pentameroidea, 43, 154 Pentameroides, 164,* 166, 178, 179 pentameroides (Enteletina), 148 Pentamerus, 10, 27, 162, 163, 164,* 165, 166, 177, 178 acutolobata, see Gypidula procerula, see Gypidula caducus, see Gypidula globus, see Gypidula knappi, see Rhipidium linguifera, see BarrandeDa microcamerus, see Stricklandia optata, see Brooksina pelagica, see Gypidula proxima, see Liocoelia subrectus, see Pentameroides ventricosus, see Porambonites pepina (Billingsella), 49 peraltus (Hemipronites), 116 Perditocardinia, 133, 135 perelegans (Isorthis), 36, 150 pergibbosa (Lissocoelina), 179 perilla (Syntrophina), 156 perlatus (Hemipronites), 116 perminuta (Rhipidomella), 134 perpasta (Jamesella), 46 macra (Jamesella), 46 subquadrata (Jamesella), 46 pcrsonata (Proschizophoria), 123 pesovis (Pentamerus), 178 Pionodema, 22, 23, 30, 37, 38, 139, 140, 141, 144 globosa, see Mimella minuscula, see Mimella PlONORTHIS, 94, 95 Pl.esiomys, 10, 31, 33, 35, 94, 95, 96 brevis, see Multicostella elongata, see Multicostella plana (Productorthis), 83 planareas, 165 Planidorsa, 24, 94, 100 planissimus (Productorthis), 83 planoconvexa Butts (Taffia), 85 planoconvexa Hall (Platyorthis), 36, 135 planus (Clitambonites), 113 planus (Gonambonites), 118 planus (Porambonites), 102 retroflexus (Billingsella), 49 Platymerella, 163, 166, 178, 184 Platyorthis, 36, 133, 135 platys (Multicostella), 98 Platystrophia, 19, 31, 36, 55, 64 fernvalensis, see Mcewanella lineolata Platystrophiinx, 56, 58, 64 playfairi (Orthis), 76 Plectambonites, 38 Plectambonitins, 112 Plectella, 38, 107, 112 Plectellinx, 107, 112 Plectorthids, 34, 36, 38, 39, 44, 48, 56 Plectorthinx, 57, 58 Plectorthts, 26, 36, 38, 54, 57, 58, 68 crassiplicata, see Cyclocoelia sectistriata, see Cyclocoelia sordida multiplicata, see Cyclocoelia whitfieldi, see Austinella Plica, 10 plicata Bancroft (Horderleyella), 127 INDEX 267 plicata Schuchcrt and Cooper (Rhvnchocamara), 189 plicata Sowcrbv (Hesperorthis), 86 plicatclla Hall^Plcctorthis), 57, 58 trcntonensis (Plcctorthis), 58 plicatclla Walcott (BillingscUa), 49 plummcri (Entelctes), 146 polita ("Camcrella"), 168 poloi (Yangtzeella), 157, 158* polonicum (Mystrophora), 131 polygramma (Rhipidomclla), 134 pentlandica (Rhipidomclla), 134, 152 PoLYTtECHIA, 21, 107, 110 montanensis, sec ClarkcUa POMATOTREMA, 24, 38, 107, 109 ponderosa (Platystrophia), 66 arnheimensis (Platystrophia), 66 auburnensis (Platystrophia), 66 PoRAMBONiTES, 7, 11,' 28, 29, 39, 101, 102 costata, see Platystrophia dentata, see Plat)strophia recta, see Angusticardinia striata, see Angusticardinia Porambonitida:, 44, 101, 103 porcata (Plcesiomys), 95 porcias (Taffia), 47, 85 porrecta (Dalmanella), 120 poststriatula (Schizophoria), 143 priceps (Clitambonites), 113 prsculta (Heterorthina), 124 praK:ursor (Platystrophia), 65 angustata (Platystrophia), 65 latiformis (Platystrophia), 65 praerupta (Clitambonites), 113 precedens (Platystrophia), 65 preoblata (Rhipidomclla), 134 preponderosa (Platystrophia), 66 prima (Schizophoria), 143 primordialis (? Syntrophina), 156 proavita (Dinorthis), 94 Productorthinse, 74, 81 Productorthis, 17, 24, 31, 35, 38, 39, 74, 82 Productus abscissus, see Panderina aculeatus, see Productorthis brevis, sec Productorthis costatus, sec Productorthis eminens, sec Productorthis cxtensus, see Panderina intermedius, see Productorthis latissimus, see Paurorthis latus, see Productorthis minimus, see Paurorthis minutus, see Paurorthis obtusus, sec Productorthis ovatus, see Productorthis parallelus, sec Productorthis cf. plana, sec Productorthis planissimus, see Productorthis pterygoideus, see Nicolella quinqueradiatus, sec Productorthis tenuis, sec Productorthis tetragona, see Panderina profundosulcata (Platystrophia), 66 hopensis (Platystrophia), 66 promincns (Hcmlpronites), 116 Pronites, sec Clitambonites adsccndcns, sec Clitambonites altus, see Clitambonites convexa, see Clitambonites cxcclsa, sec Clitambonites humilis, sec Clitambonites latus, sec Clitambonites propinqua (Isorthis), 150 Proschizophoria, 119, 123 Protegulum, 10 Protorthidas, 37, 44, 46 Protorthis, 28, 29, 44, 46 dougaldensis, see Loperia provulvaria (Schizophoria), 143 proxima (Liocoelia), 189 Pseudocruralium, 10, 37, 39 pseudogaleata (? Gypidula), 166, 172, 174 rccurrens (Gypidula), 174 Pseudointerarca, 20 pseudolinguifera (Clorinda), 172* Pseudoresupination, 10, 19 Pseudospondylium, 10, 27, 42 pterygoideus (Nicolella), 77 Ptychopleurella, 23, 74, 92 Punctx, 10, 41, 42, 170 punctata (Linoporella), 150, 151 punctostriata (Linoporella), 151 putilla (ArchaMrthis), 81 pygmaea (Isorthis), 150 pyramidalis Hall (Skenidioides), 72, 73 pyramidalis Opik (Vellamo), 114 arcuata (Vellamo), 114 pahleni (Vellamo), 114 simplex (Vellamo), 114 pyramidalis Twenhofel (Hesperorthis), 86 pyriformis (Stricklandia), 187 elongata (Stricklandia), 187 varicosa (Stricklandia), 187 pyron (Estlandia), 115 quacoensis (Protorthis), 47 quadrans (Levcnea), 123 quadriplicata (Dinorthis), 94, 96 Qucnstedt, F. A., 103 quinqucradiata (Productorthis), 83 radians (Hemipronites), 116 Rafincsquina (?) atava, see Orusia Raymond, P. E., 80, 96, 121 raymondi (Mcewanella), 69 recta Conrad (Valcourea), 97 recta Pander (Angusticardinia), 69, 84 redux (Pionodema), 141, 142 Reed, F. R. C, 152, 170 regularis (Platystrophia), 65 remnicha (Eoorthis), 51, 52 sulcata, see sulcata texana, sec texana winfieldensis, see winfieldensis Resserella, 125, 126 resseri (BillingscUa), 49 resupinata (Dalmanella), 120 268 INDEX resupinata (Schizophoria), 143, 144 lata (Schizophoria), 143 rotundata (Schizophoria), 143 Resupination, 10, 19 resupinoides (Schizophoria), 143 reticulatus (Porambonites), 102 retrorsa (Retrorsirostra), 95 Retrorsirostra, 2 1 , 94, 95 retrostriata (Heterorthis), 137 Reuschella, 39, 137, 139 reuschi (Dalmanella), 120 reversa Billings (Parastrophinella), 169 reversa McCoy (Rhipidomella), 134 Rhipidium, 165, 166, 178, 180 Rhipidomella, 25, 30, 31, 32, 35, 36, 39, 133 circulus, see Mendacella rhynchonelliformis, see Mendacella tenuilineata Foerste, see Mendacella tenuilineata Hall, see Thiemella Rhipidomellidas, 133 Rhynchocamara, 189 Rhynchonella frenum, see Lycophoria globosa, see Lycophoria sordida, see Cyclocoelia ? Rhynchonellacea, 189 rhynchonelliformis McEwan (Platystrophia), 65 rhynchonelliformis Shaler (Mendacella), 127 Rhynchotrema, 84 ribeiro (Porambonites), 102 rigida (Schizoramma), 88 rockhousensis (Isorthis), 150 rockymontana (Plisiomys), 94 roemeri Hall and Clarke 1893 (Gypidula), 174 roemeri Hall and Clarke 1893 (Sieberella) (in part), 175 rogata (Dalmanella), 120, 121 rogersensis Foerste 1909 (Eridorthis), 91 rogersensis Foerste 1910 (Vellamo), 114 romingeri Barrande (Bohemiella), 52, 53 romingeri Hall and Clarke (Gypidula), 174 rossicus (Enteletoides), 148 Rostration, 165 rotunda (Orthis), 40,* 76 rotundata Pander (Orthis), 76 rotundata Walcott (Syntrophina), 156 rotundiformis (Camerella), 168 rotundus Kirk (Harpidium), 180 rotundus Pander (Clitambonites), 113 rotundus Pander (Hemipronites), 116 rudis (Nicolella), 77 ruedemanni (Vellamo), 114 rugiplicata (Ptychopleurella), 92 rugosicostatus (Oligomys), 53 rustica (Dolerorthis), 89 osiliensis (Dolerorthis), 89 sadewitzensis (Orthis), 76 saflFordi Foerste (Rhipidomella), 134 safFordi Hall and Clarke (Multicostella), 98 saffordi Walcott (Syntrophioides), 155 Sahni, M. R., 14 salme (Nicolella), 77 salteri (Stricklandia), 187 samojedicus (Pentamerus), 178 sandbergi (Otusia), 52 Sardeson, F. W., 32, 41, 121, 165 Savage, T. E., 183 Schellwien, E., 148, 149 Schizophorella, 17, 58, 62 arisaigensis, see Mendacella Schizophoria, 6, 17, 26, 30, 31, 35, 37, 38, 40, 40,* 41, 88, 141, 143, 146, mesoloba, see Aulacophoria Schizophoriidje, 22, 34, 38, 139 Schizophoriinse, 140 Schizoramma, 26, 32,* 35, 74, 87 schmidti LebedefF (Pentamerus), 178 schmidti Noetling (Porambonites), 102, 103 schmidti Pahlen (Clitambonites), 113 schmidti Wysogorsky (Orthis), 76 schuchertensis (Orthotichia), 144 scofieldi (Camerella), 168 scoparium (Conchidium), 181 scotica (? Plectorthis), 58, 59 scovillei (Austinella), 99, 100 sectistriata (Cyclocoelia), 64 sedaliensis (Schizophoria), 143 sedgwicki (Fascicostella), 130 semele (Rhipidomella), 134 semicircularis Eichwald (Cyrtonotella), 77 semicircularis Pander (Orthis), 76 semiconvexum (Pomatotrema), 110 semiglobata (Reuschella), 139 semiovalis (Dinorthis), 94 senecta (Schizophoria), 143, 144, 150 senex (Platystrophia), 65 Septa, 10, 29, 39, 164, 166 Serial sectioning, 162 Shaft, 10, 35 shallochiensis (Vellamo), 114 shelbyensis (Wimanella), 50 Sieberella, 163, 165, 171, 174, 175 sieberi (Sieberella), 175 rectifrons (Sieberella), 175 simon (? Huenella), 160 simplex Foerste (Gypidula), 174 simplex Walcott (Wimanella), 49, 50, 51 sinuata (Hebertella), 59, 60 sinuatis (Plectorthis), 58 sinuatus (Porambonites), 102 Skenidiidse, 39, 44, 56, 71 Skenidioides, 17, 38, 71, 73, 112 Skenidium, 71, 73, 112 anthonense, see Skenidioides craigensis, see Skenidioides fallax, see Mystrophora grayiae, see Vellamo greenoughi, see Skenidioides merope, see Skenidioides nodocostata, see Ptychopleurella polonicum, see Mystrophora pyramidalis, see Skenidioides shallochiensis, see Vellamo Solaris, see Pionorthis solitaria, see Levenea Smeathenella, 39, 137, 139 INDEX 269 Socket plates, 37 Sockets, 10, 25, 34, 35* sob (Pionorthis), 95 Solaris (Plxsiomys), 95 solitaria (r Lcvenea), 123 solus (Zdimir), 177 sordida (Cyclococlia), 64 multiplicata (Cyclococlia), 6+ SowerbvcUin.'c, 112 spcncci (? Nisusia), 45 spherica (Hemipronites), I 16 • Spirifer, 22, 87 aperturata, see Platystrophia chama, see Platystrophia liratus, see Stricklandia tridens, see Platystrophia tscheffkini, see Noetlingia Spirifera biforata fissicostata, see Platystrophia Spondylium, 10, 26, 162, 164 Spondyloid, 1 1 springfieldensis (Parmorthis), 129 squamata (V'ellamo), 114 stracheyi (Austinella), 99 Stris, 1 1 striata Pander (Angusticardinia), 84 striata Pander (Nicolella), 77 striata Twenhofel (Stricklandia), 187 striata Walcott (Billingsella), 49 striato-costatus (PL-Esiomys), 95 striatula (Schizophoria), 143, 144 marjlandica (Schizophoria), 143 Stricklandia, 161, 187 balclatchiensis, see Metacamerella Stricklandidx, 161, 186 strigosa (Platystrophia), 65 Stropheodonta, 22 Strophomena, 19, 97, 165 aurora, see Eostrophomena ventrocarinata, see Valcourea Strophomenacea, 21, 22, 48 strophomenoidcs Hall (Orthostrophia), 33,* 70 strophomenoides Raymond (Valcourea), 97 subaequata (Pionodema), 141, 142 gibbosa (Dolcroides), 63 perveta (Doleroides), 63 subsequivalvis (Enteletes), 146 subcarinata Dunbar (Levenea), 123 subcarinata Hall (Levenea), 36, 123 subcordiformis (Rhipidomella), 134 subdivisus (PK-esiomys), 95 subelliptica (Schizophoria), 143 subfissicosta (Schizoramma), 88 subglobosa (Gypidula), 174 subjugata (Hebertella), 60 sublsevis (Enteletina), 148 sublamellosa (Ptychopleurella), 92 sublaticosta (Plat)-strophia), 65 sublimis (Platystrophia), 66 sublinguifer ("Penfamerella"), 176 suborbicularis (Rhipidomella), 134 subplicata Bancroft (Harknessella), 138 subplicata Reed (Orthis), 76 subquadrata Bancroft (Harknessella), 138 subquadratus Hall (Plxsiomys), 32, 32,* 94, 95, 96 subrcctus (Pcntamcroides), 178, 179 subrossicus (Enteletoidcs), 149 Suess, E., 103 sucssi (Parentclctes), 147, 148 acuticosta (Parentclctes), 147, 148 sulcata (Eoorthis), 51 Sulcus, 1 1 swallow! (Schizophoria), 143 Swantonia, 44, 159 sweeneyi (Dinorthis), 23, 94 symmetrica (Polytoechia), 110 Syntrophia, 40, 154, 158 campbelli, see Syntrophina isis, see Syntrophina nona, see Clarkella nundina, see Syntrophina palmata, see Syntrophina perilla, see Syntrophina rotundata, see Syntrophina Syntrophiacea, 43, 48, 154 Syntrophiids, 154, 158, 161, 167 Syntrophina, 154, 155 Syntrophinella, 156 Syntrophioides, 40, 154, 155 szajnochai (Isorthis), 149 tacens (Glossorthis), 78 Taffia, 85 Taffiinx, 74, 85 Teeth, 1 1 , 24 Teichert, C, 104 tenuicosta (Rhipidium), 181 tenuicostata Eichwald (Platystrophia), 66 tenuicostata Weller (Rhipidomella), 134 tenuilineata Foerste (Mendacella), 127 tenuilineata Hall (Thiemella), 136 tenuilineata Savage (Rhipidomella), 134 tenuis (Productorthis), 83 tenuistriatum (Conchidium), 181 tenuistriatus (? Hemipronites), 116 Terebratula lynx, see Platystrophia tenuicostata, see Platystrophia Terebratulitcs xquirostris, see Porambonites biforatus, see Platystrophia teretior (Porambonites), 102, 103 tersa (? Dalmanella), 120, 121, 133, 135 tesluJinaria Hall and Clarke, see Orthis rogata testudinaria Dalman (Wattsella), 120, 121, 125, 126 tetragonum Pander (Clitambonites), 113 tetragonum Pander (Orthis), 76 tetragonum Pander (Panderina), 82 lata (Panderina), 82 tetragonum Roemcr (Isorthis), 150 texana Girty (? Orthotichia), 144 texana Walcott (Eoorthis), 51 texana Walcott (Huenella), 159, 160* Ixviuscula (Huenella), 159 thakil (Dinorthis), 94 convexa (Dinorthis), 94 trifida (Dinorthis), 94 Thiemei.i-a, 19, 133, 136 thiemi (Rhipidomella), 134 Thomas, I., 25, 26 Thomson, J. Allan, 14, 30, 32, 33 270 INDEX thomsoni (Estlandia) ,115 tioga (Cariniferella), 122 transversa King (Rhipidomella), 134 transversa Pander (Orthis), 76, 84 latestriata (Orthis), 84 transversa Walcott (Nisusia), 45 transversus Pander (Clitambonites), 113 transversus Pander (Hemipronites), 116 transversus Willard (Plassiomys), 94, 96 trempealeauensis (Billingsella), 49 trentonensis McEwan (Platystrophia), 65, 68 champlainensis (Platystrophia), 65 perplana (Platystrophia), 65 trentonensis Raymond (Vellamo), 114 tricenaria (Hesperorthis), 23, 86, 87 tridens (Platystrophia), 66 trigeri (Isorthis), 150 trigona (Paurorthis), 79 trigonula (Pahlenella), 117 Trilobation, 165 trilobatum (Rhipidium), 181 Triplesia lateralis, see Syntrophia poloi, see Yangtzeella primordialis, see Syntrophina triplesiana (Stricklandia), 187 triplicata (Huenellina), 160 triplicatella (Plectorthis), 58 Tropidoleptidx, 152 Tropidoleptus, 152 truncatus (Porambonites), 102 tscheffkini (Noetlingia), 105 tschernyscheffi (Enteletes), 146 tullbergi (Archa^orthis), 81 tulliensis (Schizophoria), 143, 150 tumidula (Clorinda), 172 tumidus (Hemipronites), 115 turgida (? Linoporclla), 151, 152 Twenhofel, W. H., 92, 95, 165 typa (Marionella), 96 typica (Syntrophinella), 156 uberis (Mendacella), 127 Ulrich, E. O., 27, 43, 68, 109 ulrichi (Plassiomys), 94, 96 ultima (Vellamo), 114 umbo (Orthis), 76 unicostata (Platystrophia), 66 crassiformis (Platystrophia), 66 unguiforme (Conchidium), 181 unguis Bancroft (Wattsella), 125 unguis Sowerby (Nicolella), 77 uniplicata Cooper (Pionodema), 141 uniplicata McEwan (Platystrophia), 65, 68 uticana (Platystrophia), 65 uniplicata Nettelroth (Gypidula), 174 uralica Gorsky (Aulacophoria), 145 uralicum Tschernyschew (? Skenidium), 73 utahensis Walcott 1905 (? Nisusia), 45 utahensis Walcott 1912 (Otusia), 52 Valcourea, 10, 19, 22, 23, 25,* 94, 97, 98 vanuxemi (Rhipidomella), 134 pulchella (Rhipidomella), 134 variabilis (Rhipidomella), 134 varians (Rhynchocamara), 189 varicus (Bilobites), 130 Vellamo, 17, 23, 38, 107, 114 ventricosa Hall (Barrandella), 173 Ventricosity, 162 ventricosus Kutorga (Porambonites), 102 ventrocarinata (Valcourea), 97 vermontana (? Huenella), 160 Verneuil, E. de, 1 1 3, 1 17, 1 18 verneuili Eichwald (Vellamo), 1 14 wesenbergensis (Vellamo), 114 verneuili Hall (Anastrophia), 169 verneuilianus (Bilobites), 130 vespertilio (HarknesseUa), 138 villenovia (ThiemeUa), 136 ViRGiANA, 165, 166, 171, 178, 185 Virginia (Cariniferella), 122 beta (Cariniferella), 122 visbyensis (Parmorthis), 36, 129 volborthi Billings (Camerella), 167, 168 volborthi Pahlen (Estlandia), 115 vulgaris (? MimeUa), 61, 62 vulvaria (Schizophoria), 143 Waagen, W., 147, 148 waageni (Enteletes), 146 Walcott, C. D., 22, 27, 41, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 80 waldronensis (Parmorthis), 129 wallowayi (Platystrophia), 66 Wattsella, 39, 125 Wattsellidse, 34, 124 wattsi (Wattsella), 121, 125 weedi (? Huenella), 160 weeksi (Swantonia), 159 wemplei (Finkelnburgia), 55 wesenbergensis (Porambonites), 102 whitfieldi (Austinella), 99 whittakeri (Dalmanella), 120 wichitaensis (Eoorthis), 51 wilkinsoni (Clorinda), 171, 172 Williams, H. S., 122, 132, 136 Wimanella, 50 wimani (Archiorthis), 81 Winchell, N. H., 99 Winchell and Schuchert, 23, 95, 96, 142 winfieldensis (Eoorthis), 51 wingi ("Protorthis"), 47 wolfcampensis (Enteletes), 146 wordensis (Enteletes), 146 Wysogorsky, J., 69, 78, 80 Yangtzeella, 30, 154, 157 Zdimir, 177