EXCHANGE

191921
MEMOIRS OF

THE CONNECTICUT ACADEMY
OF ARTS AND SCIENCES

VOLUME VII DECEMBER, 1920

The Appendages, Anatomy, and Rela-
tionships of Trilobites

BY

PERCY E. RAYMOND, Ph.D.

ASSOCIATE PROFESSOR OF PALAEONTOLOGY, AND CURATOR OF INVERTEBRATE

PALAEONTOLOGY IN THE MUSEUM OF COMPARATIVE ZOOLOGY,

HARVARD UNIVERSITY

NEW HAVEN, CONNECTICUT

PUBLISHED BY THE

CONNECTICUT ACADEMY OF ARTS AND SCIENCES

AND TO BE OBTAINED ALSO FROM THE

YALE UNIVERSITY PRESS

MEM. CONN. ACAD., VOL. VII.

FRONTISPIECE.

MEMOIRS OF

THE CONNECTICUT ACADEMY
OF ARTS AND SCIENCES

VOLUME VII DECEMBER, 1920

The Appendages, Anatomy, and Rela-
* tionships of Trilobites

BY

PERCY E. RAYMOND, Ph.D.

ASSOCIATE PROFESSOR OF PALAEONTOLOGY, AND CURATOR OF INVERTEBRATE

PALAEONTOLOGY IN THE MUSEUM OF COMPARATIVE ZOOLOGY,

HARVARD UNIVERSITY

NEW HAVEN, CONNECTICUT

PUBLISHED BY THE

CONNECTICUT ACADEMY OF ARTS AND SCIENCES

AND TO BE OBTAINED ALSO FROM THE

YALE UNIVERSITY PRESS

THE TUTTLE, MOREHOUSE ft TAYLOR COMPANY

TO THE MEMORY OF

CHARLES EMERSON BEECHER

SKILLFUL WITH HAND, BRAIN, AND PEN; REVEALER OF THE MYSTERIES

OF TRILOBITES;

THIS MEMOIR IS DEDICATED

r

FOREWORD.

By CHARLES SCHUCHERT.

Trilobites are among the most interesting of invertebrate fossils and have long attracted
the attention of amateur collectors and men of science. These "three-lobed minerals" have
been mentioned or described in books at least since 1698 and now several thousand species
are known to palaeontologists. To this group of students they are the most characteristic
animals of the seas of Palaeozoic time, and even though they are usually preserved as dis-
membered parts, thousands upon thousands of "whole ones" are stored in the museums of
the world. By "whole ones" perfect individuals are not meant, for before they became
fossils the wear and tear of their time and the process of decomposition had taken away all
the softer parts and even most of the harder exterior covering. What is usually preserved
and revealed to us when the trilobites weather out of the embrace of their entombing rocks
is the test, the hard shell of the upper or dorsal side. From time to time fragments of the
under or limb-bearing side had been discovered, first by Elkanah Billings, but IDC fore 1876
there was no known place to which one could go to dig out of the ground trilobites retain-
ing the parts of the ventral side.

Students of trilobites have always wanted specimens to be delivered to them weath-
ered out of the rock by nature and revealing the ventral anatomy without further work
than the collecting, but the wish has never been fulfilled. In the Utica black shales, near
Rome, New York, there was finally discovered in 1892 a layer less than ten millimeters
thick, bearing hundreds of Triarthrus becki with most of the ventral anatomy intact.
The collector's first inkling that such were present in the Utica formation came to him
in a chance find in 1884, and for eight years he sought off and on for the stratum whence
this specimen came. His long search was finally rewarded by the discovery of the bed,
and lo! here were to be had, in golden color, prostrate specimens with the breathing and
crawling legs and the long and beautifully curved feeling organs all replaced by iron
pyrites. Fool's gold in this case helped to make a palaeontologic paradise. The bed con-
tained not only such specimens of Triarthrus becki, but also, though more rarely, of Cryp-
tolithus tesscllatus and exceptionally of Acidaspis trentonensis. This important discovery,
which has figured so largely in unraveling the evolution of the Crustacea and even has a
bearing on that of most of the Arthropoda, was made by Mr. W. S. Valiant, then curator
of the Museum of Rutgers College.

There were, however, great material difficulties to overcome before the specimens
revealed themselves with all of their information exposed for study. No surgeon was
needed, but a worker knowing the great scientific value of what was hidden, and with end-
less patience and marked skill in preparation of fossils. Much could be revealed with the
hammer, because specimens were fairly abundant. A chance fracture at times showed con-
siderable portions, often both antennae entire, and more rarely the limbs protruding beyond
the test, but the entire detail of any one limb or the variation between the limbs of the
head, thorax, and tail was the problem to be solved. No man ever loved a knotty problem
more than Charles E. Beecher. Any new puzzle tempted him, and this one of Triarthrus
becki interested him most of all and kept him busy for years. From the summer of 1893.
when he quarried out two tons of the pay stratum at Rome, until his death in 1904, his

6 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

time was devoted in the main to its solution by preparing these trilobites and learning their
anatomical significance.

The specimens of Triarthrus becki from Rome are pseudomorphs composed of iron
pyrites, as has been said, and are buried in a gray-black carbonaceous shale. A little rub-
bing of the specimens soon makes of them bronze images of the former trilobite and while
under preparation they are therefore easily seen. However, as the average individual is
under an inch in length and as all the limbs other than the antennre are double or biramous,
one lying over the other, and the. outer one fringed with a filamentous beard, the parts to
be revealed by the preparator are so small and delicate that the final touch often obliterates
them. These inherent difficulties in the material were finally overcome by endless trials on
several thousand specimens, each one of which revealed something of the ventral anatomy.
Finally some 500 specimens worthy of detailed preparation were left, and on about 50 of
these Beecher's descriptions of Triarthrus and Cryptolithus were based.

The black shale in which the specimens are buried is softer than the pseudomorphous
trilobites, a condition that is of the greatest value in preparation. With chisel and mallet
the trilobites are sought in the slabs of shale and then with sharp chisels of the dental type
they are revealed in the rough. At first Beecher sought to clean them further by chemical
methods, and together with his friends, the chemist Horace L. Wells, and the petrologist
Louis V. Pirsson, several solutions were tried, but in all cases the fossils were so much
decomposed as to make them useless in study. Therefore Beecher had to depend wholly oh
abrasives applied to the specimens with pieces of rubber. Much of this delicate work was
done on a dental lathe, but in the final cleaning most of it was done with patient work by
hand. Rubber has the great advantage of being tough and yet much softer than either speci-
men or shale. As the shale is softer than the iron pyrites, the abrasives (carborundum,
emery, or pumice) took away the matrix more quickly than the trilobite itself. When a
part was fully developed, the rubbers were cut to smaller and smaller dimensions and the
abrading reduced to minute areas. So the work went on and on, helped along from time to
time by the dental chisels. Finally Beecher became so expert with these fossils that after
one side was developed he would imbed the specimen in Canada balsam and fix it on a glass
slide, thus enabling him to cut down from the opposite side. This was done especially with
Cryptolithus because of the great scarcity bf material preserving the limbs, and two of these
revealed both sides of the individuals, though they were then hardly thicker than writing
paper.

Then came illustrations, which at first were camera-lucida drawings in pencil smoothed
out with pen and ink. "In some quarters," however, it has been said, "his methods unknown,
their results were not accepted; they were regarded as startling, as iconoclastic, and even
unreliable." He therefore decided to rework his material and to illustrate his publications
with enlarged photographs. The specimens were black, there was little relief between fossil
and matrix, and the ammonium chloride process of coating them white and photographing
under artificial light was unsuitable. Nevertheless, after many trials, he finally succeeded
in making fine enlarged photographs of the trilobites immersed in liquid Canada balsam, with
a contact cover of glass through which the picture was taken, the camera standing vertically
over the horizontal specimen. Beecher had completed this work in 1903 and in the winter of
1903-1904 was making the drawings, nearly all of which are here reproduced. On Sunday
morning, February 14, 1904, as he was working at home on a large wash drawing of Cryp-
tolithus, death came to him suddenly, leaving the trilobite problem but partially solved.

FOREWORD. 7

When the writer, in the autumn of 1904, succeeded Professor Beecher in the chair of
Pakeontology at Yale, he expected to find considerable manuscript relating to the ventral
anatomy of the trilobites, but there was only one page. It was Beecher's method first to
prepare and thoroughly study the material in hand, then to make the necessary illustrations,
and between times to read what others had written. There was no written output until
everything had been investigated and read, certain passages being marked for later reference.
Then when all was assimilated, he would write the headings of topics as they came to him,
later cutting them apart and arranging them in a logical sequence. When the writer visited
him in his home in January 1904, he was primed for his final trilobite memoir, but the
writing of it had not been begun.

The writer has never made the trilobites his special subjects for study as he has the
brachiopods, and therefore felt that he should not try to bring to light merely the material
things that Beecher had so well wrought out. It seemed at first an impossible task to
find the specialist and friend to do Beecher justice, but as the years have passed, one of
Beecher's students, always especially interested in trilobites, has grown into a full appre-
ciation of their structures and significance, and to him has fallen the continuation of his
master's work. If in the following pages he departs here and there from the accepted inter-
pretation and the results of others, it is because his scientific training, in desiring to see with
his own eyes the structures as they are, has led him to accept only those interpretations
that are based on tangible evidence as he understands such. Furthermore, in seeking the
relationship of the trilobites to the rest of the Arthropoda, his wide study of material and
literature, checked up by the ontogeny of fossil and recent forms, has led him in places from
the beaten path of supposedly ascertained phylogenies. His results, however, have been won
through a detailed study of the interrelations of the Arthropoda, starting from the fact
that the Trilobita are chronogenetically the oldest and most primitive. The trilobites are
held by him to be the most simple, generalized, ancient Crustacea known, and the progen-
itors,'- directly and indirectly, of all Arthropoda.

It is now twenty-six years since Professor Beecher began his publications on the class
Trilobita, and in commemoration of him and his work, Professor Percy E. Raymond of Har-
vard University presents this memoir, to bring to fruition the studies and teachings of his
honored guide. It has been with Professor Raymond a labor of love, and it is for the
writer of this foreword a long-desired memorial to the man to whose position in the Museum
and University he had the privilege of succeeding.

Yale University, New Haven, Connecticut.

PREFACE.

The primary object of this memoir is, as has been stated by Professor Schuchert, to.
rescue from oblivion the results of the last few years of Professor Beecher's investigations
on the ventral anatomy of trilobites. Since he left his data in the form of drawings and
photographs, without even rough notes, it became necessary, in order to write a text to accom-
pany the plates, to restudy the entire subject. Under these circumstances, it seemed best to
include all that is known about the appendages of trilobites, thus bringing together a
summary of present information on the subject.

The growth of the memoir to its present size has been a gradual one. As first com-
pleted in 1917, it contained an account of the appendages only. Thoughts upon the prob-
able use of the appendages led to the discussion of possible habits, and that in turn to a
consideration of all that is known or could be inferred of the structure and anatomy of
the trilobite. Then followed an inquiry into the relationships to other Arthropoda, which
ultimately upset firmly established preconceptions of the isolated position of the group, and
led to a modification of Bernard's view of its ancestry.

During the progress of the work, I have had the opportunity of examining most of
the known specimens retaining appendages. From the Marsh collection in the Yale Univer-
sity Museum were selected the forty-six specimens showing best the appendages of Triarthrus,
Cryptolithus, and Acidaspis. Dr. Charles D. Walcott very kindly returned to the Museum
of Comparative Zoology the slices of Ceraurus, Calymcnc, and Isotelus which were the
basis of his paper of 1881, and which had been loaned him for further study. He loaned
also eight of the more important specimens of Neolenus serratus, and two of Triarthrus
becki. At the United States National Museum I saw the specimens of Isotelus described by
Mickleborough and the isolated limbs of Calymene from near Cincinnati. The Isotelus at
Ottawa I had already studied with some care while an officer of the Geological Survey of
Canada.

This memoir consists, as shown in the table of contents, of four parts. The appen-
dages of Neolenus, Isotelus, Ptychoparia, Kootcnia, Ceraurus, Calymene, and Acidaspis are
discussed, as fully as circumstances warrant, in the first part, and new restorations of the
ventral surfaces of Neolenus, Isotelus, Triarthrus, Ceraurus and Cryptolithus are included
It is not supposed that these restorations will be of permanent value in all of their detail,
but they are put forward as the best approximations to the real structure that the writer
is able to present from the materials so far discovered. I am greatly indebted to Doctor
Elvira Wood for the care and skill with which she has worked up these restorations from
my rather sketchy suggestions. She has put into them not only a great amount of patient
work, but also the results of considerable study of the specimens.

Part II is a discussion of the internal anatomy of the trilobite and a brief statement of
some of the possible habits and methods of life of these animals. Part III, which begins
with a survey of the relationships of the trilobites to other Arthropoda, is largely taken up
with an attempt to demonstrate the primitive characteristics of the former, and their probable
ancestral position. The form of the ancestor of the trilobite is deduced from a study of
the morphology, ontogeny, and phytogeny of the group, and evidence adduced to indicate
that it was a depressed, flattened, free-swimming animal of few segments.

IO THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

In Part IV are included somewhat detailed descriptions of a few of the best specimens
of Triarthrus and Cryptolithus. Professor Beecher, while an observer of the minutest
details, believed in publishing only the broader, more general results of his investigations.
This method made his papers brief, readable, and striking, but it also resulted in leaving in
some minds a certain amount of doubt about the correctness of the observations. In a mat-
ter so important as this, it has seemed that palaeontologists are entitled to the fullest possible
knowledge of the specimens on which the conclusions are based. The last part is, therefore,
a record of the data for the restorations of Triarthrus and Cryptolithus.

The illustrations in the plates were nearly all made by or under the supervision of
Professor Beecher, as were also text figures 45 and 46.

In conclusion, I wish to express my thanks to Mrs. Charles E. Beecher for the use of
drawings which were the personal property of Professor Beecher; to Doctor Charles D.
Walcott for photographs of the limbs of Calyrncnc, and for his kindness in sending me the
slices of trilobites from Trenton Falls and specimens of Neolenus and Triarthrus; to Doctor
R. V. Chamberlin for suggestions and criticisms in regard to the relationship of trilobites
to Insecta, Arachnida, Chilopoda, and Diplopoda; to Mr. Samuel Henshaw, Director of the
Museum of Comparative Zoology, for permission to use the time which has been devoted to
this work; and to Miss Clara M. Le Vene, for assistance in the preparation of the manu-
script. My greatest debt is to Professor Charles Schuchert, to whom the work owed
its inception, who has assisted in many ways during its prosecution, and who read the manu-
script, and arranged for its publication. To him I can only express my warmest thanks for
the favors which I have received and for the efforts which he has put forth to make this
a worthy memorial to our friend and my teacher, Professor Charles Emerson Beecher.

Harvard University, Cambridge, Mass.
November, 1919.

TABLE OF CONTENTS.

Historical review 17

Part I. The appendages of trilobites 20

Terminology 20

The appendages of Ncolemts 21

Historical 21

Neolenus scrratus (Rominger) 21

Cephalon ; 21

Thorax 22

Pygidium 23

Epipodites and exites 23

Description of individual specimens 23

Restoration of Neolenus 30

Nathorstia transitans Walcott 31

The appendages of Isotelus 32

Historical 32

Isotelus latus Raymond 34

Isotelus maximus Locke 35

Restoration of Isotelus 37

Isotelus gigas Dekay 37

Isotelus arenicola Raymond 39

The appendages of Triarthrus (see also Part IV) 39

Triarthrus becki Green 39

Historical 40

Restoration of Triarthrus 42

Relation of cephalic appendagess to marking on dorsal surface of glabella 43

Anal plate 44

The appendages of Ptychoparia 45

Ptychoparia striata (Emmrich) 45

Ptychoparia cordillcra (Rominger) 45

Ptychoparia permulta Walcott 45

The appendages of Kootenia 46

Kootenia dawsoni Walcott 46

The appendages of Calymene and Ceraurus 46

Historical . . 46

Comparison of the appendages of Calymene and Ceraurus with those of

Triarthrus 47

Spiral branchiae 48

Ventral membrane 50

Appendifers . 51

Calymene senaria Conrad 52

Cephalic appendages 52

Thoracic appendages 53

Pygidial appendages 54

12 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

Relation of hypostoma to cephalon in Calymcne 55

Restoration of Calymcne 56

Calymcne sp. ind 56

Ceraurus pleurexanthemus Green 57

Cephalic appendages 58

Thoracic appendages 59

Pygidial appendages 59

Relation of hypostoma to cephalon 59

Restoration of Ceraurus pleurexanthemus 60

The appendages of Aridaspis trentonensis Walcott 61

The appendages of Cryptolithus (see also Part IV) 61

Cryptolithus tcssellatus Green 61

Restoration of Cryptolithus 62

Summary on the ventral anatomy of trilobites 64

Comparison of appendages of different genera 64

Coxopodite 64

Cephalon 64

Thorax 66

Pygidium ' 67

Caudal rami 68

Homology of cephalic appendages with those of other Crustacea 69

Functions of the appendages 70

Antennules 70

Exopodites 70

Endopodites 71

Use of the pygidium in swimming 72

Coxopodites 74

Position of the appendages in life 74

Part II. Structure and habits of trilobites 77

Internal organs and muscles 77

Alimentary canal 77

Ceraurus pleurexanthemus 79

Calymenc senaria 80

Cryptolithus goldfussl 80

Summary 81

Gastric glands 82

Summary 84

Heart 85

Illcenus 85

Ceraurus and Calymene 85

The median "ocellus" or "dorsal organ" 86

Nervous system 89

Various glands 89

Dermal glands 89

Renal excretory organs 90

Reproductive organs 90

TABLE OF CONTENTS. 13

Panderian organs 90

Musculature 91

Flexor muscles 92

Extensor muscles 92

Hypostomial muscles 94

Eyes 96

Summary 97

Sex 98

Eggs 98

Methods of life (See also under "Functions of the Appendages") 98

Habits of locomotion 99

Food and feeding methods 103

Tracks and trails 104

Part III. Relationship of the trilobites to other Arthropoda 106

Crustacea : 106

Branchiopoda 106

Burgcssia bclla Walcott 108

Waptia fieldcnsis Walcott 108

Yohoia tenuis Walcott 109

Opabina regalis Walcott 109

Summary 109

Copepoda no

Archicopepoda in

Ostracoda 112

Cirripedia 113

Malacostraca 113

Phyllocarida 113

Syncarida 114

Isopoda 114

Marrella splendens Walcott 115

Restoration of Marrella 116

Arachnida 1 17

Trilobites not Arachnida 117

Merostomata 119

Sidneyia ine.vpectans Walcott 119

Emeraldella brocki Walcott 119

Malaria and Habelia 120

Araneae 121

Insecta 122

Chilopoda 123

Diplopoda 124

Primitive characteristics of trilobites 125

Trilobites the most primitive arthropods 125

Limbs of trilobites primitive 125

Summary 128

Number of segments in the trunk 128

14 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

Form of the simplest protaspis 132

Origin of the pygidium 134

Width of the axial lobe 137

Presence or absence of a "brim" 137

Segmentation of the glabella 137

Summary 138

The simplest trilobite 138

Naraoia compacta Walcott 139

The ancestor of the trilobites, and the descent of the Arthropoda 140

Evolution within the Crustacea 142

Summary , . 144

Evolution of the Merostomata 146

Evolution of the "Tracheata" 147

Summary on lines of descent 147

Final summary 151

Part IV. Description of the appendages of individual specimens 152

Triarthrus becki Green 152

Cryptolithus tessellatus Green 158

Bibliography 163

LIST OF ILLUSTRATIONS.

1. Triarthrus becki Green. Diagram of limb to show nomenclature employed ... 20

2. Neolenus serratus (Rominger). Two thoracic appendages 24

3. The same. An exopodite 26

4. The same. A so-called "epipodite" 26

5. The same. The so-called "exites" 29

6. The sanm A cephalic limb 29

7. The same. Restoration of a transverse section 30

8. The same. Restoration of the ventral surface 31

9. Isotelus. Restoration of the ventral surface 38

10. Triarthrus becki Green. Restoration of the ventral surface 41

1 1 . The same. Median appendage 44

12. Ceraurus pleurexanthemus Green. Slice showing an exopodite 49

13. Calymene senaria Conrad. Slice showing cephalic coxopodites 53

14. The same. Another similar slice 53

15. The same. Slice showing method of articulation of the appendages 53

16. The same. Restoration of the ventral surface 55

17. Ceraurus pleurexanthemus Green. Slice showing the method of articulation of

the appendages 58

18. The same. Slice showing an exopodite above an endopdite 58

19. The same. Restoration of a transverse section 60

20. Cryptolithus tessellatus Green. Restoration of the ventral surface 63

21. Ceraurus pleurexanthemus Green. Slice showing the abdominal sheath 79

22. The same. Slice showing the large alimentary canal 79

TABLE OF CONTENTS. 15

23. Calymene senaria Green. Slice showing the large alimentary canal 79

24. Ceraurus pleurexanthemus Green. Restoration of a longitundinal section 81

25. Cryptolithus tessellatus Green. Cheek showing the genal caeca 84

26. Illcenus. Volborth's figure of the heart 85

27. Heart of Apus 85

28. Isotelus gigas Dekay. The Panderian organs 91

29. Ceraurus pleurexanthemus Green. Restoration, showing heart, alimentary canal,

and extensor muscles 93

30. The same. Longitudinal section of cephalon 95

31. Nilcus armadillo Dalman. Moberg's figure of the muscle-scars 95

32. Marrella splendens Walcott. Restoration of the ventral surface 116

33. Triarthrus bccki Green. Appendage of the anterior part of the thorax 126

34. Apus. Appendage from the anterior part of the trunk 127

35. Weymouthia nobilis (Ford) 138

36. Naraoia compacta Walcott 145

37. Pagetia clytia Walcott 145

38. Asaphiscus wheelcri Meek 145

39. Pcedeumias robsonensis Burling 145

40. Robcrgia sp 145

41. Diagram showing possible lines of descent of the Arthropoda 150

42. Triarthrus bccki Green. Thoracic appendages 155

43. The same. Pygidial appendages 157

44. The same. Pygidial appendages 158

45. Cryptolithus tessellatus Green. Drawing of the best single specimen 159

46. The same. Part of the thorax and pygidium, with appendages 162

Frontispiece. Charles Emerson Beecher, 1896.

Plates 1-5. Photographs of Triarthrus bccki, made by C. E. Beecher.

Plate 6. Photographs of Triarthrus bccki (figs. 1-3), Acidaspis trcntonensis (fig.
6), and Cryptolithus tessellatus (fig. 7), made by C. E. Beecher. Photo-
graphs of the endopodites of a probable species of Calymene (figs. 4, 5)

Plates 7-8. Photographs of Cryptolithus tessellatus, made by C. E. Beecher.

Plate 9. Drawings of Cryptolithus tessellatus, made by C. E. Beecher or under his
direction.

Plate 10. Photographs of Isotelus latus and /. maximus, made by C. E. Beecher.

Plate n. Drawing of a restoration of Ceraurus pleurexanthemus, made by Elvira
Wood.

HISTORICAL REVIEW.

The beginning of the search for the limbs of trilobites was coeval with the beginning
of scientific study of the group, knowledge of the appendages being essential to the proper
systematic allocation of the animals.

The early search was so barren of results that negative evidence came to be accepted
as of positive value, and it was for many years generally believed that such organs as may
have been present beneath the dorsal test were so soft as to be incapable of preservation.
This view is best expressed by Burmeister (1846, p. 43) :

There is good proof that the feet of trilobites must have been soft membranous organs, for the absence
of the slightest remains of these organs in the numerous specimens observed is of itself evidence of the fact,
and it can indeed scarcely be supposed that hard horny extremities should be affixed to a soft membranous
abdominal surface; since they would not have possessed that firm basis, which all solid organs of locomotion
require, in order that they may be properly available.

Very well reasoned, and were it not for the discovery of new material in American local-
ities, Burmeister's views would probably never have been proved incorrect. One can not
escape the suspicion that some of the accepted hypotheses of today, founded on similar
"proof," may yield in time to the weight of bits of positive evidence.

The history of the study of appendages of trilobites may be divided into two periods.
The first, in which there was a general belief that the appendages were soft organs, but
during which numerous "finds" of limbs were reported, extended from the time of Linne
to the year (1876) in which Walcott demonstrated the fact that the animals possessed
jointed ambulatory and breathing organs.

The second, much more fruitful period, began with Walcott's publication of 1881,
descriptive of the appendages of Ceraurus and Calymene, and for the purposes of this
memoir, closes with his great contribution on the anatomy of Neolcnns (1918). Beecher's
brilliant productions came in the middle of the second period.

In the first period, there were at least two authentic discoveries of appendages, those
of Eichwald (1825) and Billings (1870), but since neither of these men convinced his con-
freres of the value of his finds, the work of neither can be considered as having marked an
especial epoch in the history.

As all the authentic finds will be treated in detail on later pages, only a brief resume
of the first period will be given here. This has already been done by Burmeister (1843,
1846) and Barrande (1852, 1872), whose works have been my primary sources of informa-
tion, but I have looked up the original papers, copies of nearly all of which are to be seen
in the libraries in Cambridge and Boston. Brig.-Gen. A. W. Vogdes, U. S. A. (retired),
has very kindly placed at my disposal a number of references and notes.

Linne (1759) was the first to report the discovery of appendages of trilobites. Torn-
quist (1896) has pressed for a recognition of the contribution of the great Swedish natu-
ralist to this problem, but Beecher (1896 B) doubted the validity of the find. Linne figured
a specimen of Parabolina spinulosa (Wahlenberg), with what he interpreted as a pair of
antennae attached. He states (translation quoted from Tornquist) : "Most remarkable in
this specimen are the antennae in the front, which I never saw in any other sample, and
which clearly prove this fossil to belong to the insects." Beecher has shown as conclusively
as can be shown without access to the original specimen that the supposed antennje were
really only portions of the thickened anterior border, the appearance being due to imperfect
preservation. Briinnich as early as 1781 called attention to the imperfection of this speci-

1 8 VTHE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

men, and it is also referred to by Wahlenberg (1821, pi 39), Brongniart (1822, p. 42),
Dalman (1828, p. 73), and Angelin (1854, p. 46).

Audouin (1821) seems to have been the first naturalist with sufficient knowledge of
the Arthropoda to be competent to undertake the study of the trilobites. He concluded that
the absence of ventral appendages was probably a necessary consequence of the skeletal
conformation, and thought if any were discovered, they would prove to be of a branchial
nature.

Wahlenberg (1821) in the same year expressed his belief that the trilobites were nearly
allied to Limulus and in particular tried to show that the trilobites could have had masti-
catory appendages attached about the mouth as in that modern "insect" (p. 20). Wahlen-
berg was also the first to describe an hypostoma of a trilobite (p. 37, pi. I, fig. 6), but
did not understand the nature of his specimen, which he described as a distinct species.

Brongniart (1822, p. 40) devoted five pages of his monograph to a discussion of the
affinities of trilobites, concluding that it was very probable that the animals lacked antennae
and feet, unless it might be that they had short soft feet which would allow them to creep
about and fix themselves to other bodies.

Schlotheim (1823) thought that the spines on Agnostus pisiformis were segmented
and compared them with the antennae of Acarus.

Stokes (1823) was the first who, with understanding, published an illustration of the
ventral side of a trilobite, having figured the hypostoma of an Isotelus. He was followed
in the next year (1824) by Dekay, who also figured the hypostoma of an Isotelus, and
added some observations on the structure of trilobites. The researches of Barrande, Novak,
Broegger, Lindstroem, and others have dealt so fully with the hypostoma that further refer-
ences to that organ need not be included here.

Dalman (1826, 1828) reviewed the opinions of his predecessors, and thought it not
impossible that organs of mastication may have been present under the head shield of the
trilobite as in Limulus (1828, p. 18). In this he of course followed Wahlenberg.

Goldfuss (1828) figured sections of Dalmanites hausmanni, Phacops macro phthalina,
and Calymcnc tristani, which remind one of some of Doctor Walcott's translucent slices.
So far as one can judge from the illustrations, it is probable that what he took for limbs
were really fragments of other trilobites. Such is certainly the case in his figures 9 and
10, where a number of more or less broken thoracic segments are present. The section of
Encrinurus punctatus shown in figure 7 may possibly exhibit the position and folds of the
ventral membrane beneath the axial lobe, and also, perhaps, the appendages. His figures 4,
5 and 8 show the hypostoma in section.

Pander (1830) described the hypostoma in greater detail than had been done by previ-
ous authors, but otherwise added nothing to the subject.

Sternberg (1830) thought he had individuals showing appendages, but judging from
his poor figures, he was deceived by fragmentary specimens.

Green (1839 A, B, C) described specimens of Phacops from Berkeley Springs, West
Virginia, which had the hypostoma in position, and appear to have had a tubular opening
under the axial lobe. While appendages were not actually present, these specimens sug-
gested fairly correct ideas about the swimming and breathing organs of trilobites. They
were similar to the ones which Castelnau obtained, and all were perhaps from the same
locality.

It is not worth while to do more than enumerate the other authors of this period :
Hisinger 1837, Emmerich 1839, Milne-Edwards 1841, for they all shared the same views,
and added nothing to what was already known.

HISTORICAL REVIEW. I 9

Castelnau (1843) described and figured a Phacops said to come from Cacapon Springs,
West Virginia, which he thought possessed remains of appendages. There is nothing in the
description or figures to indicate exactly what was present, but it is very unlikely that any
limbs were preserved. The broad thin "appendage" figured may have been a fragment of
a thoracic segment. This specimen was evidently described by Castelnau before 1843, as
is inferred from a reference in the Neues Jahrbuch, 1843, P- 5°4> but I have not seen the
earlier publication.

Burmeister (1843-1846), in his "Organization of the Trilobites," reviewed in cxtenso
the history of the search for appendages, and concluded that they must have been so soft
as to preclude the possibility of their being preserved as fossils. "Their very absence in
fossils most distinctly proves their former real structure" (p. 10). In figures 7 and 8 on
plate 6 he gave a restoration of the ventral surface of an Asaphus, the first restoration of
the ventral anatomy to be attempted. Since he chose modern branchiopods as his model,
he did not go so far wrong as he might have done. Still, there is little in the figure that
would now be accepted as correct. The following quotation will serve to give the opinion
of this zoologist, who from his knowledge of the Crustacea, was the most competent of the
men of his time to undertake a restoration of the appendages of the trilobites:

. . . in giving a certain form to the feet in the restored figure, I have done so rather intending to
indicate what they might have resembled, than with any idea of assuming their actual form. I merely assert
that these organs were soft, membranous, and fringed, adapted for locomotion in water, placed on the
abdominal portion of the body, and extending sidewise beneath the lateral lobes of the rings, as shown in
the ideal transverse section. These feet were also indented, and thus divided into several lobes at the open
lower side, and each separate lobe was furnished at the margin with small bristles serving as fins. The last
and external lobe was probably longer, smaller, and more movable, and reached to the termination of the
projecting shell lobe, bearing a bladder-shaped gill on the inner side (1846, p. 45).

McCoy (1846) observed in several trilobites a pair of pores situated in the dorsal fur-
rows near the anterior end of the glabella. He showed that the pits occupy precisely the
position of the antennae of insects and suggested that they indicated the former presence
of antennae in these trilobites (chiefly Anipyx and "Trinucleus"). The evidence from Cryp-
tolithus, set forth on a later page, indicates the correctness of McCoy's view.

Richter (1848, p. 20, pi. 2, fig. 32) described and figured what he took to be a phyl-
lopod-like appendage found in a section through a Phacops. Without the specimen it is
impossible to say just what the structure really was. The outline figure is so obviously
modeled on an appendage of Apus that one is inclined to think it somewhat diagrammatic.
In calling attention to this neglected "find," Clarke (1888, p. 254, fig.) interprets the
appendage as similar to the spiral branchiae of Calymcne senaria, and adds that he himself
has seen evidence of spiral branchiae in the American Phacops rana.

Beyrich (1846) described a cast of the intestine of "Trinucleus," and Barrande (1852)
further elaborated on this discovery.

Corda (1847) made a number of claims for appendages, but all were shown by Bar-
rande (1852) to be erroneous.

Barrande (1852, 1872) gave a somewhat incomplete summary of the various attempts
to describe the appendages of trilobites, concluding that none showed any evidence of other
than soft appendages, until Billings' discovery of 1870.

Volborth (1863) described a long chambered tubular organ in Illccnus which be believed
to represent a cast of the heart of a trilobite, but which has since been likened by writers to
the intestinal tract in "Trinucleus."

PART I.

THE APPENDAGES OF TRILOBITES.
TERMINOLOGY.

The terminology employed in the succeeding pages is essentially the same as that used
by Beecher, with two new terms added. Beecher assigned to the various segments of the
limbs the names suggested by Huxley, but sometimes used the name protopodite instead of
coxopodite for the proximal one. It is obvious that he did not use protopodite in the cor-
rect sense, as indicating a segment formed by the fusion of the coxopodite and basipodite.
The usage employed here is shown in figure i.

FIG. i. — Triarthrus becki Green. Diagram of
one of the limbs of the thorax, viewed from
above, with the endopodite in advance of the exo-
podite. i, coxopodite, the inner extension being
the endobase (gnathobase on cephalon) ; 2, basip-
odite, springing from the coxopodite, and sup-
porting the exopodite, which also rests upon the
coxopodite; 3, ischiopodite ; 4, meropodite; 5,
carpopodite ; 6, propodite ; 7, dactylopodite, with
terminal spines.

The investigation of Ceraurus showed that the appendages were supported by processes
extending downward from the dorsal test, and on comparison with other trilobites it appeared
that the same was true in Calymene, Cryptolitlms, Ncolcnus, and other genera. Thin sec-
tions showed that these processes were formed by invagination of the test beneath the dorsal
and glabellar furrows. While these processes are entirely homologous with the entopo-
physes of Limulus, I have chosen to apply the name appendifcr to them in the trilobites.

The only other new term employed is the substitution of endobase for gnathobase in
speaking of the inner prolongation of a coxopodite of the trunk region. The term gnatho-
base implies a function which can not in all cases be proved.

The individual portions of which the limbs are made up are called segments, and the
articulations between them, joints. Such a procedure is unusual, but promotes clearness.

NEOLENUS. 2 1

THE APPENDAGES OF NEOLENUS.
HISTORICAL.

The first mention of Neolenus with appendages preserved was in Doctor Walcott's
paper of 1911, in which two figures were given to show the form of the exopodites in com-
parison with the branchiae of the eurypterid-like Sidneyia. In 1912, two more figures were
presented, showing the antennules, exopodites, and cerci. The specimens were found in the
Burgess shale (Middle Cambrian) near Field, in British Columbia. This shale is exceedingly
fine-grained, and has yielded a very large fauna of beautifully preserved fossils, either
unknown or extraordinarily rare elsewhere. It was stated in this paper (1912 A) that
trilobites, with the exception of Agnostus and Microdiscus, were not abundant in the shale.

In discussing the origin of the tracks known as Protichnites, Walcott presented four
figures of Neolenus with appendages, and described the three clawlike spines at the tip of
each endopodite.

Three new figures of the appendages were also contributed to the second edition of the
Eastman-Zittel "Text-book of Paleontology" (1913, p. /oi). Later (1916, pi. 9) there
was published a photograph of a wonderful slab, bearing on its surface numerous Middle
Cambrian Crustacea. Several of the specimens of Neolenus showed appendages.

Finally, in 1918, appeared the "Appendages of Trilobites," in which the limbs of
Neolenus were fully described and figured (p. 126), and a restoration presented. Organs
previously unknown in trilobites, epipodites and exites, attached to the coxopodites, were
found.

Neolenus serratus (Rominger).
(Text fig. 2-8.)

Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1911, p. 20, pi. 6, figs. I, 2 (exopodites of thorax and
cephalon) ; — Ibid., vol. 57, 1912, p. 191, pi. 24, figs, i, la (antennules, caudal rami, and endopodites of
thorax) ; — Ibid., vol. 57, 1912, p. 277, pi. 45, figs. 1-4 (antennules, endopodites of cephalon and thorax, caudal
rami) ; — Text-book of Paleontology, edited by C. R. Eastman, 2d ed., vol. i, 1913, p. 701, fig. 1343 (exopo-
dites), p. 716, fig. 1376 (abdominal appendages), fig. 1377 (appendages of thorax and pygidium) ; — Ann.
Rept. Smithson. Inst. for 1915, 1916, pi. 9; — Smithson. Misc. Coll., vol. 67, 1918, pp. 126-131 et al., pi. 14,
fig. i; pis. 15-20; pi. 21, fig..6; pis. 22, 23; pi. 31 (restoration); pi. 34, fig. 3 (restored section); pi. 35,
fig. 4; pi. 36, fig. 3 (hypostoma).

The following description of the appendages of Neolenus is summarized from Walcott's
paper of 1918, and from a study of the eight specimens mentioned below.

Cephalon.

The antennules are long, slender, and flexible, and lack the formal double curvature so
characteristic of those of Triarthrus. There are short fine spines on the distal rims of the
segments of the proximal half of each, thus giving great sensitiveness to these organs. In
the proximal portion of each, the individual segments are short and wider than long, and in
the distal region they are narrow and longer than wide.

There are four pairs of biramous cephalic appendages, which differ only very slightly
from the appendages of the thorax. All are of course excessively flattened, and they are here
described as they appear.

The coxopodites, shown for the first time in Walcott's paper of 1918, are broad, longer
than wide, and truncated on the inner ends, where they bear short, stout, unequal spines

THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

similar to those along the anterior margin. The gnathobases are but slightly modified to
serve as mouth parts, much less so than in Triarthrus, but the coxopodites of the cepha-
lon are shorter and wider than those of the thorax.

At the distal end of the coxopodite arise the endopodite and exopodite. The endo-
podite consists of six segments, the distal ones, propodite and dactylopodite, more slender
than the others, the last bearing three terminal spines. The first endopodite is shorter than
the others and slightly more slender (pi. 16, fig. i)1 and the anterior appendages turn
forward more or less parallel to the sides of the hypostoma (pi. 22). The basipodite,
ischiopodite, meropodite, and carpopodite are, in their flattened condition, roughly rectan-
gular, only a little longer than wide, taper gradually distally, each bears small spines on the
outer rim, and some of the proximal ones usually have a row along the margin.

The exopodites of the cephalon, as of the body of Ncolcnus, are very different from
those of any other trilobite whose appendages were previous!)' known. As shown in the
photographs (pi. 20, fig. 2; pi. 22), each exopodite consists of a single long, broad, leaf-
like blade, not with many segments as in Triarthrus, but consisting of a large basal and
small terminal lobe. It bears on its outer margin numerous relatively short, slender, flat
setae. The long axes of the exopodites point forward, and the setae are directed forward
and outward. They stand more nearly at right angles to the shaft on the cephalic exopo-
dites than on those of the thorax. This same type of broad-bladed exopodite is also found
on the thorax and pygidium.

The number of functional gnathobases on the cephalon is unknown. That four endo-
podites were present on one side is shown pretty clearly by specimen 58591 (pi. 16, fig. 3)
and while no more than two well preserved exopodites have been seen on a side, there
probably were four. Specimen 65513 (pi. 16, fig. i) shows gnathobases on the second and
third appendages of that 'individual as preserved, but there is no positive evidence that these
are really the second and third appendages, for they are obviously displaced. The hypos-
toma of Neolenus is narrow but long, several specimens showing that it extended back to
the horizon of the outer ends of the last pair of glabellar furrows. It is not as \vide as the
axial lobe, so that, while gnathobases attached beneath the first pair of furrows would prob-
ably not reach back to the posterior end of the hypostoma, they might lie parallel to it and
not extend beneath. It seems possible, then, that there were four pairs of endobases but that
the second rather than the first pair served as mandibles, as seems to be the case in
Ceraurus.

Thorax.

The thorax of Ncolcnus consists of seven segments, and the appendages are well shown
(pi. 17, fig. i; pi. 18, figs, i, 2; pi. 20, fig. i.), The endopodites of successive segments
vary but little, all are slender but compact, and consist of a long coxopodite with six short,
rather broad segments beyond it. In the figures, the endopodites extend some distance in
a horizontal direction beyond the edges of the dorsal test, as many as four segments being
in some cases visible, but measurements show that the appendages tended to fall outward on
decay of the animal. The dactylopodites are provided with terminal spines as in Triarthrus.
The coxopodites are long, straight, and slender. They are well shown on only one speci-
men (pi. 18), where they are seen to be as wide as the basipodite, and the endobases are set
with spines on the posterior and inner margins. They are so long that those on opposite

1 Nota bene! All references in this section are to the plates of Doctor Walcott's paper in 1918.

NEOLENUS. 23

sides must have almost met on the median line. The segments of the endopodites are mostly
but little, if any, longer than broad, and at the distal end each shows two or more spines.
The propodite and dactylopodite are notably more slender than the others. The exopodites
of the thorax are broad and flat, and each shaft has two distinct parts with different kinds
of setie. The posterior edge of the proximal lobe is fringed with a slender, flat, overlapping
hairs which are a little longer than the width of the lobe, and stand at an angle of about
60 degrees with the direction of the axis of the appendage. The outer lobe is at an angle
with the main one, and has short, very fine setae oh the margin. One or two specimens show
some evidence of a joint between the inner and outer lobes, but in the great majority of
cases they seem to be continuous; if originally in two segments, they have become firmly
united. The exopodites of the thorax, like those of the cephalon, are directed diagonally
forward and outward. (PI. 21, fig. 6; pi. 22.)

Pygidium.

The pygidium of Neolcnus serratus is large, and usually shows five rings on the axial
lobe and four pairs of ribs on the sides. There are five pairs of biramous appendages be-
longing to this shield, and behind these a pair of jointed cerci. That the number of abdomi-
nal appendages should correspond to the number of divisions of the axial lobe rather than
to the number of ribs on the pleural lobes is of interest, and in accord with other trilobites,
as first shown by Beecher.

The endopodites of the pygidium have the same form as those of the thorax, are long,
and very much less modified than those of any other trilobite whose appendages are known.
On some specimens, they extend out far beyond the dorsal test, so that nearly all the seg-
ments are visible (pi. 17, fig. 3; pi. 18; pi. 19; pi. 20, fig. i), but in these cases are prob-
ably displaced. The segments are short and wide, the whole endopodite tapering gradually
outward. The dactylopodite bears terminal spines, and the individual segments also have
outward -directed spines.

The cerci appear to have been long, slender, very spinose organs much like the anten-
nules, but stiff rather than flexible. They are a little longer than the pygidium (pi. 17, figs,
i, 2), and seem to be attached to a plate on the under surface of the posterior end and in
front of the very narrow doublure. The precise form of this attachment can not be deter-
mined from the published figures. They bear numerous fine spines (pi. 17, fig. 3).

Epipodites and Exiles.

Doctor Walcott has found on several specimens of Neolenns remains of organs which
he interprets as epipodites and exites attached to the coxopodites. A study of the specimens
has, however, convinced me that both the large and small epipodites are really exopodites,
and that the exites are badly preserved and displaced coxopodites. Detailed explanation of
this interpretation is given below in the description of the several specimens involved.

Description of Individual Specimens.

Doctor Walcott was kind enough to send me eight of the more important specimens
of Neolenus figured by him, and since my interpretation of them does not agree in all re-
spects with his, I have thought it fairer to the reader to present here rather full notes
explaining the position I have taken. I understand that since I communicated my interpre-
tation of the epipodites and exites to him, Doctor Walcott has submitted the specimens to

24 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

several palaeontologists, who consider that epipodites are really present. Since I am not able
to convince myself that their conclusion is based upon sound evidence, I give here my own
interpretation. There is of course, no a priori reason why trilobites should not have had
epipodites.

Specimen No. 58589.

Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, pi. 45, fig. 2; — Zittel-Eastman Text-book of
Paleontology, vol. I, 1913, fig. 1377; — Smithson. Misc. Coll., vol. 67, 1918, pi. 18, fig. i; pi. 20, fig. I.

This is one of the most important of the specimens, as it shows the coxopodites of
three thoracic limbs and the well preserved endopodites of six thoracic and five pairs of
pygidial appendages.

The appendages are all shifted to the left till the articular socket of the coxopodite is.
about 8 mm. outside of its proper position. The endopodites extend a corresponding amount
beyond the edge of the dorsal test and are there so flattened that they are revealed as a

Fig. 2. — Neolcnus serratus (Rominger). A sketch of the coxopodites
and endopodites of two thoracic segments. Note notch for the reception
of the lower end of the appendifer. X 3-

mere impression. The coxopodites, which are beneath the test, seem to have been somewhat
protected by it, and while hopelessly crushed, are not flattened, but rather conformed to the
ridges and grooves of the thorax.

The coxopodite of the appendage of the last thoracic segment is best preserved. It is
rectangular, about one third as wide as long, with a slight notch in the posterior margin
near the outer end. The inner end is obliquely truncated and shows about ten sharp spines
which do not appear to be articulated to the segment, but rather to be direct outgrowths
from it. There are similar spines along the posterior margin, but only two or three of
what was probably once a continuous series are now preserved. On the opposite margin
of the coxopodite from the slight depression mentioned above, there is a slight convexity in
the outline, which is better shown and explained by the coxopodite just in front of this.
That basal segment has the same form as the one just described, but as its posterior margin
is for the greater part of its length pushed under the one behind it, the spines are not shown.
On the posterior margin, two-thirds of the length from the proximal end, there is a shallow
notch, and corresponding to it, a bulge on the anterior side. From analogy with Ceraurus
and Calymene it becomes plain that the notch and bulge represent the position of the socket
where the coxopodite articulated with the appendifer. Since these structures have not been
shown in previous illustrations, a drawing giving my interpretation of them is here inserted

NEOLENUS. 25

(fig. 2). It is evident from the position of the notch that the row of spines was on the
dorsal (inner) side of the coxopodite and that the truncation was obliquely downward and
outward.

The endopodite of the last thoracic appendage is well preserved and may be described
as typical of such a leg in this part. The basipodite is as wide as the coxopodite, and it
and the three succeeding segments, ischiopodite, meropodite, and carpopodite, are all parallel-
sided, not expanded at the joints, and decrease regularly in width. The propodite and
dactylopodite are also parallel-sided, but more slender than the inner segments, and on the
end of the dactylopodite there are four little spines, three of them — one large and two small
—articulated at the distal end, and the fourth projecting from the posterior outer angle.
Each segment has one or more spines on the outer articular end, and the ischiopodite has
several directed obliquely outward on the posterior margin. All of the four proximal segments
show a low ridge parallel to and near the anterior margin, and several endopodites of the py-
gidium have a similar ridge and a row of spines along the posterior margin of some of the
segments. These features indicate that the segments in question were not cylindrical in life,
but compressed. From the almost universal location of the spines on the posterior side of
the limbs as preserved, it seems probable that in the natural position the segments were held
in a plane at a high angl'e with the horizontal, the ridge was dorsal and anterior and the
row of spines ventral and posterior. Because the spines on the endobases are dorsal it
does not follow that those on the endopodites were, for the position of the coxopodite in a
crushed specimen does not indicate the position of the endopodite of even the same appendage.

The endopodites of the pygidium are similar to the one just described, except that
some of them have spines on the posterior margin of the segments, and a few on the right
side have extremely fine, faintly visible spines on the anterior side. The specimen shows
fragments of a few exopodites, but nothing worth describing. In the middle of the right
pleural lobe there is a small organ which Walcott has interpreted as a small epipodite. It
is oval in form, broken at the end toward the axial lobe, and has exceedingly minute short
setre on the posterior margin. From analogy with other specimens, it appears to me to be
the outer end of an exopodite.

Measurements: The entire specimen is about 64 mm. long and 52 mm. wide at the
genal angles. The thorax is about 41 mm. wide (disregarding the spines) at the seventh
segment, and the axial lobe about 13 mm. wide at the same horizon. The measurements
of the individual segments of the seventh left thoracic limb are :

Coxopodite, 9 mm. long, 3 mm. wide, the middle of the notch 8 mm. from
the inner end, measured along the bottom, and 6 mm. measured
along the top.

Basipodite, 5 mm. long, 3 mm. wide

Ischiopodite, 4 3

Meropodite, 3.5 " !' 2.5

Carpopodite, 3.5 " 2

Propodite, 3 " " 1.25 "

Dactylopodite, 2 1.25 "

The five distal segments of the last pygidial endopodite are together 10.5 mm. long.
The whole six segments of the endopodite of the third thoracic segments are together 21 mm.
long. The distance from the appendifer of the third segment to the outer end of the spine
is 17 mm. From the center of the notch in the coxopodite to the outer end is 1.5 mm.,

26

THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

which, added to the length of the endopodite, 21 mm., makes a distance of 22.5 mm. from
the appendifer to the tip of the dactylopodite, showing that if projected straight outward,
the endopodi'tes of the thorax would project 5.5 mm. beyond the test, including spines.

The distance across the axial lobe from appendifer to appendifer on the seventh thoracic
segment is 12.5 mm. Measured along the top of the coxopodite, it is 6 mm. from the middle
of the notch to the inner end, and measured along the bottom it is 8 mm. From the trun-
cated form of the ends it is evident that the coxopodites extended inward and downward
from the appendifers, and with the dimensions given above, the inner toothed ends would
practically meet on the median line.

Measurements on the appendages of the pygidia show that on this specimen they extend
back about twice as far beyond the edge of the pygidium as they should, all being displaced.

Specimen No. 65514.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, pi. 19, figs. 1-3.

This specimen is so twisted apart that it is not possible to determine to what segments
the appendages belong, but it exhibits the best preserved exopodites I have seen. The

Fig. 3. — Exopodite of Neo-
lenus serratus (Rominger), to
show form of the lobes of the
shaft, and the setae. X 4-

Fig. 4. — Ncolcnus serratus (Rominger).
One of the so-called epipodites of specimen
65515, showing that it has the same outline
as an exopodite (compare figure 3) and
fragments of setae on the margin. X 3-

best one is just in front of the pygidium on the matrix, and shows a form more easily seen
than described (our fig. 3). There is a broad, flat, leaf-like shaft, the anterior side of
which follows a smooth curve, while in the curve on the posterior side, which • is convex
backward, there is a re-entrant, setting off a small outer lobe whose length is about one
third the length of the whole. This lobe seems to be a continuation of the shaft, and the
test of the whole is wrinkled and evidently very thin. The main and distal lobes of the
shaft both bear numerous delicate setae, but those of the outer lobe are much shorter and
finer than those on the main portion. The latter are flattened and blade-like.

The anterior edge of the shaft shows a narrow stiffening ridge and the setre are but little
longer than its greatest width. The second segment of the pygidium' has another exopodite
like this one, but shows faintly the line between the two lobes, as though there were two

segments.

This specimen also shows some very well preserved endopodites, but they differ in no way
from those described from specimen No. 58589. Walcott mentions two large epipodites pro-
jecting from beneath the exopodites. I judge that he has reference to the distal lobes
of the exopodites, but as these are continuous with the main shaft, there can be no other
interpretation of them than that which I have given above.

NEOLENUS. 27

• Measurements: The pygidium is 19 mm. long (without the spines) and about 34 mm.
wide at the front. The exopodites show faintly beneath the pygidial shield, but their proxi-
mal ends are too indistinct to allow accurate measurement. Apparently they were just
about long enough to reach to the margin of the shield. The best preserved one, that of
the second segment in the pygidium, is about u mm. long, 2.5 mm. wide at the widest;
the distal lobe is 2.5 mm. long, and the longest seta: of the main lobe 3.5 mm. long. The
pleural lobe of the pygidium is just 1 1 mm. wide at this point.

The endopodites project from 8 to 12 mm. beyond the pygidium, showing about four

segments.

The thoracic exopodite described above is n mm. long and 2.75 mm. wide at the widest
part. The distal lobe is 3.5 mm. long and 2.25 mm. wide, and the longest setae on the main

lobe 3 mm. long.

Specimen No. 65519.

Illustrated: Walcott, Zittel-Eastman Text-book of Paleontology, vol. I, 1913, fig. 1343; — Smithson. Misc.
Coll., vol. 67, 1918, pi. 21, fig. 6.

This specimen is somewhat difficult to study but is very valuable as showing the natural
position of the exopodites of the anterior part of the thorax. Walcott's figures are excel-
lent and show the broad leaf-like shafts, the distal lobes with the re-entrant angles in the pos-
terior margin, and the long fine set;e of the main lobes. None of the distal lobes retains its
setae. All extend back to the dorsal furrows, but the proximal ends are not actually shown.

The specimen is especially important because it shows the same distal lobes as speci-
men No. 65514, and demonstrates that they are a part of the exopodite and not of any other
structure.

Measurements: The exopodite belonging to the fourth thoracic segment is 23 mm.
long and 4 mm. wide at the widest part. The longest setae are 7 mm. in length.

Specimen No. 65520.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, pi. 20, fig. 2; pi. 22, fig. I.

This is a practically entire specimen, on two blocks, one showing the interior of the shell,
and the other the one figured by Walcott, a cast of the interior. The first shows the low
rounded appendifers at the anterior angle of each axial tergite. They are almost entirely
beneath the dorsal furrows and do not project so far into the axial lobe as those of Ccraurus
and Calynicnc. In fact, only those at the anterior end of the thorax project inward at all.
As expected, there are five pairs on the pygidium. The cephalon is unfortunately so exfol-
iated that the appendifers there are not preserved. The doublure of the pygidium is ex-
tremely narrow.

The cast of the interior shows, rather faintly, the exopodites of the right side of the
thorax and of the left side of the cephalon, and, still more faintly, the caudal rami and
a few pygidial endopodites. The exopodites on the right side are in what seems to be the
customary position, directed obliquely forward and out\vard, and the tips of their distal
lobes project slightly beyond the edge of the test. These lobes were interpreted by Walcott
as epipodites, but after comparing them with the terminal lobes of the exopodites of speci-
mens No. 65519 and 65514 I think there can be no doubt that they represent the same
structure. The pleura of the individual thoracic segments on this side of the specimen
have an unusual appearance, for they are bluntly rounded or obtusely pointed, instead of
being spinose.

28 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

The interpretation of the appendages of the cephalon is somewhat difficult. At -the
left of the glabella there are two large exopodites, the anterior of which lies over and par-
tially conceals the other. These show by their position that they belong to the fourth and
fifth cephalic appendages. In front of these lie two appendages which may be either endop-
odites or exopodites, but which I am inclined to refer to the latter. Both are narrow and
shaped like endopodites, but bear on their (kiter edges close-set fine sete. They also show
what might be considered as faint traces of segmentation. If the first of these ran under
the end of the exopodite behind it, as shown in Walcott's figure (pi. 22), then it would
be necessary to interpret it as an endopodite, but it really continues down between the exop-
odite and the glabella, and seems to be attached opposite the middle of the eye. The
specimen does not indicate clearly whether this appendage is above or below the exopo-
dite behind it, but one's impression is that it is above, in which case it also must be an
exopodite. The appendage in front, being similar, is similarly interpreted. If this be cor-
rect, then the exopodites of the second and third cephalic appendages are much shorter and
narrower than those of the fourth and fifth. • All of these appendages are obviously out of
position, for the cheek has been pushed forward away from the thorax, though still pivot-
ing on its inner angle at the neck-ring, till the eye has been brought up to the dorsal fur-
row. In this way the anterior exopodites have been thrust under the glabella and all the
appendages have been moved to the right of their original position. The anterior exopo-
dite is very poorly shown, but seems to be articulated in front of the eye. The posterior
exopodites are very similar to those on the thorax. The distal lobe is shown only by the
second from the last. It has the same form as the distal lobes on the thoracic exopodites,
and like them has much finer setae than the main lobe, but it does not stand at so great an
angle with the axis of the main lobe, nor yet is it so straight as shown in Walcott's
figure.

Measurements: The specimen is about 72 mm. long and 54 mm. wide at the genal
angles. The pygidium is 22 mm. long and 37 mm. wide. The doublure is 1.5 mm. wide.
The exopodite of the third thoracic segment is 19.5 mm. long. The pleural lobe at this point
is 13 mm. wide without the spines and 18.5 mm. wide with them. The third exopodite of
the cephalon was apparently about 15 mm. long when complete.

Specimen No. 65515.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, pi. 20, figs. 3, 4.

This is a small piece of the axial portion of a badly crushed Neolenus, showing appen-
dages on the left side as viewed from above. On the posterior half there are three large
appendages which have the exact form of the exopodites of other specimens. There is a
broad, oval, proximal lobe and a distal one at an angle with it. The proximal part of the
shaft has fine setae or the bases of them, and the distal lobe faint traces of much finer ones.
The form, and the setje so far as they are preserved, are exactly like those of the exopodites
on the specimens previously described. (See fig. 4, page 26.) Beneath them there are
slender, poorly preserved endopodites.

In front of the exopodites and endopodites lie a series of structures which Walcott
has called exites, but for which I can see another explanation. Walcott has shown them
as four broad rounded lobes, but his figure must be looked upon as a drawing and not as a
photograph, for it has been very much retouched.

NEOLENUS.

29

For convenience of discussion, these lobes may be called Nos. i, 2, 3, and 4, the last
being the posterior one (fig. 5). This lobe is best shown on the matrix, where the anterior
end is seen to be margined by stout spines, while the posterior end lies over the endopodite
and under the exopodite behind it. No. 3 is sunk below the level of the others, and only
a part of it has been uncovered. Its margin bears strong spines of different sizes. Its
full shape can not be made out, but it has neither the shape nor the form of spines shown
in figure 3, plate 20 (1918). Lobes 2 and i and another lobe in front of i seem to form
a continuous series and to be part of a single appendage. They are all in one plane, arc
so continuous that the joints between them can be made out with difficulty and if they do
belong together, can easily be explained.

Fig. 5. — A sketch
of the so-called
exites of Neolenus
serratus (Rom-
inger), to show the
form and the char-
acter of the spines.
X2.

Fig. 6. — Endop-
odite of a cephalic
appendage of Neo-
lenus serratus
(Rominger), show-
ing the very broad
coxopodite. X 2.

Before calling these structures new organs not previously seen on trilobites, it is of
course necessary to inquire if they can be interpreted as representing any known structures.
That they can not be exopodites is obvious, since they are bordered by short stout spines
instead of setas. The same stout spines that negate the above possible explanation at once
suggest that they are coxopodites (compare fig 6). At first sight, the so-called exites seem
too wide and too rounded to be so interpreted, but if reference be had to the specimens
rather than the figures, it will be noted that the only well preserved structure (No. 2) is
longer than wide, has spines only on one side and one end, and does not differ greatly
from the coxopodite of specimen No. 58589 (pi. 18, 1918). If structures 2, i, and the
segment ahead of i are really parts of one appendage, it can only be an endopodite, of
which No. 2 is the coxopodite, No. I the basipodite, and the next segment the ischiopo-
dite. If one looks carefully, there are no traces of spines on either end of No. i, but only
on the margin. The extreme width of No. 2 is against this interpretation as a coxopo-
dite (see, however, fig. 6), but it may be rolled out very flat, as this is an unusually

3O THE APPENDAGES, ANATOMY, AND RKI.ATIONS OF TKTLOBITES.

crushed specimen. No. 2 is 10 mm. long and 6 mm. wide at the widest point. No. i is
5 mrn. long and 3.5 mm. wide.

The crucial point in this determination is whether 2 and i are parts of the same appen-
dage. I believe they are, but others may differ.

Specimen No. 65513.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, pi. 45, fig. 3; — Ibid., vol. 67, 1918, pi. 16, figs, i, 2.

This is nearly all of the right half of an entire specimen, but the only appendages of
any interest are those of the cephalon. Five endopodites emerge from beneath that shield,
but as all are displaced it is not possible to say how many belong to the head. When held
at the proper angle to the light, the second and third from the front show faintly the par-
tial outlines of the coxopodites. The anterior side and end of the best preserved one
shows irregular stout spines of unequal sizes, and the inner end is truncated obliquely (fig.
6). These coxopodites are like those on the thorax of specimen No. 58589, but shorter
and wider. This of course suggests that the "exite" No. 2 of specimen No. 65515 may
be a cephalic coxopodite. The endopodite of this appendage, like the others on this cepha-
lon, is shorter and stouter than the thoracic or pygidial endopodites of the others described.

Fig. 7. — A restored section across the thorax of Neolcnus
scrratus, showing the probable form of attachment of the ap-
pendages, their relation to the ventral membrane, and the jaw-
like endobases of the coxopodites.

Measurements: The cephalon is 24 mm. long and about 60 mm. wide. The coxopodite
of the third appendage is about 10 mm. long and 5.5 mm. wide at the widest point. The
corresponding endopodite is 19 mm. long and projects n mm. beyond the margin, which is
about 5 mm. further than it would project were the appendage restored to its proper position.

RESTORATION OF NEOLENUS.

(Text fig.. 7, 8.)

This restoration is based upon the information obtained from the studies which have
been detailed in the preceding pages, and differs materially from that presented by Doctor
Walcott. The appendages are not shown in their natural positions, but as if flattened nearly
into a horizontal plane. The metastoma is added without any evidence for its former
presence.

The striking features of the appendages are the broad unsegmented exopodites which
point forward all along the body, and the strong endopodites, which show practically no
regional modification. Although the exopodites have a form which is especially adapted
for use in swimming, their position is such as to indicate that they were not so used. The
stout endopodites, on the other hand, probably performed the double function of natatory
and ambulatory legs.

NEOLENUS.

Fig. 8. — Neolcnus scrrafus (Rominger). A restoration of the ventral
surface, with the endopodites omitted from one side, to permit a better
exposition of the exopodites. The position and number of the appendages
about the mouth are in considerable doubt. Restored by Doctor Elvira
Wood under the supervision of the writer. About one-half larger than the
average specimen.

Nathorstia transitans Walcott.
Illustrated : Walcott, Smithson. Misc. Coll., vol. 57, 1912, pi. 28, fig. 2.

The badly preserved specimen on which this genus and species was based is undoubt-
edly a trilobite, but for some reason it does not find a place in Walcott's recent article
on "Appendages" (1918). The preservation is different from that of the associated trilo-
bites, being merely a shadowy impression, indicating a very soft test. The general outline

32 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

of the body, the position of the eye, and even a trace of spines about the pygidium (in
the figure) are similar to those of Neolenns, and I would venture the suggestion that
Nathorstia transitans is a recently moulted Neolenus serratus, still in the "soft-shelled" con-
dition. Even if not a Neolenus, it is probable, from the state of preservation, that it is
an animal which had recently cast its shell.

Walcott describes such fragments of appendages as remain, as follows :

Head. A portion of what may be an antenna projects from beneath the right anterior margin ; from near
the left posterolateral angle a large four-jointed appendage extends backward. I assume that this may be the
outer portion of the large posterior appendage (maxilla) of the head.

Thorax. Traces of several slender-jointed thoracic legs project from beneath the anterior segments and
back of these on the right side more or less of six legs have been pushed out from beneath the dorsal shield;
these are composed of three or four long slender joints; fragments of the three proximal joints indicate that
they are shorter and larger and that they have a fringe of fine setae. Indications of a branchial lobe (gill) are
seen in two specimens where the legs are not preserved. This is often the case both among the Merostomata
(pi. 29, fig. 3, Malaria) and Trilobita (pi. 24, fig. 2, Ptychoparia).

Two caudal rami project a little distance beneath the posterior margin of the dorsal shield.

This latter feature of course suggests Neolenus' The other appendages are too poorly
preserved to allow comparison without seeing the specimen.

The specific name was given "on account of its suggesting a transition between a
Merostome-like form, such as Malaria spinifera, and the trilobites." In what respect it
is transitional does not appear.

Formation and locality: Same as that of Neolenus serratus. One nearly complete
specimen and a few fragments were found.

THE APPENDAGES OF ISOTELUS.
HISTORICAL.

The first specimen of Isotclus with appendages was described orally by Billings before
the Natural History Society of Montreal in 1864, and in print six years later (1870, p.
479, pis. 31, 32). The specimen is described in detail on a later page. Billings recog-
nized the remains of eight pairs of legs on the thorax, a pair for each segment, and he
inferred from the fact that the appendages projected forward that they were ambulatory
rather than natatory organs. He was unable to make out the exact number of the seg-
ments in the appendages, but thought each showed at least four or five.

Having examined the individual sent to London by Billings, Woodward (1870, p. 486,
fig, i) reviewed the collection from the American Trenton in the British Museum and
found a specimen in the "Black Trenton limestone," from Ottawa, Ontario, in which, along-
side the hypostoma, was a jointed appendage, which he described as the "jointed palpus of
one of the maxilla;." This has always been considered an authentic "find," but I am in-
formed by Doctor Bather that the specimen does not show any real appendage. For
further discussion, see under Isotclus gigas.

In 1871, Billings' specimen was examined by Professors James D. Dana (1871, p.
320), A. E. Verrill, and Sydney I. Smith, who agreed that the structures identified by
Billings as legs were merely semicalcified arches of the membrane of the ventral surface,
which opinion seems to have been adopted by zoologists generally in spite of the fact that
the most elementary consideration of the structure of the thorax of a trilobite should have
shown its falsity. While the curvature of the thoracic segments was convex forward, that
of the supposed ventral arches was convex backward, and the supposed arches extended

ISOTELUS. 33

across so many segments as to have absolutely prevented any great amount of motion of
the segments of the thorax on each other. Enrollment, a common occurrence in Isotclus,
would have been absolutely impossible had any such calcified arches been present.

Walcott, in his study of trilobites in thin section (1881, pp. 192, 206, pi. 2, fig. 9),
obtained eleven slices of Isotclus gigas which showed remains of appendages. He figured
one of the sections, stating that it "shows the basal joint of a leg and another specimen
not illustrated gives evidence that the legs extended out beneath the pygidium, as indicated
by their basal joints."

The second important specimen of an Isotclus with appendages was found by Mr.
James Pugh in strata of Richmond age 2 miles north of Oxford, Ohio, and is now in the
U. S. National Museum. It was first described by Mickleborough (1883, p. 200, fig. 1-3).
In two successive finds, a year apart, the specimen itself and its impression were recov-
ered. Since I am redescribing the specimen in this memoir (see p. 35), it only remains to
state here that Mickleborough interpreted the structures essentially correctly, though not
using the same terminology as that at present adopted. His view that the anterior appen-
dages were chelate can not, however, be supported, nor can his idea that the sole appendages
of the pygidium were foliaceous branchial organs.

Walcott (1884, p. 279, fig. i) studied the original specimens and presented a figure
which is much more detailed and clear than those of Mickleborough. By further cleaning
the specimen he made out altogether twenty-six pairs of appendages. He stated that one
of these belonged to the cephalon, nine to the thorax,1 and the remaining sixteen to the
pygidium. He showed that the endopodites of the pygidium were of practically the same
form as those on the thorax, and stated that the "leg beneath the thorax of the Ohio
trilobite shows seven joints in two instances; the character of the terminal joint is unknown."
His figure shows, and he mentions, markings which are interpreted as traces of the fringes
of the exopodites.

In the same year Woodward (1884, p. 162, fig. 1-3) reproduced all of Micklebor-
ough's figures, and suggested that the last seven pairs of appendages on the pygidium of
Calymene and Isotclus were probably "lamelliform branchiferous appendages, as in Liinuliis
and in living Isopoda."

Professor Beecher published, in 1902, an outline taken from Mickleborough's figure of
this specimen, to call attention to certain discontinuous ridges along the axial cavity of the
anterior part of the pygidium and posterior end of the thorax. These ridges are well shown
in Mickleborough's figure, though not in that of Walcott, and their presence on the speci-
men was confirmed by a study by Schuchert, who contributed a diagrammatic cross-section
to Beecher's paper (1902, p. 169, pi. 5, figs. 5, 6). Beecher summarized in a paragraph
his interpretation of this specimen :

The club-shaped bodies lying within the axis are the gnathobases attached at the sides of the axis; the
curved members extending outward from the gnathobases are the endopodites ; the longitudinal ridges in
the ventral membrane between the inner ends of the gnathobases are the buttresses and apodemes of the
mesosternites; the slender oblique rod-like bodies shown in the right pleural region in Walcott's figure are
portions of the fringes of the exopodites.

In 1910, Mr. W. C. King of Ottawa, Ontario, found at Britannia, a few miles west of
Ottawa, the impression in sandstone of the under surface of a large specimen of Isotelus
arenicola, described on a later page (p. 39).

1 The posterior one of these he believed to have been crowded forward from beneath the pygidium.

34 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

Finally (1918, p. 133, pi. 24, figs. 3, 3a; pi. 25), Walcott has redescribed the speci-
men from Ohio, presenting a new and partially restored figure. He refers also to the speci-
men from Ottawa under the name Isotelus covingtoncnsis? Foerste (not Ulrich). He
advances the view, which I am unable to share, that the cylindrical appearance of the
segments of the appendages of Isotclus is due to post-mortem changes.

Isotelus latus Raymond.
(PI. 10, fig. i.)

Illustrated: Asaphus platycephalus Billings, Quart. Jour. Geol. Soc., London, vol. 26, 1870, pi. 31, figs. 1-3;
pi. 32, figs, i, 2. — Woodward, Geol. Mag., vol. 8, 1871, pi. 8, figs. I, la. — Gerstacker, in Bronn's "Klassen u.
Ordnungen d. Thier-Reichs," 1879, pi. 49, fig. i. — von Koenen, N. Jahrb. f. Min., etc., vol. i,' 1880, pi. 8,
fig. 8. — Milne-Edwards, Ann. Sci.- Nat., Zoologie, ser. 6, vol. 12, 1881, pi. 12, fig. 45.

Isotelus latus Raymond, Bull. Victoria Mem. Mus., Geol. Survey Canada, No. i, 1913, p. 45 (species
named).

Isotelus covingionensis? Walcott (not Foerste), Smithson. Misc. Coll., vol. 67, 1918, p. 134.

Knowledge of the appendages of this species is derived from the specimen which
Billings described in 1870. It was found in the Trenton, probably the Middle Trenton,
near Ottawa, Ontario, and is preserved in the Victoria Memorial Museum at Ottawa.

Viewed from the upper surface, it shows a large part of the test, but is broken along
the sides, so that parts of the free cheeks, considerable of the pleural lobes of the thorax,
and one side of the pygidium are missing. Viewed from the lower surface, the appendages
are practically confined to the cephalon and thorax.

A short time before his death, Professor Beecher had this specimen and succeeded
in cleaning away a part. of the matrix so that the appendages show somewhat more clearly
than in Billings' time, but they are not so well preserved as on the Mickleborough speci-
men, found in Ohio somewhat later.

The hypostoma is in place and well preserved ; the posterior points are but 3 mm. in
advance of the posterior margin of the cephalon. Behind the hypostoma there are only two
pairs of cephalic appendages, the first of which is represented by the coxopodite and a trace
of the endopodite. The outer end of the coxopodite is close to the outer margin of one
of the prongs of the hypostoma and about 3 mm. in front of its posterior end. The gnatho-
base curves backward and inward, and appears to pass under the tip of the hypostoma.
There were probably two appendages in front of this, whose gnathobases projected under
the hypostoma, but the specimen shows nothing of them unless it be that one small frag-
ment about 2 mm. back of the center is really a part of a gnathobase.

The specimen retains only the coxopodite and basipodite of the posterior cephalic ap-
pendage on the left side. The coxopodite is long and apparently cylindrical, the cross-
section being of uniform diameter throughout the length. The inner portion is nearly
straight, while the outer part is curved gently forward.

It is possible to make out remains of eight pairs of appendages on the thorax, some of
them represented by coxopodites only, but most with more or less poorly preserved endop-
odites as well. No exopodites are visible. The coxopodites of the thorax seem to be of
the same form as the last one on the cephalon, but slightly less curved. All are long and
heavy, and there seems to be no decrease in size toward the pygidium. The endopodites are
very imperfectly shown. They seem to be longer than those of Isotelus ma.vimns, and the
segments, while of less diameter than the coxopodites, do not show so great a contrast to

ISOTELUS. 35

them as do those of that species. The direction of the endopodites is diagonally forward,
and the outer portions do not appear to be curved backward as in Isotelus maximus. It
would appear also that the endopodites were nearly or quite long enough to reach the outer
margin of the dorsal test. On no endopoclite can more than three segments be definitely dis-
tinguished, but the longest ones are the most obscurely segmented.

No appendages are preserved on the pygidium, but at one side of the median groove
there are two projections which may be processes to which the appendages were attached.

Measurements: Total length of specimen, 109 mm. Probable length when complete,
116 mm. Length of cephalon, 40 mm.; width at genal angles, restored, about 62 mm.
(Billings' restoration). Width of doublure of front of cephalon on median line, 17 mm.;
length of hypostoma, 20 mm. Length of coxopodite of last appendage on left side of
cephalon, 10.5 mm.; length of basipodite of the same appendage, 5 mm. Diameter of cox
opodite, 2 mm.; diameter of basipodite, 1.5 mm. Length of coxopodite on left side oi
the second segment of the thorax, u mm.; diameter, about 2.5 mm. Length of basipodite
of the same, 5 mm.; diameter, about 1.5 mm. Length of ischiopodite, 3.5 mm.; diameter,
about 1.5 mm. Length of meropodite, 2.5 mm. (this may be less than the total length as
the segment is not completely exposed.) Distance between proximal ends of gnathobase."
of the fifth thoracic segment, about 7 mm. Distance between outer ends of the coxopo-
dites of the first thoracic segment (estimated from measurements on the left side), 27 mm
Distance apart of the dorsal furrows at the first thoracic segment, 27 mm. Length of the
longest exopodite which can be traced, about 20 mm.

Isotelus maximus Locke.
(PI. 10, fig. 2.)

Illustrated: Mickleborough, Jour. Cincinnati Soc. Nat. Hist., vol. 6, 1883, p. 200, figs. 1-3 (endopodites
and coxopodites).— Walcott, Science, vol. 3, 1884, p. 279, fig. I (endopodites, coxopodites, and traces of
exopodites).— Woodward, Geol. Mag., dec. 3, vol. I, 1884, p. 162, figs. 1-3 (copies of Mickleborough's
figures).— Bernard, The Apodidx, 1892, text fig. 49.— Beecher, Amer. Jour. Sci., vol. 13, 1902, p. 169, pi. 5.
figs. 5, 6 (outline from one of Mickleborough's figures and an original figure). — Walcott, Smithson. Misc.
Coll., vol. 67, 1918, p. 133, pi. 24, figs. 3, 3a ; pi. 25, fig. I.

This specimen, which conies from the Richmond strata 2 miles north of Oxford, Ohio,
is the best preserved of the specimens of Isotelus with appendages which has so far been
found. The individual consists of two parts, the actual specimen, and the impression of
the ventral side.

To describe it I am using very skillfully made plaster reproductions of both parts,
presented to the Museum of Comparative Zoology by Doctor Charles D. Walcott, and pre-
sumably made after he cleaned the specimen as described in Science (1884). I have also
an enlarged photograph (pi. 10, fig. 2) which seems to have been made after some later
period of cleaning, probably by Professor Beecher, and I have examined the original speci-
mens in Washington.

Viewed from the dorsal side, it is seen that the individual is very imperfect, the greater
part of the cephalon being removed by a diagonal bre.ak which cuts off the anterior third
of the left eye and extends to the front of the second thoracic segment on the right side.
The ends of the pleura of both sides of the thorax are broken away, as are also the greater
parts of the pleural lobes and the posterior end of the pygidium. On the ventral side, merely
the posterior tips of the hypostoma remain, but the distal ends of the appendages were so
far within the outer margin that the appendagiferous area is quite fully retained.

36 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOB1TES.

The most conspicuous feature of this specimen is the presence of nine pairs of large
coxopodites behind the hypostoma, and of the remains of ten pairs of endopodites, mak-
ing in all ten pairs of appendages which are easily seen. The apportionment of these seg-
ments to cephalon, thorax, and pygidium is not agreed upon by the people who have
examined the specimens, but if one remembers that it is the outer and not the inner end
of the coxopodite which articulates with the appendifer, it at once becomes evident that
the first two pairs of appendages on the specimen are the last two pairs belonging to the
cephalon, and that the next eight pairs are those of the thorax.

The impressions of fourteen pairs of coxopodites are readily counted on the pygidium,
and as Doctor Walcott noted sixteen pairs on the actual specimens, his number was prob-
ably correct.

Cephalon.

Projecting the line of the back of the cephalon through from the dorsal side, it is
found that the posterior tips of the hypostoma are 7 mm. in front of the posterior mar-
gin of the cephalon, and that the points of attachment of the posterior pair of cephalic ap-
pendages (the second pair shown on the specimen) are just within the posterior margin.
The gnathobases of this pair of appendages extend back some distance beneath the thorax,
and so give the impression that they belong to that part of the body. So far as can
be determined, the cephalic appendages do not differ in any way from those of the thorax.
On the mould of the ventral surface, just outside of the lateral edge of the right lobe of the
hypostoma, is the somewhat imperfectly shown impression of the endopodite of the third
cephalic appendage. The point of junction of the endopodite and coxopodite is about 2 mm.
in front of the tip of the adjacent branch of the hypostoma, and the gnathobase is curved
around just behind it. This accounts for three of the pairs of cephalic appendages. The
second cephalic appendages must have thrust their gnathobases under the prongs of the
hypostoma, and the endopodites were probably close to its edge. No trace of this pair ap-
pears on the specimen.

Thorax.

The thoracic appendages are the best preserved of any, and show the large coxopodites
and the more slender endopodites which do not extend to the outer margin of the test.
The latter extend forward and outward for about one half their length, then turn backward
in a graceful curve.

Walcott's figure in Science shows hair-like markings on the under side of the right
half of the thorax. These were interpreted by both Walcott and Beecher as fringes of the
exopodites, but since the setas of those organs on all other trilobites are always above the
endopodites, while these are represented as below them, it would seem doubtful if this in-
terpretation can be sustained. Furthermore, I find no trace of them on either cast or mould,
and the actual specimen does not now show them.

Pygidium.

The coxopodites and endopodites of the pygidium seem to be similar to those on
the thorax, but both are shorter and more slender, and the former decrease in length
rapidly toward the posterior end. As mentioned above, it is not perfectly plain how many
appendages are present, but I have accepted Doctor Walcott's count of sixteen pairs. Of the
endopodites only the barest traces are seen, and of exopodites nothing.

ISOTELUS. 37

One point of considerable interest in this specimen is the thickness, as it probably gives
some measure of the space occupied by the animal. In Triarthncs and other trilobites from
Rome, New York, the appendages are pressed directly against the dorsal test, but in
this specimen a considerable space intervenes between the plane of the appendages and the
shell. Between the central furrow and the inner surface of the dorsal test at the anterior
end of the thorax is a distance of 13 mm. and under the dorsal furrows the thickness is
about 7 or 8 mm., no accurate measurement being possible in the present state of the
specimen.

Measurements: Length of specimen on median line, 121 mm.; probable original length,
about 195 mm. (Walcott's restoration). Length of thorax, 58 mm.1 Width of axial lobe
at the first thoracic segment, 45 mm. ; total width as preserved, 92 mm. ; width as esti-
mated 'from the mould of the ventral surface, no mm.; Walcott's restoration, 105 mm.

Length of coxopodite of fourth left cephalic appendage, about 18 mm.; diameter,
about 2.5 mm. Length of coxopodite of last left cephalic appendage, about 18.5 mm. Dis-
tance apart of inner ends of gnathobases of fourth cephalic appendages, about 4 mm. Dis-
tance apart of inner ends of endobases of first thoracic segment, about 6 mm. Distance
apart of outer ends of coxopodites of first thoracic segment, about 43 mm.

Length of coxopodite of seventh left thoracic appendage 16 mm., diameter about
3.5 mm. ; length of basipodite of the endopodite of the same appendage 6 mm. ; diameter
about 2 mm.; length of ischiopodite 5 mm.; length of meropodite 4.5 mm.; length of car-
popodite 4.5 mm.; length of propodite 3 mm.; length of dactylopodite 2.75 mm.; total
length of endopodite 25.75 mm-

Length of coxopodite of fourth left thoracic appendage 20 mm., diameter 4 mm.; length
of five proximal joints of the endopodite 25 mm. ; diameter of basipodite about 2 mm.

RESTORATION OF ISOTELUS.

(Text fig. 9.)

The exopodites have been omitted from this restoration since nothing is known of their
actual form. The chief reason for the figure is to contrast the greatly developed coxopo-
dites of the posterior part of the cephalon and thorax with those of other trilobites. The
antennules and first two pairs of biramous appendages of the cephalon are more or less hy-
pothetical, and less is known of the appendages of the pygidium than is shown here. The
restoration is based somewhat upon Walcott's figure in Science. The outline is that of
a specimen of Isotelus maximus from Toronto, Ontario.

Isotelus gigas Dekay.

Illustrated : Woodward, Quart. Jour. Geol. Soc., London, vol. 26, 1870, text fig. I ; — Geol. Mag., dec. 3,
vol. i, 1884, p. 78, text fig. — Milne-Edwards, Ann. Sci. Nat, Zoologie, ser. 6, vol. 12, 1881, pi. 12, fig. 46. —
Walcott, Bull. Mus. Comp. Zool., Harvard Coll., vol. 8, 1881, pi. 2, fig. 9;— Geol. Mag., dec. 4, vol. I, 1894, pi.
8, fig. 9;— Proc. Biol. Soc. Washington, vol. 9, 1894, pi. I, fig. 9.

The specimen in the British Museum which Woodward called Asaphus platycephalus,
is, in all probability, an Isotelus gigas. Woodward says of it:

1 If this specimen had the same proportions as specimens of Isotelus maximus from Toronto, the total
length would be only 174 mm. The cephalon would be about 52 mm. long, the thorax 58 mm., and the pygidium
about 64 mm. long.

THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

Fig. 9. — A restored composite of Isotelus maximus and /. latus.
The exopodites are left out because entirely unknown. Drawn by
Doctor Elvira Wood. Natural size.

I was at once attracted by a specimen of Asaphus, from the Black Trenton Limestone (Lower Silurian),
which has been much eroded on its upper surface, leaving the hypostoma and what appear to be the appendages
belonging to the first, second, and third somites, exposed to view, united along the median line by a longitudinal
ridge. The pseudo-appendages, however, have no evidence of any articulations. But what appears to me
to be of the highest importance, as a piece of additional information afforded by the Museum specimen, is
the discovery of what I believe to be the jointed palpus of one of the maxillae, which has left its impression
upon the side of the hypostoma — just, in fact, in that position which it must have occupied in life, judging
by other Crustaceans which are furnished with an hypostoma, as Apus, Serolis, etc.

The palpus is 9 lines in length, the basal joint measures 3 lines, and is 2 lines broad, and somewhat
triangular in form.

There appear to be about 7 articulations in the palpus itself, above the basal joint, marked by swellings
upon its tubular stem, which is I line in diameter.

ISOTELUS. 39

Desiring to know more of this individual, I wrote to Doctor Bather and was surprised
to learn that the specimen which was the basis of Woodward's observations is so badly pre-
served as to be of no real value. With his permission, I append a note made by Doctor
Bather some years ago when selecting fossils to be placed on exhibition:

Asaphus gigas Dekay. Ordovician, Trenton Limestone. N. America, Canada. Descr. H. Woodward,
1870, Q. J. G. S., XXVI, pp. 486-488, text fig. I, as Asaphus platycephalus. Coll. and presd. J. J. Bigsby,
1851. Regd. I 14431.

This specimen is in the Brit. Mus. Geol. Dept. I 14431. The supposed hypostome is exceedingly doubt-
ful; it lies dorsad of the crushed glabellar skeleton. The "appendage" is merely the edge of a part in the
head-shield; the maxilla is some calcite filling, between two such laminae.

13 Sept. 1911. (Signed) F. A. BATHER.

Walcott figured a slice of Isotelus gigas from Trenton Falls, New York, which shows
a few fragments of appendages, but is of particular importance because it shows the pres-
ence of well developed appendifers beneath the axial lobe.

Isotelus arenicola Raymond.
Illustrated: Ottawa Nat, vol. 24, 1910, p. 129, pi. 2, fig. 5.

The following quotations from my paper are inserted here to complete the record of
appendage-bearing specimens :

A rather remarkable specimen of this species was found by W. C. King, Esq., on the shore of Lake
Deschenes at Britannia [near Ottawa, Ontario]. This specimen is an impression of the lower surface of
the trilobite, and shows a longitudinal ridge corresponding to the central furrow along the axis of the ventral
side of the animal, ten pairs of transverse furrows, and the impression of the hypostoma. The doublure of
the pygidium has also left a wide smooth impression, but in the cephalic region the hypostoma is the only
portion of which there are any traces remaining. The specimen was found on a waterworn surface of the
beach, partially covered by shingle. . . .

The transverse furrows are the impressions left by the gnathobases of the basal joints of the legs. They
were evidently long and very heavy, but the specimen has been so abraded that all details are obscured.
The first six pairs of impressions are longer and deeper than the four behind. The first eight pairs seem
to pertain to the thoracic appendages, while the last two belong to the pygidium. From the posterior tips
of the hypostoma to the first gnathobases of which traces are present there is a distance of about 22 mm.
without impressions. In Isotelus gigas the hypostoma normally extends back to the posterior margin of the
cephalon, so that it seems that in this specimen the impressions of the first two pairs of gnathobases under
the thorax may not have been preserved. In that case, the six pairs of strong impressions may represent
the last six pairs of thoracic segments, and the pygidium might begin with the first of the fainter ones.

Horizon and locality: From the sandstone near the base of the Aylmer (Upper Chazy)
formation at Britannia, west of Ottawa, Ontario. Specimen in the Victoria Memorial Mu-
seum, Geological Survey of Canada, Ottawa.

THE APPENDAGES OF TRIARTHRUS.

Triarthrus becki Green.

(Pis. 1-5; pi. 6, figs. 1-3; text figs, i, 10, n, 33, 42.)
(Also see Part IV.)

Illustrated: Matthew, Amer. Jour. Sci., vol. 46, 1893, pi. I, figs. 1-7; — Trans. N. Y. Acad. Sci., vol. 12,
pi. 8, figs. 1-7. — Beecher, Amer. Jour. Sci., vol. 46, 1893, text figs. 1-3; — Amer. Geol., vol. 13, 1894, pi. 3; —
Amer. Jour. Sci., vol. 47, pi. 7, text fig. I ;— Amer. Geol., vol. 15, 1895, pis. 4, 5; — Ibid., vol. 16, 1895, pi. 8,

4-O THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

figs. 12-14; pl- IO> fig- !> — Amer. Jour. Sci., vol. i, 1896, pj. 8; — Geol. Mag., dec. 4, vol. 3, 1896, pi. 9; —
Eastman-Zittel Text-book of Paleontology, vol. I, 1900, text figs. 1267-1269; — 2d ed., 1913, fig. 1375; — Studies
in Evolution, 1901, reprint of all previous figs.; — Amer. Jour. Sci., vol. 13, 1902, pi. 2, figs. 1-5; pi. 3, fig. i;
pi. 4, fig. i; pi. 5, figs. 2-4; — Geol. Mag., dec. 10, vol. 9, 1902, pis. 9-11, text figs. 1-3. — Walcott, Proc.
Biol. Soc. Washington, vol. 9, 1894, P'- *> fiss- J"6; — Geol. Mag., dec. 4, vol. i, 1894, pi. 8; — Smithson. Misc.
Coll., vol. 67, 1918, pi. 29, figs. l-ii; pi. 30, figs. 17-20; pi. 32; pi. 34, figs. 4-7; pi. 35, fig. 5. — Bernard, Quart.
Jour. Geol. Soc., London, vol. 50, 1894, text figs, n, 12. — CEhlert, Bull. Soc. Geol. France, ser. 3, vol. 24, 1896,
text figs. 1-17, 34. — Jaekel, Zeits. d. d. geol. Gesell., vol. 53, 1901, text fig. 24. — Moberg, Geol. Foren. Forhandl.,
vol. 29, pt. 5, 1907, pi. 4, fig. 2; pi. 5, fig. i. — Handlirsch, Foss. Insekten, 1908, text fig. 6. — Tothill, Amer.
Jour. Sci., vol. 42, 1916, p. 380, text fig. 5. — Crampton, Jour. N. Y. Entomol. Soc., vol. 24, 1917, pi. 2, fig. 20.

HISTORICAL.

Specimens of Triarthrus retaining appendages were first obtained by Mr. W. S. Valiant
from the dark carbonaceous Utica shale near Rome, New York, in 1884, but no consid-
erable amount of material was found until 1892. The first specimens were sent to
Columbia University, and were described by Doctor W. D. Matthew (1893). This article
was accompanied by a plate of sketches, showing for the first time the presence of antennules
in trilobites and indicating something of the endopodites and exopodites of the appendages of
the cephalon, thorax, and pygidium. Specimens had not yet been cleaned from the lower
side, so that no great amount could then be learned of the detailed structure. Matthew con-
cluded that "The homology with Limulus seems not to be as close in Triarthrus as in the
forms studied by Mr. Walcott; but the characters seem to be of a more comprehensive type,
approaching the general structure of the other Crustacea rather than any special form."

Professor Beecher's first paper, dated October 9, 1893, merely mentioned the fact that
the Yale University Museum had obtained material from Valiant's locality, but was quickly
followed by a paper read before the National Academy of Sciences on November 8, and
published in December, 1893. This paper described particularly the thoracic appendages.

This was followed in January (1894 A) by an article in which some information about
the mode of occurrence of the specimens was added, and in April (1894 B), the limbs of
the pygidium were described and figured. The determination of the structure of the appen-
dages of the head evidently presented some difficulty, for the article describing this portion
of the animal did not appear until the next February (1895 A). This cleared up the ven-
tral anatomy of Triarthrus, and was followed by a short article (1896 A) accompanied by
a restoration of the trilobite showing all the appendages.

This ended Professor Beecher's publications on Triarthrus until his final paper in 1902,
although he contributed some of his results and figures to his chapter on the trilobites in
the Eastman-Zittel Text-book of Paleontology in 1900.

The discovery of these excellent specimens had of course excited very great interest.
Doctor Walcott also studied a number of specimens from Valiant's locality, and published
in 1894, with some original figures, the results of his comparison of the appendages of
Triarthrus with those of Calymene and Ceraurus,

In his article on the "Systematic Position of the Trilobites," Bernard (1894) used
the results of Professor Beecher's studies of 1893, and also quoted the papers by Matthew
(1893) and Walcott (1894), though the article by the latter appeared too late to be used
except for a note added while Bernard's paper was in press. A final footnote quoted from
Professor Beecher's paper of April, 1894 (1894 B).

CEhlert (1896) gave an excellent summary in French of the work of Beecher and Wal-
cott on Triarthrus, with reproductions of many of their figures.

TRIARTHRUS. 41

Valiant (1901) in a non-technical article described his long search for trilobites with
antennas. The discovery of the wonderful pyritized trilobites at Cleveland's Glen near Rome

Fig. 10. — Triarthrus becki Green. A new restoration, modified
from Professor Beecher's, to incorporate the results of his later
work. The inner ends of the endobases are probably too far apart,
as it was not discovered until after the drawing had been made that
the appendifers projected within the dorsal furrows. Drawn by
Doctor Elvira Wood. X about 3.8.

was not the result of a lucky accident, but the culmination of eight years of labor in a local-
ity especially selected on account of the fineness of grain of the shale.

After 1896, Professor Beecher turned his attention largely to the problem of the classi-
fication of trilobites, and while he continued the arduous task of cleaning the matrix from

42 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

specimens of Triarthrus and Cryptolithus he did not again publish upon the subject of
appendages until forced to do so by the doubts cast by Jaekel (1901) upon the validity of
his earlier conclusions. Because of certain structures which he thought he had interpreted
correctly from a poorly preserved specimen of Ptychoparia, Jaekel came to the conclusion
that Beecher's material was not well preserved. Professor Beecher would have taken much
more kindly to aspersions upon his opinions than to any slight upon his beloved trilobites,
and his article on the "Ventral Integument of Trilobites" of 1902 was designed not only
as an answer to Jaekel, but also to show by means of photographs the unusually perfect
state of preservation of the specimens of Triarthrus. This article, like so many describ-
ing the appendages of trilobites, beginning with Matthew's, was published in two places
(Beecher 1902).

Most of Beecher's papers, except the last one, were reprinted in the volume entitled
"Studies in Evolution," published by Charles Scribner's Sons at the time of the Yale Bi-
centennial in 1901. The part pertaining particularly to Triarthrus is on pages 197 to 219.

Moberg (1907), in connection with a specimen of Eurycare angustatum which he thought
preserved some appendages, described and illustrated some of the appendages of Triarthrus.

The most recent discussion of Triarthrus, with some new figures, is by Walcott (1918,
p. 135, pis. 29, 30). He gives a summary of Beecher's work with numerous quotations.
The principal original contribution is a discussion of the form and shape of the appendages
before they were flattened out in the shale. He found also what he thought might possibly
be the remains of epipodites on three specimens, one of which he illustrated with a photo-
graph. I have seen nothing which could be interpreted as such an organ in the many speci-
mens I have studied.

A point in which Walcott differs from Beecher in the interpretation of specimens is
in regard to the development of the endopodites of small pygidia. Beecher (1894 B, pi.
7, fig. 3) illustrated a series of endopodites which he likened to the enclites of a thoracic
limb of Apus. Doctor Walcott finds that specimens in the United States National Museum
show slender endopodites all the way to the back of the pygidium, and thinks that Beecher
mistook a mass of terminal segments of exopodites for a series of endopodites. On care-
ful examination, however, the specimen shows, as Beecher indicated, a series of endopodites
in undisturbed condition (No. 222, our pi. 4, fig. 5).

Restoration of Triarthrus.

One of the more important points noted in the later studies of Triarthrus is that the
gnathites of the cephalic appendages are much less like the endobases under the thorax than
Beecher earlier thought, and showed in his restored figures and in his model. The four
gnathites of each side are curved, flattened, not club-shaped, and so wide and so close together
that they overlap one another. The metastoma is somewhat larger and more nearly cir-
cular than Beecher's earlier preparations led him to suppose.

The restoration here presented is modified only slightly from the one designed by Pro-
fessor Beecher, and the modifications are taken principally from figures published by him.
The gnathites are drawn in form more like that shown by the specimens and his figures in
the American Geologist (1895 A), and the metastoma is taken from one of the specimens.
On the thorax the chief modification is in the addition of a considerable number of spines
to the endopodites. In spite of the trivial character of most of these changes, they empha-

TRIARTHRUS. 43

size one of the important characteristics of Triarthrus — the regional differentiation of the
appendages.

It should be pointed out that although Triarthrus is usually considered to be a very
primitive trilobite, its appendages are more specialized than those of any of the others
known. This is shown in their great length, the double curvature of the antennules, the
differentiation of four pairs of endobases on the cephalon as gnathites, and the flatten-
ing of the segments of the posterior enclopodites. These departures from the uniformity
existing among the appendages of the other genera lead one to question whether the genus
is really so primitive as has been supposed.

Relation of the Cephalic Appendages to the Markings on the Dorsal Surface of the Glabella.

Triarthrus bccki is usually represented as having four pairs of glabellar furrows, but
the two pairs at the front are exceedingly faint and the first of them is hardly ever visible,
though that it does exist is proved by a number of authentic specimens. The neck furrow
is narrow and sharply impressed, continuing across the glabella with a slightly backward
curvature. In front of it are two pairs of linear, deeply impressed furrows which in their
inward and backward sweep are bowed slightly forward, the ends of the corresponding
furrows on opposite sides nearly meeting along the crest of the glabella. In front of these,
near the median line, is a pair of slight indentations, having the appearance and position of
the inner ends of a pair of furrows similar to those situated just behind them.

In front of and just outside this pair are the exceedingly faint impressions of the
anterior pair of furrows, these, as said above, being but seldom seen. They are short, slightly
indented linear furrows which have their axes perpendicular to the axis of the cephalon,
and do not connect with each other or with the dorsal furrows. The latter are narrow,
sharply impressed, and merge into a circtimglabellar furrow at the front. In front of the
circumglabellar furrow is a very narrow rounded ridge, but the anterior end of the glabella
is very close to the margin of the cephalon.

Specimen No. 214, which was cleaned from the dorsal side, shows the posterior tip of
the hypostoma, apparently in its natural position, 3.5 mm. back from the anterior margin.
The entire length of the cephalon is 6 mm., so that the hypostoma reaches back slightly over
one half the length (0.583). The greater part of it has been cleaned off, and one sees the
proximal portions of the antennules, which are apparently attached just at the sides of the
hypostoma, 2.5 mm. apart and 2.25 mm. back from the anterior edge of the cephalon. This
position is distinctly within the outline of the glabella and corresponds approximately to
the location of the second pair of glabellar furrows. Specimens 214, 215, 216, 217, and
219 all seem to show the same location for the bases of the antennules. Specimen 220 is
the one in which the basal shafts are best preserved and the points of attachment seem to
be further apart in it than in any of the others. This specimen is 38 mm. long, and the
bases of the antennules are 5.5 mm. apart and 4 mm. behind the anterior margin. As the
specimen is cleaned from the ventral side, the dorsal furrows do not show distinctly, but
another specimen of about the same size (No. 228, 38.5 mm. long) has the dorsal furrows
8 mm. apart 4 mm. back of the anterior margin.

On the same slab with specimens 209 and 210 there is an individual which, although
retaining the test, has had the proximal ends of the antennules so pressed against it that
the course of the one on the left side is readily visible. It originates in a small oval mound

44 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRTLOBITES.

whose posterior margin impinges upon the third glabellar furrow near the middle of its
course, and just outside the outer end of the second glabellar furrow. The cephalon of this
specimen is 5 mm. long, and the point of origin of the left antennule is 2.75 mm. in front
of the posterior margin and 0.75 mm. from the dorsal furrow.

It is therefore evident that the antennules in this species are not attached beneath the
dorsal furrows, but within them and opposite the second pair of glabellar furrows.

All cephalic appendages behind the antennules are attached somewhat within the dorsal
furrows, the first pair as far forward as the antennules and the last pair apparently under
the anterior edge of the neck ring. They do not appear to correspond in position to the
posterior glabellar furrows and neck ring, being more crowded. The last pair is attached
to appendifers beneath the nuchal segment, and the first pair beneath the third glabellar fur-
rows. There are no depressions on the dorsal surface corresponding to the points of at-
tachment of the mandibles.

Anal Plate.

Professor Beecher, during his first studies of Triarthrus, found no appendages pertain-
ing to the anal segment, but later evidently came upon a spinose anal plate which he caused

Fig. ii. — Triarthrus becki
Green. Anal plate of specimen
65525 in the U. S. National
Museum. Drawn by Doctor
Wood. X 20.

to be figured. The specimen (No. 201) on which this appendage is preserved is cleaned
from the dorsal side, and the anal plate is a small, bilaterally symmetrical, nearly semicir-
cular structure margined with small spines. Specimen 202 also shows the same plate (pi. 5,
fig. 6), but it is imperfectly preserved. It has a large perforation in the anterior half.
Both of these specimens are in the Yale University Museum.

The anal plate is especially well shown by specimen 65525 in the United States National
Museum (fig. u). This specimen is from Rome, New York, and two photographs of it
have been published by Walcott (1918, pi. 29, fig. 6; pi. 30, fig. 19). It is developed from
the dorsal side, and the anal plate is displaced, so that it projects behind the end of the
pygidium. It is semicircular in shape, with a hemispheric mound at the middle of the an-
terior half. Two furrows starting from the anterior edge on either side of the mound
border its sides, and, uniting back of it, continue as an axial furrow to the posterior mar-
gin. The mound is perforated for the opening of the posterior end of the alimentary canal.
The lateral borders of the plate bear five pairs of short, symmetrically placed spines. The
plate is I mm. wide and 0.5 mm. long, and the entire trilobite is 11.5 mm. long.

PTYCHOPARIA. 45

THE APPENDAGES OF PTYCHOPARIA.

Ptychoparia striata (Emmrich).
Illustrated : Jaekel, Zeits. d. d. geol. Gesell., 1901, vol. 53, part I, pis. 4, 5.

Jaekel has described a specimen of this species obtained from the Middle Cambrian near
Tejrovic, Bohemia, which on development showed beneath the test of the axial lobe, cer-
tain structures which he believed represented the casts of proximal segments of appendages.
On the basis of this specimen he produced a new restoration of the ventral surface of the
trilobite, in which he showed three short wide segments in the place occupied by the coxopo-
dite of an appendage of Triarthrus. He also made the mouth parts considerably differ-
ent from those of the latter genus. Beecher (1902) showed that the structures which
Jaekel took for segments of appendages were really the fillings between stiffening plates
of chitin on the ventral membrane, and demonstrated the fact that similar structures ex-
isted in Triarthrus. It cannot be said, therefore, that any appendages are really known
in Ptychoparia striata, but some knowledge of the internal anatomy of the species is sup-
plied by the specimen.

Ptychoparia cordillerae (Rominger).

Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 192, pi. 24, fig. 2; — Ibid., vol. 67, 1918, pi.

21, figs. 3-5 (corrected figure).

Walcott has figured a single individual of this species showing appendages, the accom-
panying description being as follows (1918, p. 144) :

Ventral appendages. — Only one specimen has been found showing the thoracic limbs. This indicates very
clearly the general character of the exopodite and that it is situated above the endopodite, although there are
only imperfect traces of the latter. . . .

The exopodites are unlike those of any trilobite now known. They are long, rather broad lobes extending
from the line of the union of the mesosternites and the pleurosternites. At the proximal end they appear to
be as wide as the axial lobe of each segment, and to increase in width and slightly overlap each other nearly
out to the distal extremity. . . . They are finely crenulated along both the anterior and dorsal margins,
which indicates the presence of fine setae.

The specimen is quite imperfectly preserved, but seems to indicate that the exopodite
of Ptyclwparia had a long, rather narrow unsegmented shaft.

Measurements (from Walcott's figure) : The specimen is a small one, about 9.5 mm.
long, an individual exopodite is about 2 mm. long and the shaft 0.33 mm. wide.

Horizon and locality: Middle Cambrian, Burgess shale, between Mount Field and
Wapta Peak, above Field, British Columbia.

Ptychoparia permulta Walcott.

Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, p. 145, pi. 21, figs, i, 2.

Walcott figured one individual of this species showing long slender antennules pro-
jecting in front of the cephalon. It is of especial interest because one of the antennules
shows almost exactly the same sigmoid curvature which is so characteristic of the related
Triarthrus. The individual segments are not visible.

Measurements: The specimen is 23 mm. long and the direct distance from the front
of the head to the anterior end of the more perfect antennule is 9.5 mm. Measured along
the curvature, the same antennule is about 1 1 mm. long.

Horizon and locality: Same as the preceding.

46 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

/

THE APPENDAGES OF KOOTENIA.

Kootenia dawsoni Walcott.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, pi. 14, figs. 2, 3.

One specimen figured by Doctor Walcott shows the distal ends of some of the exopo-
clites and endopodites of the right side. He compares the exopodites with those of Nco-
lenus, stating that the shaft consists of two segments, the proximal section being long and
flat, fringed with long setae, while the distal segment has short fine setje. The endopodite
best shown is very slender, and the segments are of uniform width and only slightly longer
than wide.

Measurements (from Walcott's figures) : Length of specimen, about 41 mm. Length
of five distal segments of an endopodite, 7.5 mm. Since the pleural lobe is only 7 mm.
wide, the endopodites, and probably the exopodites also, must have projected a few milli-
meters beyond the dorsal test when extended straight out laterally.

Formation and locality: Burgess shale, Middle Cambrian, on the west slope of the
ridge between Mount Field and Wapta Peak, above Field, British Columbia.

THE APPENDAGES OF CALYMENE AND CERAURUS.
HISTORICAL.

All of the work on these species has been done by Doctor Walcott, who summarized
his results in 1881.

In the first of his papers (1875, p. 159), Walcott did not describe any appendages
but paved the way for further work by a detailed and accurate description of the ventral
surface of the dorsal shell of Ceraurus. He demonstrated the presence in this species
of strongly buttressed processes which extend directly downward from the test just within
the line of the dorsal furrows. One pair of these is seen beneath each pair of the gla-
bellar furrows, each segment of the thorax has a pair, and there are four pairs on the
pygidium. He pointed out also that these projections- were but poorly developed on that
part of the glabella which is covered by the hypostoma. He called them axial processes, the
only name which appears to have been suggested thus far.

The first announcement of the discovery of actual appendages in Ceraurus and Calym-
ene was made by the same investigator in a -pamphlet published in 1876 in advance of the
28th Report of the New York State Museum of Natural History, the publication of the
whole report being delayed till 1879. The results were obtained by the process of cut-
ting translucent slices of enrolled trilobites derived from the Trenton limestone at Trenton
Falls, New York. Since he summarized all the results of this study in one paper at a
later date, it is not necessary to follow the stages of the work.

A second preliminary paper was published in pamphlet form in September, 1877, and
in final form in 1879, when the first figures were presented.

In his important paper of 1881, Walcott reviewed all that was known of the appen-
dages of trilobites to that time, and gave the results of seven years of study of sections of
enrolled specimens. Slices had been made of 2,200 individuals from Trenton Falls, which
resulted in obtaining 270 which were worthy of study. Of these, 205 were from Ceraurus
pleurexanthemus, 49 from Calymcnc senaria, n from Isotelus gigas, and 5 from Acidaspis
trentonensis.

CALYMENE AND CERAURUS. 47

Walcott's views on certain portions of the anatomy can best be set forth in the form
of a few extracts (1881, pp. 199-208) :

The Ventral Membrane. — In those longitudinal sections in which the ventral membrane is most perfectly
preserved^ it is shown to have been a thin, delicate pellicle or membrane, strengthened in each segment by a
transverse arch, to which the appendages were attached. These arches appear as flat bands separated by a
thin connecting membrane, somewhat as the arches in the ventral surface of some of the Macrouran
Decapods. . . .

In by far the greater number of sections, both transverse and longitudinal, the evidence of the former
presence of an exterior membrane, protecting the contents of the visceral cavity, rests on the fact that the
sections show a definite boundary line between the white calcspar, filling the space formerly occupied by the
viscera, and the dark limestone matrix. Even the thickened arches are rarely seen.

The mode of attachment of the leg to the ventral surface is shown [in transverse and longitudinal
sections of Ceraurus and Calymene}. These illustrations are considered as showing that the point of articula-
tion was a small,- round process projecting from the posterior surface of the large basal joint, and articulating
in the ventral arch somewhat as the legs of some of the Isopods articulate with the arches in the ventral
membrane. The arches of the ventral membrane in the trilobite . . . afford a correspondingly firm basis for
the attachment of the legs.

Branchial appendages. — The branchiae have required more time and labor to determine their true structure
than any of the appendages yet discovered. They were first regarded as small tubes arranged side by side,
like the teeth in a rake; then as setiferous appendages, and finally as elongate ribbon-like spirals and bands
attached to the side of the thoracic cavity, the epipodite being a so-called branchial arm. All of these parts
are now known to belong to the respiratory system, but from their somewhat complex structure, and the
various curious forms assumed by the parts when broken up and distorted, it was a long time before their
relations were determined.

The respiratory system is formed of two series of appendages, as found beneath the thorax. The first
is a series of branchiae attached to the basal joints of the legs, and the second, the branchial arms, or epipodites.

The branchix, as found in Calymene, Ceraurus. and Addaspis, have three forms. In the first they
bifurcate a short distance from the attachment to the basal joint of the leg, and extend outward and downward
as two simple, slender tubes, or ribbon-like filaments. In the second form they bifurcate in the same mariner,
but the two branches are spirals. These two forms occur in the same individual but, as a rule, the more
simple ribbon-like branchia is found in the smaller or younger specimens, and the spiral form in the adult.
. . . The spiral branchiae of Ceraurus are usually larger and coarser than those of Calymene.

The third type of the branchiae [consists of rather long straight ribbons arranged in a digitate manner
on a broad basal joint]. As far as yet known, this is confined to the anterior segments of the thorax.

The epipodite or branchial arm was attached to the basal joints of the thoracic legs and formed of two
or more joints. This has been called a branchial arm, not that it carried a branchia, but on account of its
relation to the respiratory system. It is regarded as an arm or paddle, that, kept in constant motion, produced
a current of water circulating among the branchiae gathered close beneath the dorsal shell. . . .

Of the modification the respiratory apparatus underwent beneath the pygidium, we have no evidence.

In his latest publication (1918, pp. 147-153, pis. 26-28, 33), Walcott has reviewed his
earlier work on Calymene and Ceraurus, and presented a new restoration of the former.
The coxopodites are now interpreted as being similar to those of Triarthrus and Neolenus,
but the exopodites are still held to be spiral and the setiferous organs labelled as epipo-
dites rather than exopodites.

COMPARISON OF THE APPENDAGES OF CALYMENE AND CERAURUS WITH THOSE OF

TRIARTHRUS.

As one may see by reading the above quotations from Doctor Walcott's descriptions,
he found certain branchial organs in Ceraurus and Calymene which have not been found
in other trilobites but otherwise the essential features of the appendages of all are in agree-
ment.

4^ THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

Spiral Branchice.

It is now necessary to inquire if the thin sections can not be interpreted on the basis
of trilobites with the same organs as Triarthrus. The interpretation of the structures seen
in these translucent slices is exceedingly difficult, and Doctor Walcott deserves the utmost
praise for the acumen with which he drew his deductions. Even with the present knowl-
edge of Triarthrus, Isotelus, and Neolenus as a guide, I do not think it is safe to speak
dogmatically about what one sees in them.

Walcott has summarized his results in his restoration of the appendages of Calymcne
(1918, pi. 33). The coxopodite supports a slender six-jointed endopodite as in Triarthrus,
dorsal to which is a short setiferous epipodite which differs from the exopodite of Triar-
thrus, in being less long, unsegmented, and in having shorter setae. Arising from the same
part of the coxopodite with this epipodite is the bifurcate spiral branchia which has not been
seen in this form in other trilobites. The evidence on which the existence of this organ is
postulated consists of a series of sections across the thorax, the best of them figured by Wal-
cott m his plates 2 and 3 (1881) and plate 27 (1918).

The specimens sliced were all partially or quite enrolled, and in that position one would
expect to find the appendages so displaced that it would be only rarely that a section would
be cut, either by chance or design, in such a direction as to show any considerable part of
any one appendage. This expectation has proved true in regard to the endopodites, the
sections rarely showing more than two or three consecutive segments. Sections like those
shown in figures I and 2 in plate 2 (1881) seem to be unique. On the other hand, there are
numerous slices showing the so-called spiral branchiae. They show for the most part as
a succession of rectangular to kidney-shaped spots of clear calcite.1 Usually these clear spots
are isolated, not confluent, but in a small number of specimens, perhaps three or four, the
spots are connected in such a way as to show a zig-zag band which suggests a spiral. Such
an explanation is of course entirely reasonable, but it would be surprising if so slender a
spiral should be cut in such a way as to exhibit the large series of successive turns shown
in many of these thin sections. Continuous sections of such organs should be no more
common than continuous sections of endopodites.

One of the arguments against the interpretation of these series of spots as sections
across spiral arms is that of probabilities. It is known from flattened specimens that Neo-
Icnus, Kootenia, Ptychoparia, Triarthrus, and Cryptolithus all have a single type of exopo-
dite, consisting of a simple setiferous shaft. All these genera have been examined in a
way that permits no doubt about the structure, and no trace of spiral arms has been de-
tected. On the other hand, Walcott found spiral arms in three unrelated genera, Calym-
enc, Ceraurus, and Acidaspis, all of the trilobites in which he found exopodites by the
method of sectioning. What are the probabilities that genera of three different families,
studied by means of sections, should agree in having a type of exopodite different from
that of the five genera about whose interpretation there can be no doubt?

Another argument against the interpretation of the sections as spirals is that in any
one line the individual spots are of roughly uniform size. This means of course that the
spiral has been cut by a plane parallel to the tangent plane. This might happen once, just
as once Doctor Walcott cut all six segments of a single endopodite, but that it should happen

1 In looking at Walcott's figures of 1881, it should be remembered that the dark portions of the figures
are clear calcite in the specimens, while the light part is the more or less opaque matrix.

CALYMENE AND CERAURUS. 49

repeatedly is highly improbable. Moreover, there is a limit to the diameter of the section
which may be made from these slender spirals. Most of the spots have one diameter about
one half greater than the other, but others are from three to six times as long as wide.
These last could obviously be cut only from a very large spiral, and they are therefore
interpreted by Walcott as seta? of epipodites. Yet all gradations are found among the sec-
tions, from the long setae to the short dots. (See pi. 27, 1918.) In referring to one slice,
Walcott says (1918, p. 152) :

In the latter figure and in figure 13, plate 27, the setae of several epipodites appear to have been cut across
so as to give the effect of long rows of setx. The same condition occurs in specimens of Marrella when
the setae of several exopodites are matted against each other.

Fig. 12.— A slice of Ceraurus
pleurexanthemus in which the
exopodite happened to be cut
in such a way as to show a part
of the shaft and some of the
setae in longitudinal section.
Specimen 80. X 4-

This is certainly an apt comparison, and equally true if Neolcnus, Triarthrns, or Cryp-
tolithus were substituted for Marrella.

Now consider the "epipodites." They are well shown in Calymenc in the specimens
illustrated on plate 27, figure n (1918), and plate 3, figure 3 (1881), and less clearly in
one or two others. Slices 22 (pi. 27, fig. 12, 1918) and 80 (our fig. 12) show what is
called the same organ in Ceraurus. It will be noted that all of these slices are cut in
the same way, that is, more or less parallel to the under surface of the head, or, at any rate,
on a plane parallel to a plane which would be tangent to the axial portion of the coiled shell.
The sections which show the spirals best are those which are cut by a plane perpendicular to
the long axis of the body. If one were to attempt to cut an enrolled Triarthrus in such a
way as to get a section showing the length of the setae,, one would not cut a section per-
pendicular to the axis of the animal, nor, in fact, would he cut one parallel to the ventral
plane, but it is obvious that in this latter type of section he would stand a better chance of
finding a part of the plane of the exopodite coincident with the plane of his section than in
the former. And that seems to be what has happened in these sections of Calymene and
Ceraurus. If the exopodites were preserved, transverse sections were bound to cut across
many sets of fringes, and the resultant slice would show transverse sections of the setae as a
series of overlapping spots. A few fortunately located sections in a more nearly hori-

5O THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

zontal plane might cut the setae and occasionally the shaft of one or more exopodites in
the longitudinal plane, and the resulting effect would produce the so-called "epipodites."
A careful study has shown that no one of these epipodites is complete, and they do not have
the palmate form shown in Walcott's figures.

And the last and most important argument against the spiral appendages is that cer-
tain slices, of both Calymene and Ccraurus, show definitely exopodites of exactly the type
found in other trilobites. These are discussed later in the detailed description of the vari-
ous slices.

If these series of spots are interpreted on the basis of the known structure of Triarthrus,
they are of course a series of sections through the seta? of the exopodites. It will be shown
in Part IV that these setae are not circular in section, but flattened, in Cryptolithus even
blade-like, and that they overlap one another. A section across them would give the same
general appearance as, for instance, that shown in figures 4, 6, 9, and 10 of Walcott's plate
3 (1881).

When both endopodites and the "spiral branchiae" are present in the same section
(pi. I, fig. 4; pi. 2, figs, i, 2), the "spiral branchiae" are dorsal to the endopodites, as the
setae of the exopodites would be expected to be. The specimens which show the clear
spots connected, and which suggest a spiral (pi. 3, fig. 5), may seem at first sight to bear evi-
dence against this interpretation, but one has only to think of the effect of cutting a sec-
tion along the edge where the setae are attached to the shaft of the exopodite of Triar-
thrus to see that such a zigzag effect is entirely possible. One would expect to cut just
this position only rarely, and, in fact, the zigzags are seen in only three or four sections.
The bifurcation of the basal segment of the "spiral branchiae" (pi. 3, fig. 10, 1881) is
probably more apparent than real, if indeed these basal segments have anything to do with
the succeeding one.

A second peculiarity of Calymene, shown in Walcott's restoration, is the great enlarge-
ment of the coxopodites and of the distal segments of the endopodites of the fifth pair of
appendages of the cephalon. This is based on the sections of plate 3, figures 6, 7, 8, 9,
10 (1881). After a study of the specimens I regret to find myself still unconvinced that
the posterior cephalic appendages were any larger than those in front.

Ventral Membrane.

The most striking value of the thin sections of Ccraurus and Calymene, and therein
they have a great superiority over all the other forms so far investigated, is that they show
the extent of the body cavity and the position, though not- the substance, of the ventral
membrane. Transverse sections through Ccraurus (Walcott's pi. i, figs. 1-5; pi. 2, figs,
i, 3, 1881) and Calymene (pi. 3, figs. 9, 10, 1881) show that the body cavity was
almost entirely confined to the axial lobe. The longitudinal sections of Ccraurus (pi. 2,
figs. 6, 8; pi. 4, fig. 8) and of Calymene (pi. 2, figs. 5, 7; pi. 5, figs. 1-4) show that the
ventral membrane was exceedingly thin and was wrinkled transversely when the shell was
enrolled.

The specimens of figures 1-3, plate 5 (1881) show the form of the ventral membrane
more distinctly than any of the others. The section of figure i was cut just inside the
dorsal furrow on the right side, and figure 2, which is on the opposite side of the same
slice, is almost exactly on the median line. Figure 3 shows a section just inside the left dor-
sal furrow. Section 2 did not cut any of the appendages, and the ventral membrane is

CALYMENE AND CERAURUS. 5 l

shown as a thickened, probably chitinous sheet thrown into low sharply crested folds equal
in number to, and pointing in a direction just the reverse of, the crests of the segments
of the thorax. Under the pygidium, where there would of course be less wrinkling, the
folds are hardly noticeable. In the actual specimens one sees more plainly than in the
figures the line of separation between the ventral membrane and the appendages, but the state
of preservation of everything beneath the dorsal shell is so indefinite that one does not feel
sure just what the connection between the appendages and the membrane was. In the origi-
nal of figure 5, plate 2, which seems to have been cut so as to cross tlie appendages at their
line of junction with the ventral membrane, there appear to be narrow chitinous (?) plates
extending from the ventral membrane to the dorsal test.

Appcndifers.

In Ceraurus there are regular calcareous processes which extend down from the dor-
sal test just inside the line of the dorsal furrow, and which undoubtedly serve as points
of attachment of the appendages. These processes, which for convenience I have desig-
nated as "appendifers," are broken off in most specimens showing the lower surface of
Ceraurus pleurexanthemus, but on certain ones cleaned with potash they are well preserved.
Doctor Walcott showed them well in his figures of the lower surface of this species (1875,
pi. ii ; 1881, pi. 4, fig. 5), while the attempt of Raymond and Barton (1913, pf. 2, fig. 7)
to show them by photography was not so successful.

There is one pair of appendifers on each of the thoracic segments and four pairs on
the pygidium. On the cephalon there is one pair under the neck furrow, and a pair under
the posterior glabellar furrows. These are not concealed by the hypostoma. Further for-
ward, and completely covered by the hypostoma, are two much less strongly developed but
similar ones, so that there are in all four pairs of appendifers on the cephalon, though it
is extremely doubtful if the appendages were articulated directly to all of them. On a
specimen of Ceraurus pleurexanthemus 30 mm. long on the median line, the dorsal furrows
are 7.5 mm. apart at the anterior end of the thorax, and the tips of the appendifers of
this segment are only 4 mm. apart. Each consists of a straight slender rod with a knob-
like end projecting directly downward from the dorsal test, and supported by a thin cal-
careous plate which runs diagonally forward to the anterior edge of the segment directly
under the dorsal furrow. On the pygidium three pairs of the appendifers have this form,
while the fourth pair consist of low rounded tubercles which are concealed by the doublure.
These appendifers are probably cut in many of Walcott's sections of Ceraurus, but owing
to the state of preservation it is not always possible to determine what part is appendage,
what part is body cavity, and what part is appendifer.

Nearly forty years ago Von Koenen (1880, p. 431, pi. 8, figs. 9, 10) described
and figured the appendifers of Phacops latifrons. He found them to be calcareous pro-
jections on the hinder margin of each segment, converging inward, and about 1.5 mm. long.
He correctly considered them as supports (Stiitzpunkte) for the feet.

Appendifers are well developed also in Pliomerops, and in well preserved specimens
of Calymene senaria from Trenton Falls they are present, but instead of being rod-like
processes, they are rather thick, prominent folds of the shell. They are also well shown
in some of the thin sections. A specimen of Triarthr.its (No. 229, our pi. 5, fig. 2) has
broad processes extending downward from the lower side of the test below the dorsal
furrows, much as in Calymene, and the individual of Cryptolithus shown in plate 8, figure

52 THE APPENDAGES, AN ATOM Y^ AND RELATIONS OF TRILOBITES.

i, possesses slender appendifers. Two other specimens (Nos. 237 and 242) show them
quite well. They were probably present in all trilobites, but seldom preserved. The appen-
difers have the same origin as the entopophyses of Limulus, and like them, may have rela-
tively little effect on the dorsal surface.

Calymene senaria Conrad.
(Text figs. 13-16, 23.)

Illustrated: Walcott, Bull. Mus. Comp. Zool., .Harvard Coll., vol. 8, 1881, pi. I, figs. 6-10; pi. 2, figs. 5-7,
10; pi. 3, figs, i, 3, 8-10; pi. 4, figs, 3, 7; pi. 5, figs. 1-6; pi. 6, figs. I (restoration), 2; — Proc. Biol. Soc.
Washington, vol. 9, 1894, pi. i, fig. 7 (restoration); — Geol. Mag., dec. 4, vol. i, 1894, pi. 8, figs. 7, 8; —
Smithson. Misc. Coll., vol. 67, 1918, pi. 26, figs. 1-7, 9-13; pi. 27, figs. 4, 5 (not sa), 11 (not 12, Ceraurus),
13, 14, 15 (not Ceraurus") ; pi. 28, figs. 7, 8; pi. 33, fig. i (restoration); pi. 34, fig. 2; pi. 35, fig. 6.— Dames,
N. Jahrb. f. Min., etc., vol. i, 1880, pi. 8, figs. 1-5. — Milne-Edwards, Ann. Sci. Nat., Zoologie, ser. 6, vol. 12,
1881, pi. n, figs. 19-32; pi. 12, figs. 33-41. — Packard, Amer. Nat., vol. 16, 1882, p. 796, fig. 12. — Bernard, The
Apodidse, 1892, text figs. 50, 52, 54; — Quart. Jour. Geol. Soc., London, vol. 50, 1894, text figs. 13, 15, 17. —
CEhlert, Bull. Soc. Geol. France, ser. 3, vol. 24, 1896, fig. 12.— Beecher, Amer. Jour. Sci., vol. 13, 1902, pi.
5, fig- 7.

In both of Walcott's accounts (1881, 1918) of the appendages of Calymene and
Ceraurus, he has described them together, so that those who have not taken time to study
the illustrations and disentangle the descriptions are very apt to have a confused notion
in regard to them. I have therefore selected from the original specimens those slices of
Calymene which are most instructive, and bearing in mind the probable appearance of the
appendages of an enrolled Triarthrus, have tried to interpret them. In such a method of
study, I have of course started with a pre-formed theory of what to expect, but have
tried to look for differences as well as likenesses.

Cephalic Appendages.

Antcnnules. — The evidence of antennules rests on a single slice (No. 78). The appen-
dage in question is exceedingly slender and arises at the side of the hypostoma near its
posterior end. It shows fine, slender segments, and curves first outward and then forward.
If it is in its natural position, it is not an antennule, but the endopodite of the second or
third pair of cephalic appendages. It is short, only about one-third the length of the hy-
postoma, but is doubtless incomplete. The two distal segments show a darker filling, indi-
cating that they were hollow. Judging from analogy with other trilobites, the appendage
is probably an endopodite and not an antennule. There can be no reasonable doubt, how-
ever, that Calymene possessed antennules.

Some idea of the form of the coxopodites of the cephalic appendages may be obtained
from sections which cut in approximately the plane of the hypostoma. Such sections are
shown in Walcott's photographs (pi. 26, figs. 4, 6, n, 1918). Specimens 50 (fig. 4, our
fig- :3)> 51 (fig- 6), 6 (fig. n), and 40 (our fig. 14) agree in showing two pairs of slender
coxopodites which are attached at the sides of the hypostoma and run backward parallel and
close to it, and two pairs of larger coxopodites which are behind the hypostoma, although the
point of attachment of the third pair is in front of its tip. The anterior pair are appar-
ently under-developed and no longer function as mouth parts, while the posterior two pairs
are large and armed on their inner ends with spines. Specimen 78, which has already been
mentioned in connection with the antennules, shows a second very slender appendage back
of the so called antennule, which is equally slender, but is directed outward instead of for-

CALYMENE.

53

ward. It seems not improbable, from their position and similarity, that these two are the
endopodites of the first two appendages on one side of the hypostoma. Specimen 6 shows
rather inadequately the endopodites of the second and third cephalic appendages. I have
not found other slices showing endopodites of the cephalon. Walcott, in both his restora-
tions, has shown enlarged, paddle-shaped dactylopodites on the distal ends of the fourth
cephalic endopodites. The evidence for this rests principally on three slices, No. 38 (pi.
26, figs. 9, 10), 53 (pi. 26, fig. 12), and 43 (pi. 26, fig. 13). Of these, No. 43 may be dis-
missed at once as too poorly preserved to be interpreted. No. 53 does show a section of
an appendage which seems to have an unusually wide dactylopodite, but this slice presents
no evidence at all as to the appendage to which the dactylopodite appertains, nor can one
even be sure that there has not been a secondary enlargement. Specimen 43 shows this

Fig. 13. — Slice through Caylmene
scnaria in the plane of the hypostoma,
showing the very slender coxopodites
beside that organ, the spines on the
inner end of one of the maxillulae,
and the anterior position of the at-
tachment of all these appendages.
From a photographic enlargement.
Specimen 50. X 4-

Fig. 15. — Transverse section
of Calymene, showing method
of articulation with the ap-
pendifer. The shell is in solid
black, the filling of the append-
age and appendifer stippled.
Traced from a photographic
enlargement of the slice.
Specimen 63. X 7.

Fig. 14. — Slice
through the hypo-
stoma and tho-
rax of Calymene
senaria Conrad,
showing the small
size of the coxop-
odites nearest the
hypostoma. Shell
in black, append-
ages and filling of
abdominal cavity
dotted. From a
photographic en-
largement. Speci-
men 40. X 3.8.

feature much less definitely than is indicated by the published photograph and drawing.
The segment in question is strongly curved, with a constriction possibly dividing it into
two. If it is in its natural position in this section, it obviously belongs to one of the
thoracic segments and not to the cephalon. With evidence of difference so unsatisfactory,
I prefer to reconstruct the posterior cephalic endopodites on the same plan as those of the
thorax.

Exopodites. — Walcott admits that there is no direct evidence of spiral exopodites in
the cephalon of Calymene. No one of the sections cutting through the plane of the hypos-
toma shows any trace of appendages which could be interpreted as exopodites.

Thoracic Appendages.

The large coxopodites of the anterior thoracic appendages are well shown in many speci-
mens cut longitudinally, of which Nos. 23, 50, and 55 may be mentioned, since photographs
of them have been published by Walcott (pi. 26, figs. 1-4, 1918). The endobases of all
taper toward the proximal ends. Transverse slices show sections of the coxopodites which

54 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

are no wider than those in longitudinal sections, indicating that they were not compressed
but probably cylindrical. This is borne out by an individual (pi. 28, fig. 7, 1918) which is
not a slice but an actual specimen, the body cavity of which was hollow, and, opened from
above, shows the impressions of the last two coxopodites of the cephalon, and the first four
of the thorax.

One transverse section (No. 63, see our fig. 15) is especially valuable, as it shows
the method of articulation of the coxopodites with the dorsal skeleton. Another specimen
(No. 73) shows that appendifers are present in Calymene, and while the appendifer does
not retain its original form in slice No. 63, the section does show clearly that there was a
notch in the inner (upper) side of the coxopodite into which the lower end of the appen-
difer fitted, thus giving a firm, articulated support for the appendage. This notch appears
to be slightly nearer the outer than the inner end of the coxopodite, and since it must have
made a kind of ball-and-socket joint, considerable freedom of movement was allowed. The
appendage must have been held in place by muscles within the coxopodite and attached to the
appendifer.

No slice which I have seen shows a continuous section through all the segments of an
endopodite, but many, both longitudinal and transverse, show one, two, or as many as three
segments.

Such sections as No. 120 show that the endopodites of the thorax were slender and
composed of segments of rather uniform diameter. Other sections, notably No. 83, 154,
and in, show that they tapered distally, and bore small spines at the outer end of each
segment.

The exopodites of course furnish the chief difficulty in interpretation. Doctor Wal-
cott finds two sets of structures attached to the coxopodite, a long, slender, spiral exopo-
dite, and a short, broad epipodite with a fringe of long setae. Since he has given the same
interpretation for Calymene, Ceranrus, and Acidaspis, I have considered the question of
all three together on a preceding page (p. 48), and given my reasons for regarding both struc-
tures as due to sections in different directions across setiferous exopodites.

Sections like those shown in figures n, 13, and 14 of plate 27 (1918) happen to be cut
in or near the plane of the setae of an exopodite, and so show hairs of considerable length.
Such sections are, as would be expected, very few in number, while sections like those shown
on figures 4, 5, 7, and 9 of plate 27, which cut the setae more nearly at right angles, are
very common. Slices which give any definite idea of the form of the shaft of the exopodite
are exceedingly rare. Perhaps the most satisfactory one is No. 23 (pi. 3, fig. 3, 1881),
which shows the proximal part of a long, slender, unsegmented shaft, with the bases of a
number of slender setae. The organ is not complete, as would be inferred from the pub-
lished figure, but the section cuts diagonally across it, and the total length is unknown.
It is directed forward, like the exopodites of Ncolcnus, but whether or not this is a natural
position is yet to be learned.

The proximal, non-setiferous portion of the exopodite is evidently at an angle with
the setiferous part. Another similar exopodite is apparently shown by specimen 29 (pi. 3,
fig. 9, 1881, which has a similar angulated shaft and just a trace of the bases of the setae.

Pygidial Appendages.

That appendages were present under the pygidium is shown by longitudinal sections,
but nothing is known of the detail of structure.

CALYMENE.

55

Relation of Hypostoma to Cephalon in Calymene.

In Calymene the shape of the hypostoma bears little relation to the shape of the gla-
bella, and it is relatively smaller, both shorter and narrower, than in Ceraurus. In shape,
neglecting the side lappets at the front, it is somewhat rectangular, but rounded at the back,
where it is bifurcated by a shallow notch. The anterior edge has a narrow flange all
across, which is turned at almost right angles to the plane of the appendage, and which

Fig. 16. — Restoration of Calymene senaria Con-
rad, based upon data obtained from the study of
the translucent sections made by Doctor Walcott.
Prepared by Doctor Elvira Wood, under the su-
pervision of the author. About twice natural size.

fits against the doublure of the free cheeks at the sides and against the epistoma in the
middle. The side lappets show on their inner (upper) surface shallow pits, one on each
lappet, which fit over projections that on the dorsal surface show as deep pits in the bottom
of the dorsal furrows in front of the anterior glabellar furrows. The appendifers on
the head in Calymene take the form of curving projections of shell underneath the gla-
bellar and neck furrows, and owing to the narrowness of the hypostoma, all these are visi-
ble from the ventral side, even with it in position. This shield extends back about 0.6 of

5 6 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

the length of the cephalon, and to a point a little behind the second glabellar furrow from
the back of the head.

In Doctor Walcott's restoration of Calymene he has represented all four pairs of bira-
mous appendages as articulating back of the posterior end of the hypostoma. I think his
sections indicate that the gnathobases of two pairs of these appendages rested alongside or
beneath it, and in particular, the longitudinal sections (1881, pi. 5) would appear to show
that the mouth was some distance in advance of its posterior end.

Restoration of Calymene.
(Text fig. 1 6.)

From what has been said above, it is evident that for a restoration of the appendages
of Calymene considerable dependence must be placed upon analogy with other trilobites.
Nothing is positively known of the antennules, the exopodites of the cephalon, or any ap-
pendages, other than coxopodites, of the pygidium, but all were probably present. It is
inferred from the slices that the first two pairs of cephalic appendages were poorly devel-
oped, the endopodites short and very slender, the coxopodites lying parallel to the sides of
the hypostoma and nearly or quite functionless. The gnathites of the last two pairs of
cephalic appendages are large, closely approximated at their inner ends, and bear small
tooth-like spines. The endopodites are probably somewhat better developed than the an-
terior ones and more like those on the thorax.

The coxopodites of the thorax appear to have had nearly cylindrical endobases which
tapered inward. The endopodites were slender, tapering gradually outward, and probably
did not extend beyond the dorsal test. Small spines were present on the distal end of
each segment. Each exopodite had a long, slender, unsegmented shaft, to which were at-
tached numerous long, overlapping, flattened setas. The shaft may have been angulated
near the proximal end, and may have been directed somewhat forward and outward as
in Neolenus, but the evidence on this point is unsatisfactory. The number of pairs of ap-
pendages is that determined by Walcott from longitudinal sections, namely, four pairs on
the cephalon beside the antennules, thirteen pairs in the thorax, and nine pairs on the
pygidium.

Calymene sp. ind.

(PI. 6, figs. 4, 5-)

Illustrated: Walcott, Bull. Mus. Comp. Zool., Harvard Coll., vol. 8, 1881, pi. 6, figs, sa, b;— Proc. Biol.
Soc. Washington, vol. 9, 1894, pi. I, fig. 10; — Geol. Mag., dec. 4, vol. I, 1894, pi. 8, fig. 10; — Smithson. Misc.
Coll., vol. 67, 1918, pi. 36, figs. I, 2, 2a-d. — Milne-Edwards, Ann. Sci. Nat., Zoologie, ser. 6, vol. 12, 1881; pi.
12, figs. 443, b.

In the United States National Museum there is a thin piece of limestone, about 3
inches square, which has on its surface eight jointed objects that have been called legs of
trilobites. Two of these were figured by Walcott (1881, pi. 6, fig. 5). The slab contains
specimens of Dalmanella and Cryptolithus, in addition to the appendages of trilobites, and
is said by Doctor Ulrich to have come from the tipper part of the Point Pleasant formation
(Trenton) on the bank of the Ohio River below Covington, Kentucky.

The specimens are all endopodites of long slender form, similar to those of Triarthrus,
but since that genus does not occur in the Point Pleasant, it is necessary to look upon some
other trilobite as the former possessor of these organs. Both Isotelus and Calymene occur

CERAURUS. 57

at this horizon, and as the specimens obviously do not belong to Isotelus or Cryptolithus,
it is probable that they were formerly part of a Calymcne.

All the endopodites are of chitinous material, and the various specimens show, accord-
ing to the perfection of their preservation, from four to six segments. The endopodite as
a whole tapers but slightly outward, and the individual segments are of nearly equal length.
They appear to be but little crushed, and are oval in section, with a crimped anterior and
posterior margin. One or two show a median longitudinal ridge, such as is seen in some
appendages of Triarthrus. Each segment is parallel-sided, with a slight expansion at the
distal end, where the next segment fits into it.

Under the heading "Ordovician Crustacean Leg," Walcott (1918, p. 154, pi. 36, figs.
1,2) has recently redescribed these specimens, and thinks that they do not belong to Calym-
ene, nor, indeed, to any trilobite. He concludes that they were more like what one would
expect in an isopod. Passing over the fact that the oldest isopod now known is Devonian,
the fossils in question seem to me quite trilobite-like. Walcott says :

The legs are associated with fragments of Calymene mecki but it is not probable that they belong to
that species; if they did, they are unlike any trilobite leg known to me. The very short coxopodite and
basopodite are unknown in the trilobites of which we have the legs, as they are fused into one joint forming
the long protopodite in the trilobite. The distal joint is also unlike that of the trilobite legs known to us.

A great deal of Doctor Walcott's difficulty probably arises from his homology of the
coxopodite of the trilobite with the protopodite of the higher Crustacea. The coxopodite
of the trilobite is not fused with the basipodite, this latter segment always remaining free.
Indeed, Walcott himself says of Neolenus (1918, p. 128) :

Each thoracic leg (endopodite) is formed of a large elongate proximal joint (protopodite), four strong
joints each about 1.5 times as long as wide (basopodite, ischiopodite, meropodite and carpopodite) ; two
slender elongate joints (propodite and dactylopodite) and a claw-like, more or less tripartite termination.

Walcott's drawing (pi. 36, fig. i) is a composite one, and while it shows eight seg-
ments, I was not able to count more than seven on any of the specimens themselves. In
regard to the terminal segment, the dactylopodite of the limb shown in his plate 36, figure 2,
is unusually long, and a comparison with other photographs published on the same plate
shows that such long segments are unusual.

Proof that these are appendages of a Calymene is of course wanting, but there is no
particular reason so far to say that they are not.

Measurements: Two of the more complete specimens, each showing six segments, are
each 8 mm. long.

Somewhat similar to the specimens from Covington are the ones described by Eich-
wald (1825, p. 39, 1860, pi. 21 ) , the specimens being from the Silurian of Gotland. The figure
copied by Walcott (1881, pi. 6, fig. 4) has never been looked upon as entirely satisfactory
evidence of the nature of the specimen, and so far as I know, the fossil has not been seen
by any modern investigator.

Ceraurus pleurexanthemus Green.
(PI. II ; text figs. 12, 17-19, 21, 22, 24, 29, 30.)

Illustrated: Walcott, Ann. Lye. Nat. Hist. New York, vol. II, 1875, pi- «;— 3ist Ann. Kept. New York
State Mus. Nat. Hist, 1879, pi. i, fig. 3;— Bull. Mus. Comp. Zool., Harvard Coll., vol. 8, 1881, pi. i, figs.
1-5; pi. 2, figs. 1-4, 6-8; pi. 3, figs. 2, 4-7; pi. 4, figs, i, 2, 4-6, 8; pi. 6, fig. 3;— Smithson. Misc. Coll., vol. 67,

THE APPENDAGES., ANATOMY, AND RELATIONS OF TRILOBITES.

1918, pi. 26, figs. 8, 14, 15; pi. 27, figs. 1-3, sa, 6-9, 12 (not Calymene), (not 15, Calyinene) ; pi. 28, figs. 1-5;
pi. 34, fig. i; pi. 35, fig. 7. — Milne-Edwards, Ann. Sci. Nat., Zoologie, ser. 6, vol. 12, 1881, pi. 10, figs. 1-18.—
Bernard, The Apodidae, 1892, text figs. 46, 51.

Cephalic Appendages.

No trace of antennules has yet been found.

I. find only three sections cut through the plane of the hypostoma of Ceraurus which
show anything of the cephalic appendages, and no one of them is very satisfactory. The
best is No. 22, the one figured by Walcott (pi. 3, fig. 2, 1881 ; pi. 27, fig. 12, 1918), but
one should remember that this section is not actually cut in the plane of the hypostoma but
is a slice diagonally through the head, cutting through one eye and the posterior end of
the hypostoma. It shows what seem to be the coxopodites of the second, third, and fourth
pairs of cephalic appendages, the exopodites of the third and fourth pairs, and the metas-
toma. If this interpretation is correct, the first pair of gnathites lay alongside the hypos-

Fig. 17. — Transverse section of Ceraurus
pleurexanthemus, showing the relation of
the coxopodite to the appendifer. Traced
from a photographic enlargement of the
slice. Specimen 128. X 4-5-

Fig. 18. — Slice of Ceraurus pleurex-
anthemus, showing a nearly continu-
ous section of an endopodite and an
exopodite above it. The latter is so
cut as to show only the edge of the
shaft and the bases of a few seta:.
Traced from a photographic enlarge-
ment. Specimen in. X 4-

toma or under its edge, and were feebly developed, the second pair were attached in front
of the tip of the hypostoma, curved back close to it, and their inner ends reached the sides
of the metastoma. The third and fourth pairs were back of the metastoma, the third
pair was stronger than the second, and the fourth probably like the third.

Specimen 92 shows traces of the slender endopodites belonging to the cephalon, but no
details. Specimen 22 shows on one side exopodites (epipodites of Walcott) belonging to
the third and fourth cephalic appendages. That belonging to the third shows some long
sete and a trace of the shaft, while the one on the fourth appendage (third coxopodite) has
a portion of a broad shaft and a number of long setse. It should again be remembered
that the slice does not cut through the plane of the exopodite, but across it at a low angle,
so that a part but not all of the shaft is shown. On the other side of this slice there is a
fairly good section of one of the thoracic exopodites. It is, however, turned around in
the opposite direction from the others, as would be expected in an enrolled specimen.

Specimens 4 and 5 (pi. i, figs. 4, 5, 1881) are slices cut diagonally through the head
of Ceraurus, in front of the posterior tip of the hypostoma. They show fragments of
endopodites and exopodites which may be interpreted as practically identical in form with
those of the thorax. Due to the diagonal plane in which the section is cut, slice 5 shows

CERAURUS. 59

the coxopoclites of two pairs of appendages, one lying nearer the median cavity than the
other. It is extremely difficult to visualize the interpretation of such sections.

Thoracic Appendages.

A transverse section through a thoracic segment (No. 128, our fig. 17) shows the re-
lation of coxopodite to appendifer to be the same as in Calymene, the upper side of the
coxopodite having a notch a little outward from the middle. After seeing that specimen,
it is possible to understand slice No. 168, which shows longitudinal sections through a num-
ber of coxopodites of the thorax, with fragments of both exopodites and endopodites artic-
ulated at the distal ends. These and longitudinal vertical sections like No. 18 (pi. 2, fig.
8, 1881) show that the endobases taper inward, and the general uniformity in width in
sections taken at various angles indicates that the coxopodites were not greatly flattened.

A unique slice (No. in, pi. 2, fig. 2, 1881 ; pi. 27, fig. i, 1918; our fig. 18) shows a
nearly complete thoracic endopodite, and above it a part of the proximal end of the exopo-
dite of the same segment. When one considers that out of over two thousand sections only
this one shows the six successive segments of an endopodite, one realizes how futile it is
to expect that dozens of the equally slender "spirals" should be cut so as to show prac-
tically all their turns.

This endopodite is slender, all the segments have nearly the same length and diameter,
though there is a slight taper outward, each segment is expanded distally for the articula-
tion of the next, and there are small spines on the distal ends of some of them. There is
probably a terminal spine present, though it is neither so long nor so plainly visible as in
Walcott's photograph.

The exopodite on this same specimen was evidently cut diagonally across near the setif-
erous edge, showing a section through the shaft and the bases of seven setae (fig. 18). This
section is so exactly what would be obtained by cutting similarly an exopodite of either
Neolenus or Triarthrus that it should in itself dispose of the "spiral-exopodite" theory.

Several sections have already been illustrated showing sections across the setae of the
exopodites (pi. 3, figs. 4-6, 1881 ; pi. 27, figs. 3, 4, 9, 1918), and similar sections are not
uncommon. Only a very few, however, show sections in the plane of the exopodite. If
only No. in, described above, were known, it would be inferred that the exopodite had a
slender shaft as in Calymene, but another good slice, No. 80 (fig. 12, ante) shows that the
blade was rather broad, though not so broad as in Neolenus. The other specimen is No. 22,
which has already been discussed. The thoracic exopodite of this specimen has been very
incorrectly figured by Walcott, as it shows no such palmate shaft as he has indicated, but a
long blade-like one is outlined, though its entire width is not actually shown.

Pygidial Appendages.

Sections 14 and 18 (pi. 2, figs. 4, 8, 1881) prove the presence under the pygidium of
three pairs of appendages, the coxopodites and fragments of endopodites of which are shown.
Nothing is known of the exopodites.

Relation of Hypostoma to Cephalon.

In Ceraurus the body portion and posterior end of the hypostoma are roughly oval,
about as wide as the glabella at its broadest part, and the posterior edge extends back to

6O THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

within 0.5 to i mm. of the neck furrow. The posterior pair of appendifers are behind the
hypostoma, while the second pair are in front of its posterior end but escape being covered
by it on account of its oval shape. At the anterior end the hypostoma is widened by the
presence of two side lappets which extend beyond the boundaries of the glabella. In both
Ceraurus and Cheirurus the anterior edge of the hypostoma fits against the doublure at the
anterior margin of the head and the epistoma is either entirely absent or is so narrow as not
to be seen in specimens in the ordinary state of preservation. A section across the cephalon
of Ceraurus pleurexanthemus at the horizon of the eyes shows the sides of the hypostoma
fitting closely against the sides of the glabella (Walcott's pi. i, fig. i). Further back on
the head it is not in contact with the dorsal test, and the gnathobases extend beneath it.

Restoration of Ceraurus pleurexanthemus.
(PI. ii ; text fig. 19.)

The restoration of the appendages of Ceraurus pleurexanthemus is a tentative one, based
upon a careful study of the translucent sections prepared by Doctor Walcott. In no case

Fig. 19. — Restoration of a transverse section of the thorax
of Ceraurus pleurexanthemus Green, showing the relation of the
appendages to the appendifers and the ventral membrane. The
probable positions of the heart and alimentary canal are
indicated.

among these sections is the actual test of any appendage preserved, and the real form of each
part is generally obscured by the crystallization of the calcite which fills the spaces formerly
occupied by animal matter.

No section shows anything which can be identified as any part of the antennules, so
that these organs have been supplied from analogy with Triarthrus.

There are undoubtedly four pairs of biramous Cephalic appendages, but their points of
attachment are not so obvious. There are two pairs of conspicuous appendifers on the
posterior part of the cephalon and another pair almost concealed by the hypostoma. It is
probable that the appendages of the cephalon were not attached directly beneath them, as
the four pairs have to be placed within the space occupied by the three pairs of appendifers.
As the mouth is in front of the posterior end of the hypostoma, the gnathites of the first
pair of biramous appendages may have extended beneath that organ, or they may have lain
beside it, and only become functional when the hypostoma was dropped down in the feed-
ing position. The second pair of gnathites reached just to the tip of the hypostoma, and the
other two pairs seemingly curved backward behind it.

The points of attachment on the thorax, as shown clearly in sections, were directly be-
neath the lower ends of the appendifers. The endopoclites were long enough to reach to
or a little beyond the outer extremities of the pleural spines, while the exopodites were
apparently somewhat shorter. Each endopodite consisted of six short, fairly stout seg-
ments, each with at least two spines on the somewhat expanded distal ends. The exact

CERAURUS. 6 1

form of the exopodites could not be made out. The shaft was apparently rather short, unseg-
mented, and fairly broad. The setae appear from the sections to have been more or less
blade-shaped and to have overlapped, as do those of the exopodites of Cryptolithus. Judg-
ing from their position in the sections, the setae not only bordered the posterior side of the
shaft, but radiated out from the end as well.

The pygidium shows three pairs of functional appendifers, hence three pairs of appen-
dages have been supplied. There is a fourth pair of rudimentary appendifers, but as they
are beneath the doublure they could not have borne ambulatory appendages.

THE APPENDAGES OF ACIDASPIS TRENTONENSIS WALCOTT.
(PI. 6, fig. 6.)

A single individual of Acidaspis trentonensis, obtained from the same locality and hori-
zon as the specimens of Triarthrus and Cryptolithus, when cleaned from the ventral side
shows a number of poorly preserved endopodites which seem very similar in shape and
position to those of Triarthrus. One endopodite on the right side of the head and the first
five on the right side of the thorax are the best shown. All are slender, are directed first
forward at an angle of about 45° with the axis, then, except in the case of the cephalic
appendage, turn backward on a gentle curve and extend a little distance beyond the margin
of the test, but not as far as the tips of the lateral spines of the thoracic segments.

The individual segments of the endopodites can not be seen clearly enough to make
any measurements. On the fourth and fifth endopodites of the thorax, some of the seg-
ments seem to be broad and triangular as in Triarthrus. All that can be seen indicates that
Acidaspis had appendages entirely similar to those of Triarthrus, but perhaps not quite
so long, as they seem not to have projected beyond the limits of the lateral spines. There
are no traces of antennules nor, unfortunately, of exopodites.

Measurements: Length 8 mm.

Walcott (1881, p. 206) stated that his sections had shown the presence in this species
of legs "both cephalic and thoracic" and also the "spiral branchiae." His specimens were
from the Trenton at Trenton Falls, New York.

THE APPENDAGES OF CRYPTOLITHUS.

Cryptolithus tessellatus Green.
(PI. 6, fig. 7; pis. 7-9; text figs. 20, 25, 45, 46.)

(See also Part IV.)
Illustrated: Beecher, Amer. Jour. Sci., vol. 49, 1895, pi. 3.

When Professor Beecher wrote his short article on the "Structure and Appendages of
Trinucleus" (1895), he had only three specimens showing appendages. In his later work
he cleaned several more, so that there are now thirteen specimens of Trinucleus = Cryp-
tolithus available for study, though some of these do not show much detail. In his last
and unpublished study, Beecher devoted the major part of his attention to this genus, and
summarized his findings in the drawings which he himself made of the best individuals (text
figs. 45, 46). Valiant (1901) stated that he had found a Trinucleus with antennae in the
Frankfort shale south of Rome, New York. The specimen has not been figured.

62 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

None of the specimens shows much more of the appendages of the cephalon than, the
hypostoma and the antennules, so that we are still in ignorance about the mouth parts.

The most striking characteristics of the appendages are as follows : the antennules are
long, and turn backward instead of forward; none of the limbs projects beyond the margin
of the dorsal test; the exopodites extend beyond the endopodites, reaching very nearly to
.the margin of the test; the endopodites are not stretched out at right angles to the axis, but
the first three segments have a forward and outward direction as in Triarthrus, while the
last four turn back abruptly so that they are parallel to the axis; the limbs at the anterior
end of the thorax are much more powerful than the others; the dactylopodites of the endop-
odites show a fringe of setae instead of three spines as in Triarthrus and Neolenus. All
these would, as Beecher has already suggested, seem to be adaptations to a burrowing habit
of life, the antennules being turned backward and the other appendages kept within the
shelter of the dorsal test in order to protect them, and the anterior endopodites enlarged and
equipped with extra spines to make them more efficient digging and pushing organs.

Restoration of Cryptolithus.
(Text fig. 20.)

It should be definitely understood that the present figure is a restoration and not a
drawing of a specimen, and that there are many points in the morphology of Cryptolithus
about which no information is available, especially about the appendages under the central
portion of the cephalon. The information afforded by all the figures published in this
memoir is combined here. As gnathites are preserved on none of the specimens, those rep-
resented in the figure are purely conventional.

A person who is acquainted only with Cryptolithus preserved in shale, or with figures,
usually has a very erroneous idea of the fringe It is not a flat border spread out around
the front of the head, but stands at an angle about 45° in uncrushed specimens of most
species. When viewed from the lower side, there is a single outer, concentric row of the
cup-shaped depressions, bounded within by a prominent girder. This row is in an approxi-
mately horizontal plane, while the remainder of the doublure of the fringe rises steeply into
the hollow of the cephalon. Since the front of the hypostoma is attached to this doublure,
it stands high up within the vault and under the glabella. Two specimens, Nos. 231 and
233, show something of the hypostoma, and they are the only ones known of any Ameri-
can trinucleid. That of specimen 233, the better preserved, is very small, straight across the
front, and oval behind. It seems that it is abnormally small in this specimen and I should
not be surprised if in other specimens it should be found to be larger.

In the Bohemian Trinucleoidcs rcussi (Barrande), the oldest of the trinucleids, the hy-
postoma is very commonly present, and is of the proper size to just cover the cavity of the
glabella, seen from the lower side, and has, toward the anterior end, side flaps which reach
out under the prominent glabellar lobes. This large size of the hypostoma would cause the
antennules to be attached outside the dorsal furrows, and the position in which they are at-
tached in the American species of Cryptolithus may be explained as an inherited one, since
with the small hypostoma they might have been within the glabella, as in Triarthrus.

The antennules are seen in three specimens, and in all cases are directed backward. The
particular course in which they are drawn in the restoration is purely arbitrary. The sec-
ond pair of cephalic appendages are represented as directed downward and forward, since

CRYPTOLITHUS. 63

in one or two specimens fragments of forward -pointing endopoclites were seen near the front
of the cephalon, and because in other trilobites the second pair of appendages is always
directed forward. The remaining three pairs have a more solid basis in observed fact, for
the two or three specimens retaining fragmentary remains of them indicate that they turn
backward like those on the thorax, and that the individual segments are longer and more

Fig. 20. — Cryptolithus tessellatus Green. A restoration of the appendages drawn by Doctor
Elvira Wood from the original specimens and from the photographs made by Professor
Beecher. X 9-

nearly parallel-sided than those of the more posterior appendages. The gnathites of all
the cephalic appendages are admittedly purely hypothetical. None of the specimens shows
them. As drawn, they are singularly inefficient as jaws, but if, as is suggested by the casts
of the intestines of trinucleids found in Bohemia, these trilobites were mud-feeders, ineffi-
cient mouth-parts would be quite in order.

The appendages of the thorax and pygidium can fortunately be taken quite directly
from the photographs of the dorsal and ventral sides of well preserved specimens. There

64 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

is of course a question as to the number and the exact form of those on the pygidium, but
I think the present restoration is fairly well justified by the specimens. As would be ex-
pected from the narrow axial lobe, the gnathobases of the coxopodites are short and small.

SUMMARY ON THE VENTRAL ANATOMY OF TRILOBITES.
COMPARISON OF APPENDAGES OF DIFFERENT GENERA.

Since the appendages of Triarthrus, Cryptolithus, Neolenus, Calymene, and Ceraurus
are now known with some degree of completeness, those of Isotclus somewhat less fully,
and something at least of those of Ptychoparia, Kootenia, and Acidaspis, these forms being
representatives of all three orders and of seven different families of trilobites, it is of some
interest to compare the homologous organs of each.

All in which the various appendages are preserved prove to have a pair of antennules,
four pairs of biramous limbs on the cephalon, as many pairs of biramous limbs as there
are segments in the thorax, and a variable number of pairs on the pygidium, with, in the
case of Neolenus alone, a pair of tactile organs at the posterior end. Each limb, whether
of cephalon, thorax, or pygidium, consists of a coxopodite, which is attached on its dorsal
side to the ventral integument and supported by an appendifer, an exopodite, and an endopo-
dite. The exopodite is setiferous, and the shaft is of variable form, consisting of one, two,
or numerous segments. The endopodite always has six segments, the distal one armed with
short movable spines.

Coxopodite.

The coxopodite does not correspond to the protopodite of higher Crustacea, the basip-
odite remaining as a separate entity. The inner end of the coxopodite is prolonged into
a flattened or cylindrical process, which on the cephalon is more or less modified to assist
in feeding, and so becomes a gnathobase or gnathite. The inner ends of the coxopodites of
the thorax and pygidium are also prolonged in a similar fashion, but are generally some-
what less modified. These organs also undoubtedly assisted in carrying food forward to the
mouth, but since they probably had other functions as well, I prefer to give them the more
non-committal name of cndobases.

In Triarthrus and Neolenus the endobases are flattened and taper somewhat toward
the inward end. In Isotelus, Calymene and Ceraurus, they appear to have been cylindrical.
In other genera they are not yet well known. In all cases, particularly about the mouth,
they appear to have been directed somewhat backward from the point of attachment. As it
is supposed that these organs moved freely forward and backward, the position in which they
occur in the best preserved fossils should indicate something of their natural position when
muscles were relaxed.

Cephalon.

Antennules. — Antennules are known in Triarthrus, Cryptolithus, Neolenus, and Ptycho-
paria. In all they are long, slender, and composed of numerous segments, which are spinif-
erous in Neolenus, and very probably so in the other genera.

In Triarthrus, Neolenus, and Ptychoparia they project ahead of the cephalon, emerg-
ing quite close together under the front of the glabella, one on either side of the median
line. In Cryptolithus they turn backward beneath the body, but since only three or four
specimens are known which retain them, it is possible that other specimens would show

r

COMPARISON OF APPENDAGES. 65

that these organs were capable of being turned forward as well as backward. The proxi-
mal ends of the antennules being ball-like, it is probable, as Doctor Faxon has suggested to
me, that these "feelers" had considerable freedom of motion. The antennules of Triarthrus
are apparently somewhat less flexible than those of the other genera, and have a double cur-
vature that is seen among the others only in Ptyclioparia. The proximal end of an an-
tennule in Triarthrus is a short cylindrical shaft, apparently articulating in a sort of ball-
and-socket joint. The proximal end in the other genera is still unknown. The points of
attachment in Triarthrus seem to be under the inner part of the second pair of glabellar fur-
rows. In Cryptolithus they appear to be beside the anterior lobe of the glabella under what
have long been known as the antennal pits. In the other genera the location is not definitely
known, but in Neolcnus it seems to be under the dorsal furrows near the anterior end of the
glabella. Viewed from the under side, the point of attachment is probably always beside the
middle or anterior part of the hypostoma, just behind the side wings.

Paired biramous appendages. — Behind the antennules all the appendages except those
on the anal segment are biramous, consisting of a coxopodite with an inward-directed endo-
base and an outward-directed pair of branches, the exopodite above, and the six-jointed en-
dopodite beneath. The basipodite really bears the exopodite, but the latter also touches
the coxopodite. This structure has been seen in Triarthrus, Cryptolithus, Neolenus, Koo-
tenia, Calymenc, Ceraurus, and Ptychoparia. In Triarthrus, Neolenus, Acidaspis, Ptyclio-
paria, and Kootenia, the appendages extend beyond the margins of the dorsal test. In
Cryptolithus and Isotelus none (other than antennules) does so. In Isotelus and Acidaspis
only the endopodites have been seen. In Triarthrus, Calymene, Ceraurus, and Neolcnus
there are four pairs of appendages behind the antennules. The other genera probably had
the same number, but the full structure of the under part of their cephala is not known. In
Triarthrus the endopodites of the cephalon are slender, the individual segments parallel-sided,
the inner ones flattened, the outer ones cylindrical in section. They project slightly
beyond the edge of the cephalon when fully extended, and each terminates in three small
spines. In Cryptolithus the endopodites of the cephalon are longer than those of
the thorax, but with the possible exception of the first pair, are bent backward at
the carpopodite, and do not ordinarily project beyond the brim of the test. In Neolenus
the endopodites of the cephalon are rather thick and wide, but are long, project for-
ward, and extend beyond the brim. The individual segments are flattened, probably com-
pressed oval in section. The terminal segment of each is furnished with three strong spines
at its distal end. In Calymene and Ceraurus the endopodites appear to consist of slender
segments which are oval or circular in section. In Calymene Walcott believed the three
distal segments of the last endopodites of the head to be greatly enlarged, giving these ap-
pendages a paddle-like form similar to some of the appendages of eurypterids. The evi-
dence for this does not seem to me to be good. The cephalic endopodites of Isotelus are
entirely similar to those of the thorax, and are rather short, consisting of a series of short
cylindrical segments which do not taper greatly toward the distal end. The endopodites of
the cephalon of Acidaspis, Kootenia, and Ptychoparia are still unknown.

The exopodites of the cephalon seem in all known cases {Triarthrus, Cryptolithus, Neo-
lenus, and Ceraurus) to be like those of the thorax. They point more directly forward in
most cases, project beyond the margin of the head normally only in Triarthrus, and usually
occupy the region under the cheeks (fixed and free).

The endobases of the coxopodites of the appendages of the cephalon probably in all cases

66 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

function as mouth-parts (gnathites), and are especially modified for this purpose in Triar-
thrus, being flattened, shoe-shaped in outline, and so arranged that they work over one an-
other in a shearing fashion. While the more anterior of the coxopodites are attached in
front of the posterior tip of the hypostoma, the gnathites of Triarthrus bend backward so
that all are behind the hypostoma. In Calymcne and Ccraurus, two or three pairs of the
gnathites are back of the hypostoma, and one or more pairs may be beside or under the
hypostoma. In these genera the mouth is probably in front of the tip of the upper
lip. In Isotclus, the mouth seems to have been situated in the notch between the
two branches of the hypostoma, and the gnathites of two or three pairs of the appendages
probably worked under its forks. Since the length of the hypostoma differs in the various
species of Isotclus, there would be a variable number of gnathites projecting under its forks,
according to the species. In this genus the gnathites are of the same long form, cylindrical
in cross-section, as the endobases of the thoracic segments, but each is bowed back consider-
ably from the point of attachment.

The gnathites of Ncolcnus are like the endobases of the thorax, but broader. The great
length of the hypostoma makes it probable that the mouth was far back and that some of
the gnathites were in front of it. The gnathites of Cryptolithus are unknown. Professor
Beecher in his drawing shows some fragments with toothed ends near the hypostoma, and
it may be that they are inner ends of gnathites, but I see nothing to substantiate such an in-
terpretation. If, as some suppose, Cryptolithus was a mud feeder, the gnathites were prob-
ably poorly developed. Of the gnathites of Kootcnia, Ptychoparia, and Acidaspis also

nothing is known.

Thorax.

In each genus there is a pair of appendages for each segment of the thorax. When
the axial lobe is narrow, the endobases of the coxopodites are small and short (Cryptolithus,
Ceraurus, Calymene). When the axial lobe is wide, the endobases are long and stout (Isot-
clus. Triarthrus). The exopodites always lie above and in front of the corresponding endop-
odites. In Triarthrus the two branches are of practically equal length. In Cryptolithus the
exopodites are much the longer. In Neolenus, Calymcne, Ccraurus, Kootcnia, and Pty-
choparia, the exopodites are shorter than the endopodites.

The exopodites in Triarthrus consist of a proximal shaft, succeeded by numerous short
segments, and ending distally in a long, grooved, somewhat spatula-shaped segment. Along
the anterior margin of the shaft there are many small spines. Along the posterior margin
there are numerous flattened seta-, which all lie in one plane and which seem to be more or
less united to one another like the barbs of a feather. The setae are short, not much longer
than the width of one of the thoracic segments, and point backward and outward. In Cryp-
tolithus the shaft does not seem to be made up of small segments, and is narrow, with a
decided backward curve. The seta; are considerably longer and much more flattened than
in Triarthrus. In Calymcne the state of preservation does not allow a very full knowledge
of the exopodites, but they appear to have a slender, unjointed shaft and short and delicate
sete. The coiled branches of the exopodites as described by Walcott seenfi to me to be
only ordinary Triarthrus-like organs, and this, as I understand from Schuchert, was also the
view of Beecher. In Ccraurus the exopodite seems to have been somewhat paddle-shaped,
expanded at the distal end, and to have had rather thick, blade-like setse.

The exopodite of Ncolcnus is decidedly leaf-like, and reminds one somewhat of the exites

COMPARISON OF APPENDAGES. 67

of some of the phyllopods. The shaft is a broad unsegmented blade. The setse are slender,
delicate, flattened, and a little longer than the width of the shaft. The exopodites of
this genus point forward all along the body. In Kootenia the exopodites are like those
of Ncolcnus, but with a narrower shaft. The exopodites of Ptychoparia appear to be very
much like those of Triarthrus, but the shaft is probably not segmented.

The endopodites of the thorax of Triarthrus, Cryptolithus, and Acidaspis show pro-
gressive modification from front to back in the broadening of the individual segments and
the assumption by them of a triangular form. Not only do the individual segments become
more triangular from front to back, but more of the segments of each endopodite become tri-
angular. This modification has so far been seen in these three genera only. The individ-
ual segments, except the distal ones, seem to be flattened in all these genera. The distal
end of the terminal segment of each endopodite of Triarthrus bears three small movable
spines, and each of the segments usually bears three or more spines, located in sockets along
the dorsal surface and at the anterior distal angle of each segment. The endopodite of
Cryptolithus is bent backward at the carpopodite and this segment is always thickened. At
the distal end of the dactylopodite there is a tuft of spines, the triangular segments have
tufts of spines on their posterior corners, and there are groups of spines also in the neigh-
borhood of the articulations.

The endopodites of Ceraurus, Calymcne, and Isotclus are all relatively slender, the seg-
ments are parallel-sided, and there seems to be no particular modification from front to back
of the thorax. The endopodites of Isotelus are short, the entire six segments of one being
but little longer than the coxopodite of the same appendage. The segments of the endopo-
dites of Neolenus are mostly short and wide, and at the distal end of the terminal segment
there are three stout spines. In Kootenia the endopodites are long and very slender. The
endopodites of Ptychoparia are too poorly preserved to show details, and those of the thorax
of Acidaspis likewise reveal little structure, but they seem to have the triangular modifica-
tion, and to turn back somewhat sharply at about the position of the carpopodite.

Pygidium.

Beecher showed that in Triarthrus there was a pair of appendages on the pygidium for
every segment of which it is composed except the last or anal segment (protopygidium).
Walcott has since shown that in Neolenus this segment bears a pair of cerci, and Beecher's
drawings show that in his later studies he recognized a spinous plate, the possible bearer
of cerci, on the anal segment of Triarthrus. The appendages of the anal segment have not
yet been seen on other species of trilobites.

The appendages of the pygidium do not show any special modifications, but seem in
all cases to be similar to those of the posterior part of the thorax. In Cryptolithus all the
pygidial appendages are short and remain beneath the cover of the dorsal test, while in
Triarthrus and Neolenus they extend behind it.

In the latter genus the endopodites of the pygidial appendages appear to be practi-
cally identical in form with those of the thorax, the individual segments being perhaps a
little more nearly square in outline. Like those of the thorax, the segments of the pygidial
endopodites bear numerous short spines. The caudal cerci are richly segmented, slightly
flexible, spinous tactile organs. They are symmetrically placed, nearly straight when in their
natural position, and make an angle of about 75° with one another. They appear to be

68 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

attached to a narrow rim-like plate which seems to fit in just ahead of the doublure of the
pygidium, or perhaps over it.

In Ceraurus, Calymene, and Isotclus, the endopodites of the pygidium are similar to
those of the thorax, but seemingly more slender, with less well developed coxopodites, and
with, in the last-named genus, slender cylindrical segments. Exopodites are not known on
the pygidia of any of these genera, but since they are present and like those of the thorax
in Triarthrus, Cryptolithns, Neolenus, and Ptychoparia, there is little reason to think that
they were absent in Ceraurus or Calymene, though there is some question about Isotclus.

The limbs are largest and longest on the anterior part of the thorax of a trilobite, and
diminish regularly in length and strength to the posterior end of the pygidium. This reg-
ular gradation shows, as Beecher'was the first to 'point out, that the growing point of the
trilobites is, as in other arthropods, in front of the anal segment. New free segments are
introduced into the thorax at the anterior end of the pygidium, and this has led to some
confusion between the growing point and the place of introduction of free segments.

If a new segment were introduced at a moult in front of the pygidium, that segment
would probably have less fully developed appendages than those adjacent to it, and so make
a break in the regular succession. The condition of the appendages corroborates the evi-
dence derived from the ontogeny of the pygidium, and proves that the new segments are
introduced at the same growing point as in other Arthropoda.

Caudal Rami.

Bernard, who believed that the Crustacea had been derived through an Apus-like an-
cestor (1892, pp. 20, 85, 274), pointed out that four or less than four anal cirri were to
be expected. Two well developed cirri and two rudimentary ones are present in Apus, and
they are also to be found in other phyllopods and some isopods. It is, however, character-
istic of the Crustacea as a whole to lack appendages on the anal segment. Caudal cirri
(cerci) are much more freely developed in the hexapods than in the Crustacea, particularly
in the more primitive orders, Palaeodictyoptera, Apterygota, Archiptera, and Neuroptera.
They are supposed, in this case, to be modified limbs, and therefore not homologous with
the bristles on the anal segment of an annelid. Doctor W. M. Wheeler of the Bussey In-
stitution has kindly allowed me to quote the following excerpt from a letter to me, as
expressing the opinion of one who has made an extensive study of the embryology of insects :

I would say that I have no doubt that the cerci of insects are directly inherited from the insect ancestors.
They are always highly developed in the lower insects, and only absent or vestigial in a few of the most
highly specialized orders such as the Hemiptera, Diptera, and Hymenoptera. I have further no doubt
concerning their being originally ambulatory in function. They are certainly not developed independently in
insects. Embryologically they arise precisely like the legs, and each cercus contains a diverticulum of the
mesoblastic somite precisely as is the case with the ambulatory legs and mouth parts.

The "pygidial antennae" seem to be as fully developed in Neolenus as in any of the
other arthropods, and may suggest a common ancestry of the phyllopods, isopods, and
hexapods, in the trilobites. They were doubtless tactile organs, and while the evidence is
chiefly negative, it would seem that they proved useless, and were lost early in the phylog-
eny of this group. Possibly the use of the pygidium as a swimming organ proved de-
structive to them.

COMPARISON OF APPENDAGES. 69

HOMOLOGY OF THE CEPHALIC APPENDAGES WITH THOSE OF OTHER CRUSTACEA.

The head of the typical crustacean bears five pairs of appendages, namely, the antennules,
antennas, mandibles, and first and second maxillae, or, as they are more properly called, the
maxillulae and maxillae.

As Beecher has pointed out, the "antenme" of the trilobites, on account of their pre-
oral position and invariably uniramous character, are quite certainly to be correlated with
the antennules.

The second pair of appendages, the first pair of biramous ones, Beecher homologized
with the antenna; of other crustaceans, and that homology has been generally accepted,
though Kingsley (1897) suggested that it was possible that no representatives of the true
antennae were present.

In preparing the restorations in the present study, the greatest difficulty has been to
adjust the organs about the mouth. In Triarthrus, numerous specimens show that without
question there are four pairs of gnathites back of the hypostoma, and that all four belong
to the cephalon. In forms with a long hypostoma, however, there was no room on the
cephalon for the attachment of four pairs of gnathites, neither were there enough appet>
difers to supply the requisite fulcra. At first I supposed I had solved the difficulty by
assuming the mouth to be in front of the posterior tip of the hypostoma, as it really is in
Ceraurus and Calyinenc, and allowing the gnathites to play under the hypostoma as Wal-
cott (1912) has shown that they do in Marrella. Finally, when I came to study in greater
detail the slices of Calymene and Ceraurus, they seemed to show that the anterior one or two
pairs of appendages became degenerate and under-developed. This was probably a special-
ization due to the great development of the hypostoma in trilobites, that organ being much
more prominent in this than in any other group. As the hypostoma lengthened to accom-
modate the increasing size of sub-glabellar organs (stomach, heart, etc.), the mouth mi-
grated backward, leaving the anterior appendages ahead of it, with their gnathobases, at
least, f unctionless. That such migration has taken place, even in Triarthrus, is shown by
the fact that the points of articulation of the first biramous appendages are pre-oral, and it
is more obviously true of Ceraurus. Correlated with the weakening of the appendages
on the lower surface is the loss of glabellar furrows on the upper surface. The glabellar
furrows mark lines of infolding of the test to form the appendifers and other rugosities for
the attachment of tendons and muscles. It is conceivable that this migration backward of
the mouth began very early in the history of the race, and that even before Cambrian times,
the antennae, probably originally biramous appendages like those on the remainder of the
body, had dwindled away and become lost. If this is the case, then the first pair of bira-
mous appendages of Triarthrus would be mandibles, the second pair maxillulne, and the third
pair maxillae.

There remain the last pair of cephalic appendages, and they bring up the whole head
problem of the trilobites. Beecher has stated (1897 A, p. 96) his conviction that the head
of the trilobite is made up of five segments, representing the third, fourth, fifth, sixth, and
seventh neuromeres of the theoretical crustacean. As a matter of fact, he really made up
the head of seven segments, since he stated that the first neuromere was represented by the
hypostoma and the second by the epistoma and free cheeks.

Jaekel (1901, p. 157) nearly agreed with Beecher, but made eight segments, as he saw

70 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

five segments in the glabella of certain trilobites. In his table (p. 165) he has listed the
segments with their appendages as follows: I. Acron, with hypostoma; 2, rostrum (epis-
toma), with free cheeks; 3, first frontal lobe, with (?) antennules; 4, second frontal lobe,
with antennae; 5, mandibles; 6, first, or pre-maxillae ; 7, second maxilla; 8, occipital seg-
ment with maxillipeds.

Jaekel refused to believe that the antennae of trilobites were really entirely simple, and
so homologized them with the antennae and not the antennules of other Crustacea. In this
he was obviously incorrect, but it accounts for his homology of the remainder of the cephalic
appendages.

It is, at present, impossible to demonstrate the actual number of somites in the cephalon
of the trilobite, but I believe that Beecher was correct in holding that the glabella was
composed of four segments. There are, it is true, a number of trilobites (Mesonacidse, Para-
doxidae, Cheiruridae, etc.) which show distinctly four pairs of glabellar furrows, indicat-
ing five segments in the glabella. This is, however, probably due to a secondary division
of the first lobe.

The correspondence of the five segments on the dorsal side with the five pairs of appen-
dages makes it unlikely that any pair of limbs has been lost. The condition in Marrella,
where a trilobite-like cephalon bears five pairs of appendages, the second pair of which are
tactile antenme, is favorable to the above interpretation. In spite of the apparent degener-
ation of the first two pairs of appendages in Calymene, no limbs are actually missing, and
if some are dropped out in the later trilobites it would not affect the homology of those
now known. I therefore agree with Beecher in homoldgizing the appendages, pair for pair,
with those of the higher Crustacea.

FUNCTIONS OF THE APPENDAGES.

Antennules.

The antennules were obviously tactile organs, probably freely movable in most trilo-
bites, but in the case of Triarthrus perhaps rather rigid, judging from the great numbers of
specimens which show the characteristic sigmoid curve made familiar by Professor Beecher's
restoration. The proximal end of the shaft of each antennule of Triarthrus is hemispheric
and doubtless fitted into a socket, thus suggesting great mobility of the whole organ. In
spite of this, I have seen no specimens in which they did not turn in toward each other and
cross the anterior margin very near the median line. In front of the margin, various
specimens show evidence of flexibility, but from the proximal end to the margin the position
is the same in all specimens.

In all the few specimens of Cryptolithus retaining the antennules, these organs are
turned directly backward, but it is entirely within the range of probabilities that while its
burrowing habits made this the more usual position, the animal had the power of turning
them around to the front when they could be used to advantage in that direction.

Exopodites.

It has been the opinion of most observers that the exopodites of trilobites were swim-
ming organs, while others have thought that they functioned also in aerating the blood.
To the present writer it seems probable that the chief function was that of acting as gills,
for which the numerous thin, flattened or blade-like setae are particularly adapted. That

FUNCTION OF APPENDAGES. 7 1

they were also used in swimming is of course possible, but that was not their chief function.
It should be remembered that the exopodites are always found dorsal to or above the endopo-
dites, and in a horizontal plane. For use in swimming it would have been necessary to
rotate each exopodite into a plane approximately perpendicular to or at least making a con-
siderable angle with the dorsal test. In this position, the exopodites would have been thrust
down between the endopodites, and one would expect to find some specimens in which a
part at least of the exopodites were ventral to the endopodites. Specimens in this condi-
tion have not yet been seen among the fossils. To avoid having the exopodites and endopo-
dites intermingled in this way, the animal would have to bring all the endopodites together
along the axial line in a plane approximately perpendicular to the dorsal test, in which case
the exopodites would be free to act as swimming organs. The fact that the setae of an
exopodite stay together like the barbs on a feather would of course tend to strengthen the
idea that the exopodites could be used in swimming, but that is not the only possible ex-
planation of this condition. The union of the basipodite and exopodite shows that the two
branches of the appendage acted together. Every movement of one affected the other, and
the motion of the endopodites in either swimming or crawling produced a movement of the
exopodites which helped to keep up a circulation of water, thus insuring a constant supply
of oxygen.

Although Neolenus is usually accounted a less primitive form than Ptychoparia or
Triarthrns, it has much the most primitive type of exopodite yet known. It would appear
that the exopodites were originally broad, thin, simple lamellae, which became broken up,
on the posterior side, into fine cylindrical setae. As development progressed, more and more
of the original lamella was broken up until there remained only the anterior margin, which
became thickened and strengthened to support the delicate filaments. The setae in turn be-
came modified from their original simple cylindrical shape to form the wide, thin, blade-like
filaments of Cryptolithus and Ccraurus.

Another possible use of the exopodites is suggested by the action of some of the bar-
nacles, which use similar organs as nets in gathering food and the endopodites as rakes
which take off the particles and convey them to the mouth. The exopodites of the trilo-
bite might well set up currents which would direct food into the median groove, where it
could be carried forward to the mouth.

Endopodites.

The endopodites were undoubtedly used for crawling; in some trilobites, probably most
of them, for swimming; in the case of Cryptolithus, and probably others, for burrowing; and
probably in all for gathering food, in which function the numerous spines with which they
are arrayed doubtless assisted.

Various trails have been ascribed to the action of trilobites, and many of them doubtless
were made by those animals (see especially Walcott, 1918). Some of these trails seem to
indicate that in crawling the animal rested on the greater part of each endopodite, while
others, notably the Protichnites recently interpreted by Walcott (1912 B, p. 275, pi. 47),
seem to have touched only the spinous tips of the dactylopodites to the substratum. The
question of the tracks, trails, and burrows which have been ascribed to trilobites is dis-
cussed briefly on a later page; but can not be taken up fully, as it would require another
monograph to treat of them satisfactorily.

The flattened, more or less triangular segments of the endopodites of the posterior part

72 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

of the thorax and pygidium in Triarthrus, Cryptolithus, 'and Acidaspis probably show an
adaptation of the endopodites of the posterior part of the body both as more efficient push-
ing organs and as better swimming legs. The fact that these segments are pointed below
enabled them to get a better grip on whatever they were crawling over, and the flatten-
ing allowed a much greater surface to be opposed to the water in swimming. In this con-
nection it might be stated that it seems very probable that the trilobites with large pygidia
at least, perhaps all trilobites, had longitudinal muscles which allowed them to swim by an
up and down motion of the fin-like posterior shield, the pygidium acting like the caudal fin
of a squid. Such a use would explain the function of the large, nearly flat pygidia seen
in so many of the trilobites beginning with the Middle Cambrian, and of those with wide
concave borders. It should be noted here that it is in trilobites like Isotclus, with pygidia
particularly adapted to this method of swimming, that the endopodites are most feebly de-
veloped, and show no flattening or modification as swimming organs.

The relatively strong, curved, bristle-studded endopodites of Cryptolithus, combined
with its shovel-shaped cephalon, indicate Limulus-\ike burrowing habits for the animal, and
the mud-filled casts of its intestine corroborate this view. That it was not, however, en-
tirely a mud groveller is indicated by its widespread distribution in middle Ordovician times.

Use of the Pygidium in Swimming.

The idea that the use of the pygidium as a swimming organ is a possible explanation
of that caudalization which is so characteristic of trilobites has not been developed so far
as its merits seem to deserve. Two principal uses for a large pygidium of course occur to
one : either it might form a sort of operculum to complete the protection when the trilo-
bite was enrolled, or it might serve as a swimming organ. That the former was one of its
important functions is shown by the nicety with which the cephalon and pygidium are
adapted to one another in such families as the Agnostid;e, Asaphida;, Phacopidse, and others.
That a large pygidium is not essential to perfect protection on enrollment is shown by
an equally perfect adjustment of the two shields in some families with small pygidia, notably
the Harpedida; and Cheiruridas. That the large pygidial shields are not for protective pur-
poses only is also shown by those forms with large pygidia which are not adjusted to the
conformation of the cephalon, as in the Goldiidse and Lichadidre. It is evident that a large
pygidium, while useful to complete protection on -enrollment, is not essential.

It would probably be impossible to demonstrate that the trilobites used the pygidium
in swimming. The following facts may, however, be brought forward as indicating that they
probably did so use them.

1. The appendages, both exopodites and endopodites, are relatively feebly developed
as swimming organs. This has been discussed above, and need not be repeated. It must
in fairness be observed, however, that many modern Crustacea get about very well with
limbs no better adapted for swimming than those of the trilobites.

2. The articulations of the thoracic segments with each other and with the two shields
are such as to allow the pygidium to swing through an arc of at least 270°, that is, from
a position above the body and at right angles to it, around to the plane of the bottom
of the cephalon. Specimens are occasionally found in which the thorax and pygidium are
so flexed that the latter shield stands straight above the body. A well preserved Dipleura
in this position is on exhibition in the Museum of Comparative Zoology, and Mr. Narraway
and I have figured a Bumastus milleri in the same attitude (Ann. Carnegie Mus., vol. 4,
1908, pi. 62, fig. 3).

FUNCTION OF APPENDAGES. 73

3. What little can be learned of the musculature (see under musculature, seq.) indi-
cates that the trilobites had powerful extensor and flexor muscles, such as would be required
for this method of swimming. It may be objected that the longitudinal muscles were too
small to permit the use of a caudal fin. In the lobster, where this method of progression
is most highly developed, there is a large mass of muscular tissue which nearly fills the pos-
terior segments. Trilobites have not usually been thought of as powerfully muscled, but it
may be noted that in many cases broad axial lobes accompany large pygidia. As the chief
digestive region appears to have been at the anterior end, and other organs are not largely
developed, it seems probable that the great enlargment of the axial lobe was to accommo-
date the increased muscles necessary to properly operate the pygidium. It may be noted that
in all these genera the axial lobe of the pygidium is either short or narrow.

4. The geological history of the rise of caudalization favors this view. With the ex-
ception of the Agnostidae and Eodiscidse, all Lower Cambrian trilobites had small pygidia,
and the same is true of those of the Middle Cambrian of the Atlantic realm (except for the
Dorypyge of Bornholm). In Pacific seas, however, large-tailed trilobites of the families
Oryctocephalidae, Bathyuridae, and Asaphidse then began to be fairly common, though mak-
ing up but a small part of the total fauna of trilobites. In the Upper Cambrian of the
Atlantic province the Agnostidae were the sole representatives of the isopygous trilobites,
while in the Pacific still another family, the Dikelocephalidae, was added to those previously
existing.

With the Ordovician, caudalization reached its climax and the fashion swept all over
the world. It is shown not so much in the proportion of families with large pygidia, as in
the very great development of the particular trilobites so equipped. Asaphidae and Illaenidse
were then dominant, and the Proetidas, Cyclopygidae, Goldiidae, and Lichadidae had begun
their existence. A similar story is told by the Silurian record, except that the burden of
the Asaphidse has been transferred to the Lichadidae and Goldiidas. All the really old (Cam-
brian) families of trilobites with small pygidia had now disappeared. In the general dwin-
dling of the subclass through the Devonian and later Palaeozoic, the few surviving species
with small pygidia were the first to go, and the proetids with large abdominal shields the
last.

The explanation of this history is probably to be found in the rise of the predatory
cephalopods and fishes, the natural enemies of the trilobites, against whom they could have
no other protection than their agility in escaping. While the records at present known carry
the fishes back only so far as the Ordovician (fishes may have arisen in fresh waters and
have gone to sea in a limited way in the Ordovician and more so in Silurian time) and the
cephalopods to the Upper Cambrian, the rise of the latter must have begun at an earlier
date, and it is probably no more than fair to conjecture that the attempt to escape swim-
ming enemies caused an increase in the swimming powers of the trilobites themselves. At
any rate, the time of the great development of the straight cephalopods coincided with the
time of greatest development of caudalization; both were initiated in the Pacific realm, and
both spread throughout the marine world during the middle Ordovician. And since, in the
asaphids, a decrease in swimming power of the appendages accompanied the increase in the
size of the pygidium, it seems probable that the swimming function of the one had been
transferred to the other. A high-speed, erratic motion which could be produced by the
sudden flap of a pygidium would be of more service in escape than any amount of steady
swiftness produced by the oar-like appendages of an animal of the shape of a trilobite.

74 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

Coxopodites.

The primary function of the endobases of the coxopodites was doubtless the gathering,
preparation, and carrying of food to the mouth. Although the endobases of opposite sides
could not in all cases meet one another, they were probably spinose or setiferous and could
readily pass food from any part of the axial groove forward to the mouth, and also send
it in currents of water. The endobases of the cephalic coxopodites were probably modified
as gnathites in all cases, but little is known of them except in Triarthrus, where they were
flattened and worked over one another so as to make excellent shears for slicing up food,
either animal or vegetable. In some cases the proximal ends of opposed gnathites were
toothed so as to act as jaws, but a great deal still remains to be learned about the oral
organs of all species.

The writer has suggested (1910, p. 131) that a secondary function of the endobases
of the thorax of Isotelus and probably other trilobites with wide axial lobes was that of loco-
motion. In Isotehis the endobases of the thorax are greatly over-developed, each being much
stouter and nearly as long as the corresponding endopodite, and the explanation seemed to
me to lie in the locomotor or crawling use of these organs instead of the endopodites. Cer-
tain trails which I figured seemed to support this view.

POSITION OF THE APPENDAGES IN LIFE.

In almost all the specimens so far recovered the appendages are either flattened by
pressure or lie with their flat surfaces in or very near the plane of stratification of the sedi-
ment. This flattening is extreme in Neolcnus, Ptychoparia, and Kootenia, moderate in Triar-
thrus and Cryptolithus, and apparently slight or not effective in Isotelus, Ceraurus, and
Calymcne. These last are, however, from the conditions of preservation, least available
for study.

In Part IV, attention is called to a specimen of Triarthrus (No. 222) in which some
of the endopodites are imbedded nearly at right angles to the stratification of the shale.
This specimen is especially valuable because it shows that the appendages in the average
specimen of Triarthrus have suffered very little compression, and it also suggests the prob-
able position of the endopodites when used for crawling.

In considering the position of the appendages in life, one must always remember one
great outstanding feature of trilobites, the thinness and flexibility of the ventral membrane.
The appendages were not inserted in any rigid test but were held only by muscular and con-
nective tissue. Hence we must premise for them great freedom of motion, and also rela-
tively little power. The rigid appendifers, and the supporting apodemes discovered by
Beecher, supplied fulcra against which they could push, but their attachment to these was
rather loose.

Considering, first, the position of the appendages in crawling, it appears that different
trilobites used their appendages in different ways. Neolcnus had compact stocky legs, which
allowed little play of one segment on another, as is shown by the wide joints at right angles
to the axis of the segment. Such limbs were stiff enough to support the body when the
animal was crawling beneath the water, where of course it weighed but little. That such a
crawling attitude was adopted by trilobites has been shown by Walcott in his explanation
of the trails known as Protichnites (1912 B, p. 278). Many trilobites probably crawled in

POSITION OF APPENDAGES. 75

this way, on the tips of the toes, so to speak. In such the limbs would probably extend
downward and outward, with the flattened sides vertical.

The limb of Triarthrus, however, is of another type. The endopodites are long, slender,
flexibly jointed, the whole endopodite probably too flexible to be used as a unit as a leg
must be in walking on the "toes." The proximal segments of" the thoracic and pygidial
endopodites are, however, triangular instead of straight-sided, and, the spine-bearing apex
of the triangle being ventral, it enabled the endopodites to get a grip on the bottom and
thus push the animal forward. This method of progression was more clumsy and less rapid
than that of Ncolcnus, but it sufficed. The natural position of the endopodite when used
in this way would seem to be with the flattened sides of the segments standing at an angle
of 30° to 45° with the vertical, thus allowing a good purchase on the bottom and at the
same time offering the minimum resistance to the water when moving the appendages
forward.

Isotelus has endopodites different from those of either Neolenus or Triarthrus. They
are composed of cylindrical segments, the joints indicating a certain amount of flexibility.
Since there is no method by which the segments may get a purchase on the bottom other
than by pushing with the distal ends, it would seem at first thought that Isotelus, like Neo-
lenus, crawled on its "toes." The endopodites of Isotelus are however, short and feeble
when compared with the size of the test, while the endobases of the coxopodites are ex-
traordinarily developed. These facts, together with certain trails, strongly suggest the use
of the coxopodites as the primary ambulatory organs, the endopodites probably assisting.
In this event, the position of the endopodites and coxopodites would be downward, both
outward and inward from the point of attachment, and the motion both backward and
forward. The fact that in the specimens as preserved the coxopodites point backward and
the endopodites forward indicates that the limb as a whole swung on a pivot at the appen-
difer. It is of course natural to suggest that the coxopodites and endopodites of all the
trilobites with wide axial lobes, Nileus, Bumastus, Homalonotus, etc., were developed in
this same way.

Cryptolithus presents still another and very peculiar development of the endopodites
where ability to get purchase on the sea floor is obtained by a stout limb of slight flexibility,
bowed and turned backward in the middle, where an enlarged segment insures stiffness.
The segments are flattened, and since the greatest strength when used in pushing and crawl-
ing is in the long axis of the oval section of the flattened limb, it seems probable that these
limbs did not hang directly down, with their sides vertical, but that their position in life
was very much the same as that in which they are preserved as fossils. By moving these
bowed legs forward and backward in a plane at a small angle to the surface of the body, a
powerful pushing impetus could be obtained. They may, however, have occupied much the
same position as do those of Limulus.

In the case of the endopodites, therefore, it is necessary to study the structure and prob-
able method of their use in each individual genus before suggesting what was the probable
position in life. In the act of swimming, the position was probably more uniform. When
the endopodites were used in swimming, as they undoubtedly could be with more or less
effect in all the trilobites now known, those with flattened surfaces probably had them at
such an angle as to give the best push against the water on the back stroke, while on the
forward stroke the appendage would be turned so that' the thin edge opposed the water.
The great flexibility of attachment would certainly permit this, though unfortunately nothing

76 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

is as yet known of the musculature. The coxopodites of course had less freedom of move-
ment in this respect, and probably could not turn their faces. For this reason, it seems to
me likely that those coxopodites which are compressed did not stand with their flattened
faces vertical, but in a position which was nearly horizontal or at least not more than 45°
from the horizontal. If the flattened faces were vertical, they would be in constant oppo-
sition to the water during forward movements and would be of no use in setting up cur-
rents of water toward the mouth, as every back stroke would reverse the motion.

The position of the exopodites in life seems to have been rather uniform in all the
genera now known. I have set forth on a previous page my reasons for thinking that they
took little part in swimming, and I look upon them as being, in effect, leaf-gills. It seems
probable that in all genera the exopodites were held rather close to the test, the shaft more
or less rigid, the filamentous setae gracefully pendent, but pendent as a sheet and not individ-
ually, there having been some method by which adjoining setae were connected laterally.
Free contact with the water was thus obtained without the mingling of endopodites and ex-
opodites which would have been so disastrous to progression.

PART II.

STRUCTURE AND HABITS OF TRILOBITES.
INTERNAL ORGANS AND MUSCLES.

Granting that the trilobite is a simple, generalized, ancient crustacean, it appears justifi-
able to attribute to it such internal organs as seem, from a study of comparative anatomy,
to be primitive.

The alimentary canal would be expected to be straight and simple, curving downward
to the mouth, and should be composed of three portions, stomodaeum, mesenteron, and proc-
todseum, the first and last with chitinous lining. In modern Crustacea, muscle-bands run from
the gut to part of the adjacent body wall, so that scars of attachment of these muscles
may be sought. At the anterior end of the stomodseum, they are usually especially strong.
From the mesenteron there might be pouch-like or tubular outgrowths.

The heart would probably be long and tubular, with a pair of ostia for each somite.

In modern Crustacea, the chief organs of renal excretion are two pairs of glands in the
head, one lying at the base of the antennae and one at the base of the maxillae. Only one
pair is functional at a time, but these are supposed to be survivors of a series of segmen-
tally arranged organs, so that there might be a pair to each somite of a trilobite.

The nervous system might be expected to consist of a supracesophageal "brain," com-
prising at least two pairs of ganglionic centers, and a double ventral chain of ganglia with
a ladder-like arrangement.

Besides these organs, a variety of glands of special function might be predicted.

Reproductive organs probably should occur in pairs, and more than one pair is to be
expected. There is little to indicate the probable location of the genital openings, but they
may have been located all along the body back of the cephalon.

It may be profitable to summarize present knowledge of such traces of these organs
as have been found in the fossils, if only to point out what should be sought.

ALIMENTARY CANAL.

Beyrich (1846, p. 30) first called attention to the alimentary canal of a trilobite, (Cryp-
tolithus goldfussi,) and Barrande (1852, p. 229) confirmed his observations. A number
of specimens of this species have been found which show a straight cylindrical tube or
its filling, extending from the glabella back nearly to the posterior end of the pygidium. It
lies directly under the median line of the axial lobe, and less than its own diameter beneath
the dorsal test. At the anterior end it apparently enlarges to occupy the greater part of the
space between the glabella and the hypostoma, but was said by the early observers to extend
only a little over halfway to the front. Beyrich thought the position of the median tubercle
indicated the location of the anterior end.

Walcott (1881, p. 200) stated that in his experience in cutting sections of trilobites it
was a very rare occurrence to find traces of the alimentary canal. The visceral cavity was
usually filled with crystalline calcite and all vestiges of organs obliterated. There were,
however, some slices which showed a dark spot under the axial lobe, which probably rep-
resented the canal. In his restoration he showed it as of practically uniform diameter
throughout, and extending but slightly in front of the mouth.

78 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

Jaekel (1901, p. 168, fig. 28) has produced a very different restoration. His discus-
sion of this point seems so good, and has been so completely overlooked, that I will append
a slightly abridged version of a translation made some years ago for Professor Beecher.
The idea was, however, not original with Jaekel, as it was suggested by Bernard (1894, p.
417), but not worked out in detail.

While considering the problem as to what organ could hav; lain beneath the glabella of the trilobite, and
while studying the organization of living Crustacea for the purpose of comparison, I found in the collections
of the Geological Institute preparations of Limulus which seemed to me to directly solve the entire question.

From the mouth, which lies at about the middle of the head shield, the oesophagus bends forward, swells
out at the frontal margin of the animal at a sharp upward bend in order to take a straight course backward
after the formation of an enlarged stomach. Still within the head shield there branch out from each' side
of the canal two small vessels which pass over into the richly branched mass of liver lying under the broad
lateral parts of the head shield. After seeing this specimen, I no longer had the least doubt that the head
shield of the trilobites is to be interpreted in a similar manner. The position of the hypostoma and
gnathopods makes it necessary to assume that the position of the mouth of the trilobite lay pretty far back.
If, therefore, this depends upon the secondary ventral deflection of the oral region, as seems to be the case,
then it is a priori probable that the anterior part of the canal has also shared in this ventral inflection.

The posterior part of the canal in the region of the segmented thorax and pygidium is comparatively
narrow, as shown long ago by Beyrich; he represents only a thin tube which shows no swellings whatever,
and such are usually missing in Arthropoda.

As the glabella of most trilobites is regularly convex, there must lie beneath it an organ running from
front to back, which presses the bases of the cephalic legs away from each other and down from the dorsal
test. An organ so extensive and unpaired, running thus from front to back, can, among the Arthropoda, be
regarded only as an alimentary canal, for the swellings of the cephalic ganglia and the heart are by far too
small to produce such striking elevations on the front and upper surface of the glabella. The canal might
then have consisted of a gizzard belonging to the oesophagus, and a stomach proper or main digestive canal.

. . . Among the trilobites there are two pairs of vessels on both sides of the glabella which have
precisely the same position with reference to the supposed course of the alimentary canal as the ducts of
the hepatic lobes in Limulus. One observes in numerous trilobites, although in different degrees of clearness
and under various modifications, a dendritic marking of the inner surface of the cheeks which takes its
rise at the lateral margins of the glabella and spreads thence like a bush over the entire surface of the
. cheeks. Exactly the same position is taken by the richly branched hepatic lobes of Limulus on the lower
surface of the head shield; a fact of special weight in favor of the homology and similar significance of the
two phenomena, is that in the trilobites also, the anterior of the two main ducts is the larger, the posterior
the smaller. The striking similarity of the two structures is shown by a comparison of the head shield of
Eurycare [Elyx] from the Cambrian of Sweden, in which the course of the canals is shown with remarkable
clearness [with those of Limulus].

I have been able to convince myself that the existence of the two canals on each side is also the rule in
other genera, even though the posterior pair is frequently but feebly developed or completely obscured by
the anterior pair. In Dionide formosa, for example, I find only the anterior pair, which is very large and
divided into two principal branches. From all these considerations it seems to me no longer doubtful that
the median elevation was caused by the stomach and gizzard, and that the cheeks have principally served to
cover the hepatic appendages of the alimentary canal.

The cause of the incomplete development of the glabellar lobes lies, hence, in the intrusion of the
alimentary canal, and it makes naturally the most effect where the gizzard spreads out and bends into the
stomach. This spot lies behind the frontal lobe, which is hence increased in size according as the stomach
increases in size; in this way not only the foremost segments of the glabella become enlarged, but also the
following ones more or less pressed aside. This process is easily followed phylogenetically and ontogenetically.

From the latter point of view, the development of Paradoxides is very instructive. In a head shield
2.5 mm. long the whole anterior part of the glabella is broadened, but the five pairs of lateral impressions
are clearly marked and the six segments of the head bounded by them are all of about the same size. In a
head shield about 13 mm. long, the foremost segment is very much increased in size, the jaw lobes pressed
still further apart; in adult forms both anterior segments are combined into the frontal swellings of the
glabella. In other groups this process proceeds phylogenetically still further, so that among the Phacopidae
and in Trinucleus, behind the frontal swelling of the glabella only the last cephalic segment retains a certain
independence. The frontal lobe is thus no definite part, although it is as a rule composed of the mesotergites
of the first two cranidial segments.

ALIMENTARY CANAL.

79

This idea of an enlarged mesenteron certainly has much to commend it, and such actual
evidence as exists seems in favor of rather than against it. The strongest, firmest, best-
protected place in the whole body of the trilobite is the cavity between the vaulted glabella
and the hypostoma. As Jaekel has said, it is far too large a cavity for the brain, larger
than would seem to be required for a heart, and what else could be there but a stomach?
As has already been pointed out, Beyrich and Barrande found a pear-shaped enlargement of
the alimentary canal under the glabella of Cryptolithus. Longitudinal sections through
the glabella of Calymcnc and Ceraurus practically always show the cavity there filled with
clear crystalline calcite. One actual specimen of Ceraurus (Walcott 1881, pi. 4, fig. i)
shows the cavity between the glabella and hypostoma entirely empty. The vacant spaces in
these two classes of specimens do not, however, necessarily mean anything more than im-
perfect preservation.

Fig. 21. — Transverse slice through
Ceraurus plcurex.anthemus, to show
the dorsal sheath above the abdomi-
nal cavity. Specimen 118. Traced
from a photographic enlargement.
X4.

Fig. 22. — -Transverse section through
the cephalon of Ceraurus fleurexan-
themus, showing the abdominal sheath
and the large mud-filled alimentary
canal (clear white). Traced from a
photographic enlargement. Specimen
97- X 3-3-

Fig. 23. — Trans-
verse section of
the thorax of Ca-
lymene senaria,
showing the large
size of the mud-
filled alimentary
canal(clear white).
Traced from a
photographic en-
largement One
appendifer (also
clear white) is
shown. Specimen
153- X 3-3-

Ceraurus pleurexanthemus.

This species is taken up first, as it is the one shown in Walcott's often-copied figure
(1881, pi. 4, fig. 6). It is to be feared that too many have looked at this figure without
reading the accompanying explanation, and have taken it for a copy of an actual specimen
and not a mere diagram, which it admittedly is. The evidence on which it is based is com-
prised in eight transverse slices, one through the glabella and seven through the thorax.
Three of these have been figured by Walcott: No. 27, 1881, pi. 3, fig. 7; No. 13, 1881, pi. 2,
fig. 3, 1918, pi. 26, fig. 14; No. 202, 1918, pi. 27, fig. 8. In all, as can be seen by reference to
the figures, the canal is partially collapsed, and is much larger than is indicated in Walcott's
restoration. The other sections bear out the testimony of those figured. One of these figured
specimens (No. 27) and another figured herewith (No. 118, see fig. 21) show an exceedingly
interesting structure which has previously escaped notice. The body cavity seems to have
had, in this region at least, a chitinous sheath on the dorsal side. As shown especially in
figure 21, this sheath impinges dorsally and laterally against the axial lobe and thus fur-
nishes a special protection for the soft organs beneath, probably protecting them from the
strain of the dorsal muscles.

80 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

While there is no way in which the location of these sections in the thorax can be posi-
tively determined, it is probable that they came from the anterior end. In sections further
back, supposed to be in the posterior region of the mesenteron, no sheath is shown, but the
canal is nearly if not quite as large in relation to the size of the axial lobe.

The single section through the glabella (specimen 97) is of course important and for-
tunately well preserved (fig. 22). It shows the dorsal sheath pressed against the inner sur-
face of the axial lobe along its middle portion, but diverging from it at the sides. The
section of the canal is oval, nearly twice as wide as high, but it is obviously somewhat de-
pressed. The original canal evidently filled nearly the whole of the dorsal part of the glabella
in this particular region. Unfortunately, the connection with the mouth is not shown, and
the form of the hypostoma indicates that the section cut the glabella diagonally, either in
the anterior or posterior part, probably the latter. In all these cases it should be remem-
bered that the specimens were found lying on their backs, and the canal has fallen in (dor-
sally) since death.

The sections show that in Ceraurus pleurcxanihemus the anterior part of the alimentary
canal was large, filling the part of the glabella below the heart; that the body cavity was
provided with a chitinous dorsal sheath extending back into the thorax; and that the pos-
terior portion of the mesenteron was likewise large and oval in section. Since the alimen-
tary canal must be connected with the mouth and anus, some such restoration as that of
Jaekel is indicated. No chitinous lining of the stomodjeum or proctodasum was found, but
it is not certain that any of the sections cut either of those regions.

Calymene senaria.

Ten transverse sections and one longitudinal slice show the form of the alimentary canal
in Calymene. One of these has been figured by Walcott (1881, pi. I, fig. 9) but without
showing the organ in question.

The only section cutting the cephalon which shows any trace of the canal is a longi-
tudinal one (No. 141), which is not very satisfactory. It has a large, nearly circular,
opaque spot under the anterior part of the glabella which may or may not represent a sec-
tion across the anterior end of the mesenteron. Three sections (No. 9, 115, 143) show
the dorsal sheath, the latter having the mud-filled canal beneath it. The sheath arches
across the axial lobe as in Ceraurus, leaving room for the dorsal muscles at the sides and
above it. In this region the canal is large and oval in section. Six slices cut the mesen-
teron behind the abdominal sheath (Nos. 39, 117, 148, 153, 62, 65) (see fig. 23). In the
first four of these it is oval in section and large, but not so large as in No. 143. In the
last two, it is small and circular in section, from which it is inferred that the canal tapers
posteriorly.

Cryptolithus goldfussi (Barrande).

Illustrated: Beyrich, Untersuch. iiber Trilobiten, Berlin, 1846, pi. 4, fig. ic. — Barrande, Syst. Sil. Boheme,
vol. I0 1852, pi. 30, figs. 38, 39.

Both Beyrich and Barrande have shown that from the posterior end of the axial lobe
to the neck-ring on the cephalon, the alimentary canal in Cryptolithus has a nearly uniform
diameter of less than half the width of the axial lobe. In front of the neck-ring, it enlarges,
and while its original describers state that it extends only about halfway to the front of

ALIMENTARY CANAL. 8l

the glabella, Barrande's figure 39 shows it extending quite to the front, and his figure 38 shows
it fully two thirds of the distance to the anterior end, as does Beyrich's figure of 1846.

The Museum of Comparative Zoology contains a single specimen of this species from
Wesela, Bohemia, which shows the course of the canal from the middle of the pygidium to
the anterior part of the glabella. The enlargement appears to begin about halfway to the
front of the glabella and to be greatest at the anterior end. At the anterior end of the
glabella, the anterior end of the thorax, and the posterior end of the pygidium, the canal is
still packed full of a material somewhat darker in appearance than the matrix, while the re-
mainder of it is open. A well defined constriction is present under the middle of the next
to the last thoracic segment, but whether this is accidental or whether it indicates the point
where the mesenteron discharges into the proctodseum can not be determined. The inside
of the canal has somewhat of a lustre and there are three conical projections into it on the
median ventral line, a very small one in front of the neck furrow, a larger one under the
anterior part of the second segment, and a third between the fourth and fifth segments.

Summary.

The specimens of Cryptolithus from Bohemia and of Ceraurus and Calymene from
New York seem to substantiate the claim of Bernard and Jaekel that at the anterior end

Fig. 24. — Longitudinal section of Ceraurus pleurexanthemus, show-
ing the probable outline of the alimentary canal and the heart above
it. A restoration based on the slices described above.

of the canal there was an enlarged organ which occupied the greater part of the cavity of
the glabella. It appears that it extended into the thorax, and that above it and the heart
was a chitinous dorsal sheath. Behind the enlarged portion, the mesenteron appears to have
been of practically uniform diameter in Cryptolithus, but to have tapered posteriorly in
Ceraurus and Calymene. The proctodeeum can not yet be differentiated from the mesen-
teron, and only in Cryptolithus has the posterior portion of the alimentary canal been seen.
It is, there, merely a continuation of the mesenteron. The stomodasum likewise has not been
identified, but was probably a short gullet leading up from the mouth into the enlarged
digestive cavity.

The principle of the enlargement of the latter and its influence on the dorsal shell once
established, the significance of different types of glabellre becomes apparent. It will be re-
membered that the glabella of the protaspis of most trilobites is narrow, and that the same
is true of the glabellae of most ancient and all primitive trilobites. The free-swimming larvae
and the free-swimming ancestors of the trilobites were probably strictly carnivorous, lived
on concentrated food, and needed but a small digestive tract. As the animals "discovered
the ocean bottom" and began to be omnivorous or herbivorous, larger stomachs were re-
quired, and so in the later and more specialized trilobites the glabella became expanded lat-
terally or dorsally, or both, to meet the requirement for more space, until, in such Devonian
genera as Phacops, the cephalon was nearly all glabella.

82 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

GASTRIC GLANDS.

Taekel's suggestion, quoted above, that the so called "nervures" seen on the under sur-
faces of the heads of some trilobites are really glands for the secretion of digestive juices,
is at least worthy of consideration. Moberg, however (1902, p. 299), suggested that these
markings probably had something to do with the eyes rather than the stomach. He says in
part (translation) :

In general we can now say that such features are common to all the eyeless Conocoryphidae. With the
conocoryphs I include Elyx and consider Harpides as at least closely related. Similar impressions are also
found in forms with eyes, as, for instance, in the Olenidse, but here such radiate partly from the border
of the eye, partly from the front end of the glabella, partly from the [visual surface of the] eye, and some-
times from the angle between the occipital ring and the glabella. They therefore go out from such different
points that they can not possibly be -branches of the liver. It would also be very remarkable if such an
important organ should have been developed in a few eyeless forms, but have failed to leave the least
trace in the rest of the trilobites.

Lindstroem (1901, pp. 18, 19, 33; pi. 5. figs- 29. 31! P1- 6, figs- 43-45) has discussed
these markings and given beautiful figures showing their appearance in Olcnus, Pambolina,
Efax, Conocoryphe, and S oleno pleura. He decided that they were to be explained as branches
of the circulatory system, comparing them with the veins and arteries of Limulus. He
pointed out that there was a coincidence between these markings and the position of the eyes,
and suggested a causal connection with the latter.

Beecher (1895 B, p. 309), also from a comparison with Limulus, suggested that the
eye-lines of Cryptolithus, Harpes, Conocoryphe, Olenus, Ptychoparia, Arethusina, etc., prob-
ably represented the optic nerves, and since the eye-lines are usually the main trunks of the
dendritic markings, it is fair to assume that he considered the whole as due to branches of
nerves.

Reed has recently (1916, pp. 122, 173) discussed -these lines as developed in the Tri-
nucleidse, and seems to accept Beecher's explanation.

Three explanations of the "nervures" are thus current, and the authors of all of them
refer us to Limulus as proving their claims! So far as general appearance goes, the mark-
ings on the trilobites more closely resemble the veins of a Limulus than either the nerves or
"liver" of that animal. The veins, however, are not in contact with the dorsal shell, but
are buried in the liver and muscles, while the arrangment of the arteries, which are dorsal
in position, is quite unlike what is seen in the trilobites.

The term nervures, as applied to these markings, is not only misleading, but an incor-
rect use of one of Barrande's words, for by nervures. he meant delicate surface markings.
Until the real function of the organs which made these markings is definitely established, it
may be well to call them genal cceca, for they obviously were open tunnels ending blindly,
whatever they contained.

The question of the function of the genal caeca can not, in any case, be settled by an
appeal to Limulus, and it is doubtful if it can be settled at all at the present time. Cer-
tain things tend to show that Jacket's explanation is the most plausible, and these may be
briefly set forth.

Walcott (1912 A, pp. 176, 179, pis. 27, 28) has described specimens of Naraoia and
Burgcssia in which similar markings are well shown, and where they are obviously con-
nected with the alimentary canal just at the anterior end of the mesenteron. In Burgessia,
which seems to be a notostracan branchiopod, the trunk sinuses are very wide, and the ap-

GASTRIC GLANDS. 83

pearance is on the whole unlike that of any known trilobite. In Naraoia, however, the
markings are much finer and directly comparable with those of Elyx. If my contention that
Naraoia is a trilobite should be sustained, it might almost settle the question of the "ner-
vures." In Burgcssia these lateral trunks enter the main canal behind the fifth pair of ap-
pendages. In the trilobites they debouch much further forward.

The principal argument in favor of the interpretation of these markings as nerves lies
in their connection with the eyes. There is considerable evidence to indicate that the eye-
lines and the genal caeca are two distinct structures, but because both originate from the
sides of the anterior lobe of the glabella, and both extend outward at nearly right angles
to the axis, or obliquely backward, they are, when both present, coincident. Genal caeca
occur on blind trilobites, on trilobites with simple eyes, and on trilobites with compound eyes.
Eye-lines occur on trilobites with both simple and compound eyes, and genal caeca may or
may not be present in both cases. The morphology of the ridge forming the eye-line in
trilobites with compound eyes is well known. It is abundantly proved by ontogeny that it
is the continuation of the palpebral lobe, and a development of the pleura of the first dor-
sal segment of the cephalon. Lake, Swinnerton, and Reed have tried to show that the eye-
lines of the Harpedidae and Trinucleidse are homologous with the eye-lines of the trilobites
with compound eyes, and that the ocelli on the cheeks are therefore degenerate compound
eyes.

The simplest form of the genal caecum is seen in the blind Elyx (Lindstroem 1901, pi.
6, fig. 43). The main trunk is at nearly right angles to the axis, the increase in its width
is gradual in approaching the glabella, and an equal number of branches diverge from both
sides.

Ptychoparia striata (Barrande 1852, pi. 14, figs. I, 3) is an excellent example of a trilo-
bite with compound eyes and genal caeca. It will be noted that the main trunk and the eye-
line are coincident, and that both on the free and fixed cheeks the branches are all on
the anterior side of the eye-line. Compare this with the condition in Conocoryphe
(Barrande, pi. 14, fig. 8; Lindstroem, pi. 6, fig. 44), and one sees there a main branch
having the same direction as in Ptychoparia and likewise with all the branches on the anterior
side. At first sight this would seem to support the contention that these lines do lead out
to the eyes, since Conocoryphe is blind, and the main trunk leads practically to the margin.
But although Conocoryphe is blind, it has free cheeks, and the main trunk does not lead to
the point on those free cheeks where eyes are to be expected, but back into the genal angles.
And this direction holds in such diverse genera (as to eyes and free cheeks) as Harpes, Crypto-
lithus, Dionide, and Endymionia. In all these the genal caeca fade out in the genal angles, and
in none of them would compound eyes be expected in that region. The coincidence of the
eye-lines with the trunks of the genal caeca in Ptychoparia seems to be merely a coincidence.
That the markings which radiate from the eyes of Ptychoparia and Solcnopleura are not im-
pressions made by nerves is obvious. That they are of the same nature as the similar mark-
ings in the eyeless trilobites is equally obvious. Ergo, they can not be nerves in either case,
and that they have anything to do with the eyes is highly improbable. The eye was merely
superimposed upon these structures.

The relation of the genal caeca to the ocelli on the cheeks is best shown in the Trinu-
cleidae. In all species of Tret as pis simple eyes are present, and in most of them there are
very narrow eye-lines. The latter are occasionally continued beyond the ocular tubercle back
to the genal angle. A similar course is seen in Harpes. If the simple eye is the homologue

84 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

of the compound eye, and the eye-line here the homologue of the eye-line in Ptychoparia,
why does it continue beyond the eye? In any case, it can not be interpreted as a nerve.
Cryptolithus tessellatus, when the cephalon is 0.45 mm. to 0.65 mm. long, shows short eye-
lines and a small simple eye on each cheek. In some half-grown specimens, traces of the ocelli
can be seen, but the eye-lines are absent. In the adult, both the eye-lines and the ocelli are
entirely wanting. Reed states that "nervures" are also absent, and so they are from most
specimens, but well preserved casts of the interior from the Upper Trenton opposite Cincin-
nati show them, and one cheek is here figured (fig. 25). As apparent from the figure, the
main trunk is very short and gives rise to two principal branches, the first of which in
its turn sends off lines from the anterior side. It was a specimen showing these lines which
Ruedemann (1916, p. 147) figured as showing facial sutures. The interest lies in the fact
that while the ocelli and eye-lines were lost in development, the genal cjeca are present
in the adult, showing that they are different structures.

Fig. 25. — Cryptolithus
tessellatus Green. Side
view of the cheek of a
specimen from the top of
the Trenton opposite Cin-
cinnati, Ohio, to show the
branching genal caeca.
These are the "facial
sutures" of Ruedemann.

Harpides is another genus in which genal caeca are strikingly shown, and in this case
they completely cover the huge cheeks, radiating from two main trunks to the front and
sides. I have seen no good specimens, but it would appear from Angelin's figure (1854,
pi. 41, fig. 7) that the rather large, simple eyes are not situated exactly on the vascular trunks.
In the Harpides from Bohemia, the main trunks extend out with many branches beyond the
simple eyes. It should be stated that the courses of the genal casca are not correctly figured
by Barrande (Supplement, 1872, pi. I, fig. n), as shown by casts of the original specimen
in the Museum of Comparative Zoology. From Barrande's figure, one would suppose that
the eye-lines and their continuation beyond the "ocelli" were superimposed upon the genal
c?eca without having any definite connection with them, but as a matter of. fact the radial
markings really diverge from the main trunks as in Elyx and similar forms.

Summary.

As Reed has said, these lines are not mere ornamentation, but rather represent traces
of structures of some functional importance. They probably can not be explained as traces
of nerves and more likely represent either traces of the gastric caeca or of the circulatory

HEART. 05

system. While they are known chiefly in Cambrian and Lower Ordovician trilobites, there
is no evidence that the organs represented were not present in later forms, even if the shell
may not have been affected by them. While they indicate very fine, thread-like canals, the
present evidence seems to be in favor of assigning to them the function of lodging the glands
which secreted the principal digestive fluids.

HEART. ,

Illcenus.

Volborth (1863, pi. I, fig. 12 = our fig. 26) has described the only organ in a trilobite
which suggests a heart. A Russian specimen of Illcenus with the shell removed shows a
somewhat flattened, tubular, chambered organ extending from under the posterior end of
the cephalon to the anterior end of the pygidium. The posterior nine chambers were each
1.5 mm. long and 1.5 mm. wide, while the two anterior chambers were respectively 2.5 mm.

Fig. 26. — Copy of Vol-
borth's figure of the heart
of Illanus.

Fig. 27. — Heart
of Apus. Copied
from Gerstacker.

and 3 mm. wide. These were all under the thorax, and at least two more chambers are
shown under the cephalon, but rather obscurely. The species of the Illcenus is not stated,
but since no Illcenus has more than ten segments in the thorax, and this tube has at least
thirteen chambers, it is evident that its constrictions are inherent in it, and are not due to
the segmentation of the thorax. Beecher has made a passing allusion to this organ as an
alimentary canal. This was the original opinion of Volborth. Pander, however, suggested
to him that it might be a heart. The alimentary canal of Cryptolithus does not show any
constrictions, while the heart of Apus (see fig. 27) and other branchiopods does show them.
It should be noted, further, that while this heart enlarges toward the front, it is everywhere
very small as compared with the width of the axial lobe, and much narrower than sections
of Ccraurus and Calymene would lead one to expect the alimentary canal of Illcenus to be.
Where the heart is 1.5 mm. to 3 mm. wide, the axial lobe is n mm. wide.

While this may be merely a cast of the alimentary canal it is sufficiently like a heart to
deserve consideration as such an organ.

Ceraurus and Calymene.

Nothing suggesting a heart has been seen in the sections of Ceraurus and Calymene.
The mesenteron and its sheath crowd so closely against the dorsal test in the anterior part

86 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

of the thorax that there seems to be no room for the heart, but it must have been located
beneath the sheath and above the alimentary canal. If the latter were filled with mud, and
the animals lay on their backs, as most of them did at death, the canal would drop down
into the axial lobe and the soft heart would naturally disappear and leave 110 trace of its
presence in the fossils.

The Median "Ocellus" or "Dorsal Organ."

Many trilobites, otherwise smooth, bear on the glabella a median pustule which is usually
referred to as a simple eye or median ocellus, but whose function can not be said to have
been certainly demonstrated. Ruedemann (1916, p. 127), who has recently made a careful
study of this problem, lists about thirty genera, members of ten families, Agnostid;e, Eodi-
scidae, Trinucleidae, Harpedidce, Remopleuridae, Asaphida% Illaenidre, Goldiidre, Cheiruridse,
and Phacopidas, in which this tubercle is present, and had he wished he might have cited
more, for it is of almost universal occurrence in Ordovician trilobites.

I have not especially searched the literature for references to this median tubercle. It
is often mentioned by writers in descriptions of species, but apparently few have tried to
explain it. Beyrich (1846, p. 30) suggested that it indicated the beginning of the alimentary
canal. Barrande mentioned it, but if he gave any explanation, it has escaped me. McCoy
(Syn. Pal. Foss. 1856, p. 146) called it an ocular (?) tubercle, and that seems to have been
the interpretation which most writers on trilobites have assigned to it, if they suggested any
function at all. Beecher (1895 B, p. 309) concurred in this opinion.

Bernard (1894, p. 422) ascribed to this tubercle, as well as to the median tubercle on
the nuchal segment, an excretory function, comparing it with the "dorsal organ" in Apus.

Reed (1916, p. 174) states that it may be either the representative of the "dorsal"
organ of the branchiopods, or a median unpaired ocellus.

Ruedemann (1916) has made the only real investigation of the subject. He came to
the conclusion that it was a parietal eye, without a crystalline lens, but corresponding to the
"parietal eye of other crustaceans, and especially of the phyllopods, which is a lens-shaped
or pear-shaped sac, usually filled with sea water." He found that above the "ocellus" the
test was usually thin or even absent, and in a few cases a dark line beneath seemed to out-
line the original form of the sac. His summary follows :

It is claimed that most, if not all, trilobites possessed a median or parietal eye on the glabella. [In
proof of this assertion the following facts are stated:]

1. A great number of species, belonging to more than thirty genera, possess a distinct tubercle on the
glabella. This tubercle occurs alone in many genera, otherwise smooth, as in the Asaphidae, and is hence of
functional importance.

2. In certain cases, as in Cryptolithus tessellatus, distinct lenticular bodies [not lenses] were recognized ;
in others, as in Asaphus expansus, only a thinner, probably transparent test. Many other species show a
distinct pit in interior casts of the tubercle, indicating a lens-like thickening of the top of the tubercle. The
median eye therefore probably possessed all the different stages of development seen in other crustaceans.

3. As in the parietal eyes of the crustaceans and the eurypterids, the tubercles are most prominent and
distinct in the earlier growth-stages, notably so in Isotelus gigas.

4. The tubercle is especially well developed in the so called blind forms where the lateral eyes are
abortive, as in Cryptolithus (Trinucleus), Dionide, Ampyx.

5. The tubercles always appear on the apex on the highest part of the glabella, where their visual
function would be most useful.

6. The tubercle is generally situated between the lateral eyes, like the parietal eye in crustaceans and
eurypterids, on account of its close connection with the brain.

7. Frequently it forms the posterior termination of a short crest, also as in certain eurypterids (Sty-
lonurus), indicating the direction of the nerve.

HEART. . 87

V

8. The median eye is borne on a tubercle or mound in the Ordovician and Silurian trilobites, while the
tubercle is rarely noticed in the Devonian and in few Cambrian forms. In the Devonian forms, similarly
as in many crustaceans and in later growth-stages of some asaphids, the strong development of the lateral
eyes may have led to a loss of the parietal eyes. In the Cambrian genera evidence is present to suggest that
the parietal eyes consisted only of transparent spots or lens-like thickenings of the exoskeleton, hardly
noticeable from the outside.

9. It is a priori to be inferred that the trilobites should, as primitive crustaceans, have possessed median
or parietal eyes.

As a student, I accepted Professor Beecher's dictum that this tubercle represented a
median ocellus, but more recently a number of things have led me to the view that it is
the point of attachment of the ligament by which the heart is supported.

The chief arguments against its interpretation as a parietal eye seem to be that its
structure is not absolute proof, being capable of other explanation; its position is variable,
in front, between, or back of the eyes; it is exactly like other tubercles on the median line,
especially the nuchal spine or tubercle, and the similar ones along the axial lobe of the
thorax; and it is not present in the protaspis or very young trilobites.

1. The structure disclose'd by Ruedemann's sections, a sort of sac-like cavity beneath a
thinned test, can be explained as a gland, a ligamentary attachment, or a vestigial spine, as
well as an eye. In a section of Asaphus expansus, which I made some years ago when try-
ing to get some light on this problem, there is a similar cavity under the pustule, but a
secondary layer of shell lay beneath it and apparently cut it off from the glabellar region,
thus indicating that it had lost its function in the adult of this animal. Sections through
the tubercles of the glabella of Ceraurus show all of them hollow, with very thin upper
covering or none at all, and their structure is not unlike that of the tubercle of Cryptolithus.
In fact, sections can be seen in Doctor Walcott's slices which are practically identical with
the one Ruedemann obtained from Cryptolithus. Since it is obvious that not all of the
pustules of a Ceraurus could have been eyes, the evidence from structure is rather against
than for the interpretation of the median pustule as such an organ.

2. The position of the tubercle varies greatly in different genera. Where furthest for-
ward (Tretaspis, Goldius), it is just back of the frontal lobe, while in some species of asa-
phids it is in the neck furrow. In species with compound eyes it is frequently between the
eyes, but more often back of them. If its history be traced in a single family, it is gen-
erally found farthest forward in the more ancient species and moves backward in the more
recent ones. The eyes do this same thing, but the median tubercle goes back further than
the eyes. This can be seen, for example, in the American Asaphidre, where the pustule is
up between the eyes of Hemigyraspis and Symphysurus of the Beekmantown and back of the
eyes of the Isotclus of the Trenton. Turning now to the under side of the head, it appears
that the tubercle bears a rather definite relation to the hypostoma. If the hypostoma is short,
the tubercle is well forward. If long, it is far back on the head. It seems in many cases
to be just back of the posterior tip of the hypostoma, or just behind the position of the
mouth, while in others it is not as far back as the tip of the hypostoma.

The median tubercle is in many cases developed into a long spine. This is usually in
an ancient member of a tubercle-bearing family, and suggests that in most cases the tubercle
is a vestigial organ. An example of this occurs in Trinude aides, the most ancient
of the Trinucleida:. Trinudeoides reussi (Barrande) (Supplement, 1872, pi. 5, figs. 17, 18)
has a very long slender spine in this position. It could be explained as an elevated median
eye, but it also very strongly suggests the zoasal spine of modern brachyuran Crustacea.

88 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

Gurney (Quart. Jour. Mic. Sci., vol. 46, 1902, p. 462) supports Weldon in the conclusion
that the long spines of the zosea are directive, and states that the animal swims in the direction
of the long axis of the spine. He also suggests that, since the period of their presence cor-
responds to the period before the development of the "auditory" organs, the spines may
perform the functions of balancing and orientation. It is generally admitted that the spine
of the zosea is also protective, and the obvious function, first pointed out by Spence Bate
in 1859, is that it contains a ligament which helps suspend the heart, which lies beneath the
spine. This latter function may have been that of the median tubercle in the trilobite. Such
an explanation would account for the backward migration mentioned above, for as the
stomach enlarged and the mouth moved backward on the ventral side, the heart may have
been pushed backward on the upper side.

There is also a curious parallelism between the ontogenetic history of the zoieal spine
and the phylogenetic history of the Trinucleidae or Cheiruridae (Nieszkowskia is the ancient
member of this family in which the spine replaces the tubercle). When first hatched, the
larval crab shows no trace of the spine, but very quickly it evaginates, lying dorsally on the
median line, pointing forward (Faxon, Bull. Mus. Comp. Zool., vol. 6, 1880, pi. 2). With
the splitting of the original envelope, the spine becomes erect, but persists only a short time,
and is reduced to a vestigial tubercle toward the end of the zoosal stages, its disappearance be-
ing, as pointed out by Gurney, coincident with the development of the balancing organs. This
manner of suspension of the heart by a long tendon certainly does suggest that Gurney is
right in his interpretation of the function. Briefly, the zoaeal spine served for a short time
a function later taken over by other organs. It was not present in the youngest stages, it
became prominent at a very early stage, was soon vestigial, and then lost.

Take now the trilobites. There is no trace of the median pustule in the protaspis of
any form, and in many primitive trilobites it is absent. It appears first as a long spine in
certain families, and later becomes vestigial and disappears. Very few trilobites of Silurian
and later times show it at all.

In the particular case of the Trinucleidse, which were burrowers, the spine is present
on only the oldest and most primitive of the group, a form which has only a most rudimen-
tary fringe. It is obvious from the large size of the pygidium in the larval trinucleid that
this family is derived from a group of free swimmers. Trinucleoides reussi was perhaps in
the transitional stage, just leaving the swimming mode of life, and belonged to a group
which had not developed any other "statocyst" than the median spine. Among the later
Trinucleidaa the spine became a vestigial tubercle, and in some cases entirely disappeared. A
similar history can be traced in the Cheiruridae, starting from some such forms as the Ameri-
can Lower Ordovician Nieszkowskia (N- perforator p. ex.).

Another example of a median spine instead of a tubercle is in Goldius rhinoceros (Bar-
rande). Since this species is not from the oldest Goldius-bearing rocks, but from the Lower
Devonian, it does not follow what seems to be the general rule, but makes an interesting ex-
ception. Goldius rhinoceros (Barrande) (Supplement, 1872, pi. 9, figs. 12, 13) has the
median tubercle elevated into a stubby, recurved spine very suggestive of the horn of a
rhinoceros. Since the eyes of this species are very well developed, there seems no especial
reason for the elevation of a parietal eye, and the example certainly does not support that
interpretation.

3. This tubercle is essentially similar to other tubercles on the median line of cephalon,
thorax, and even pygidium. This has been discussed sufficiently under section i above, but

VARIOUS GLANDS. 89

it may perhaps be justifiable to point out that in some of the Agnostidas there is a median
tubercle on both shields, and since it has not yet been demonstrated beyond question which
shield is the cephalon, to say which one is a parietal eye and which one is a tubercle is im-
possible. In other words, the parietal eye can not be differentiated from any other tubercle
except by its position.

4. One of the as yet unexplained features of the protaspis of trilobites is the absence
of the "nauplius eye." Beecher (1897 B, p. 40) explained this on the ground of the
extremely small size of the protaspis and the imperfection of the preservation. If the me-
dian tubercle were really a median eye, it should be present in the protaspis and the earlier
stages of the ontogeny, even if not in the adult, and should certainly appear before the com-
pound eyes. (In Liiniilns, however, the compound eyes appear first.) The median eye has
not so far been seen in any young trilobite in any stage previous to that in which compound
eyes are present. The full ontogeny is not known of any species with compound eyes in
which the median tubercle is present in the adult, but theoretically the median eye should be
most prominent in the young of just those primitive trilobites about whose development most
is known.

NERVOUS SYSTEM.

There has been a rather general impression among students of trilobites that the eye-
lines, which should be differentiated from the genal caeca, denote the course of the optic
nerves, but no other evidence of the nervous system has been found, save the so called
nervures which have been discussed above. In Apus the nerves leading to the eyes come
off from the anterior ganglion or "brain" and run directly to the eyes. If conditions were
similar in the trilobites, the "brain" was beneath the anterior glabellar lobe, provided, of
course, that the eye-lines do indicate the course of the optic nerve.

The ontogenetic history of the eye-lines of trilobites with compound eyes is instructive,
and has already been discussed by Lindstroem (1901, pp. 12-25), Dut ne did not cite the
case of Ptychoparia, which is particularly interesting, because in this genus both eye-lines
and "nervures" are present. Beecher (1895 C, p. 171, pi. 8, figs. 5-7) has shown that in
Ptychoparia kingi the eye-lines of a specimen in the metaprotaspis stage run forward at a
low angle with the glabella, while in the adult their course is nearly at right angles to it. They
have therefore swung through an arc of at least 60° and in so doing have had ample oppor-
tunity to become coincident with the primary trunks of the genal caeca. Once that was ac-
complished, it is quite likely that the one fold in the shell would continue to house both
structures. In other trilobites, there is a similar backward progression of the eye-lines.

As would be expected, the ventral ganglia and the longitudinal cords left no trace in
the test. Since each segment has appendages, there was probably a continuous chain of
ganglia back to the posterior end of the pygidium.

VARIOUS GLANDS.

Dermal glands. — The surface of many trilobites is "ornamented" with pustules and
spines which on sectioning are nearly always found to be hollow, and in many cases have a
fine opening at the tip. While it is generally believed that the purpose of these spines was
protective, yet it is possible that many of them were merely outgrowths which increased
the area through which the respiratory function could be carried on. It will be recalled

9O THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

that most of the smooth trilobites are punctate, some of them very conspicuously so, and
the spines and pustules of ornamented trilobites may merely subserve the same function as
the pores of smooth ones.

If the spines were protective, it would not be surprising if some of them, hollow and
open at the top, were poisonous also, and had glands at the base. These are, however,
purely matters of speculation so far.

Renal excretory organs. — Nothing has been seen of any such organs, unless the genal
caeca may possibly be of that nature. The main trunks of these always lead to the sides
of the anterior glabellar lobe, which is not the point of attachment of either antennae or
biramous limbs, so that there seems little chance that they will bear this interpretation.

Reproductive organs. — Nothing is yet positively known about the reproductive organs
or the position of their external openings. If the "exites" of Neolenns could be interpreted
as brood-pouches, which does not seem probable, then the genital openings were located near
the base of some pair of anterior thoracic appendages.

The Panderian Organs: Internal Gills or Poison Glands?

At a meeting of the Mineralogical Society at St. Petersburg, Volborth (1857) announced
that Doctor Pander had two years before discovered certain organs on the lower side of
the doublure of the pleural lobes of the thorax of a specimen of Asaphus expansus. These
organs were oval openings in the doublure, one near the posterior margin of the cephalon,
and one on each thoracic segment of the half-specimen figured by Volborth in 1863. They
were explained by Volborth and by Eichwald (1860, 1863) as the points of attachment of
appendages. Billings (1870) described and figured the "Panderian organs" of "Asaphus
platycephalus" and stated that he had seen them in Asaphus [Ogygites] canadensis and
A. megistos [Isotclus maximus\ as well. He thought some sort of organ was attached to
them, but could not suggest its function. Woodward (1870) thought that the openings were
"only the fulcral points on which the pleurae move." Their position outside the fulcra shows
that this explanation is impossible.

So far as I am aware, the Panderian organs have been seen only in the Asaphidas.
Barrande figured them in "Ogygia" [Hemigyraspis] desiderata (1872) and Schmidt in two
species of Pseudasaphus. They seem to occupy the same position in Bohemian, Russian,
and American specimens. There is always one pair of openings on each thoracic segment,
and one pair in line with them on the posterior margin of the cephalon. They occur near
the anterior margin of the segment, and near the inner end of the doublure. In some cases
they are surrounded by a ventrally projecting rim, while in others they have a thin edge.
There seem to be no markings on the interior of the shell which are connected with them.

While thinking over the trilobites in connection with the origin of insects, it occurred
to me that these hitherto unexplained Panderian organs might possibly be openings to internal
gills and that the Asaphidse might have been tending toward an amphibious existence. On
mentioning this to Doctor R. V. Chamberlin of the Museum of Comparative Zoology, he
called my attention to the possibility that they might be openings similar to those of the
repugnatorial glands of Diplopoda. While no definite decision as to the function can be
made, the explanation offered by Doctor Chamberlain seems more plausible than my own,
and has suggested still a third, namely, that they might be the openings of poison glands.

If one were to argue that these apertures are really connected with respiration, it might
be pointed out that they are ventral in position, while the foramina repugnatoria are always

MUSCULATURE. 91

dorsal or lateral, even in diplopods with broad lateral expansions. If offensive secretions
were poured out beneath a concave shell like that of a trilobite, they would be so confined
as to be but slightly effective against an enemy. This would indicate that if these open-
ings were the outlets of glands, the substance secreted might be a poison used to render prey
helpless. On the other hand, openings to gills are normally ventral in position, and if the
pleural lobes were folded down against the body, they would be brought very close to the
bases of the legs.

A further curious circumstance is that so far no traces of exopodites have been found
on Isotclus. The endopodites of both Isotelus latus and /. maximus are fairly well pre-
served in the single known specimen of each, yet no authentic traces of exopodites have
been found with them. Moreover, Walcott sliced specimens of Isotelus from Trenton Falls
and found only endopodites. It may also be recalled that the finding of the specimen of
Isotelus arcnicola at Britannia and the tracks which I attributed to it, suggested to me that
it was a shore-loving animal (1910). It offers a field for further inquiry, whether the

Fig. 28. — Side view of a specimen of
Isotelus gigas Dekay, from which the test
of the pleural lobes has been broken to
show the position of the Panderian organs.
Natural size. Specimen in the Museum of
Comparative Zoology.

Asaphidae may not have had internal gills, and whether some primitive member of the family
may not have given rise to tracheate arthropods.

The explanation of the Panderian organs as openings of poison glands is less radical
than the one just set forth, and so possibly lies nearer the truth. One would expect poison
glands to lie at the bases of fangs, and so they do in specialized animals like chilopods and
scorpions, but the trilobites may have had the less effective method of pouring out the poison
from numerous glands. The purpose may have been merely to paralyze the brachiopod or
pelecypod which was incautious enough to open its shell in proximity to the asaphid.

MUSCULATURE.

This is. a field which is rather one for investigation than for exposition. Very little
has been done, though probably much could be. The chief obstacle to a clearer understand-
ing of the muscular system lies in the difficulty of getting at the inner surface of the test
without obscuring the faint impressions in the process.

There exist in the literature a number of references to scars of attachment of muscles,
and any study of the subject should of course begin by the collection of such data. I shall
at this time refer to only a few observations on the subject.

The structure and known habits of trilobites make it obvious that strong flexor and
extensor muscles must have been present, and some trace of them and of their points of
attachment should be found. It is likely that their proximal ends were tough tendons. The
muscles holding up the heart and alimentary canal would be less likely to reveal their pres-

92 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

ence by scars, but there must have been at least one pair of strong muscles extending from
the under side of the head across to the hypostoma. Judging from the method of attach-
ment, the muscles moving the limbs were short ones, chiefly within the segments of the legs
themselves.

Flexor Muscles.

Since the majority of trilobites had the power of enrollment, and seem also to have used
the pygidia in swimming, the flexors must have been important muscles. Beecher (1902,
p. 170) appears to have been the only writer to point out any tangible evidence of their
former presence. Walcott (1881, p. 199) had shown that the ventral membrane was
reinforced in each segment by a slightly thickened transverse arch. Beecher showed that
on this thickened arch in Triarthrus, Isotelus, Ptychoparia, and Calymene, there are low lon-
gitudinal internal ridges or folds. One of these is central, and there is a pair of diagonal
ridges on either side. Beecher interpreted these ridges as separating the strands of the
flexor muscles, and believed that a line of median ridges divided a pair of longitudinal
muscles, while the outer ridges showed the place of attachment of the pair of strands which
was set off to each segment. He did not discuss the question as to where the anterior and
posterior ends were attached. In trilobites with short pygidia, the attachment would prob-
ably have been near the posterior end, and it is possible that the two scars beneath the dou-
blure and back of the last appendifers in Ceraurus may indicate the point of attachment in
that genus.

There is as yet no satisfactory evidence as to where the anterior ends of the flexors
were attached. In Apus these muscles unite in an entosternal sinewy mass above the mouth,
but no evidence of any similar mass has been found in the trilobites and it is likely that
the muscles were anchored somewhere on the test of the head.

Extensor Muscles.

The exact position of these muscles has not been previously discussed. The interior of
the dorsal test shows no such apodemes as are found on the mesosternites, but, as I have
shown in the discussion of the alimentary canal of Calymene and Ceraurus, there is an
opening on either side of the axial lobe between the dorsal test and the abdominal sheath,
and it is entirely probable that an extensor muscle passed through each of these. The ab-
dominal sheath extends only along the posterior region of the glabella and the anterior
part of the thorax, and probably served to protect the soft organs from the strain of the
heavy muscles. The extensors (see fig. 29) probably lay along the top of the axial lobe
on either side of the median line of the thorax to the pygidium, where they appear to
have been attached chiefly on the under side of the anterior ring of the axial lobe, although
strands probably continued further back. This is above and slightly in front of the fulcral
points on the pleura, and meets the mechanical requirements. Ceraurus (Walcott, 1875, and
1881, p. 222, pi. 4, fig. 5) shows a pair of very distinct scars on the under side of the first
ring of the pygidium, and in many other trilobites (Illatnus, Goldius, etc.) distinct traces of
muscular attachment can be seen in this region, even from the exterior. The anterior ends
were probably attached by numerous small strands to the top of the glabella, and, principally,
to the neck-ring.

On enrolling, the sternites of all segments are pulled forward and the tergites backward.
In straightening out, the reverse process takes place. The areas available for muscular at-

MUSCULATURE. 93

taclnnent are so disposed as to indicate longitudinal flexor and extensor muscles rather than
short muscles extending from segment to segment. Indeed, the tenuity of the ventral mem-
brane is such as to preclude the possibility of enrollment by the use of muscles of that sort,
while powerful longitudinal flexors could have been anchored to cephalon and pygidium. The
strongly marked character of the neck-ring of trilobites is probably to be explained as due

Fig. 20. — Restoration of a part of the internal organs of
Ceraurus pleurexanthemus as seen from above. At the sides
are the extensor muscles, and in the middle, the heart overlying
the alimentary canal. Drawn by Doctor Elvira Wood, under
the supervision of the author.

to the attachment of the extensor muscles, rather than to its recent incorporation in the
cephalon. The same is true of the anterior ring on the pygidium.

Possible preservations of extensors and flexors in Ceraurus. — Among Doctor Walcott's
sections are four slices which I should not like to use in proving the presence of longitudinal
muscles, but which may be admitted as corroborative evidence. Two of these transverse
sections (Nos. 114 and 199) show a dorsal and a ventral pair of dark spots in positions
which suggest that they represent the location of the dorsal and ventral muscles, while two
others (Nos. 131 and 140) show only the upper pair of spots. The chief objection to this

94 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

interpretation is that it is difficult to imagine how the muscles could be so replaced that they
happen to show in the section. Both the sections showing all four spots are evidently from
the anterior part of the thorax, as they show traces of the abdominal sheath, which seems
to be squeezed against the inside of the axial lobe, with the muscles (?) forced out to the
sides. The ventral pair lie just inside the appendifers, but even if they are sections of muscles,
all four are probably somewhat out of place.

Hypostomial Muscles.

The hypostoma fits tightly against the epistoma, or the doublure when the epistoma is
absent, but in no trilobite has it ever been seen ankylosed to the dorsal test, and its rather
frail connection therewith is evidenced by the relative rarity of specimens found with it
in position. That the hypostoma was movable seems very probable, and that it was held in
place by muscles, certain. The macula: were always believed to be muscle scars until Lind-
stroem (1901, p. 8) announced the discovery by Liljevall of small granules on those of
Goldius polyactin (Angelin). These were interpreted as lenses of eyes by Lindstroem, who
tried to show that the macuhe of all trilobites were functional or degenerate eyes. Most pa-
laeontologists have not accepted this explanation, and since the so called eyes cover only a
part of the surface of the maculae, it is still possible to consider the latter as chiefly muscle-
scars.

In Lindstroem's summary (1901, pp. 71, 72) it is admitted that the globular lenses
are found only in Bronteus (Goldius) (three Swedish species only) and Cheirurus spinu-
losus Nieszkowski, while the prismatic structure supposed to represent degenerate eyes was
found in eleven genera (Asaphidas, Illaenidas, Lichadidae). All of these are forms with well
developed eyes, and Lindstroem himself points out that the appearance of actual lenses in
the hypostoma was a late development, long after the necessity for them would appear to
have passed.

The use of the hypostoma has been discussed by Bernard (1892, p. 240) and extracts
from his remarks are quoted :

The earliest crustacean-annelids possessed large labra or prostomia projecting backward, still retained
in the Apodidae and trilobites. This labrum almost necessitated a very deliberate manner of browsing. The
animal would creep along, and would have to run some way over its food before it could get it into its
mouth, the whole process, it seems to us, necessitating a number of small movements backwards and forwards.
Small living prey would very often escape, owing to the fact that the animal's mouth and jaws were not
ready in position for them when first perceived. The labrum necessitates the animal passing forwards over
its prey, then darting backward to follow it with its jaws. We here see how useful the gnathobases of
Apus must be in catching and holding prey which had been thus passed over. Indeed the whole arrangement
of the limbs of Apus with the sensory endites forms an excellent trap to catch prey over which the labrum
has passed.

In alcoholic specimens of Apus the labrum is not in a horizontal plane, as it is in most
well preserved trilobites, but is tipped down at an angle of from 30° to 45°, and the big
mandibles lie under it. It has considerable freedom of motion and is held in .place by muscles
which run forward and join the under side of the head near its posterior margin. It seems
entirely possible that the hypostoma of the trilobite had as much mobility as the labrum of
Apus, and that by opening downward it brought the mouth lower and nearer the food. It
wil| be recalled that the hypostomata of practically all trilobites are pointed at the posterior
margin, there being either a central point or a pair of prongs. By dropping down the hypos-
toma until the point or prongs rested on or in the substratum, and sending food forward

MUSCULATURE.

95

to the mouth by means of the appendages, a trilobite could make of itself a most excellent
trap, and if the animal could dart backward as well as forward, the hypostoma would be still
more useful. There is no reason to suppose that they could not move backward, and the
"pygidial antennae" of Ncolenus indicate that animals of that genus at' least did so. This
habit of dropping down the hypostoma would also permit the use of those anterior gnatho-
bases which seem too far ahead of the mouth in the trilobites with a long hypostoma.

For actual evidence on this point, it is necessary to have recourse once more to Doctor
Walcott's exceedingly valuable slices. From such sections of Ceraurus as his Nos. 100, 106,
108, 170, and 173, it is evident that the hypostoma of that form could be dropped con-
siderably without disrupting the ventral membrane (fig. 30). Sections of Calymene already
published (Walcott 1881, pi. 5, figs, i, 2) show the hypostoma turned somewhat downward,
and the slices themselves show sections of the anterior pair of gnathobases beneath the

Fig. 30. — Longitudinal
section of cephalon of
Ceraurus pleurexanthe-
mus, to show position of
the mouth and folds of
the ventral membrane
between the glabella and
the hypostoma. The test
is in solid black and the
part within the ventral
membrane dotted.
From a photographic
enlargement. Specimen
169. X 3-9-

Fig. 31. — A copy of Doctor Moberg's figure of
Nileus armadillo, showing the position of the
muscle scars.

hypostoma. When the hypostoma was horizontal, these gnathobases were crowded out at
the sides.

If the hypostoma were used in the manner indicated, the muscles must have been more
efficient than those of the labrum of A pus, and it is probable that they crossed to the dorsal
test. Just where they were attached is an unsolved problem. Barrande (1852, pi. i, fig. i)
has indicated an anterior pair of scars and a single median one on the frontal lobe of
Dalmanitcs that may be considered in this connection, and also three pairs of scars on the
last two lobes of the glabella of Proctus (1852, pi. i, fig. 7). Moberg (1902, p. 295, pi. 3,
figs. 2, 3, text fig. i) has described in some detail the muscle-scars of a rather remarkable
specimen of Nileus armadillo Dalman. While, as I shall point out, I do not agree wholly
with Professor Moberg's interpretation, I give here a translation (made for Professor
Beecher) of his description, with a copy of his text figure :

The well preserved surface of the shell permits one to note not only the tubercle (t) but a number of
symmetrically arranged glabellar impressions. And because of their resemblance to the muscular insertions
of recent crustaceans, I must interpret them as such. They appear partly as rounded hollows (k and i), also
as elongate straight or curved areas (a, b, c, e, g, h) made up of shallow impressions or furrows about
i mm. long, sub-parallel, and standing at an angle to the trend of the areas. Impression e is especially well

g6 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

marked, inasmuch as the perpendicular furrows are arranged in a shallow crescentic depression; and impres-
sion d shows besides the obscure furrows a number of irregularly rounded depressions. Larger similar ones
occur at f, and in part extend over toward g.

The meaning of these impressions, or their myologic significance, could be discussed, although such
discussion might rather be termed guessing.

Inner organs, such as the heart and stomach, might have been attached to the shell along impressions a
and b. Also along or behind c and h, which both continue into the free cheeks, ligaments or muscular
fibers may have been inserted. From d, e, f, and g, muscles have very likely gone out to the cephalic
appendages. Against this it may be urged that impression d is too far forward to have belonged to the
first pair of feet. Again, the impression h may in reality represent two confluent muscular insertions, from
the first of which, in that case, arose the muscles of the fourth pair of cephalic feet. Were this the case, the
muscles of the first pair of cheek feet should be attached at e. And d in turn may be explained as the
attachment of the muscles of the antennae, k those of the hypostoma, and from i possibly those of the epistoma.
That k is here named as the starting point of the hypostomial muscles and not those of the antennae, depends
partly on the analogous position of i and partly on the fact that the hypostoma of Nileus armadillo (text
figure, in which the outline of the hypostoma is dotted), by reason of it? wing-like border, could not have
permitted the antennae to reach forward, but rather only outward or backward.

My own explanation would be that impressions e, f, and g correspond to the glabellar
furrows, h the neck furrow, and all four show the places of attachment of the appendifers.
Those at d may possibly be connected with the antennae, although I should expect those
organs to be attached under the dorsal furrows at the sides of the hypostoma. It will
be noted that either b, k, or i correspond well with the macula; of the hypostoma and some
or all of them may be the points of attachment of hypostomial muscles. They correspond
also with the anterior scars of Dalmanites.

EYES.

While I have nothing to add to what has been written about the eyes of trilobites, this
sketch of the anatomy would be incomplete without some reference to the little which has
been done on the structure of these organs.

Quenstedt (1837, p. 339) appears to have been the first to compare the eyes of trilo-
bites with those of other Crustacea. Johannes Miiller had pointed out in 1829 (Meckel's
Archiv) that two kinds of eyes were found in the latter group, compound eyes with a smooth
cornea, and compound eyes with a facetted coat. Quenstedt cited Trilobites esmarkii Schlo-
theim (=Illcentis crassicauda Dalman) as an example of the first group, and Calymcnc ma-
cro phthalma Brongniart (-—Phacops latifrons Bronn) for the second. Misreading the some-
what careless style of Quenstedt, Barrande (1852, p. 133) reverses these, one of the few
slips to be found in the voluminous writings of that remarkable savant.

Burmeister (1843; 1846, p. 19) considered the two kinds of eyes as essentially the
same, and accounted for the conspicuous lenses of Phacops on the supposition that the cornea
was thinner in that genus than in the trilobites with smooth eyes.

Barrande (1852, p. 135) recognized three types of eyes in trilobites, adding to Quen-
stedt's smooth and facetted compound eyes the groups of simple eyes found in Harpes. In
his sections of 1852, pi. 3, figs. 15-25, which are evidently diagrammatic, he shows sepa-
rated biconvex lenses in both types of compound eyes, Phacops and Dalmanites on one hand,
and Asaphus, Goldius, Acidaspis, and Cyclopyge on the other. Clarke ( 1888), Exner ( 1891 )
and especially Lindstroem (1901) have since published much more accurate figures and
descriptions. The first person to study the eye in thin section seems to have been Packard
(1880), who published some very sketchy figures of specimens loaned him by Walcott. He

EYES. 97

studied the eyes of Isotclus gigas, Bathyurus longispinus, Calymenc/and Phacops, and decided
that the two types of eyes were fundamentally the same. He also compared them with the
eyes of Limulus.

Clarke (1888), in a careful study of the eye of Phacops rana, found that the lenses
were unequally biconvex, the curvature greater on the inner surface. The lens had a cir-
cular opening on the inner side, leading into a small pear-shaped cavity. The individual
lenses were quite distinct from one another, and separated by a continuation of the test of
the cheek.

Exner (1891, p. 34), in a comparison of the eyes of Phacops and Limulus, came to
the opinion that they were very unlike, and that the former were really aggregates of
simple eyes.

Lindstroem (1901, pp. 27-31) came to the conclusion that besides the blind trilobites
there were trilobites with two kinds of compound eyes, trilobites with aggregate eyes, and
trilobites with stemmata and ocelli. His views may be briefly summarized.

I. Compound eyes.

1. Eyes with prismatic, plano-convex lenses.

"A pellucid, smooth and glossy integument, a direct continuation of the common test of the body, covers
the corneal lenses, quite as is the case in so many of the recent Crustacea. The lenses are closely packed,
minute, usually hexagonal in outline, flat on the outer and convex on the inner surface. Such eyes are best
developed in Asaphus, Illanus, Nileus, Bumastus, Proctus, etc."

2. Eyes with biconvex lenses.

The surface of the eye is a mass of contiguous lenses, covered by a thin membrane which is frequently
absent from the specimens, due to poor preservation. The lenses are biconvex, and being in contact with one
another, are usually hexagonal, although in some cases they nearly retain their globular shape. Such eyes
are found in Bury care, Peltura, Spharopthalmus, Ctenopyge, Goldius, Cheirurus, and probably others.

II. Aggregate eyes.

The individual lenses are comparatively large, distinct from one another, each lying in its own socket.
There is, however, a thin membrane, which covers all those in any one aggregate, and is a continuation of
the general integument of the body. This membrane is continued as a thickened infolding which forms the
sockets of the lenses.

Such eyes are known in the Phacopidx only.

III. Stemmata and ocelli.

The stemmata are present only in Harpes, where there may be on the summit of the cheek two or three
ocelli lying near one another. Each, viewed from above, is nearly circular in outline, almost hemispheric,
glossy and shining. In section they prove to be convex above and flat or slightly concave beneath. The
test covers and separates them, as in the case of the aggregate eyes.

The ocelli of the Trinucleidse and Eoharfes are smaller, and the detailed structure not yet investigated.

Lindstroem concludes that so far as its facets or lenses are concerned, the eye of the trilobite shows the
greatest analogy with the Isopoda, and the least with Limulus.

SUMMARY.

The simplest eyes found among the Trilobita are the ocelli. These consist of a Simple
thickening of the test to form a convex surface capable of concentrating light. The simi-
larity in position of the paired ocelli of trilobites and the simple eyes of copepods has per-
haps a significance.

The schizochroal eyes may well be compared with the aggregate eyes of the chilopods
and scorpions. The mere presence of a common external covering is not sufficient to prove
this a true compound eye, especially as the covering is merely a continuation of the general
test.

98 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

The holochroal eyes are of two kinds, one with plano-convex and one with biconvex
lenses. The latter would seem to be mechanically the more perfect of the two, and it is
worthy of note that the trilobites possessing the biconvex lenses have, in general, much smaller
eyes than those with the other type.

If, as some investigators claim, the parietal eye of Crustacea originates by the fusion of
two lateral ocelli, trilobites show a primitive condition in lacking this eye, which may have
originated through the migration toward the median line of ocelli like those of the Trinu-
cleidoe.

SEX.

That the sexes were separate in the Trilobita there can be very little doubt, but the
study of the appendages has as yet revealed nothing in the way of sexual differences. One
of the most important points still to be determined is the location of the genital openings.

In many modern Crustacea, the antennae or antennules are modified as claspers, and it
is barely possible that the curious double curvature of the antennules of Triarthrus indi-
cates a function of this sort. The antennules of many specimens have the rather formal
double curvature, turning inward at the outer ends, and retain this position of the frontal
appendages, no matter what may be the condition of those on the body. Other specimens
have the antennules variously displaced, indicating that they are quite flexible. It is conceiv-
able that the individuals with rigid antennules are males, the others females.

It is interesting to note that the antennules of Ptychoparia pcrmulta Walcott (1918, pi.
21, fig. i) have the same recurved form. All the specimens of Neolenus, however, show very
flexible antennas.

Barrande and Salter laid great stress upon the "forme longue" and "forme large"
as indicating male and female. This was based upon the supposition that the female of
any animal would probably have a broader test than the male, a hypothesis which seems to
be very little supported by fact. In practical application it was found that the apparent dif-
ference was so often due to the state of preservation or the confusion of two or more
species, that for many years little reference has been made to this supposed sex difference.

EGGS.

In his classic work on the trilobites of Bohemia, Barrande described three kinds of spheri-
cal and one of capsule-shaped bodies which he considered to be the eggs of trilobites. After
a review of the literature and a study of specimens in the collections of the Museum of
Comparative Zoology, it can be said that none of these fossils has proved to be a trilobite
egg, but that they may be plants. A full account of them will be published elsewhere.

Walcott (1881) and Billings (1870) have described similar bodies within the tests of
Calymenc and Ceraurus, but without showing positive evidence as to their nature.

METHODS OF LIFE.

This is a subject upon which much can be inferred, but little proved. Without trying
to cover all possibilities, it may be profitable to see what can be deduced from what is known
of the structure of the external test, the internal anatomy, and the appendages. This can,
to a certain extent, be controlled by what is inferred from the strata in which the specimens
are found, the state of preservation, and the associated animals. (For other details, see
the discussion of "Function of the Appendages" in Part I.)

LOCOMOTION. 99

HABITS OF LOCOMOTION.

The methods of locomotion may be deduced with some safety from a study of the ap-
pendages, and, as has repeatedly been pointed out, all trilobites could probably swim by their
use. This swimming was evidently done with the head directed forward, and could prob-
ably be accomplished indifferently well with either the dorsal (gastroneetic, Dollo) or the
ventral (notonectic) side up. If food were sought on the bottom by means of sight, the animal
would probably swim dorsal side up, for by canting from side to side it could see the bottom
just as easily as though it were ventral side up, and at the same time it would be in position
to drop quickly on the prey. In collecting food at the surface, it might swim ventral side up.

All trilobites could probably crawl by the use of the appendages, and, as has already
been pointed out, there are great differences in the adjustment of the appendages to different
methods of crawling. Some crawled on their "toes," some by means of the entire endopo-
dites, and some apparently used the coxopodites to push themselves along. That the normal
direction of crawling was forward is indicated by the position of the eyes and sensory anten-
nules. There is no evidence that their mechanism was irreversible, however, and the position
of the mouth and the shape of the hypostoma indicate that they usually backed into feeding
position. The caudal rami of Neolenus were evidently sensory, and the animal was pre-
pared to go in either direction.

The use of the pygidium as a swimming organ, suggested by Spencer (1903, p. 492) on
theoretical grounds, developed by Staff and Reck (1911, p. 141) from a mechanical stand-
point, and elaborated in the present paper by evidence from the ontogeny, phylogeny, and
musculature, provided the animal with a swifter means of locomotion. By a sudden flap
of this large fin, a backward darting motion could be obtained, which would be invaluable
as a means of escape from enemies. Staff and Reck seem to think that in this movement
the two shields were clapped together, and that the animal was projected along with the hinge-
like thorax forward. This might be a very plausible explanation in the case of the bivalve-
like Agnostidae, and it is one I had suggested tentatively for that family before I read Staff
and Reek's paper. In the case of the large trilobites with more segments, however, it would
be more natural to think of a mode of progression in which there was an undulatory move-
ment of the body and the pygidium, up-and-down strokes being produced by alternately
contracting the dorsal and ventral muscles. Bending the pygidium down would tend to pull
the animal backward, while bringing it back into position would push it forward. It fol-
lows, therefore, that one of these movements must have been accomplished very quickly, the
other slowly. If the muscle scars have been interpreted properly, the ventral muscles were
probably the more powerful, an indication that the animal swam backward, using the cephalon
and antennules as rudders.

The chief objection to the theory of swimming by clapping the valves together is that
where the thorax consists of several segments it no longer acts like the hinge of a bivalve,
and a sudden downward flap of the pygidium would impart a rotary motion to the animal.
Take, for example, such nearly spherical animals as the Illsenidae, and it will readily be seen
that there is nothing to give direction to the motion if the pygidium be brought suddenly
against the lower surface of the cephalon. A lobster, it is true, progresses very well by
this method, but it depends upon its great claws and long antenna; to direct its motions.
The whole shape of the trilobite is of course awkward for a rapidly swimming animal. It
could keep afloat with the minimum of effort and paddle itself about with ease, but it was
not built on the correct lines for speed.

100 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

Dollo (1910, p. 406), and quickly following his lead, Staff and Reck (1911, p. 130), have
published extremely suggestive papers, showing that by the use of the principle of correlation
of parts, much can be inferred about the mode of life of the trilobites merely from the
structure of the test.

Dollo studied the connection between the shape of the pygidium and the position and
character of the eyes. As applied by him, and later by Clarke and Ruedemann, to the euryp-
terids, this method seems most satisfactory. He pointed out that in Eurypterida like Sty-
lonurus and Eurypterus, where there is a long spine-like telson, the eyes are back from the
margin, so that a Limulns-l\ke habit of pushing the head into the sand by means of the limbs
and telson was possible. Ercttopterus and Pterygotus, on the other hand, have the eyes on
the margin, so that the head could not be pushed into the mud without damage, and have
a fin-like telson, suggesting a swimming mode of life.

In carrying this principle over to the trilobites, Dollo was quite successful, but Staff
and Reck have pointed out some modifications of his results. The conclusions reached
in both these papers are suggestive rather than final, for not all possible factors have been
considered. The following are given as examples of interesting speculations along this line.

Homalonotus dclphinoccphalus, according to Dollo, was a crawling animal adapted to
benthonic life in the euphotic region, and an occasional burrower in mud. This is shown by
well developed eyes in the middle of the cephalon, a pointed pygidium, and the plow-like
profile of the head. This was as far as Dollo went. From the very broad axial lobe of
Homalonotus it is fair to infer that, like Isotclus, it had very long, strong coxopodites which
it probably vised in locomotion, and also very well-developed longitudinal muscles, to be used
in swimming. From the phylogeny of the group, it is known that the oldest homalonotids
had broad unpointed pygidia of the swimming type, and that the later species of the genus
(Devonian) are almost all found in sandstone and shale, and all have wider axial lobes
than the Ordovician forms. It is also known that the epistoma is narrower and more
firmly fused into the doublure in later than in earlier species. These lines of evidence tend
to confirm Dollo's conclusion, but also indicate that the animals retained the ability to swim
well.

On the same grounds, Olenellus thompsoni and Dalmanites limulurus were assigned the
same habitat and habits. Both were considered to have used the terminal spine as does
Limulus.

Olenellus thompsoni is generally considered to be unique among trilobites in having a
Limulus-like telson in place of a pygidium. This "telson" has exactly the position and
characteristics of the spine on the fifteenth segment of Mesonacis, and so long ago as 1896,
Marr (Brit. Assoc. Adv. Sci., Rept. 66th Meeting, page 764) wrote :

The posterior segments of the remarkable trilobite Mesonacis vermontana are of- a much more delicate
character than the anterior ones, and the resemblance of the spine on the fifteenth "body segment" of this
species to the terminal spine of Olenellus proper, suggests that in the latter subgenus posterior segments of a
purely membranous character may have existed devoid of hard parts.

This prophecy was fulfilled by the discovery of the specimens which Walcott described
as Pcedeumias transitans, a species which is said by its author to be a "form otherwise
identical with O. thompsoni, [but] has rudimentary thoracic segments and a Hohnia-like
pygidium posterior to the fifteenth spine-bearing segment of the thorax." A good speci-
men of this form was found at Georgia, Vermont, associated with the ordinary specimens
of Olenellus thompsoni, and I believe that it is merely a complete specimen of that species.

LOCOMOTION. IOI

Olenellus gilberti, which was formerly supposed to have a limuloid telson, has now been
shown by Walcott (Smithson. Misc. Coll., vol. 64, 1916, p. 406, pi. 45, fig. 3) to be a
Mesonacis and to have seven or eight thoracic segments and a small plate-like pygidium
back of the spine-bearing fifteenth segment. All indications are that the spine was not in
any sense a^pygidium. Walcott states that Olenellus resulted from the resorption of the
rudimentary segments of forms such as Mesonacis and Pcedewnias, leaving the spine to
function -as a pygidium. This would mean the cutting off of the anus and the posterior
part of the alimentary canal, and developing a new anal opening on the spine of one of
the thoracic segments !

If the spine of the fifteenth segment is not a 'pygidium, could it be used, as Dollo
postulates, as a pushing organ? Presumably not, for though in entire specimens of Olenellus
(Pccdcumias*) it extends back beyond the pygidium, it probably was borne erect, like the
similar spines in Elliptocephala, and not in the horizontal plane in which it is found in
crushed specimens.

While this removes some of the force of Dollo's argument, his conclusion that Olenellus
was a crawling, burrowing animal living in well lighted shallow waters was very likely cor-
rect. The long, annelid-like body indicates numerous crawling legs, there is no swimming
pygidium, and the fusion of the cheeks in the head makes a stiff cephalon well adapted for
burrowing.

Staff and Reck have pointed out that Dalmanitcs limulurus was not entirely a crawler,
but, as shown by the large pygidium, a swimmer as well. This kind of trilobite probably
represents the normal development of the group in Ordovician and later times. The Pha-
copidae, Proetidze, Calymeniche, and other trilobites of their structure could probably crawl
or swim equally well, and could escape enemies by darting away or by "digging them-
selves in."

Cryptolithus tessellatus (Trinucleus concentricus} is cited by Dollo as an example of
an adaptation to life in the aphotic benthos, permanently buried in the mud. In this case
he appealed to Beecher's interpretation of the appendages, and pointed out that while the
adult is blind, the young have simple eyes and probably passed part of their life in the
lighted zone. It needs only a glance at the very young to convince one that the embryos
had swimming habits, so that in this form one sees the adaptation of the individual during
its history to all modes of life open to a trilobite. The habits of the Harpedidse may have
been similar to those of the Trinucleidze, but the members of this family are supplied with
broad flat genal spines. It has been suggested that these served like pontoons, runners, or
snow-shoes, to enable the animal to progress over soft mud without sinking into it. Some
such explanation might also be applied to the similar development in the wholly unrelated
Bathyuridse. The absence of compound eyes and the poor development of ocelli in the Har-
pedidas suggest that they were burrowers, and from these two families, Trinucleidse and
Harpedidae, it becomes evident that a pygidial point or spine is not a necessary part of the
equipment of a burrowing trilobite. In fact, from the habits of Limulus it is known that
the appendages are relied upon for digging, and that the telson is a useful but not indis-
pensable pushing organ.

Deiphon is an interesting trilobite from many points of view. Its pleural lobes are
reduced to a series of spines on either side of the body, and its pygidium is a mere spinose
vestige. Dollo considered this animal a swimmer in the euphotic zone, because its eyes
are on the anterior margin, its body depressed, its glabella globose, and its pygidium flat

IO2 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

and spinose. That such a method of life was secondary in a cheirurid was indicated to
him by the fact that the more primitive members of the family seemed adapted for crawl-
ing. Staff and Reck have gone further and shown that the pygidium is only the vestige of
a swimming pygidium, and that the great development of spines suggests a floating rather
than a swimming mode of life. They therefore argue for a planktonic habitat. A similar
explanation is suggested for Acidaspis and other highly spinose species.

The Aeglinidae, or Cyclopygidse as they are more properly called, present the most re-
markable development of eyes among the trilobites. In this, Dollo saw, as indeed earlier
writers have, an adaptation to a region of scanty light. The cephalon is not at all adapted
to burrowing, but the pygidium is a good swimming organ, and one is apt to agree that this
animal was normally an inhabitant of the ill lighted dysphotic region, but also a nocturnal
prowler, making trips to the surface at night. Similar habits and habitat are certainly indi-
cated for Telephus and the Remopleuridas, but whether Nileus and the large-eyed Bumastus
are capable of the same explanation is doubtful.

Finch (1904, p. 181) makes the suggestion that "Nilcus" (Vogdesia) vigilans, an
abundant trilobite in the calcareous shale of the Maquoketa, was in the habit of burying itself,
posterior end first. He found a slab containing fifteen entire specimens, all of which had
the cephalon extended horizontally near the surface of the stratum, and the thorax and
pygidium projecting downward. The rock showed no evidence that they were in burrows,
and the fact that all were in the same position indicates that the attitude was voluntarily
assumed. They appear to have entrenched themselves by the use of the pygidia, which are
incurved plates readily adapted for such use, and, buried up to the eyes, awaited the coming
of prey, but were, apparently, smothered by a sudden influx of mud. The form of the eye
in Vogdesia vigilans bears out this supposition of Finch's. Not only are the eyes unusually
tall, but the palpebral lobe is much reduced, so that many of the lenses look upward and
inward, as well as outward, forward and backward. The particular food required by V. vigi-
lans must have been very plentiful in the Maquoketa seas of Illinois and Iowa, for the species
was very abundant, but that its habits were self -destructive is also shown by the great num-
ber of complete enrolled specimens of all ages now found there. The soft mud was appar-
ently fatal to the species before the end of the Maquoketa, for specimens are seen but very
rarely in the higher beds.

Vogdesia vigilans is shaped much like Bumastus, Illcenus, Asaphus, Onchometopus, and
Brachyaspis, and it may be that these trilobites with incurved pygidia had all adopted the
habit of digging in backward. As noted above, their pygidia are not very well adapted
for swimming, and most of them have large or tall eyes.

Dollo's comparison of the Cyclopygidas to the huge-eyed modern amphipod Cystosoma
is instructive. This latter crustacean, which has the greater part of the dorsal surface of the
carapace transformed into eyes, is said to live in the dysphotic zone, at depths of from 40
to 100 fathoms, and to come to the surface at night. It swims ventral side down.

The kinds of sediments in which trilobites are entombed have so far afforded little evi-
dence as to their habitat. Freeh (Lethaea palaeozoica, 1897-1902, p. 67 et seq.) who has
collected such evidence as is available on this subject, places as deeper water Ordovician
deposits the "Trinucleus-Schiefer" of the upper Ordovician of northern Europe and Bohemia,
the "Triarthrus-Schiefer" of America, the "Asaphus-Schiefer" of Scandinavia, Bohemia,
Portugal, and France, and the Dalmania quartzite of Bohemia. .

Cryptolithus and Triarthrus, although not confined to such deposits, are apt to occur

FEEDING.

103

chiefly in very fine-grained shales, in company with graptolites. These latter are distributed
by currents over great distances within short periods. It is somewhat curious that the nearly
blind burrowing Trinucleidae, the dysphotic, large-eyed Remopleuridse and Telephus, the blind
nektonic Agnostidse and Dionide, and the planktonic graptolites should go together and make
up almost the entire fauna of certain formations. Yet, when the life history of each type
is studied, a logical explanation is readily at hand, for all have free-swimming larvae.

A list of the methods of life noted above is given by way of summary, with examples.

Pelagic

Benthonic <

Planktonic

Nektonic

Crawlers and

slow swimmers
Crawlers and

active swimmers
Crawlers, slow

swimmers, and

burrowers

( Primarily
(I Secondarily
"Primarily

Secondarily

Earliest protaspis of all trildbites

Deiphon, Odontopleura, etc.

Later protaspis of all trilobites. Naraoia

Probably many thin-shelled trilobites. with large pygidia

(only partially nektonic)
Cyclopygidae ) .
.RemoplLida ( ("ekton.c dysphot.c)

Most trilobites with small pygidia. Triarthrus, Para-

doxides, etc.
Most trilobites with large pygidia. Isotelus, Dal-

manites, etc.

Trinucleidse, Harpedidae, some Mesonacidae, etc.

FOOD AND FEEDING METHODS.

This subject has been less discussed than the methods of locomotion. The study of
the appendages has shown that while the mouth parts were not especially powerful, they were
at least numerous, and sufficiently armed with spines to shred up such animal and vegetable
substances as they were liable to encounter. It having been ascertained that the shape of the
glabella and axial lobe furnishes an indication of the degree of development of the alimen-
tary canal it is possible to infer something of the kind of food used by various trilobites.

The narrow glabellas and axial lobes of the oldest trilobites would seem to indicate a
carnivorous habit, while the swollen glabellse and wider lobes of later ones probably denote an
adaptation to a mixed or even a vegetable diet. This can not be relied upon too strictly,
of course, for the swollen glabellse of such genera as Deiphon or Sphccrexochus may be due
merely to the shortening up of the cephalon.

Walcott (1918, p. 125) suggests that the trilobites lived largely upon worms and con-
ceives of them as working down into the mud and prowling around in it in search of such
prey. While there can be no doubt that many trilobites had the power of burying them-
selves in loose sand or mud, a common habit with modern crustaceans, most of them were
of a very awkward shape for habitual burrowers, and how an annelid could be successfully
pursued through such a medium by an animal of this sort is difficult to understand. In
fact, the presence of the large hypostoma and the position of the mouth were the great
handicaps of the trilobite as a procurer of live animal food, and coupled with the rela-
tively slow means of locomotion, almost compel the conclusion that errant animals of any
size were fairly safe from it. This restricts the range of animal food to small inactive
creatures and the remains of such larger forms as died from natural causes. The modern
Crustacea are effective scavengers, and it is probable that their early Palaeozoic ancestors
were equally so. It is a common saying that in the present stressful stage of the world's
history, very few wild animals die a natural death. In Cambrian times, competition for

IO4 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

animal food was less keen, and with the exception of a few cephalopods, a few large anne-
lids, and a few Crustacea like Sidneyia, there seem to have been no aggressive carnivores.
In consequence, millions of animals must have daily died a natural death, and had there been
no way of disposing of their remains, the sea bottom would soon have become so foul that
no life could have existed. For the work of removal of this decaying matter, the carniv-
orous annelids and the Crustacea, mostly trilobites, were the only organisms, and it is prob-
able that the latter did their full share. After prowling about and locating a carcass, the
trilobite established himself over it, the cephalon and hypostoma on one end and the
pygidium on the other enclosing and protecting the prey, which was shredded off and passed
to the mouth at leisure by means of the spinose endobases.

Even in Middle Cambrian times some trilobites (e. g., Parado.vides) seem to have en-
larged the capacity of the stomach and. taken vegetable matter, but later, in the Upper
Cambrian and Ordovician, when the development of cephalopods and fishes caused great
competition for all animal food, dead or alive, most trilobites seem to have become omniv-
orous. This is of course shown by the swollen glabella, with reduced lateral furrows, and,
in the case of a few species (Calymene, Ceraurus), the known enlargement of the stomach.

Cryptolithus is the only trilobite which has furnished any actual evidence as to its food.
From the fact that the alimentary tract is found stuffed from end to end with fine mud,
and because it is known to have been a burrower, it has been suggested by several that it
was a mud feeder, passing the mud through the digestive tract for the sake of what organic
matter it contained. Spencer (1903, p. 491) has suggested a modification of this view:

The phyllopods appear to feed by turning over whilst swimming and seizing with their more posterior
appendages a little mud which swarms with infusoria, etc. This mud is then pushed along the ventral
groove to the mouth. Casts, of the intestine of trilobites are still found filled with the mud.

Ceraurus and Calymene also must have occasionally swallowed mud in quantity, other-
wise the form of the alimentary canal could not have been preserved as it is in a few of
Doctor Walcott's specimens.

TRACKS AND TRAILS OF TRILOBITES.

Tracks and trails of various sorts have been ascribed by authors to trilobites since these
problematic markings first began to attract attention, but as the appendages were until re-
cently quite unknown, all the earlier references were purely speculative. The subject is a
difficult one, and proof that any particular track or trail could have been made in only one
way is not easily obtained. Since the appendages have actually been described, compara-
tively little has been done, Walcott's work on Protichnites (1912 B, p. 275) being the most
important. Since the first description of Protichnites by Owen (Quart. Jour. Geol. Soc.,
London, 1852, vol. 8, p. 247), it has been thought that these trails were made by crustaceans,
and the only known contemporaneous crustaceans being trilobites, these animals were natu-
rally suggested. Dawson (Canadian Nat. Geol., vol. 7, 1862, p. 276) ascribed them, with
reserve, to Paradoxides, and Billings (1870, p. 484) suggested Dikelocephalus or Aglaspis.
Walcott secured well preserved specimens which showed trifid tracks, and these were readily
explained when he found the legs of Neolcnus, which terminated with three large spines.
Similar trifid terminations had already been described by Beecher, and clearly pictured in his
restoration of Triarthrus, but the spines and the tracks had somehow not previously been
connected in the mind of any observer. Walcott concluded that the tracks had been made

TRACKS AND TRAILS. IC>5

by a species of Dikeloccphalus, possibly by D. hartti, which occurs both north and south of the
Adirondack^. In a recent paper, Burling (Amer. Jour. Sci., ser. 4, vol. 44, 1917, p. 387)
has argued that Protichnites was not the trail of a trilobite, but of a "short, low-lying, more
or less heavy set, approximately i2-legged, crab-like animal," which had an oval shape, toed
in, and was either extremely flexible or else short and more or less flexible in outline.
This seems to describe a trilobite.

Cliinactichnites, the most discussed single trail of all, has also been ascribed to trilo-
bites, — by Dana (Manual of Geology, 1863, p. 185), Billings (1870, p. 485), and Packard
(Proc. Amer. Acad. Arts and Sci., vol. 36, 1900, p. 64), — though less frequently than
to other animals. The latest opinion (see paper by Burling cited above) seems to be against
this theory.

Miller (1880, p. 217) described under the generic name Asaphoidichnus two kinds of
tracks which were such as he supposed might be made by an Asaphus (Isotclus). In re-
ferring to the second of the species, he says : "Some of the toe-tracks are more or less
fringed, which I attribute to the action of water, though Mr. Dyer is impressed with the
idea that it may indicate hairy or spinous feet." The type of this species, A. dyeri, is in the
Museum of Comparative Zoology, and while it may be the trail of a trilobite, it would be
difficult to explain how it was produced.

Ringueberg (1886, p. 228) has described very briefly tracks found in the upper part
of the Medina at Lockport, New York. These consisted of a regularly succeeding series of
ten paired divergent indentations arranged in two diverging rows, with the trail of the pygid-
ium showing between each series. The ten pairs of indentations he considered could have
been made by ten pairs of legs like those shown by the specimen of Isotelus described by
Micklebo rough, and the intermittent appearance of the impression of the pygidium suggested
to him that the trilobite proceeded by a series of leaps.

Walcott (1918, pp. 174-175, pi. 37-42) has recently figured a number of interesting
trails as those of trilobites, and has pointed out that a large field remains open to anyone
who has the patience to develop this side of the subject.

PART III.
. RELATIONSHIP OF THE TRILOBITES TO OTHER ARTHROPODA.

It can not be said that the new discoveries of appendagiferous trilobites have added
greatly to previous knowledge of the systematic position of the group. Probably none will
now deny that trilobites are Crustacea, and more primitive and generalized than any other
group in that class. The chief interest at present lies in their relation to the most nearly
allied groups, and to the crustacean ancestor.

Trilobites have been most often compared with Branchiopoda, Isopoda, and Merostom-
ata, the present concensus of opinion inclining toward the notostracan branchiopods (Apod-
idae in particular) as the most closely allied forms. It seems hardly worth while to burden
these pages with a history of opinion on this subject, since it was not until the appendages
were fully made out, from 1881 to 1895, that zoologists and palaeontologists were in a
position to give an intelligent judgment. The present status is due chiefly to Bernard (1894),
Beecher (1897, 1900, et seq.}, and Walcott (1912, et seq.).

The chief primitive characteristics of trilobites are: direct development from a pro-
taspis common to the subclass; variability in the number of segments, position of the mouth,
and type of eyes; and serially similar biramous appendages.

The recent study has modified the last statement slightly, since it appears that in some
trilobites there was a modification of the appendages about the mouth, suggesting the initia-
tion of a set of tagmata.

In comparing the trilobites with other Crustacea, the condition of the appendages must
be especially borne in mind, for while these organs are those most intimately in contact with
the environment, and most subject to modification and change, yet they have proved of
greatest service in classification.

Appendages have been found on trilobites from only the Middle Cambrian and Middle
and Upper Ordovician, but as the Ordovician was the time of maximum development of the
group, it is probable that trilobites of later ages would show degradational rather than pro-
gressive changes. All the genera which are known show appendages of the same plan, and
although new discoveries will doubtless reveal many modifications of that plan, general infer-
ences may be drawn now with some assurance.

The chief characteristics of the appendages are : first, simple antennules, a primitive fea-
ture in all Crustacea, as shown by ontogeny; second, paired biramous appendages, similar to
each other all along the body, the youngest and simplest in front of the anal segment, the
oldest and most modified on the cephalon. The endobases are retained on all the coxopo-
dites, except possibly, in some species, the anterior ones, and these gnathobases are modi-
fied in some genera as mouth-parts, while in others they are similar throughout the series.
With these few fundamentals in mind, other Crustacea may be examined for likenesses. The
differences are obvious.

CRUSTACEA.
BRANCHIOPODA.

The early idea that the trilobites were closely related to the Branchiopoda was rejuve-
nated by the work of Bernard on the Apodidas (1892) and has since received the support

BRANCHIOPODA.

of most writers on the subject. Fundamentally, a great deal of the argument seems to be
that Apus lies the nearest of any modern representative of the class to the theoretical crus-
tacean ancestor, and as the trilobites are the oldest Crustacea, they must be closely related.
Most writers state that the trilobites could not be derived from the Branchiopoda (see, how-
ever, Walcott 1912 A), nor the latter from any known trilobite, but both subclasses are be-
lieved to be close to the parent stem.

Viewed from the dorsal side, there is very little similarity between any of the branchi-
opods and the trilobites, and it is only in the Notostraca, with their sessile eyes and
depressed form, that any comparison can be made. The chief way in which modern Bran-
chiopoda and Trilobita agree is that both have a variable number of segments in the body,
that number becoming very large in Apus on the one hand and Mesonacis and Padeumias
on the other. In neither are the appendages, except those about the mouth, grouped in
tagmata. Other likenesses are : the Branchiopoda are the only Crustacea, other than Trilo-
bita, in which gnathobases are found on limbs far removed from the mouth ; the trunk limbs
are essentially leaf-like in both, though the limb of the branchiopod is not so primitive as
that of the trilobite; caudal cerci occur in both groups.

If the appendages be compared in a little more detail, the differences prove more strik-
ing than the likenesses.

In the Branchiopoda, the antennules are either not segmented or only obscurely so. In
trilobites they are richly segmented.

In Branchiopoda, the antennae are variable. In the Notostraca they are vestigial, while
in the males of the Anostraca they are powerful and often complexly developed claspers.
Either condition might develop from the generalized biramous antennas of Trilobita, but
the present evidence indicates a tendency toward obsolescence. Claus' observations indicate
that the antennae of the Anostraca are developments of the exopodites, rather than of the
endopodites.

The mandibles and maxillae of the Branchiopoda are greatly reduced, and grouped
closely about the mouth. Only the coxopodites of the Trilobita are modified as oral appen-
dages.

The trunk limbs of Apus are supposed to be the most primitive among the Branchio-
poda, and comparison will be made with them. Each appendage consists of a flattened axial
portion, from the inner margin of which spring six endites, and from the outer, two large
flat exites (see fig. 34). This limb is not articulated with the ventral membrane, but attached
to it, and, if Lankester's interpretation of the origin of schizopodal limbs be correct, then
the limb of Apus bears very little relation to that of the Trilobita. In Apus there is no distinct
coxopodite and the endobases which so greatly resemble the similar organs in the Trilobita
are not really homologous with them, but are developments of the first endite. Beecher's
comparison of the posterior thoracic and pygidial limbs of Triarthrus with those of Apus
can not be sustained. Neither Triarthrus nor any other trilobite shows any trace of phyl-
lopodan limbs. Beecher figured (1894 B, pi. 7, figs. 3, 4) a series of endopodites from the
pygidium of a young Triarthrus beside a series of limbs from a larval Apus. Superficially,
they are strikingly alike, but while the endopodites of Triarthrus are segmented, the limbs of
Apus are not, and the parts which appear to be similar are really not homologous. The
similarity of the thoracic limbs in the two groups is therefore a case of parallelism and does
not denote relationship.

IO8 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

Geologically, the Branchiopoda are as old as the Trilobita, and while they did not have
the development in the past that the trilobite had, they were apparently differentiated fully
as early. Anostraca, Notostraca and Conchostraca, three of the four orders, are represented
in the Cambrian by forms which are, except in their appendages, as highly organized as the
existing species. Brief notes on the principal Middle Cambrian Branchiopoda follow :

Burgessia bella Walcott.
Illustrated : Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 177, pi. 27, figs. 1-3 ; pi. 30, figs. 3, 4.

This is the most strikingly like the modern Branchiopoda of any species described by
Walcott from the Middle Cambrian, and invites comparison with A pus. The carapace is
long, loosely attached to the body, and extends over the greater part of the thorax. The eyes
are small, sessile, and close to the anterior margin.

The appendages of the head consist of two pairs of antennae, and three pairs of slender,
jointed legs. Both pairs of antennae are slender and many-jointed, the antennules some-
what smaller than the antennae. The exact structure of the limbs about the mouth has not
yet been made out, but they are slender, tapering, endopodite-like legs, with at least three or
four segments in each, and probably more.

There are eight pairs of thoracic appendages, each limb having the form of the endopo-
dite of a trilobite and consisting of seven segments and a terminal spine. The proximal three
segments of each appendage are larger than the outer ones, and have a flattened triangular
expansion on the inner side. Walcott also states that "One specimen shows on seven pairs
of legs, small, elongate, oval bodies attached near the first joint to the outer side of the leg.
These bodies left but slight impression on the rock and are rarely seen. They appear to
represent the gills." They are not figured, but taken in connection with the endopodite-like
appearance of the segmented limbs, one would expect them to be vestigial exopodites.

A small hypostoma is present on the ventral side, and several of the specimens show
wonderfully well the form of the alimentary canal and the hepatic caeca. The main branches
of the latter enter the mesenteron just behind the fifth pair of cephalic appendages.

Behind the thorax the abdomen is long, limbless, and tapers to a point. It is said to
consist of at least thirty segments.

Compared with Apus, Burgessia appears both more primitive and more specialized.
The carapace and limbless abdomen are A pus-like, but there are very few appendagiferous
segments, and the appendages are not at all phyllopodan, but directly comparable with those
of trilobites, except, of course, for the uniramous character of the cephalic limbs. A closer
comparison may be made with Marrella.

Waptia fieldensis Walcott.
Illustrated : Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 181, pi. 27, figs. 4, 5.

The carapace is short, covering the head and the anterior part of the thorax. The
latter consists of eight short segments with appendages, while the six abdominal segments,
which are similar to those of the thorax, are without limbs except for the last, which bears
a pair of broad swimmerets. The eyes are marginal and pedunculate. The antennules are
imperfectly known, but apparently short, while the antennas are long and slender, with rela-
tively few, long, segments. The mandibles appear to be like endopodites of trilobites and
show at least six segments. As so often happens in these specimens from British Columbia,

BRANCHIOPODA.

the preservation of the other appendages is unsatisfactory. As illustrated (Walcott, 1912
A, pi. 27, fig. 5), both endopodites and exopodites appear to he present, and the shaft of
the exopodite seems to be segmented as in Triarthrus.

Walcott considers Waptia as a transitional form between the Branchiopoda and the
Malacostraca.

Yohoia tenuis Walcott.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 172, pi. 29, figs. 7-13.

This species, though incompletely known, has several interesting characteristics. The
head shows, quite plainly in some specimens, the five segments of which it is composed.
The eyes are small, situated in a niche between the first and second segments, .and are
described as being pedunculate. The eight segments of the thorax all show short triangu-
lar pleural extensions, somewhat like those of Remopleuridcs or Robergia. The abdomen
consists of four cylindrical segments, the last with a pair of expanded caudal rami.

The antennules appear to be short, while the antennas are large, with several segments,
ending in three spines, and apparently adapted for serving as claspers in the male. The
third, fourth, ami fifth pairs of cephalic appendages are short, tapering, endopodite-like
legs similar to those of Burgessia.

The appendages of the thorax are not well preserved, and there seem to be none on the
abdomen.

This species is referred by Walcott to the Anostraca.

Opabina regalis Walcott.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 167, pi. 27, fig. 6; pi. 28, fig. I.

This most remarkably specialized anostracan is not well enough known to allow com-
parison to be made with other contemporaneous Crustacea, but it is worthy of mention.

There is no carapace, the eyes are pedunculated, thorax and abdomen are not differ-
entiated, and the telson is a broad, elongate, spatulate plate. There seem to be sexual dif-
ferences in the form of the anterior cephalic and caudal appendages, but this is not fully
established. The most remarkable feature is the long, large, median cephalic appendage
which is so suggestive of the proboscis of the recent Thamnoccphalus platyurus Packard.
The appendages are not well enough preserved to permit a determination as to whether
they are schizopodal or phyllopodan.

Summary.

Walcott referred Burgessia and Waptia to new families under the Notostraca, while
Yohoia and Opabina were placed with the Anostraca. Except for the development of the
carapace, there is a striking similarity between Waptia and Yohoia, serving to connect the
two groups.

The Branchiopoda were very highly specialized as early as Middle Cambrian time, the
carapace of the Notostraca being fully developed and the abdomen limbless. Some (Bur-
gessia) had numerous segments, but most had relatively few. The most striking point
about them, however, is that so far as is known none of them had phyllopodan limbs.
While the preservation is in most cases unsatisfactory, such limbs as are preserved are trilo-
bite-like, and in the case of Burgessia there can be no possible doubt of the structure. An-
other interesting feature is the retention by Yohoia of vestiges of pleural lobes. The Middle

IIO THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

Cambrian Branchiopoda are more closely allied to the Trilobita than are the modern ones,
but still the subclass is not so closely related to that group as has been thought. Modern
Apus is certainly much less like a trilobite than has been supposed, and very far from being
primitive. The Branchiopoda of the Middle Cambrian could have been derived from the
trilobites by the loss of the pleural lobes, the development of the posterior margin of the
cephalon to form a carapace, and the loss of the appendages from the abdominal segments.
Modern branchiopods can be derived from those of the Middle Cambrian by the modifica-
tion of the appendages through the reduction of the endopodite and exopodite and the
growth of the endites and exites from the proximal segments.

Carpenter (1903, p. 334), from his study of recent crustaceans, has already come to
the conclusion that the Branchiopoda are not the most primitive subclass, and this opinion
is strengthened by evidence derived from the Trilobita and from the Branchiopoda of the
Middle Cambrian.

COPEPODA.

The non-parasitic Eucopepoda are in many ways much nearer to the trilobites than any
other Crustacea. These little animals lack the carapace, and the body is short, with typi-
cally ten free segments and a telson bearing caudal furcze. The head is composed of five
segments (if the first thoracic segment is really the fused first and second), is often flat-
tened, and lacks compound eyes. Pleural lobes are well developed, but instead of being
flattened as in the trilobite, they are turned down at the sides or even incurved. A labrum
is present.

The antennules, antenme, and mandibles are quite like those of trilobites. The anten-
nules are very long and made up of numerous segments. The antennae are biramous, the
junction between the coxopodite and basipodite is well marked, and the endopodite consists
of only two segments.

The mandibles are said to "retain more completely than in any other Crustacea the
form of biramous swimming limbs which they possess in the nauplius." The coxopodites
form jaws, while both the reduced endopodite and exopodite are furnished with long setae.
The maxillulse are also biramous, but very different in form from those of the trilobite,
and the maxillae are phyllopodan.

The first thoracic limb is uniramous and similar to the maxillae, but the five following
pairs are biramous swimming legs with coxopodite, basipodite, exopodite, and endopodite.
Both the exopodite and endopodite are shorter than in the trilobites, but bear setae and spines.

The last pair of thoracic limbs are usually modified in the male into copulatory organs.
In some females they are enlarged to form plates for the protection of the eggs, in others
they are unmodified. In still others they are much reduced or disappear. The abdomen
is without appendages.

The development in Copepoda is direct, by the addition posteriorly to the larval form
(nauplius) of segments, and the appendages remain nearly unmodified in the adult.

Altogether, the primitive Copepoda seem much more closely allied to the Trilobita than
any other modern Crustacea, but unfortunately no fossil representative of the subclass
has been found. This is not so surprising when one considers the habits and the habi-
tat of most of the existing species. Many are parasitic, many pelagic in both fresh
and marine waters, and many of those living on the bottom belong to the deep sea or fresh
water. Most free-living forms are minute, and all have thin tests.

ARCHICOPEPODA. I I I

The eyes of copepods are of interest, in that they suggest the paired ocelli of the Har-
pedid^e and Trinucleidae. In the Copepoda there are, in the simplest and typical form of
these organs, three ocelli, each supplied with its own nerve from the brain. Two of these
are dorsal and look upward, while the third is ventral. In some forms the dorsal ocelli
are doubled, so that five in all are present (cf. some species of Harpes with three ocelli on
each mound). In some, the cuticle over the dorsal eyes is thickened so as to form a lens,
as appears to be the case in the trilobites. These peculiar eyes may be a direct inheritance
from the Hypoparia.

ARCHICOPEPODA.

Professor Schuchert has called my attention to the exceedingly curious little crustacean
which Handlirsch (1914) has described from the Triassic of the Vosges. Handlirsch
erected a new species, genus, family, and order for this animal, which he considered most
closely allied to the copepods, hence the ordinal name. Enthycarcinus kessleri, the species in
question, was found in a clayey lens in the Voltzia standstone (Upper Bunter). Associated
with the new crustacean were specimens of Estheria only, but in the Voltzia sandstone itself
land plants, fresh and brackish water animals, and occasionally, marine animals are found.
The clayey lens seems to have been of fresh or brackish water origin.

All of the specimens (three were found) are small, about 35 mm. long without including
the caudal rami, crushed flat, and not very well preserved. The head is short, not so wide
as the succeeding segments, and apparently has large compound eyes at the posterior lateral
angles. The thorax consists of six segments which are broader than the head or abdomen.
The abdomen, which is not quite complete in any one specimen, is interpreted by Hand-
lirsch as having four segments in the female and five in the male. Least satisfactory of
all are traces of what are interpreted by the describer as a pair of long stiff unsegmented cerci
or stylets on the last segment.

The ventral side of one head shield shows faint traces of several appendages which
must have presented great difficulty in their interpretation. A pair of antennules appear to
spring from near the front of the lower surface, and the remainder of the organs are grouped
about the mouth, which is on the median line back of the center. Handlirsch sees in these
somewhat obscure appendages four pairs of biramous limbs, antennae, mandibles, maxillulse,
and maxilhe, both branches of each consisting of short similar segments, endopodites and
exopodites being alike pedifortn.

Kach segment of the thorax has a pair of appendages, and those on the first two are
clearly biramous. The endopodites are walking legs made up of numerous short segments
(twelve or thirteen according to Handlirsch's drawing), while the exopodite is a long breath-
ing and rowing limb, evidently of great flexibility and curiously like the antennules of the
same animal. The individual segments are narrow at the proximal end, expand greatly at
the sides, and have a concave distal profile. A limb reminds one of a stipe of Diplograptus.
Both branches are spiniferous.

No appendages are actually present on the abdomen, but each segment has a pair of
scars showing the points of attachment. From the small size of these, it is inferred that
the limbs were poorly developed.

This species is described in so much detail because, if it is a primitive copepod, it has
a very important bearing on the ancestry of that group and is the only related form that
has been found fossil.

112 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

The non-parasitic copepods have typically ten (eleven) free segments, including the
telson, and the four abdominal segments are much more slender than the six in front of
them. In this respect the agreement is striking, and the presence of five pairs of appen-
dages in the head and six free segments in the thorax is a more primitive condition than
in modern forms where the first two thoracic segments are apparently fused (Caiman, 1909,

P- 73)-

The large compound eyes of this animal are of course not present in the copepods, but

as vestiges of eyes have been found in the young of Calanus, it is possible that the ancestral
forms had eyes.

The greatest difficulty is in finding a satisfactory explanation of the appendages. The
general condition is somewhat more primitive than in the copepods, for all the appendages
are biramous, while in the modern • forms the maxillipeds are uniramous and the sixth pair
of thoracic appendages are usually modified in the male as copulatory organs. In the cope-
pods the modification is in the direction of reduction, both endopodites and exopodites usu-
ally possessing fewer segments than the corresponding branches in the trilobites. The
endopodite of Euthycarcinus, on the contrary, possesses, if Handlirsch's interpretation is
correct, twice as many segments as the endopodite of a trilobite. If the Copepoda are
descended from the trilobites, as everything tends to indicate, then Euthycarcinus is certainly
not a connecting link. The only truly copepodan characteristic of this genus is the agree-
ment in number and disposition of free segments. The division into three regions instead
of two, the compound eyes, and the structure of the appendages are all foreign to that group.

With the Limulava fresh in mind, one is tempted to compare Euthycarcinus with that
ancient type. The short head and large marginal eyes recall Sidncyia, and the grouping
of the appendages about the mouth also suggests that genus and Emcraldclla. In the Limu-
lava likewise there is a contraction of the posterior segments, although it is behind the
ninth instead of the sixth. There is no likeness in detail between the appendages of the
Limulava and those of Euthycarcinus, but the composite claws of Sidncyia show that in
this group there was a tendency toward the formation of extra segments.

If this fossil had been found in the Cambrian instead of the Triassic, it would prob-
ably have been referred to the Limulava, and is not at all impossible that it is a descendant
from that group. As a connecting link between the Trilobita and Copepoda it is, however,
quite unsatisfactory.

OSTRACODA.

The bivalved shell of the Ostracoda gives to this group of animals an external appear-
ance very different from that of the trilobites, but the few appendages, though highly modi-
fied, are directly comparable. The development, although modified by the early appearance
of the bivalved shell within which the nauplius lies, is direct. Imperfect compound eyes
are present in one family.

The antennules are short and much modified by functioning as swimming, creeping, or
digging organs. They consist of eight or less segments. The antennas are also locomotor
organs, and in most orders are biramous. The mandibles are biramous and usually with,
but sometimes without, a gnathobase. The maxillulae are likewise biramous but much
modified.

The homology of the third post-oral limb is in question, some considering it a maxilla
and others a maxilliped. It has various forms in different genera. It is always much modi-

MALACOSTRACA. 1 1 3

fied, but exopodite and endopodite are generally represented at least by rudiments. The
fourth post-oral limb is a lobed plate, usually not distinctly segmented, and the fifth a uni-
ramous pediform leg. The sixth, if present at all, is vestigial.

Very little comparison can be made between the Ostracoda and Trilobita, other than
in the ground-plan of the limbs, but the presence of biramous antennae is a primi-
tive characteristic.

CIRRIPEDIA.

Like the ostracod, the adult cirriped bears little external resemblance to the trilobite.
The form of the nauplius is somewhat peculiar, but it has the typical three pairs of appen-
dages, to which are added in the later metanauplius stages the maxillae and six pairs of
thoracic appendages. In the adult, the antennules, which serve for attachment of the larva,
usually persist in a functionless condition, while the antennas disappear. The mandibles,
maxilluhe, and maxillae are simple and much modified to form mouth parts, and the six
pairs of thoracic appendages are developed into long, multisegmented, biramous appendages
bearing numerous setae which serve for catching prey. Paired eyes are present in later
metanauplius stages, but lost early in the development. The relationship to the trilobite evi-
dently is not close.

MALACOSTRACA.

1. Phyllocarida.

The oldest malacostracans whose appendages are known are species of Hymcnocaris.
One, described as long ago as 1866 by Salter, has what seem to be a pair of antennae
and a pair of jaw-like mouth-parts. Another more completely known species has recently
been reported by Walcott (1912 A, p. 183, pi. 31, figs. 1-6). This latter form is described
as having five pairs of cephalic appendages: a pair of minute antennules beside the small
pedunculated eyes, a pair of large uniramous antennae, slender mandibles and maxilluhe,
and large maxilhe composed of short stout segments. There are eight pairs of biramous
thoracic limbs, the exopodites setiferous, the endopodites composed of short wide segments
and ending in terminal claw-like spines. These appendages are like those of trilobites.

Hymcnocaris belongs to the great group of extinct ceratocarid Crustacea which are
admitted to the lowest of the malacostracan orders, Phyllocarida, because of their resem-
blance to Ncbalia, Parancbalia, Ncbaliopsis, and Nebaliella, the four genera which are at
present living. The general form of the recent and fossil representatives of the order is
strikingly similar. The chief outward difference is that in many of the fossils the telson
is accompanied by two furcal rami, while in the modern genera it is simple. It now be-
comes possible to make some comparison between the appendages of Hymcnocaris of the
Middle Cambrian and the Nebaliidas of modern seas.

In both there are five pairs of cephalic and eight of thoracic appendages, while those
of the abdomen of Hymenocaris are not known.

In both, the antennules are less developed than the antennae. In the Nebaliidae the
antennules show evidence of having been originally double (they are obviously so in the
embryo), while they are single in Hymcnocaris. In both, the antennae are simple. The
remaining cephalic organs are too little shown by the specimen from the Middle Cambrian
to allow detailed comparison. The mandibles, maxillulas, and maxillae of Ncbalia are, how-
ever, of types which could be derived from the trilobite.

I 14 THE APPENDAGES, ANATOMY, AND RELATION'S OF TRILOBITES.

In three of the genera of the Nebaliidae, the eight pairs of thoracic limbs are all simi-
lar to one another, though those of the genera differ. All are biramous. The limbs of
Hymcnocaris can apparently be most closely correlated with those of Ncbalia antarctica, in
which the endopodite consists of short flattened segments, and the exopodite is a long seti-
ferous plate. Epipodites are present in both Ncbalia and Hymcnocaris.

So far as the appendages of Hymenocaris are known, they agree very well with those
of the Nebaliidse, and since they are of the trilobite type, it may safely be stated that the
Trilobita and Malacostraca are closely related.

2. Syncarida.

Walcott (1918, p. 170) has compared the limbs of Neolenus with those of the syn-
carid genera Anaspidcs and Koonunga. These are primitive Malacostraca without a cara-
pace, but as they have a compressed test and Anaspides has stalked eyes, their gross anatomy
does not suggest the trilobite. The thoracic appendages are very trilobite-like, since the
endopodite has six segments (in Anaspides} and a multisegmented setiferous exopodite.
The coxopodites, except of the first thoracic segment, do not, however, show endobases, and
those which are present are peculiar articulated ones. The cephalic appendages are special-
ized, and the antennules double as in most of the Malacostraca. External epipodites are
very numerous on the anterior limbs.

This group extends back as far as the Pennsylvania!! and had then probably already
become adapted to fresh-water life. It may be significant that the Palaeozoic syncarids
appear to have lacked epipodites. While differing very considerably from the Trilobita,
the Syncarida could have been derived from them.

3. Isopoda.

Since the earliest times there has been a constant temptation to compare the depressed
shields of the trilobites with the1 similar ones of isopods. Indeed, when Scrolls with its
Lichadian body was first discovered about a hundred years ago, it was thought that living
trilobites had been found at last. The trilobate body, cephalic shield, sessile eyes, abdom-
inal shield, and pleural extensions make a wonderful parallel. This similarity is, however,
somewhat superficial. The appendages are very definitely segregated in groups on the vari-
ous regions of the body, and while the pleopods are biramous, the thoracic legs are with-
out exopodites (except in very early stages of development of one genus). The Isopoda
arose just at the time of the disappearance of the Trilobita, and there seems a possibility
of a direct derivation of the one group from the other. It should be pointed out that while
the differences of Isopoda from Trilobita are important, they are all of a kind which could
have been produced by the development from a trilobite-like stock. For example :

Isopoda have a definite number of segments. There is less variation in the number
of segments among the later than the earlier trilobites.

Isopoda have no facial suture. In at least three genera of trilobites the cheeks become
fused to the cranidium and the sutures obliterated.

Isopoda have one or two segments of the thorax annexed to the head. While this
is not known to occur in trilobites, it is possible that it did.

Most Isopoda have a fairly stiff ventral test. The ventral membrane of trilobites
would probably have become stiffened by impregnation of lime if the habit of enrollment
had been given up.

MARRELI.A SPLENDENS. I I 5

In Isopoda the antenna? are practically uniramous sensory organs. The second cephalic
appendages of trilobites are capable of such development through reduction of the exopodite.

In the Isopoda the. coxopodites are usually fused with the body, remaining as free,
movably articulated segments only in a part of the thoracic legs of one suborder, the Asellota.
Endobases are entirely absent. This is of course entirely unlike the condition in Trilobita,
but a probable modification.

In Isopoda there is a distinct grouping of the appendages, with specialization of func-
tion. The trilobites show a beginning of tagmata, and such development would be expected
if evolution were progressive. .

In both groups, development from the embryo is direct. Rudiments of exopodites of
thoracic legs have been seen in the young of one genus.

The oldest known isopod is Oxyuropoda ligioidcs Carpenter and Swain (Proc. Royal
Irish Acad., vol. 27, sect. B, 1908, p. 63, fig. I ), found in the Upper Devonian of County
Kilkenny, Ireland. The appendages are not known, but the test is in some ways like that of
a trilobite. The thorax, abdomen, and pygidium are especially like those of certain trilo-
bites, and there is no greater differentiation between thorax and abdomen than there is be-
tween the regions before and behind the fifteenth segment of a Pcedeumias or Mesonacis.
The anal segment is directly comparable to the pygidium of a Ccraurus, the stiff unseg-
mented uropods being like the great lateral spines of that genus.

The interpretation of the head offered by Carpenter and Swain is very difficult to under-
stand, as their description and figure do not seem to agree. What they consider the first
thoracic segment (fused with the head) seems to me to be the posterior part of the cephalon.
and it shows at the back a narrow transverse area which is at least analogous to the nuchal
segment of the trilobite. If this interpretation can be sustained, Oxyurofoda would
be a very primitive isopod in which the first thoracic segment (second of Carpenter and
Swain) is still free. According to the interpretation of the original authors, the species is
more specialized than recent Isopoda, as they claim that two thoracic segments are fused
in the head. The second interpretation was perhaps made on the basis of the number of
segments (nineteen) in a recent isopod.

MARRELLA SPLENDENS WALCOTT.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 192, pis. 25, 26.

Among the most wonderful of the specimens described by Doctor Walcott is the "lace
crab." While the systematic position was not satisfactorily determined by the describer,
it has been aptly compared to a trilobite. The great nuchal and genal spines and the large
marginal sessile eyes, coupled with the almost total lack of thoracic and abdominal test, give
it a bizarre appearance which may obscure its real relationships.

The cephalon appears to bear five pairs of appendages, antennules, and antennae, both
tactile organs with numerous short segments, mandibles, and first and second maxillae. The
last three pairs are elongate, very spinose limbs, of peculiar appearance. They seem to have
seven segments, but are not well preserved. These organs are attached near the posterior
end of the labrum.

There are twenty-four pairs of biramous thoracic appendages, which lack endobases.
The endopodites are long and slender, with numerous spines ; the exopodites have narrow,
thin shafts, with long, forward pointed setae. The anal segment consists of a single plate.

Il6 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

Further information about this fossil will be eagerly awaited. None of the illustra-
tions so far published shows biramous appendages on the cephalon. This, coupled with the
presence of tactile antenrue, makes its reference to the Trilobita impossible, but the
present interpretation indicates that it was closely allied to them.

Fig. 32. — Marrclla splendens Walcott. Restoration of the ventral surface, based upon the
photographs and descriptions published by Walcott. Although all the limbs of the trunk
appear to be biramous, only endopodites are placed on one side and exopodites on the other,
for the sake of greater clearness in the illustration. Drawn by Doctor Elvira Wood, under
the supervision of the writer. X about 6.

Restoration of Marrclla.
(Text fig. 32.)

The accompanying restoration of the ventral surface of Marrclla is a tentative one,
based on Doctor Walcott's description and figures. The outline is taken from his plate 26,
figure i ; the appendages of the head from plate 26, figures 1-3, 5, and plate 25, figures 2,
3 ; the endopodites, shown on the left side only, from figures 3 and 6, plate 25. I have not
studied actual specimens, and the original description is very incomplete. The restoration is
therefore subject to revision as the species becomes better known.

ARACHNIDA. 117

ARACHNIDA.

No attempt will be made to pass in review all of the subclasses of the arachnids. Some
of the Merostornata are so obviously trilobite-like that it would seem that their relationship
could easily be proved. The task has not yet been satisfactorily accomplished, however,
and new information seems only to add to the difficulties.

So far as I know, the Aranese have not previously been compared directly with trilobites,
although such treatment consists merely in calling attention to their crustacean affinities, as
has often been done.

Carpenter's excellent summary (1903, p. 347) of the relationship of the Arachnida to
the trilobites may well be quoted at this point :

The discussion in a former section of this essay on the relationship between the various orders of
Arachnida led to the conclusion that the primitive arachnids were aquatic animals, breathing by means of
appendicular gills. Naturally, therefore, we compare the arachnids with the Crustacea rather than with the
Insecta. The immediate progenitors of the Arachnida appear to have possessed a head with four pairs of
limbs, a thorax with three segments, and an abdomen with thirteen segments and' a telson, only six of which
can be clearly shown by comparative morphology to have carried appendicular gills. But embryological
evidence enables us to postulate with confidence still more remote ancestors in which the head carried well
developed compound eyes and five pairs of appendages, while it may be supposed that all the abdominal
segments, except the anal, bore limbs. In these very ancient arthropods, all the limbs, except the feelers,
had ambulatory and branchial branches ; and one important feature in the evolution of the Arachnida must
have been the division of labour between the anterior and posterior limbs, the former becoming specialized
for locomotion, the latter for breathing. Another was the loss of feelers and the degeneration of the com-
pound eyes. Thus we are led to trace the Arachnida (including the Merostornata and Xiphosura) back to
ancestors which can not be regarded as arachnids, but which were identical with the primitive trilobites, and
near the ancestral stock of the whole crustacean class.

TRILOBITES N9T ARACHNIDA.

While no one having any real knowledge of the Trilobita has adopted Lankester's scheme
of the inclusion of the group as the primitive grade in the Arachnida, reference to it may
not be amiss. This theory is best set forth in the Encyclopaedia Britannica, Eleventh
Edition, under the article on Arachnida. It is there pointed out that the primitive arachnid,
like the primitive crustacean, should be an animal without a fixed number of somites, and
without definitely grouped tagmata. As Lankester words it, they should be anomomeristic
and anomotagmatic. The trilobites are such animals, and he considers them Arachnida and
not Crustacea for the following reasons :

Firstly and chiefly, because they have only one pair (apart from the eyes) of pre-oral
appendages. "This fact renders their association with the Crustacea impossible, if classifi-
cation is to be the expression of genetic affinity inferred from structural coincidence."

Secondly, the lateral eyes resemble no known eyes so closely as the lateral eyes of
Limulus.

Thirdly, the trilobation of the head and body, due to the expansion and flattening of
the sides or pleura, is like that of Limulus, but "no crustacean exhibits this trilobite form."

Fourthly, there is a tendency to form a pygidial or telsonic shield, "a fusion of the pos-
terior somites of the body, which is precisely identical in character with the metasomatic
carapace of Limulus." No crustacean shows metasomatic fusion of segments.

Fifthly, a large post-anal spine is developed "in some trilobites" (he refers to a figure
of Dalinanitcs).

Il8 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

Sixthly, there are frequently lateral spines on the pleura as in Limiilus. No crustacean
has lateral pleural spines.

These points may be taken up in order.

1. If trilobites have one appendage-bearing segment in front of the mouth, they are
Arachnida; if two, Crustacea. This is based on the idea that in the course of evolution
of the Arthropoda, the mouth has shifted backward from a terminal position, and that as a
pair of appendages is passed, they lose their function as mouth-parts and eventually become
simple tactile organs. Thus arise the cheliceras of most arachnids, and the two pairs of
tactile antennae of most Crustacea. This theory is excellent, and the rule holds well for
modern forms, but as shown by the varying length of the hypostoma in different trilobites,
the position of the mouth had not become fixed in that group. In some trilobites, like Triar-
thrus, the gnathobases of the second pair of appendages still function, but in all, so far as
known, the mouth was back of the points of attachment of at least two pairs of appendages,
and in some at least, back of the points of attachment of four pairs. As pointed out in the
case of Calymene and Cerdurus, the trilobites show a tendency toward the degeneration of
the first and second pairs of biramous appendages, particularly of the gnathobases. They
are in just that stage of the backward movement of the mouth when the function of the
antennae as mandibles has not yet been lost. If the presence of functional gnathobases back
of the mouth, rather than the points of attachment in front of the mouth, is to be the guide,
then Triarthms might be classed as an arachnid and Calymene and Isotelus as crustaceans.
In other words, the rule breaks down in this primitive group.

2. Superficially, the eyes of some trilobites do look like those of Limulns, but how
close the similarity really was it is impossible to say. The schizochroal eyes were certainly
very different, and Watase and Exner both found the structure of the eye of the trilobite
unlike that of Limulus.

3. The importance of the trilobate form of the trilobite is very much overestimated.
It and the pygidium are due solely to functional requirements. The axial lobe contained
practically all the vital organs and the side lobes were mechanical in origin and secondarily
protective. That the crustacean is not trilobate is frequently asserted by zoologists, yet
every text-book contains a picture of a segment of a lobster with its axial and pleural lobes.
It is a fundamental structure among the Crustacea, obscured because most of them are com-
pressed rather than depressed.

4. The pygidium of trilobites is compared with the metasomatic shield of Limulus. No
homology, if homology is intended, could be more erroneous. The metasomatic shield of
Limulus is, as shown by ontogeny and phylogeny, formed by the fusion of segments formerly
free, and includes the segments between the cephalic and anal shields, or what would be
known as the thorax of a trilobite. No trilobite has a metasomatic shield. The pygidium
of a trilobite, as shown by ontogeny, is built up by growth in front of the anal region, and
since the segments were never free, it can not strictly be said to be composed of fused
segments. Some Crustacea do form a pygidial shield, as in certain orders of the Isopoda.

5. The post-anal spine of Dalmanites and some other trilobites is similar to that of
Limulus, but this seems a point of no especial significance. That a similar spine has not
been developed in the Crustacea is probably due to the fact that they do not have the broad
depressed shape which makes it so difficult for a Limulus to right itself when once turned
on its back. Relatively few trilobites have it, and it is probably correlated with some special
adaptation.

MEROSTOMATA. 1 19

6. There is nothing among the trilobites comparable to the movable lateral spines of
the metasoma of Limulus.

While, as classifications are made up, the Trilobita must be placed in the Crustacea
rather than the Arachnida, there is no reason why both the modern Crustacea and the Arach-
nida should not be derived from the trilobites.

MEROSTOMATA.

It has been a custom of long standing to compare the trilobite with Limulus. Packard
(1872) gave great vitality to the theory of the close affinity of the two when he described
the so called trilobite-stage in the development of Limulus polyphemus. His influence on
Walcott's ideas (1881) is obvious. Lankester has gone still further, and associated the
Trilobita with the Merostomata in the Arachnida.

The absence of antennules at any stage in development allies Limulus so closely with
the Arachnida and separates it so far from the Trilobita that in recent years there has been
a tendency to give up the attempt to prove a relationship between the merostomes and trilo-
bites, especially since Clarke and Ruedemann, in their extensive study of the Eurypterida,
found nothing to indicate the crustacean nature of that group. A new point of view is, how-
ever, presented by the curious Sidneyia inexpectans and Emeraldella brocki described by
Walcott from the Middle Cambrian.

Sidneyia inexpectans Walcott.

Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1911, p. 21, pi. 2, fig. I (not figs. 2, 3); pis. 3-5;
pi. 6, fig. 3; pi. 7, fig. i.

The body of this animal is elongate, somewhat eurypterid-like, but with a broad telson
supplied with lateral swimmerets. The cephalon is short, with lateral compound eyes. The
trunk consists of eleven segments, the anterior nine of which are conspicuously wider than
the two behind them, and the telson consists of a single elongate plate.

On the ventral side of the head there is a large hypostoma and five, pairs of appendages.
The first pair are multisegmented antennules. The second pair have not been adequately
described. The third are large, complex claws, and the fourth and fifth suggest broad,
stocky endopodites. Broad gnathobases are attached to the coxopodites of the third to fifth
pairs of appendages and form very strong jaws.

The first nine segments of the thorax have one pair each of broad filiform branchial
appendages, suggestive of the exopodites of trilobites, but no endopodites have been seen.
The tenth and eleventh segments seem to lack appendages entirely.

Emeraldella brocki Walcott.

Illustrated : Sidneyia inexpectans Walcott partim, Smithson. Misc. Coll., vol. 57, 1911, pi. 2, figs. 2, 3
(not fig. i) ; — Ibid., 1912, p. 206, text fig. 10.

Emeraldella brocki Walcott, Ibid., 1912, p. 203, pi. 30, fig. 2; text fig. 8; — Ibid., vol. 67, 1918, p. 118
(correction).

Emeraldella has much the same shape as Sidneyia and the same number of segments,
but instead of a broad flat telson, it has a long Limulus-\ike spine. The cephalon is about
as wide as long, and eyes have not yet been seen. The body consists of eleven segments and
a telson (Walcott says twelve and a telson but shows only eleven in the figures). Nine of
the segments, as in Sidneyia, are broad, the next two narrow.

I2O THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

The ventral side of the cephalon has a long hypostoma, and five pairs of appendages.
The first pair are very long multi segmented antennules and the next four pairs seem to be
rather slender, spiniferous, jointed endopodites. Whether or not gnathobases were present
is not shown by the figures, but owing to the long hypostoma the appendages are grouped
about the mouth. All the segments of the body, unless it were the telson, seem to have
borne appendages. On the anterior end, they were clearly biramous (1912, p. 206, text
fig. 10), and that they were present along the body is shown by figure 2, plate 30, 1912.

The present state of knowledge of both these peculiar animals leaves much to be desired.
The indications are that the cephalic appendages are not biramous, and that only one pair of
antennae, the first, are developed as tactile organs. The thoracic appendages of Emcraldclla
are biramous, and also possibly those of Sidneyia. In the latter, the last two abdominal seg-
ments seem to have been without appendages, while in Emcraldella at least one branch of
each appendage, and possibly both, is retained.

These animals, which may be looked upon as the last survivors of an order of pre-
Cambrian arthropods, have the appearance of an eurypterid, but their dominant character-
istics are crustacean. The features which suggest the Eurypterida are : elongate, obovate,
non-trilobate, tapering body; telson-like posterior segment; marginal, compound, sessile eyes;
claw-like third cephalic appendages ; and, more particularly, the general resemblance of the
test to that of an eurypterid like Strabops. In form, Sidneyia agrees with the theoretical
prototype of the Eurypterida reconstructed by Clarke and Ruedemann (Mem. 14, N. Y.
State Mus., vol. i, 1912, p. 124) in its short wide head with marginal eyes, and its undiffer-
entiated body. There is, moreover, no differentiation of the postcephalic appendages.

The crustacean characteristics are seen in the presence of five, instead of six, pairs of
appendages on the head, the first of which are multisegmented antennules, and in the bira-
mous appendages on the body of Emeraldella. It should be noted that these latter are
typically trilobitic, each consisting of an endopodite with six segments and a setiferous
exopodite.

Clarke and Ruedemann (1912, p. 406) have discussed Sidneyia briefly, and conclude:

It seems to us probable that the Limulava [Sidneyia and Amiella] as described are not eurypterids but
constitute a primitive order, though exhibiting some remarkable adaptive features. This order possibly
belongs to the Merostomata, but is distinctly allied to the crustaceans in such important characters as the
structure of the legs and telson, and is therefore much generalized.

The specialization of Sidneyia consists in the remarkable development of a highly com-
plex claw on each of the third cephalic appendages, and in the compound tail-fin, built up
of the last segment and one or more pairs of swimmerets. These two characteristics seem
to preclude the possibility of deriving the eurypterids from Sidneyia itself, but it seems
entirely within reason that they may have been derived from another slightly less specialized
member of the same order.

That Sidneyia is descended from any known trilobite seems highly improbable, but that
it was descended from the same ancestral stock as the trilobites is, I believe, indicated by
the presence of five pairs of appendages on the cephalon and trilobitic legs on the abdomen.

Molaria and Habelia.

Other so called Merostomata found by Walcott in the Middle Cambrian are the genera
Molaria and Habelia, both referred to the Cambrian family Aglaspidae. These genera seem
to conform with Aglaspis of the Upper Cambrian in having a trilobite-like cephalon without

AKANE^E. 121

facial sutures, a trilobite-like thorax of a small but variable (7-12) number of segments,
and a Limitlus-like telson. Neither of them has yet been fully described or figured, but
(Walcott 1912 A, p. 202) Habclia appears to have five pairs of cephalic appendages, the
first two pairs of which are multisegmented antenna;. The thoracic appendages are likewise
none too well known, but they appear to have been biramous. The endopodites are better
preserved than the exopodites, but in at least one specimen of Malaria the exopodites are
conspicuous.

If these genera are properly described and figured, their appendages are typically crus-
tacean, and fundamentally in agreement with those of Marrclla. The relation to the Trilo-
bita is evidently close, the principal differences being the absence of facial sutures and the
presence of true antennae. I am therefore transferring the Aglaspidae from the Merosto-
mata to a new subclass under the Crustacea.

ARANE^E.

The spiders have the head and thorax fused, the abdomen unsegmented except in the
most primitive suborder, and so appear even less trilobite-like than the insects. The appen-
dages likewise are highly specialized. The ' cephalothorax bears six pairs of appendages,
the first of which are the preoral cheliceraa, while behind the mouth are the pedipalpi and
four pairs of ambulatory legs. The posterior pairs of walking legs belong to the thorax,
but the anterior ones are to be homologized with the maxillae of Crustacea, so that the spiders
are like the trilobites in having functional walking legs on the head.

The chief likenesses are, however, seen in the very young. On the germ band there
appear a pair of buds in front of the rudiments of the cheliceras which later unite to form
the rostrum of the adult. At the time these buds appear, the chelicera are postoral, but
afterward move forward so that both rostrum and cheliceras are in front of the mouth.
The rostrum is therefore the product of the union of the antennules, and the chelicerse are
to be homologized with the antennae. There seems to be some doubt about the homology
of the pedipalps with the mandibles, as at least one investigator claims to have found rudi-
ments of a segment between the one bearing the chelicerse and that with the pedipalps.

Jaworowski (Zool. Anzeiger, 1891, p. 173, fig. 4) has figured the pedipalp from the
germ band of Trochosa singoriensis, and called attention to the fact that it consists of a c-ox-
opodite and two segmented branches which may be interpreted as exopodite and endopodite.
He designated as exopodite the longer branch which persists in the adult, but since the ambu-
latory legs of Crustacea are endopodites, that would seem a more likely interpretation. As
the figure is drawn, the so called endopodite would appear to spring from the proximal seg-
ment of the "exopodite." If the two terms were interchanged, the homology with the limb
of the trilobite or other crustacean would be quite perfect.

In the young, the abdomen is segmented and the anterior segments develop limb-buds,
the first pair of which become the lung books and the last two pairs the spinnerets of the
adult. There seems to be some question about the number of segments. Montgomery
(Jour. Morphology, vol. 20, 1909, p. 337). reviewing the literature, finds that from eight to
twelve have been seen in front of the anal segment. The number seem to vary with the
species studied. This of course suggests connection with the anomomeristic trilobites.

The oldest true spiders are found in the Pennsylvanian, and several genera are now
known. The head and thorax are fused completely, but the abdomen is distinctly seg-

122 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

merited. Some of the Anthracomarti resemble the trilobites more closely than do the Aranere,
as they lack the constriction between the cephalothorax and abdomen. The spiders of the
Pennsylvanian have this constriction less perfectly developed than do modern Aranerc, and
occupy an intermediate position in this respect. In the Anthracomarti, the pedipalpi are
simple, pediform, and all the appendages have very much the appearance of the coxopodites
and endopodites of trilobites. Chelicerse are not known, and pleural lobes are well devel-
oped in this group. Anthracomarti have not yet been found in strata older than the Penn-
sylvanian, but they seem to be to a certain extent intermediate between true spiders and the
marine arachnid.

INSECTA.

Handlirsch (in several papers, most of which are collected in "Die Fossilen Insekten,"
1908) has attempted to show that all the Arthropoda can be derived from the Trilobita,
and has advocated the view that the Insecta sprang directly from that group, without the
intervention of other tracheate stock. At first sight, this transformation seems almost
an impossibility, and the view does not seem to have gained any great headway among ento-
mologists in the fourteen years since it was first promulgated. If an adult trilobite be com-
pared with an adult modern insect, few likenesses will be seen, but when the trilobite is
stripped of its specializations and compared with the germ-band of a primitive insect, the
theory begins to seem more possible.

Handlirsch really presented very little specific evidence in favor of his theory. In fact,
one gets the impression that he has insisted on only two points. Firstly, that the most
ancient known insects, the Palaeodictyoptera, were amphibious, and their larvae, which lived
in water, were very like the adult. Secondly, that the wings of the Palasodictyoptera prob-
ably worked vertically only, and the two main wings were homologous with rudimentary
wing-like outgrowths on each segment of the body. These outgrowths have the appear-
ance of, and might have been derived from, the pleural lobes of trilobites.

He figured (1908, p. 1305, fig. 7) a reconstructed larva of a palaeodictyopterid as
having biramous limbs on each segment, but so far as I can find, this figure is purely schematic,
for there seems to be no illustration or description of any such larva in the body of his work.

That the insects arose directly from aquatic animals is of course possible, and Hand-
lirsch's first argument has considerable force. It may, however, be purely a chance that the
oldest insects now known to us happen to be an amphibious tribe. The Palacodictyoptera
are not yet known to antedate the Pennsylvanian, but there can be no doubt that, insects
existed long before that time, and the fact that their remains have not been found is good
evidence that the pre-Pennsylvanian insects were not aquatic. Comstock, who has recently
investigated the matter, does not believe that the Palaeodictyoptera were amphibious (The
Wings of Insects, Ithaca, N. Y., 1918, p. 91).

The second argument, that wings arose from the pleural lobes of trilobites, is exceedingly
weak. Where most fully set forth (1907, p. 157), he suggests that trilobites may occasion-
ally have left the water, climbed a steep bank or a plant, and then glided back into their native
element, taking advantage of the broad flat shape to make a comfortable and gentle descent !
This sport apparently became so engaging that the animal tried experiments with flexible
wing tips, eventually got the whole of the pleural lobes in a flexible condition, and selected
those of the second and third thoracic segments for preservation, while discarding the
remainder. The pleural lobes of trilobites are not only too firmly joined to the axial portion

INSECTA AND CHILOPODA. 123

of the test to be easily transformed into movable organs, but they are structurally too unlike
the veined wings of insects to make the suggestion of this derivation even worthy of con-
sideration.

Tothill (1916) has recently reinvestigated the possible connection between insects, chi-
lopods, and trilobites, and, from the early appearance of the spiracles in the young, came to
the conclusion that the insects were derived from terrestrial animals. He suggested that they
may have come through the chilopods from the trilobites. The hypothetical ancestor of the
insects, as restored by Tothill from the evidence of embryology and comparative anatomy,
is an animal more easily derived from the Chilopoda than from the Trilobita. Five pairs of
appendages are present on the head, and the trunk is made up of fourteen similar segments,
each with a pair of walking limbs and a pair of spiracles.

Only the maxilla; and maxilluke are represented as biramous. If the ancestor of the
Insecta was, as seems possible, tracheate, this fact alone would rule out the trilobites.
Among tracheates, the Chilopoda are certainly more closely allied to the Insecta than are
any other wingless forms. If the ancestors of the insects were not actually chilopods, they
may have been chilopod-like, and there can be little doubt that both groups trace to the
same stock.

As to the ancestry of the Chilopoda, it is probable that they had the same origin as
the other Arthropoda. Tothill has pointed out that in the embryo of some chilopods there
are rudiments of two pairs of antenna? and that the two pairs of maxillre and the maxilli-
peds are biramous. This would point rather to the Haplopoda than directly to the trilobites
as possible ancestors, and may explain why the former vanish so suddenly from the geological
record after their brief appearance in the Middle Cambrian. They may have gone on to
the land.

There seem to be no insuperable obstacles to prevent the derivation, indirectly, of the
insects from some trilobite with numerous free segments, and small pygidium. The anten-
nules and pleural lobes must be lost, the antennas and trunk limbs modified by loss of exopo-
dites. Wings and tracheae must be acquired.

Handlirsch places the date of origin of the Insecta rather late, just at the end of the
Devonian and during the "Carboniferous." By that time most families of trilobites had
died out, so that the possibilities of origin of new stocks were much diminished. If the
haplopod-chilopod-insect line is a better approximation to the truth, then the divergence began
in the Cambrian.

CHILOPODA.

The adult chilopod lacks the antennules, and all of the other appendages, with- the ex-
ception of the maxilluke, are uniramous. The walking legs are similar to the endopodites
of trilobites, and usually have six or seven segments. The appendages are therefore such as
could be derived by modification of those of trilobites by the almost complete loss of the
exopodites and shortening of the endopodites of the head. The position of the postoral ap-
pendages, the posterior ones outside those closest the mouth, is perhaps foreshadowed in the
arrangement of those of Triarthrus.

The Chilopoda differ from the Hexapoda in developing the antenna; instead of the
antennules as tactile organs, but this can not be used with any great effect as an argument
that the latter did not arise from the ancestors of the former, since it is entirely possible
that in early Palasozoic times the pre-Chilopoda possessed two pairs of antennae. The first
pair are still recognizable in the embryo of certain species.

124 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

The oldest chilopods are species described by Scudder (Mem. Boston Soc. Nat. Hist.,
vol. 4, 1890, p. 417, pi. 38) from the Pennsylvania!! at Mazon Creek, Grundy County, Illinois.
Only one of these, Latzclia primordialis Scudder (pi. 38 fig. 3), is at all well preserved.
This little animal, less than an inch long, had a depressed body, with a median carina, exceed-
ingly long slender legs, and about nineteen segments. The head is very nearly obliterated.

DIPLOPODA.

The diplopods, especially the polydesmids with their lateral outgrowths, often have a
general appearance somewhat like that of a trilobite, but on closer examination few like-
nesses are seen. The most striking single feature of the group, the possession by each seg-
ment of two pairs of appendages, is not in any way foreshadowed in the trilobites, none
of which shows any tendency toward a fusion of pairs of adjacent segments. The anten-
nules are short, antennae absent, mandibles and maxilluke much modified, the latter possibly
biramous, and the maxillze absent. The trunk appendages are very similar to those of chi-
lopods, and could readily be derived from the endopodites of trilobites.

The oldest diplopods are found in the Silurian (Ludlow) and Devonian (Lower Old
Red) of Scotland, and three species belonging to two genera are known. The oldest is
Archidesmus loganensis Peach (1889, p. 123, pi. 4, fig. 4), and the Devonian species are
Archidesmus macnicoli Peach and Kampecaris forfarensis Page (Peach 1882, p. 182, pi. 2,
fig. 2, 2a, and p. 179, pi. 2, figs. i-ig). All of these species show lateral expansions like
the recent Polydesmidae, and these of course suggest the pleural lobes of trilobites. All
three of the species are simpler than any modern diplopod, for there is only a single pair
of appendages on each segment. No foramina rcpugnatoria were observed, and the eyes of
Kampecaris forfarensis as described are singularly like those of a phacopid.

Peach says: "The eye itself is made up of numerous facets which are arranged in
oblique rows, the posterior end of each row being inclined downwards and outwards, the
facets being so numerous and so close together that the eye simulates a compound one." There
is also a protecting ridge which somewhat resembles a palpebral lobe (1882, pi. 7, fig. la).
Peach comments on the strength of the test, and from his description it appears that it must
have been preserved in the same manner as the test of trilobites. It was punctate, and gran-
ules and spines were also present. The presence of the lateral outgrowths in these ancient
specimens would seem to indicate that they are primitive features, and may have been in-
herited. While possibly not homologous with the pleural extensions of trilobites, they may
be vestiges of these structures.

The limbs are made up of seven segments which are circular in section and expand at
the distal end. The distal one bears one or two minute spines. They are most readily com-
pared with the endopodites of Isotclus. The resemblance is, in fact, rather close. The
sternal plates are wider and the limbs of opposite sides further apart than in modern diplo-
pods. Except for one pair of antenna;, no cephalic appendages are preserved.

While these specimens do not serve to connect the Diplopoda with the Trilobita, they
do show that most of the specializations of the former originated since Lower Devonian
times, and lead one to suspect that the derivation from marine ancestors took place very early,
perhaps in the Cambrian. If no very close connection with the trilobites is indicated, there
is also nothing to show that the diplopods could not have been derived from that group.

PRIMITIVE CHARACTERISTICS OF TRILOBITES. 125

i

PRIMITIVE CHARACTERISTICS OF TRILOBITES.
TRILOBITES THE MOST PRIMITIVE ARTHROPODS.

The Arthropods., to make the simplest possible definition, are invertebrate animals with
segmented body and appendages. The most primitive arthropod would appear to be one
composed of exactly similar segments bearing exactly similar appendages, the segments of
the appendages themselves all similar to one another. It is highly improbable that this most
primitive arthropod imaginable will ever be found, but after a survey of the whole phylum,
it appears that the simpler trilobites approximate it most closely.

That the trilobites are primitive is evidenced by the facts that they have been placed
at the bottom of the Crustacea by all authors and claimed as the ancestors of that group by
some; that Lankester derived the Arachnida from them; and that Handlirsch has consid-
ered them the progenitors of the whole arthropodan phylum.

Specializations among the Arthropoda, even among the free-living forms, are so numer-
ous that it would be difficult to make a complete list of them. In discussing the principal
groups, I have tried to show that the essential structures can be explained as inherited from
the Trilobita, changed in form by explainable modifications, and that new structures, not'
present in the Trilobita, are of such a nature that they might be acquired independently in
even unrelated groups.

The chief objections to the derivation of the remainder of the Crustacea from the trilo-
bites have been: first, that the trilobites had broad pleural extensions; second, that they had
a large pygidium; and lastly, that they had only one pair of tactile antennae.

It has now been pointed out that many modern Crustacea have pleural extensions, but
that they usually bend down at the sides of the body, and also that in the trilobites and more
especially in Marrella, there was a tendency toward the degeneration of the pleural lobes.
A glance at the Mesonacidse or Paradoxidae should be convincing proof that in some trilo-
bites the pygidium is reduced to a very small plate.

In regard to the second antennae standard text-books contain statements which are actu-
ally surprising. A compilation shows that the antennae are entirely uniramous in but a
very few suborders, chiefly among the Malacostraca ; that they are biramous with both
exopodite and endopodite well developed in most Copepoda, Ostracoda, and Branchiopoda;
and that the exopodite, although reduced in size, still has a function in some suborders of
the Malacostraca. The Crustacea could not possibly be derived from an ancestor with two
pairs of uniramous antenna;.

Although I have defended the trilobites, perhaps with some warmth, from the impu-
tation that they were Arachnida, my argument does not apply in the opposite direction, and
I believe Lankester was right in deriving the Arachnida from them. If the number of
appendages in front of the mouth is fundamental, then the trilobites were generalized, primi-
tive, and capable of giving rise to both' Crustacea and Arachnida. As shown on a previous
page (p. 119), the "connecting links" so far found tend to disprove rather than to prove
the thesis, but the present finds should be looked upon as only the harbingers of the greater
ones which are sure to come.

LIMBS OF TRILOBITES PRIMITIVE.

The general presence, in an adult or larva, of some sort of biramous limbs through-
out the whole class Crustacea has led most zoologists to expect such a limb in the most

126 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

primitive crustaceans, and apparently the appendage of the trilobite satisfies the expectation.
It is well, perhaps, as a test, to consider whether by modification this limb could produce
the various types of limbs seen in other members of the class. In the first place, it is
necessary to have clearly in mind the peculiarities of the appendage to be discussed.

It should first of all be remembered that the limb is articulated with the dorsal skeleton
in a manner which is very peculiar for a crustacean. The coxopodite swings on a sort of
ball-and-socket joint, and at the outer end both the exopodite and the basipodite articulate
with it. Since the exopodite articulates with the basipodite as well as with the coxopo-
dite, the two branches are closely connected with one another and there is little individual
freedom of movement. This is, of course, a necessary consequence of their articulation
with a segment which is itself too freely movable to provide a solid base for attachment
of muscles. The relation of the appendifer, coxopodite, and two rami is here shown dia-
grammatically (fig. 33), the exopodite branching off from the proximal end of the basipo-
dite at the junction with the coxopodite.

In all trilobites the endopodite consists of six segments, and the coxopodite of a single
segment the inner end of which is prolonged as an endobase. There does not seem to be
any variation from this plan in the subclass, although individual segments are variously

Fig- 33- — Diagrammatic representation of an
appendage of the anterior end of the thorax of
Triarthrus bccki Green, to show relation of exo-
podite and endopodite to each other and to the
coxopodite. Much enlarged.

modified. The exopodites are more variable, but all consist of a flattened shaft with setae on
one margin. No other organs such as accessory gills, swimming plates, or brood pouches
have yet been found attached to the appendages, the evidence for the existence of the vari-
ous epipodites and exites described by Walcott being unsatisfactory (see p. 23).

In the Ostracoda the appendages are highly variable, but it is easily seen that they
are modifications of a limb which is fundamentally biramous. In most species, both exop-
odite and endopodite suffer reduction. The exopodite springs from the basipodite and that
segment is closely joined to the coxopodite, producing a protopodite. In some cases the
original segments of the endopodites fuse to form a stiff rod. While highly diversified,
these appendages are very trilobite-like, and some Ostracoda even have biramous antenn;c.

The non-parasitic Copepoda have limbs exceedingly like those of trilobites. Many of
them are biramous, the endopodites sometimes retaining the primitive six segments. Coxop-
odite and basipodite are generally united, and endopodite and exopodite variously modified.
Like some of the Ostracoda, the more primitive Copepoda have biramous antennae.

As would be expected, the appendages of the Cirripedia are much modified, although
those of the nauplius are typical. The thoracic appendages of many are biramous, but both
branches are multisegmented.

In the modern Malacostraca the ground plan of the appendages is biramous, but in most
orders they are much modified. In many, however, the appendages of some part of the
body are biramous, and in many the endopodites show the typical six segments. From the
coxopodites arise epipodites, some of which assist in swimming, and some in respiration.

PRIMITIVE CHARACTERISTICS OF TRILOBITES.

Because of the many instances in which such extra growths arise, and because of the form
of the appendages of the Branchiopoda, it has been suggested that the primitive crustacean
leg must have been more complex than that of the trilobite. In looking over the Malacos-
traca, however, one is struck by the fact that epipodites generally arise where the exopo-
dites have become aborted or are poorly developed, and seem largely to replace them. The
coxopodite and basipodite are usually fused to form a protopodite, and a third segment
is sometimes present in the proximal part of the appendage.

In the Branchiopoda are found the most complex crustacean limbs, and the ones most
difficult to homologize with those of trilobites. In recent years, Lankester's homologies
of the parts of the limbs of Apus with those of the Malacostraca have been quite gener-
ally accepted, and the appendages of the former considered primitive. Now that it is
known that the Branchiopoda of the Middle Cambrian (Burgessia et at.) had simple trilo-
bite-like appendages, it becomes necessary to exactly reverse the opinion in this matter.
The same homologies stand, but the thoracic limbs of Apus must be looked upon as highly
specialized instead of primitive.

Fig. 34. — One of the appendages of the
anterior part of the trunk of Apus, showing
the endites (beneath) and exites (above).
The proximal endite forms a gnathobase
which is not homologous with the gnatho-
base (or endobase) of the trilobite. Copied
from Lankester. Much enlarged.

Lankester (Jour. Micros. Sci., vol. 21, 1881) pointed out that the axial part of the
thoracic limb of Apus (fig. 34) is homologous with the protopodite in the higher Crus-
tacea, that the two terminal endites corresponded to the exopodite and endopodite, and that
the other endites and exites were outgrowths from the protopodite analogous to the epip-
odites of Malacostraca. There seems to be no objection to retaining this interpretation,
but with the meaning that both endopodite and exopodite are much reduced, and their func-
tions transferred to numerous outgrowths of the protopodite. One of the endites grows
inward to form an endobase, the whole limb showing an attempt to return to the ancestral
condition of the trilobite. The limbs of some other branchiopods are not so easy to under-
stand, but students of the Crustacea seem to have worked out a fairly satisfactory compari-
son between them and Apus.

The discovery that the ancestral Branchiopoda had simple biramous appendages instead
of the rather complex phyllopodan type is another case in which the theory of "recapitu-
lation" has proved to hold. It had already been observed that in ontogeny the biramous
limb preceded the phyllopodan, but so strong has been the belief in the primitive character
of the Apodida? that the obvious suggestion has been ignored. Even in such highly special-
ized Malacostraca as the hermit crabs the development of certain of the limbs illustrates the
change from the schizopodal to the phyllopodan type, and Thompson (Proc. Boston Soc.
Nat. Hist., vol. 31, 1903, pi. 5, fig. 12) has published an especially good series of drawings

128

THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

showing the first maxilliped. In the first to fourth zoex the limb is biramous but in the
glaucothoe a pair of broad processes grow out from the protopodite, while the exopodite and
particularly the endopodite become greatly reduced. In the adult the endopodite is a mere
vestige, while the flat outgrowths from the protopodite have become very large and bear

setse.

Summary.

The limbs of most Crustacea are readily explained as modifications of a simple bira-
mous type. These modifications usually take the form of reduction by the loss or fusion of
segments and quite generally either the entire endopodite or exopodite is lacking. Modifi-
cation by addition frequently occurs in the growth of epipodites, "endites," and "exites"
from the coxopodite, basipodite, or both. A protopodite is generally formed by the fusion
of coxopodite and basipodite, accompanied by a transference of the proximal end of the
exopodite to the distal end of the basipodite. A new segment, not known in the trilobites
(precoxal), is sometimes added at the inner end.

Among modern Crustacea, the anterior cephalic appendages and thoracic appendages of
the Copepoda and the thoracic appendages of certain Malacostraca, Syncarida especially,
are most nearly like those of the trilobite. The exact homology, segment for segment, be-
tween the walking legs of the trilobite and those of many of the Malacostraca, even the
Decapoda, is a striking instance of retention of primitive characteristics in a specialized
group, comparable to the retention of primitive appendages in man.

NUMBER OF SEGMENTS IN THE TRUNK.

Various attempts have been made to show that despite the great variability, trilobites
do show a tendency toward a definite number of segments in the body.

Emmrich (1839), noting that those trilobites which had a long thorax usually had
a short pygidium, and that the reverse also held true, formulated the law that the number
of segments in the trunk was constant (20 + 0- Very numerous exceptions to this law
were, however, soon discovered, and while the condition of those with less than twenty-one
segments was easily explained, the increasing numl>er of those with more than twenty-one
soon brought the idea into total disrepute.

Quenstedt (1837) had considered the number of segments of at least specific impor-
tance, and both he and Burmeister (1843) considered that the number of segments in the
thorax must be the same for all members of a genus. As first shown by Barrande (1852.
p. 191 et seq.), there are very many genera in which there is considerable variation in the
number of thoracic segments, and a few examples can be cited in which there is variation
within a species, or at least in very closely related species.

Carpenter (1903, p. 333) has tabulated the number of trunk segments of such trilo-
bites as were listed by Zittel in 1887 and finds a steady increase throughout the Palaeozoic.
His table, which follows, is, however, based upon very few genera.

Period

No. of Genera

Average No. of body-segments

Cambrian

12

17.66

Ordovician

23

18.58

Silurian

16

19-34

Devonian

i

I 10

20.70

Carboniferous

2

20.75

1

SEGMENTS IN TRUNK. 129

Due chiefly to the efforts of Walcott, an increasingly large number of Cambrian genera
are now represented by entire specimens, and since these most ancient genera are of great-
est importance, a few comments on them may be offered.

The total number of segments can be fairly accurately determined in at least nineteen
genera of trilobites from the Lower Cambrian. These include eight genera of the Meson-
acid;e (Olcnellus was excluded) and Eodiscus, Goniodiscus, Protypus, Bathynotus, Atops,
Olenopsis, Crcpicephalus, Vanuxemella, Corynexochus, Bathyuriscus, and Poliella. The ex-
tremes of range in total segments of the trunk is seen in Eodiscus (9) and Pcedeumias
(45-)-), and these same genera show the extremes in the number of thoracic segments,
there being 3 in the one and 44+ in the other. Pcedeumias probably shows the greatest var-
iation of any one genus of trilobites, various species showing from 19 to 44+ thoracic seg-
ments. The average for the nineteen genera is 13.9 segments in the thorax, 3.7 segments
in the pygidium, or a total average of 17.6 segments in the trunk. Crcpicephalus with
12-14 segments in the thorax and 4-6 in the pygidium, and Protypus, with 13 in the thorax
and 4-6 in the pygidium, are the only genera which approach the average. All of the Mes-
onacidae, except one, Olenelloides, have far more thoracic and fewer pygidial segments than
the average, while the reverse is true of the Eodiscida;, Vanuxemella, Corynexochus, Bath-
yuriscus, and Poliella.

The eight genera of the Mesonacidse, Nevadia, Mesonacis, Elliptoccphala, Callavia, Holmia,
Wanncria, Pcedeumias, and Olenelloides, have an average of 20.25 segments in the thorax
and 1.5 in the pygidium, a total of 21.75. If> however, the curious little Olenelloides be
omitted, the average for the thorax rises to 22.14 ancl the total to 23.84. Olenelloides is,
in fact, very probably the young of an Olcnellus. Specimens are only 4.5 to n mm. long,
and occur in the same strata with Olenellus (see Beecher 1897 A, p. 191).

Thirty-three genera from the Middle Cambrian afford data as to the number of seg-
ments, the Agnostidas being excluded. The extreme of variation there is smaller than
in the Lower Cambrian. The number of thoracic segments varies from 2 in Pagetia to
25 in Acrocephalites, and these same genera show the greatest range in total number of trunk
segments, 8 and 29 respectively.

The average of thoracic segments for the entire thirty-three genera is 10.5, of pygidial
segments 5.9, a total average of 16.4. It will be noted that the thorax shows on the average
less and the pygidium more segments than in the Lower Cambrian. If the Agnostidae could be
included, this result would doubtless be still more striking. Of the genera considered,
Asaphiscus with 7-11 thoracic and 5-8 pygidial segments, Blainia with 9 thoracic and 6-n
pygidial, Zacanthoidcs with 9 thoracic and 5 pygidial, and Anomocare with n thoracic and
7-8 pygidial segments came nearest to the average. Only a few departed widely from it.
The genera tabulated were Acrocephalites, Alokistocarc, Crepicephdus, Karlia, Hamburgia,
Corynexochus, Bathyuriscus, Poliella, Agraulos, Dolichometopus, Ogygopsis, Orria, Asaphis-
cus, Ncolenus, Burlingia, Blainia, Blountia, Marjumia, Pagetia, Eodiscus, Goniodiscus, Albcrt-
ella, Oryctocara, Zacanthoidcs, Anomocare, Anomocarella, Coosia, Conocoryphe, Ctenoce-
phalus, Parddoxidcs, Ptychoparia, Sao, and Ellipsocephalus.

Enough genera of Upper Cambrian trilobites are not known from entire specimens to
furnish satisfactory data. Excluding from the list the Proparia recently described by Wal-
cott, the average total trunk segments in ten genera is 18, but as most of the genera are
Olenidse or olenid-like, not much weight can be attached to these figures.

For the Cambrian as a whole, the average for sixty-two genera is between 17 and 18
trunk segments, which is surprisingly like the result obtained by Carpenter from only twelve

13° THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOB1TES.

genera, and tends to indicate that it must be somewhere near the real average. If the 5
or 6 segments of the head be added, it appears that the "average" number of segments is
very close to the malacostracan number 21. Genera with 16 to 18 trunk segments are Cal-
lavia, Protypus, Bathynotus, Crepiccphalus, Bathyuriscus, Ogygopsis, Burlingia, Orria, Asa-
pliiscus, Blainia, Zacanthoides, Neolenus, Anomocare, Conocoryphe, Saukia, Olcnus, and
Eurycare.

The order Proparia originated in the Cambrian, and Walcott has described four genera,
one from the Middle, and three from the Upper. The number of segments in these genera
is of interest. Burlingia, the oldest, has 14 segments in the thorax and i in the pygidium.
Of the three genera in the Upper Cambrian, Norii'oodia has 8-9 segments in the thorax and
3-4 in the pygidium ; Millardia 23 in thorax and 3-4 in pygidium ; and Mcnomonia 42 in thorax
and 3-4 in pygidium. It is of considerable interest and importance to note that the very elon-
gate ones are not from the Middle but from the Upper Cambrian.

Forty genera of Ordovician trilobites known from entire specimens were tabulated, and
it was found that the range in the number of segments in the thorax and pygidium was
surprisingly large. Agnostus, which was not included in the table, has the fewest, and
Eoharpes, with 29, the most. While the range in number of segments in the thorax is
2 to 29, the range of the number in the pygidium, 2 to 26, is almost as great. A species
of Dionide has 26 in the pygidium, while Remopleuridcs and Glaphurus have evidence of
only 2. The average number of segments in the thorax for the forty genera was 10.15, m
the pygidium 8.81, and the average number for the trunk 19.

Genera with just 19 segments in the trunk appear to be rare in the Ordovician, a
species of Ampyx being the only one I have happened to notice. Calymene, Tretaspis, Triar-
thrus, Asaphus, Ogygites, and Goldius come with the range of 18 to 20. Goldius, with
10 segments in the thorax and (apparently) 8 in the pygidium, comes nearest to the averages
for these two parts of the trunk. Goldius, Amphilichas, Bmnastus, Acidaspis, Actinopeltis,
and Spharexochus are among the genera having 10 segments in the thorax, and there are
many genera which have only one or two segments more or less than 10.

In most Ordovician genera, thirty-five out of the forty tabulated, the number of seg-
ments in the thorax is fixed, and the variation is in any case small. In four of the five
genera where it was not fixed, there was a variation of only one segment, and the greatest
variation was in Pliomcrops, where the number is from 15 to 19. This of course indicates
that the number of segments in the thorax tends to become fixed in Ordovician time. The
variation in the number of segments in the pygidium is, however, considerable. It is
difficult in many cases to tell how many segments are actually present in this shield, as it
is more or less smooth in a considerable number of genera. Extreme cases of variation
within a genus are found in Encrinurus, species of which have from 7 to 22 segments in
the pygidium, Cybcloides with 10 to 20, and Dionide with 10 to 26. As the number in the
thorax became settled, the number in the pygidium became more unstable, so that not
even in the Ordovician can the total number of segments in the trunk be said to show any
tendency to become fixed.

The genera used in this tabulation were : Eoharpes, Cryptolithus, Tretaspis, Trinuclcus,
Dionide, Raphiophorus, Ampy.r, Endymionia, Anisonotus, Triarthrus, Remopleuridcs, Bath-
yurus, Bathyurcllus, Ogygiocaris, Asaphus, Ogygites, Isotelus, Goldius, Cyclopyge, Amphili-
chas, Odontopleura, Acidaspis, Glaphurus, Encrinurus, Cybelc, Cybeloides, Ectenonotus,
Calymene, Ceraurus, Pliomera, Pliomerops, Pterygometopus, Chasmops, Eccoptochile, Acti-
nopeltis, Sphcerexochus, Placoparia, Pilckia, Selenopeltis, and Calocalymene.

SEGMENTS IN TRUNK.

Only sixteen genera of Devonian trilobites were available for tabulation, and it is not
always possible to ascertain the exact number of segments in the pygidium, although genera
with smooth caudal shields had nearly all disappeared. The number of segments in the thorax
had become pretty well fixed by the beginning of the Devonian, Cyphaspis with a range of
from 10 to 17 furnishing the only notable exception. The range for the sixteen genera is
from 8 to 17, the average u, the number exhibited by the Phacopid?e which form so large
a part of the trilobites of the Devonian. The greater part of the species have large pygidia,
and while the range is from 3 to 23, the average is 11.2. Probolium, with n in the
thorax and 11-13 m tne pygidium, and Phacops, with n in the thorax and 9-12 in the
pygidium, approach very closely to the "average" trilobite, and various species of other
genera of the Phacopidje have the same number of segments as the norm. In every genus,
however, the number of segments in the pygidium is variable, the greatest variation being
in Ddmanites, with a range of from 9 to 23. The number of segments in the pygidium
was therefore not fixed and was on the average higher than in earlier periods.

The genera used in the tabulation were : Calymcne, Dipleura, Goldius, Proetus, Cyphas-
pis, Acidaspis, Phacops, Hausinania, Coronura, Odontochile, Pleur acanthus, Calmonia, Pen-
naia, Dalnianites, Probolium, and Cordania.

The trilobites of the late Paheozoic (Mississippian to Permian) belong, with two pos-
sible exceptions, to the Proetidrc, and only three genera, Proetus, Phillipsia, and Griffithides,
appear to be known from all the parts. I am, however, assuming that both Brachymetopus
and Anisopyge have 9 segments in the thorax, and so have tabulated five genera. The
range in the number of segments in the pygidium is large, from 10 in some species of
Proetus to 30 in Anisopyge, and the average, 17.3, is high, as is the average for total num-
ber in the trunk, 26.3. Anisopyge, a late Permian trilobite described by Girty from Texas,
is perhaps the last survivor of the group. It seems to have had 39 segments in the trunk,
making it, next to the Cambrian Pcedeumias and Menomonia, the most numerously segmented
of all the trilobites.

The above data may be summarized in the following table :

S

c

u
S

0)

Jl

C

V

1

y

Period

g

8

A

IM

O

X

°l

o

Sg

•o

o 2

X j=

z ^

•1

6

>• *-*

.' °-

> 1

< .5

< .a

Lower Cambrian

19

13-9

3.7

17.6

Middle Cambrian

33

10.5

s-9

16.4

Entire Cambrian

62

17-19

Ordovician

40

10.15

8.81

18.96

Devonian

16

II

II. 2

22.2

Late Paleozoic

S

9

17-3

26.3

This table confirms that made up by Carpenter, and shows even more strikingly the
progressive increase in the average number of segments in the trunk throughout the Pake-
ozoic.

While the two trilobites with the greatest number of segments are Cambrian, yet on the
average, the last of the trilobites had the more numerously segmented bodies. The multi-
segmented trilobites are:

132

THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

c

C

c

6

6

4)

B

S

be M 5

M

Period

Genus

$ X

cd

« "-5

'o o

•88

'o =

. a

c

o

6

f, £

X .B

Z .5

Lower Cambrian

Pcedeumias

44+

I

45+

Upper Cambrian

Menomonia

42

4

46

Ectenonotus

12

22

34

Ordovician

Encrinurus

II

22

33

Diomde

6

26

32

Silurian

Harpes

29

3

32

Devonian

Coronura

ii

23

34

Dalmanites

ii

23

34

Permian

Anisopyge

7+(9?)

30

39?

Anisopyge, the last of the trilobites, stands third on the list of those having great
numbers of segments, and in each period there are a few which have considerably more
than the average number. It may be of some significance that of these nine genera only
Pcedeumias and Anisopyge belong to the Opisthoparia, the great central group, and that five
are members of the Proparia, the latest and most specialized order.

FORM OF THE SIMPLEST PROTASPIS.

It would naturally be expected that the young of the Cambrian trilobites should be
more primitive than the young of species from later formations, and Beecher (1895 C)
has shown that this is the case. He had reference, however, chiefly to the eyes, free cheeks,
and spines, and by comparison of ontogeny and phylogeny, demonstrated the greater sim-
plicity of the protaspis which lacked these organs. It remains to inquire which among the
other characteristics are most fundamental.

Among the trilobites of the Lower Cambrian, no very young have been seen except
of Mesonacidae. Of these, the ontogeny of Elliptocephala asaplioides Emmons is best known,
thanks to Ford, Walcott, and Beecher, but, as the last-named has pointed out, the actual
protaspis or earliest shield has not yet been found. The youngest specimen is the one roughly
figured by Beecher (1895 C, p. 175, fig. 6). It lacks the pygidium, but if completed by
a line which is the counterpart of the outline of the cephalon, it would have been 0.766 mm.
long. The pygidium would have been 0.183 mm. long, or 23 per cent of the whole length.
The axial lobe was narrow, of uniform width along the cephalon, showed a neck-ring and
four indistinct annulations, but did not reach quite to the anterior end, there being a mar-
gin in front of the glabella about o.i mm. wide. The greatest width of the cephalon was
0.66 mm., and of the glabella 0.233 mm., or practically 35 per cent of the total width.
Other young Elliptocephala up to a length of i mm., and young Pcedeumias, Mcsonacis, and
Holmia (see Kiaer, Videnskaps, Skrifter, I Mat.-Naturv. Klasse, 1917, No. 10) show about
the same characteristics, but all these have large compound eyes on the dorsal surface and
specimens in still younger stages are expected. It may be pointed out, however, that in
these specimens the pygidium is proportionately larger than in the adult. Walcott cites one
adult 126 mm. long in which the pygidium is 6 mm. long, or between 4 and 5 per cent of
the total length, while in the incomplete specimen described above, it was apparently 23 per

THE PROTASPIS. 133

cent. In a specimen i mm. long figured by Walcott, the pygidium is 0.15 mm. long, or 15
per cent of the whole length.

The development of several species of trilobites from the Middle Cambrian is known.
Barrande (1852) described the protaspis of Sao hirsutk, Peronopsis integer, Phalacroma bib-
ullatwn, P. nudum, and Condylopyge rex. Broegger figured that of a Liostracus (Geol. For.
Forhandl., 1875, pi. 25, figs,. 1-3) and Lindstroem (1901, p. 21) has reproduced the same.
Matthew (Trans. Roy. Soc. Canada, vol. 5, 1888, pt. 4, pis. i, 2) has described the pro-
taspis of a Liostracus, Ptychoparia linnarssoni Broegger, and Soleno pleura robbi Hartt.
Beecher (1895 C, pi. 8) has figured the protaspis of Ptychoparia kingi Meek, and the writer
that of a Paradoxides (Bull. Mus. Comp. Zool., vol. 58, No. 4, 1914, pi. i).

Sao, Liostracus, Ptychoparia, and Solenopleura all have the same sort of protaspis. In
all, the axial lobe reaches the anterior margin and is somewhat expanded at that end ; in all,
the glabella shows but slight trace of segmentation; and in all, the pygidium occupies from
one fifth to one fourth the total length. There is considerable variation in the width of
the axial lobe. It is narrowest in Ptychoparia, -where in the middle it is only 14 per cent
of the whole width, and widest in Solenopleura, where it is 28 per cent. In Ptychoparia
the pygidium of the protaspis occupies from 18 to 22 per cent of the whole length. In
the adult it occupies 10 to 12 per cent. In Solenopleura it makes up about 26 per cent of
the protaspis, and in the adult about 8 per cent.

In the youngest stages of all these trilobites, the pygidium is incompletely separated
from the cephalon. The first sign of segmentation is a transverse crack which begins to
separate the cephalon and pygidium, and by the time this has extended across the full width
the neck segment has become rather well defined. In this stage the animal is prepared to
swim by means of the pygidium, and first becomes active. The coincident development of
the free pygidium and the neck-ring strongly suggests that the dorsal longitudinal muscles
are attached beneath the neck- fur row.

The single protaspis of Paradoxides now known, while only i mm. long, is not in the
youngest stage of development. It is like the protaspis of Olenellus in having large eyes
on the dorsal surface and a narrow brim in front of the glabella. The glabella is nar-
rower than in the adult.

The initial test of no agnostid has probably as yet been seen, as all the young now
known show the cephalon and pygidium distinctly separated. Phalacroma bibullatum and
P. nudum are both practically smooth and isopygous when 1.5 mm. long. P. bibullatum
shows no axial lobe at this stage, but a wide glabella and median tubercle develop later,
and when the glabella first appears, it extends to the anterior margin. In Peronopsis integer
and Condylopyge rex, the axial lobe is outlined on each of the equal shields in specimens
about i mm. long, but is without furrows and reaches neither anterior nor posterior margin.

From the foregoing brief description it appears that the pygidium of the protaspis
varies in different groups from as little as 15 per cent of the total length in the Mesonacidae
to as much as 50 per cent in the Agnostidae; that the axial lobe varies from as little as 14
per cent of the total width in one Ptychoparia to as much as 50 per cent in Phalacroma
nudum; that the glabella reaches the anterior margin in the Olenidse, Solenopleuridse, and
Phalacroma bibullatum, while there is a brim in front of it in the Olenellidse, Paradox-
ids, and three of the species of the Agnostidae. The decision as to which of these conditions
are primitive may be settled quite satisfactorily by study of the ontogeny of the various
species.

134

THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

ORIGIN OF THE PYGIDIUM.

Taking first the pygidium, it has already been pointed out that in each case the pygidium
of the adult is proportionally considerably smaller than the pygidium of the protaspis. The
stages in the growth of the pygidium are better known in Sao hirsuta than in any other
trilobite, and a review of Barrande's description will be advantageous.

Barrande recognized twenty stages in the development of this species, but there was
evidently a still simpler protaspis in his hands than the smallest he figured, for he says,
after describing the specimen in the first stage : "We possess one specimen on which the
head extends from one border to the other of the disk, but as this individual is unique
we have not thought it sufficient to establish a separate stage." This specimen is important
as indicating a stage in which there was not even a suggestion of division between cephalon
and pygidium.

In the first stage described by Barrande, the form is circular, the length is about 0.66 mm.,
and the glabella is narrow with parallel sides and no indications of lateral furrows. The
neck segment is indicated by a slight prominence on the axial lobe, and back of it a con-
striction divides the axial lobe of the pygidium into two nodes, but does not cross the
pleural lobes. The position of the nuchal segment permits a measurement of the part which
is to form the pygidium, and shows that that shield made up 30 per cent of the entire
length.

In the second stage, when the test is 0.75 mm. long, the cephalon and pygidium become
distinctly separated, and the latter shield shows 'three annulations on the axial and two
pairs of ribs on the pleural lobes. It now occupies 33^ per cent of the total length.

In the third stage, when the total length is about i mm., the pygidium has continued
to grow. It now shows five annulations on the axial lobe, and is 46 per cent of the total
length.

In the fourth stage, two segments of the axial lobe have been set free from the front
of the pygidium. The length is now 1.5 mm. and the pygidium makes up 32 per cent of the
whole. From this time the pygidium continues to decrease in size in proportion to the total
length, as shown in the following table.

Stage

Length in
mm.

Percentage
of pygidium

Segments in
thorax

Segments in
pyfjidium

I

0.66 30 o

2

2

0-75 331A 0

3

3

1. 00 46 0

5

4

1.50

32 2

5-6

5

I.SO

25 3

4

6

i-75

23 4

4

7

i. 80

21

5

3.

8

2.00 17

6

3

9 2.50

13

7

3

10 3.00

12

8

3

ii

3-50

II

9

3-4

12

4.00

II

10

3-4

13 5-00

10

ii

3

14 5-50

9

12

2-4

15 6. oo

8

13

3-4

16

6.50

8

14

3

17 7.00 7

15

3

18 7-50 7

16

3

19 7.50 6

17

2

20

10-25

6

17

2

THE PROTASPIS. 135

This table shows the rapid increase in the length of the pygidium till the time when
the thorax began to be freed, the very rapid decrease during the earlier part of its forma-
tion until six segments had been set free, and then a more gradual decrease until the entire
seventeen segments had been acquired, after which time the relative length remained constant.
From an initial proportion of 30 per cent, it rose to nearly one half the whole length,
and then dwindled to a mere 6 per cent, showing conclusively that the thorax grew at the
expense of the pygidium.

If this conclusion can be sustained by other trilobites, it indicates that the large pygid-
ium is a more primitive characteristic of a protaspis than is a small one. I have already
shown that the pygidium is proportionately larger in the protaspis in the Mesonacidae, Soleno-
pleuridas, and Olenidse, and a glance at Barrande's figures of "flydrocephalus" carens and
"///' saturnoides, both young of Paradoxides. will show that the same process of develop-
ment goes on in that genus as in Sao. There is first an enlargement of the pygidium to
a maximum, a rise from 20 per cent to 33 per cent in the case of H. carens and then, with
the introduction of thoracic segments, a very rapid falling off. All of these are, however,
trilobites with small pygidia, and it has been a sort of axiom among palaeontologists that
large pygidia were made up of a number of coalesced segments. While not definitely so
stated, it has generally been taken to mean the joining together of segments once free. The
asaphid, for instance, has been thought of as descended from some trilobite with rich seg-
mentation, and a body-form like that of a Mcsonads or Paradoxides.

The appeal to the ontogeny does not give as full an answer to this question as could
be wished, for the complete life-history of no trilobite with a large pygidium is yet known:
While the answer is not complete, enough can be gained from the study of the ontogeny of
Dalmanitcs and Cydopyge to show that in these genera also the thorax grows by the break-
ing down of the pygidium and that no segment is ever added from the thorax to the pygid-
ium. The case of Dahnanites socialis as described by Barrande (1852, p. 552, pi. 26) will
be taken up first, as the more complete. The youngest specimen of this species yet found
is 0.75 mm. long, the pygidium is distinctly separated from the cephalon, and makes up 25
per cent of the length. This is probably not the form of the shell as it leaves the egg. At
this stage there are two segments in the pygidium, but they increase to four when the test
is i mm. long. The cephalon has also increased in length, however, so that the proportional
length is the same. The subjoined table, which is that compiled by Barrande with the pro-
portional length of the pygidium added, is not as complete as could be desired, but affords
a very interesting history of the growth of the caudal shield. The maximum proportional
length is reached before the introduction of thoracic segments, and during the appearance of
the first five segments the size of the pygidium drops from 25 to 15 per cent. Several
stages are missing at the critical time between stages 8 and 9 when the pygidium had added
three segments to itself and has supplied only one to the thorax. This would appear to
have been a sort of resting or recuperative stage for the pygidium, for it increased its
own length to 20 per cent, but from this stage up to stage 12 it continued to give up seg-
ments to the thorax and lose in length itself. After stage 12, when the specimens were
8 mm. long, no more thoracic segments were added, but new ones were introduced into
the pygidium, until it reached a size equal to one fifth the entire length, as compared with
one fourth in the protaspis.

1 36

THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

Stage

Length in
mm.

Percentage
of pygidium

SL-JJIMCIUS in Segments in
thorax pygidium

I

0.75

25

0

2

2

0.75

25

0

3

3

I.OO

25

0

4

4

1. 00

22

I

3

5

1-25

20

2

3

6

1. 25

18

3

. 3

7

1. 60

IS 4

3

8

1. 60

IS

5

3

9

3-oo

20

6

6

10

3-So

20 7

6

ii

8.00

18

9

7

12

8.00

16

ii

5

13

12. OO l6

ii

7

14

19.00 18

ii

9

IS

95.00

20

ii

ii

Since the above was written, Troedsson (1918, p. 57) has described the development
of Dalmanites eucentrus, a species found in the Brachiopod shales (Upper Ordovician) of
southern Sweden. This species follows a course similar to that of D. socialis, so that the
full series of stages need not be described. The pygidium is, however, of especial interest,
for there is a stage in which it shows two more segments than in the adult. Troedsson
figures a pygidium 1.28 mm. long which has eight pairs of pleural ribs, while the adult
has only six pairs. The ends of all these ribs are free spines, and were the development
not known one would say that this was a case of incipient fusion, while as a matter of fact,
it is incipient freedom.

A further interest attaches to this case, because of the close relationship between
D. eucentrus and D. mucronatus. The latter species appears first in the Staurocephalus
beds which underlie the Brachiopod shales, so that in its first appearance it is somewhat the
older. The pygidium of the adult D. mucronatus is larger than that of D. eucentrus, having
eight pairs of pleural ribs, the same number as in the young of the latter. In short, D. eucen-
trus is probably descended from D. mucronatus, and in its youth passes through a stage in
which it has a large pygidium like that species. Once more it appears that the small pygidium
is more specialized than the large one.

The full ontogeny of Cyclapyge is not known, but young specimens show conclusively
that segments are not transferred from the thorax to the pygidium, but that the opposite
occurs. As shown by Barrande (1852) and corroborated by specimens in the Museum of
Comparative Zoology, the process is as follows : The third segment of the adult of this
species, that is, the fourth from the pygidium, bears a pair of conspicuous cavities on the
axial portion. In a young specimen, 7 mm. long, the second segment bears these cavities,
but as the thorax has only four segments, this segment is also the second instead of the
fourth ahead of the pygidium. The pygidium itself, instead of being entirely smooth, as
in the adult state, is smooth 'on the posterior half, but on the anterior portion has two well
formed but still connected segments, the anterior one being more perfect than the other.
These are evidently the two missing segments of the thorax, and instead of being in the
process of being incorporated in the pygidium, they are in fact about to be cast off from it
to become free thoracic segments. In other words, the thorax grows through the degener-

THE PROTASPIS. 137

ation of the pygiclium. That the thorax grows at actual expense to the pygidium is shown
by the proportions of this specimen. In an adult of this species the pygidium, thorax, and
cephalon are to each other as 9:11 : 13. In the young specimen they are as 10 : 6 : 12,
the pygidium being longer' in proportion both to the thorax and to the cephalon than it
would be in the adult.

This conception of the breaking down of the pygidium to form the thorax will be very
helpful in explaining many things which have hitherto seemed anomalous. For instance,
it indicates that the Agnostida;, whose subequal shields in early stages have been a puzzle,
are really primitive forms whose pygidia do not degenerate ; likewise the Eodiscidse, which,
however, show within the family a tendency to free some of the segments. The annelidan
Mesonacidae may not be so primitive after all, and their specialized cephala may be more
truly indicative of their status than has previously been supposed.

The facts of ontogeny of trilobites with both small and large pygidia do show that
there is a reduction of the relative size of the caudal shield during the growth-stages, and
therefore that the large pygidium in the protaspis is probably primitive. The same study
also shows that the large pygidium is made up of "coalesced segments" only to the extent
that they are potentially free, and not in the sense of fused segments.

WIDTH OF THE AXIAL LOBE.

That the narrow type of axial lobe is more primitive than the wide one has already
been demonstrated by the ontogeny of various species, and space need not be taken here
to discuss the question. Most Cambrian trilobites have narrow axial lobes even in the
adult so that their development does not bring this out very strikingly, though it can be
seen in Sao, Ptychoparia, etc., but 'in Ordovician trilobites such as Triarthrus and especially
Isotelus, it is a conspicuous feature.

PRESENCE OR ABSENCE OF A "BRIM."

That the extension of the glabella to the front of the cephalon is a primitive feature
is well shown by the development of Sao (Barrande, 1852, pi. 7), Ptychoparia (Beecher,
1895 C, pi. 8), and Parado.rides (Raymond, Bull. Mus. Comp. Zool., vol. 57, 1914), although
in the last genus the protaspis has a very narrow brim, the larva during the stages of intro-
duction of new segments a fairly wide one, and most adults a narrow one.

The brim of Sao seems to be formed partly by new growth and partly at the expense
of the frontal lobe, for that lobe is proportionately shorter in the adult than in the protas-
pis. In Cryptolithus and probably in Harpes, Harpides, etc., the brim is quite obviously
new growth and has nothing to do with the vital organs. Its presence or absence may not
have any great significance, but when the glabella extends to the frontal margin, it certainly
suggests a more anterior position of certain organs. In Sao, the only trilobite in which
anything is known of the position of the hypostoma in the young, the posterior end is con-
siderably further forward in a specimen a. 5 mm. long than in one 4 mm. long, thus indi-
cating a backward movement of the mouth during growth, comparable to the backward move-
ment of the eyes.

SEGMENTATION OF THE GLABELLA.

The very smallest specimens of Sao show a simple, unsegmented axial lobe, and the
same simplicity has been noted in the young of other genera. Beecher considered this as

138 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

due to imperfect preservation of the exceedingly small shells, which practically always occur
as moulds or casts in soft shale. There is, however, a very general increase in the strength
of glabellar segmentation in the early part of the ontogeny of all trilobites whose life his-
tory is known, and in some genera, like the Agnostid;e, there is no question of the compara-
tively late acquisition of glabellar furrows. Even in Paradoxidcs, the furrows appear late
in the ontogeny.

Summary.

If absence of eyes on the dorsal surface be primitive, as Beecher has shown, and if
the large pygidium, narrow axial lobe, and long unsegmented glabella be primitive, then
the known protaspis of the Mesonacidae and Paradoxidre is not primitive, that of the Olen-
idse is very primitive, and that of the Agnostidre is primitive except that in one group the
axial lobe, when it appears, is rather wide, and in the other a brim is present.

Fig- 35- — A specimen of Wey-
mouthia nobilis (Ford), col-
lected by Mr. Thomas H. Clark
at North Weymouth, Mass.
Note the broad smooth shields
of this Lower Cambrian eodis-
«id. X 6.

Subsequent development from the simple unsegmented protaspis would appear to show,
first, an adaptation to swimming by the use of the pygidium; next, the invagination of the
appendifers as shown in the segmentation of the axial lobe indicates the functioning of the
appendages as swimming legs; then with the introduction of thoracic segments the assump-
tion of a bottom-crawling habit is indicated. Some trilobites were fully adapted for bottom
life, and the pygidium became reduced to a mere vestige in the production of a worm-like
body. Other trilobites retained their swimming habits, coupled with the crawling mode of
life, and kept or even increased (Isotelus) the large pygidium.

THE SIMPLEST TRILOBITE.

In the discussion above I have placed great emphasis on the large size of the primi-
tive pygidium, because, although there is nothing new in the idea, its significance seems to
have been overlooked.

If the large pygidium is primitive, then multisegmentation in trilobites can not be primi-
tive but is the result of adaptation to a crawling life. It is annelid-like, but is not in itself
to be relied upon as showing relationship to the Quctopoda. Simple trilobites with few seg-
ments, like the Agnostidse, Eodiscidae, etc., were, therefore, properly placed by Beecher at

THE SIMPLEST TRILOBITE. 139

the base of his classification, and there is now less chance than ever that they can be called
degenerate animals.

From the phylogeny of certain groups, such as the Asaphidre, it is learned that the geo-
logically older members of the family have more strongly segmented anterior and posterior
shields than the later ones. That there has been a "smoothing out" is demonstrated by
a study of the ontogeny of the later forms. From such examples it has come to be thought
that all smooth trilobites are specialized and occupy a terminal position in their genealogi-
cal line. This has caused some wonder that smooth agnostids like P halacr oma bibullatutn
and P. niidutn should be found in strata so old as the Middle Cambrian, and was a source
of great perplexity to me in the case of Weymouthia (Ottawa Nat., vol. 27, 1913) (fig.
35). This is a smooth member of the Eodiscidre, and, in fact, one of the simplest trilo-
bites known, for while it has three thoracic segments, it shows almost no trace of dorsal fur-
rows or segmentation on cephalon or pygidium, and, of course, no eyes. Following the
general rule, I took this to be a smooth-out eodiscid, and was surprised that it should come
from the Lower Cambrian, where it is associated with Elliptocephala at Troy, New York,
and with Callaz'ia at North Weymouth, Massachusetts, and where it has lately been found
by Kircr associated with Holmia and Kjerulfia at T<£mten, Norway. It now appears it is
really in its proper zone, and instead of being the most specialized, is the simplest of the
Eodiscid;t.

What appears to be a still simpler trilobite is the form described by Walcott as Naraoia.

Naraoia compacta Walcott.
(Text fig. 36.)

Illustrated : Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 175, pi. 28, figs. 3, 4. — Cleland, Geology,
Physical and Historical, New York, 1916, p. 412, fig. 382 F (somewhat restored).

This very imperfectly known form is referred by Walcott to the Notostraca on
what appear to be wholly inadequate grounds, and while I do not insist on my interpreta-
tion, I can not refrain from calling attention to the fact that it can be explained as the
most primitive of all trilobites. It consists of two subequal shields, the anterior of which
shows slight, and the posterior considerable evidence of segmentation. It has no eyes, no
glabella, and no thorax, and is directly comparable to a very young Phalacroma bibullatum
(see Barrande 1852, pi. 49, figs, a, b). Walcott states that there is nothing to show how
many segments there are in the cephalic shield, but that on one specimen fourteen were
faintly indicated on the abdominal covering. The appendages are imperfectly unknown, as
no specimen showing the ventral side has yet been described. The possible presence of
antennas and three other appendages belonging to the cephalic shield is mentioned, and there
are tips of fourteen legs projecting from beneath the side of one specimen. As figured,
some of the appendages have the form of exopodites, others of endopodites, indicating that
they were biramous.

Naraoia is, so far as now known, possessed of no characteristics which would prevent
its reference to the Trilobita, while the presence of a large abdominal as well as a cephalic
shield would make it difficult to place in even so highly variable a group as the Branchi-
opoda. On the other hand, its only exceptional feature as a trilobite is the lack of thorax,
and all study of the ontogeny of the group has led us to expect just that sort of a trilo-
bite to be found some day in the most ancient fossiliferous rocks. Naraoia can, I think.

140 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

be best explained as a trilobite which grew to the adult state without losing its protaspian
form. It was found in the Middle Cambrian of British Columbia.

Even if Naraoia should eventually prove to possess characteristics which preclude the
possibility of its being a primitive trilobite, it at least represents what I should expect a
pre-Cambrian trilobite to look like. What the ancestry of the nektonic primitive trilobite
may have been is not yet clear, but all the evidence from the morphology of cephalon, pygid-
ium, and appendages indicates that it was a descendant of a swimming and not a crawling
organism.

Since the above was written, the Museum of Comparative Zoology has purchased a
specimen of this species obtained from the original locality. The shields are sub-equal, the
posterior one slightly the larger, and the axial lobes are definitely outlined on both. The
glabella is about one third the total width, nearly parallel-sided, somewhat pointed at the
front. There are no traces of glabellar furrows. The axial lobe of the pygidium is also
about one third the total width, extends nearly to the posterior margin, and has a rounded
posterior end. The measurements are as follows : Length, 33 mm. ; length of cephalon,

16 mm., width, 15 mm.; length of glabella, 11.5 mm., width, 5.5 mm.; length of pygidium,

17 mm., width, 15 mm.; length of axial lobe, 14 mm., width, 5.5 mm.

The species is decidedly Agnostus-like in both cephalon and pygidium, and were it not
so large, might be taken for the young of such a trilobite. The pointed glabella is com-
parable to the axial lobes of the so called pygidia of the young of Condylopyge rex and Pero-
nopsis integer (Barrande, Syst. Sil., vol,. i, pi. 49).

THE ANCESTOR OF THE TRILOBITES, AND THE DESCENT OF THE ARTHROPODA.

The "annelid" theory of the origin of the Crustacea and therefore of the trilobites,
originating with Hatschek (1877) and so a^Y championed by Bernard (1892), has now
been a fundamental working hypothesis for some years, and has had a profound influence in
shaping thought about trilobites. This hypothesis has, however, its weak points, the prin-
cipal one being its -total inhibition of the workings of that great talisman of the palaeontol-
ogist, the law of recapitulation. Its acceptance has forced the zoologist to look upon the
nauplius as a specially adapted larva, and has caused more than one forced explanation of
the protaspis of the trilobite. When so keen a student as Caiman says that the nauplius
must point in some way to the ancestor of the Crustacea (1909, p. 26), it is time to reex-
amine some of the fundamentals. This has been done in the preceding pages and evidence
adduced to show that the primitive features of a trilobite indicate a swimming animal, and
that the adaptations are those which enabled it to assume a crawling mode of existence.
It has also been pointed out that in Naraoia there is preserved down to Middle Cambrian
times an animal like that to which ontogeny points as a possible ancestor of the trilobites.
Naraoia is not the simplest conceivable animal of its own type, however, for it has built
up a pygidium of fourteen or fifteen somites. One would expect to find in Proterozoic sedi-
ments remains of similar animals with pygidia composed of only one or two somites, with
five pairs of appendages on the cephalon, one or two pairs on the pygidium, a ventral mouth,
and a short hypostoma. Anything simpler than this could not, in my opinion, be classed as
a trilobite.

What the ancestor of this animal was is mere surmise. It probably had no test, and
it may be noted in this connection that Naraoia had a very thin shell, as shown by its

THE ANCESTOR OF THE TRILOBITE. I41

state of preservation, and was in that respect intermediate between the trilobite and the
theoretical ancestor. Every analysis of the cephalon of the trilobite shows that it is made
up of several segments, certainly five, probably six, possibly seven. Every study of the tril-
obite, whether of adult, young, or protaspis, indicates the primitiveness of the lateral exten-
sions or pleural lobes. The same studies indicate as clearly the location of the vital organs
along the median lobe. These suggestions all point to a soft-bodied, depressed animal composed
of few segments, probably with simple marginal eyes, a mouth beneath the anterior margin,
tactile organs at one or both ends, with an oval shape, and a straight narrow gut running
from anterior mouth to terminal anus. The broad flat shape gives great buoyancy and is
frequently developed in the plankton. Inherited by the trilobites, it proved of great use to
the swimmers among them.

The known animal which most nearly approaches the form which I should expect the
remote ancestor of the trilobites to have had is Amiskwia sagittiformis Walcott (Smith-
son. Misc. Coll., vol. 57, 1911, p. 112, pi. 22, figs. 3, 4). This "worm" from the Middle
Cambrian is similar in outline to the recent Spadella, and is referred by Walcott to the
Chjetognatha. It has a pair of lateral expansions and a flattened caudal fin, a narrow
median alimentary canal, and a pair of rather long simple tentacles. With the exception
of a thin septum back of the head, no traces of segmentation are shown.

Some time in the late pre-Cambrian, the pre-trilobite, which probably swam by rhyth-
mic undulations of the body, began to come into occasional contact with a substratum, and
two things happened : symmetrically placed, i. e., paired, appendages began to develop on the
contact surface, and a test on the dorsal side. The first use of the appendages may have
been in pushing food forward to the mouth, and for the greater convenience in catching
such material, a fold in front of the mouth may have elongated to form the prototype of
the hypostoma. At this time the substratum may not have been the ocean bottom at all,
but the animals, still free swimmers, may have alighted at feeding time on floating algae
from the surface of which they collected their food. While the dorsal test was originally
jointed at every segment, the undulatory mode of swimming seems to have given way to the
method of sculling by means of the posterior end only, or by the use of the appendages, and
the anterior segments early became fused together.

The result of the hardening of the dorsal test was of course to reduce to that extent
the area available for respiration, and this function was now transferred in part to the limbs,
which bifurcated, one branch continuing the food-gathering process and the other becom-
ing a gill. The next step may have been the "discovery" of the ocean bottom and the
tapping of an hitherto unexploited supply of food. Upon this, there set in those adapta-
tions to a crawling mode of existence which are so well shown in the trilobite. The crawl-
ing legs became lengthened and took on a hardened test, the hypostoma was greatly elongated,
pushing the mouth backward, and new segments were added to produce a long worm-like
form which could adapt itself to the inequalities of the bottom. That the test of the appen-
dages became hardened later than that of the body is shown by the specimens of Neolcnus,
in which the dorsal shell as preserved in the shale is thick and solid, while the test of the
appendages is a mere film.

The late Proterozoic or very earliest Cambrian was probably the time of the great
splitting up into groups. The first development seems to have been among the trilobites
themselves, the Hypoparia giving rise to two groups with compound eyes, first the Opis-
.thoparia and later the Proparia. About this same time the Copepoda may have split off

142 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

from the Hypoparia, continuing in the pelagic habitat. At first, most of the trilobites seem
to have led a crawling existence, but about Middle Cambrian time they began to go back par-
tially to the ancestral swimming habits, and retained some of the trunk segments to form
a larger pygidium. The functional importance of the pygidium explains why it can not be
used successfully in making major divisions in classification. Nearly related trilobites may
be adapted to diverse methods of life.

EVOLUTION WITHIN THE CRUSTACEA.

The question naturally arises as to whether the higher Crustacea were derived from
some one trilobite, or whether the different groups have been developed independently from
different stocks. The opinion that all other crustaceans could have been derived from an
Apus-\ike form has been rather generally held in recent years, but Carpenter (1903, p. 334)
has shown that the leptostracan, Nebalia, is really a more primitive animal than Apus. He
has pointed out that in Leptostraca the thorax bears eight pairs of simple limbs with
lamelliform exopodites and segmented endopodites, while the abdomen of eight segments has
six pairs of pleopods and a pair of furcal processes, so that only one segment is limbless.
Contrasted with this are the crowded and complicated limbs of the anterior part of the
trunk of Apus, and the appendage-less condition of the hinder portion. Further, a compari-
son between the appendages of the head of Nebalia and those of Apus shows that the former
are the more primitive. The antennules of Nebalia are elongate, those of Apus greatly re-
duced ; the mandible of Nebalia has a long endopodite, and Carpenter points out that from it
either the malacostracan mandible with a reduced endopodite or the branchiopodan mandible
with none could be derived, but that the former could not have arisen from the latter.
The maxillae of Apus are also much the more specialized and reduced.

Nebalia being in all else more primitive than Apus, it follows that the numerous ab-
dominal segments of the latter may well have arisen by the multiplication of an originally
moderate number, and the last trace of primitiveness disappears.

It is now possible to add to the results obtained from comparative morphology the testi-
mony of palfeontology, already outlined above, and since the two are in agreement, it must
be admitted that the modern Branchiopoda are really highly specialized.

As has already been pointed out, Hymcnocans, the leptostracan of the Middle Cambrian,
has very much the same sort of appendages as the Branchiopoda of the same age, both
being of the trilobite type. Which is the more primitive, and was one derived from the
other?

The Branchiopoda were much more abundant and much more highly diversified in
Cambrian times than were the Leptostraca, and, therefore, are probably older. Some of the
Cambrian branchiopods were without a carapace, and some were sessile-eyed. These were
more trilobite-like than Hymcnocans. Many of the Cambrian branchiopods had developed
a bivalved carapace, though not so large a one as that of the primitive Leptostraca. The
present indications are, therefore, that the Branchiopoda are really older than the Leptos-
traca, and also that the latter were derived from them. It seems very generally agreed that
the Malacostraca are descended from the Leptostraca, and the fossils of the Pennsylvanian
supply a number of links in the chain of descent. Thus, Pygoccphalus cooperi, with its
brood pouches, is believed by Caiman (1909, p. 181) to stand at the base of the Peracaridan
series of orders, and Uronectes, Palccocaris, and the like are Paheozoic representatives of the
Syncarida. Others of the Pennsylvanian species appear to tend in the direction of the Sto-

THE EVOLUTION OF THE CRUSTACEA. 1 43

matopocla, whose true representatives have been found in the Jurassic. The Isopoda seem
to be the only -group of Malacostraca not readily connected up with the Leptostraca. Their
depressed form, their sessile-eyes, and their antiquity all combine to indicate a separate origin
for the group, and it has already been pointed out how readily they can be derived directly
from the trilobite.

While the Copepoda seem to have been derived directly from the Hypoparia, the re-
mainder of the Crustacea apparently branched off after the compound eyes became fully
developed, unless, as seems entirely possible, compound eyes have been developed indepen-
dently in various groups. Most Crustacea were derived from crawling trilobites (Lower
Cambrian or pre-Cambrian Opisthoparia), for they lost the large pygidium, and also the
major part of the pleural lobes. In all Crustacea, too, other than the Copepoda and Ostra-
coda, there is a tendency to lose the exopodites of the antenna?.

These modifications, which produced a considerable difference in the general appearance
of the animal, are easily understood. As has been shown in previous pages, the trilobites
themselves exhibit the degenerative effect on the anterior appendages of the backward move-
ment of the mouth, and the transformation of a biramous appendage with an endobase into
a uniramous antenna is a simple result of such a process. The feeding habits of the trilo-
bites were peculiar and specialized, and it is natural that some members of the group should
have broken away from them. In any progressive mode of browsing the hypostoma was a
hindrance, so was soon gotten rid of, and the endobases not grouped around the mouth
likewise became functionless. The chief factor in the development of the higher Crustacea
seems to have been the pinching claw, by means of which food could be conveyed to the
mouth. It had the same place in crustacean development that the opposable thumb is be-
lieved to have had in that of man.

An intermediate stage between the Trilobita and the higher Crustacea is at last exhibited
to us by the wonderful, but unfortunately rather specialized Marrella, already described.
It retains the hypostoma and the undifferentiated biramous appendages of the trilobite, but
has uniramous antennae, there are no endobases on the coxopodites of the thoracic appen-
dages, the pygidium is reduced to a single segment, and the lateral lobes of the thorax are
also much reduced. Marrella is far from being the simplest of its group, but is the only
example which survived even down to Middle Cambrian times of what was probably once
an important series of species transitional between the trilobites and the higher Crustacea.

In this theory of the origin of the Crustacea from the Trilobita, the nauplius becomes
explicable and points very definitely to the ancestor. According to Caiman (1909, p. 23) :

The typical nauplius has an oval unsegmented body and three pairs of limbs, corresponding to the anten-
nules, antennas, and mandibles of the adult. The antennules are uniramous, the others biramous, and all
three pairs are used in swimming. The antennx may have a spiniform or hooked masticatory process at the
base, and share with the mandibles which have a similar process, the function of seizing and masticating the
food. The mouth is overhung by a large labrum or upper lip and the integument of the dorsal surface of
the body forms a more or less definite dorsal shield. The paired eyes are as yet wanting, but the median eye is
large and conspicuous.

The large labrum or hypostoma, the biramous character of the appendages, especially
of the antennae, the functional gnathobases on the second and third appendages, and the
oval unsegmented shield are all characteristics of the trilobites, and it is interesting to note
that all nauplii have the free-swimming habit.

The effect of inheritance and modification through millions of generations is also
shown in the nauplius, but rather less than would be expected. The most important modifi-

144 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

cation is the temporary suppression of the posterior pairs of appendages of the head, so that
they are generally developed later than the thoracic limbs. The median or nauplius eye
has not yet been found in trilobites, and if it is, as it appears to be, a specialized eye, it
has probably arisen since the later Crustacea passed the trilobite stage in their phylogeny.

The oldest Crustacea, other than trilobites, so far known are the Branchiopoda and
Phyllocarida described by Walcott and discussed above. It is important to note that while
the former have already achieved such modified characteristics that they have been referred
to modem orders, they retain the trilobite-like limbs and some of them still have well devel-
oped pleural lobes.

Caiman (1909, p. 101) says of the Copepoda:

On the hypothesis that the nauplius represents the ancestral type of the Crustacea, the Eucopepoda would
be regarded as the most primitive existing members of the class, retaining as they do, naupliar characters in
the form of the first three pairs of appendages and in the absence of paired eyes and of a shell-fold. As
already indicated, however, it is much more probable that they are to be regarded as a specialized and in
some respects degenerate group which, while retaining, in some cases, a very primitive structure of the
cephalic appendages, has diverged from the ancestral stock in the reduction of the number of somites, the
loss of the paired eyes and the shell-fold, and the simplified form of the trunk-limbs.

If the Eucopepoda be viewed in the light of the theory of descent here suggested, it
is at once seen that while they are modified and specialized, they more nearly approximate
the hypothetical ancestor than any other living Crustacea. Compound eyes are absent, and
it can not be proved that they were ever present, although Grobben is said to have observed
rudiments of them in the development of Calanus. The "simplified limbs" are the simple
limbs of the trilobite, somewhat modified. The absence of the shell-fold and carapace is
certainly a primitive characteristic. Add to this the direct development of the small number
of segments, and the infolded pleural lobes, and it must be admitted that the group pre-
sents more trilobite-like characteristics than any other. It seems very likely that the primi-
tive features were retained because of the pelagic habitat of a large part of the group.

Ruedemann (Proc. Nat. Acad. Sci., vol. 4, 1918, p. 382, pi.) has recently outlined a
possible method of derivation of the acorn barnacles from the phyllocarids. Starting from
a recent Balanus with rostrum and carina separated by two pairs of lateralia, he traces back
through Calophragmus with three pairs of lateralia to Protobalanus of the Devonian with
five pairs. Still older is the newly discovered Eobalanus of the upper Ordovician, which
also has five pairs of lateralia but the middle pair is reversed, so that when the lateralia of
each side are fitted together, they form a pair of shields like those of Rhinocaris, separated
by the rostrum and carina, which are supposed to be homologous with the rostrum and
dorsal plate of the Phyllocarida. Ruedemann suggests that the ancestral phyllocarid attached
itself by the head, dorsal side downward, and the lateralia were developed from the two
valves 'of the carapace during its upward migration, to protect the ventral side exposed in
the new position.

This theory is very ingenious, but has not been fully published at the time of writing,
and it seems very doubtful if it can be sustained.

Summary.

The salient points in the preceding discussion should be disentangled from their setting
and put forward in a brief summary.

It is argued that the ancestral arthropod was a short and wide pelagic animal of few
segments, which so far changed its habits as to settle upon a sub-stratum. As a result of

SUMMARY.

change in feeding habits, appendages were developed, and, due perhaps to physiological
change induced by changed food, a shell was secreted on the dorsal surface, covering the
whole body. Such a shell need not have been segmented, and, in fact, the stiffer the shell,
the more reason for development of the appendages. Activity as a swimming and crawling
animal tended to break up the dorsal test into segments corresponding to those of the soft
parts, and, by adaptation, a floating animal became a crawling one, with consequent change

Fig. 36. — Naraoia
comfacta Walcott.
An outline of the
test, after Walcott.
Natural size.

Fig. 37—Pagetia
cfytio Walcott. An
eodiscid with com-
pound eyes. After
Walcott. X 5.

Fig. 38. — Asaphis-
cus wheeleri Meek.
A representative
trilobite of the
Middle Cambrian
of the Pacific
province. After
Meek. X Vi.

Fig. 39. — Padeumias
robsonensis Burling. Re-
stored from a photograph
published by Burling.
X'A.

Fig. 40. — Robergia sp.
Restored from fragments
found in the Athens shale
(Lower Middle Ordovi-
cian), at Saltville, Va.
Natural size.

from a form like that of Naraoia to one like Pccdeumias. (See figs. 36-40.) A contin-
uation of this line of development by breaking up and loss of the dorsal test led through
forms similar to Marrella to the Branchiopoda of the Cambrian, in which not only is there
great reduction in the test, but also loss of appendages. The origin of the carapace is still
obscure, but Bernard (1892, p. 214, fig. 48) has already pointed out that some trilobites,
Acidaspid;e particularly, have backward projecting spines on the posterior margin of the
cephalon, which suggest the possibility of the production of such a shield, and in Marrella
such spines are so extravagantly developed as almost to confirm the probability of such

146 THE APPENDAGES, ANATOMY, AND RELATIONS OF TKILOBITKS.

origin. In this line of development two pairs of tactile antenn;c were produced, while the
anomomeristic character of the trilobite was retained. From similar opisthoparian ances-
tors there were, however, derived primitive Malacostraca retaining biramous antenme, but
with a carapace and reduced pleural lobes and pygidium. From this offshoot were prob-
ably derived the Ostracoda, the Cirripedia, and the various orders of the Malacostraca, with
the possible exception of the Isopoda. I have suggested independent origins of the Copepoda
and Isopoda, but realize the weighty arguments which can be adduced against such an
interpretation.

It is customary to speak of the Crustacea and Trilobita as having had a common ances-
try, rather than the former being in direct line of descent from the latter, but when it can
be shown that the higher Crustacea are all derivable frorq the Trilobita, and that they possess
no characteristics which need have been inherited from any other source than that group,
it seems needless to postulate the evolution of the same organs along two lines of develop-
ment.

I can not go into the question of which are more primitive, sessile or stalked eyes, but
considering the various types found among the trilobites, one can but feel that the stalked
eyes are not the most simple. While no trilobite had movable stalked eyes, it is possible
to homologize free cheeks with such structures. They always bear the visual surface, and,
in certain trilobites (Cydopygc), the entire cheek is broken up into lenses. Since a free
cheek is a separate entity, it is conceivable that it might lie modified into a movable organ.

EVOLUTION OF THE MEROSTOMATA.

It has been pointed out above that the Limulava (Sidncyia, Amiella, Emeraldclla) have
certain characteristics in common with the trilobites on the one hand and the Eurypterida
on the other. These relationships have been emphasized by Walcott, who derives the Euryp-
terida through the Limulava and the Aglaspina from the Trilobita. The Limulava may be
derived from the Trilobita, but indicate a line somewhat different from that of the remain-
der of the Crustacea. In this line the second cephalic appendages do not become antennce.
and the axial lobe seems to broaden out, so that the pleural lobes become an integral part
of the body. As in the modern Crustacea, the pygidium is reduced to the anal plate, and
this grows out into a spine-like telson.

From the Limulava to the Eurypterida is a long leap, and before it can be made without
danger, many intermediate steps must be placed in position. The direct ancestor of the
Eurypterida is certainly not to be seen in the highly specialized Sidncyia, and probably not
in Emcraldella, but it might be sought in a related form with a few more segments. The
few species now known do suggest the beginning of a grouping of appendages about the
mouth, a suppression of appendages on the abdomen, and a development of gills on the
thorax only. Further than that the route is uncertain.

Clarke and Ruedemann, whose recent extensive studies give their opinion much weight,
seem fully convinced that the Merostomata could not have been derived from the Trilobita,
but are rather inclined to agree with Bernard that the arachnids and the crustaceans were
derived .independently from similar chuetopod annelids (1912, p. 148).

The greater- part of their work was, however, finished before 1910, and although they
refer to Walcott's description of the Limulava (1911), they did not have the advantage
of studying the wonderful series of Crustacea described by him in 1912. While the evi-

SUMMARY. 147

dence is far from clear, it would appear that the discovery of animals with the form of
Limiting and the eurypterids and the appendages of trilobites means something more than
descent from similar ancestors. Biramous limbs of the type found in the trilobites would
probably not be evolved independently on two lines, even if the ancestral stocks were of
the same blood.

The Aglaspidae, as represented by Malaria and Habclia in the Middle Cambrian, are
quite obvious closely related to the trilobites. easily derived from them, and retain numer-
ous of their characteristics. That they are not trilobites is, however, shown by the presence
of two pairs of antenme, the absence of facial sutures, and the possession of a spine-like
telson,

The Aglaspidnc have always been placed in the Merostomata, and nearer the Limulidse
than the Eurypterida. The discovery of appendages does not at all tend to strengthen that
view, but indicates rather that they are true Crustacea which have not given rise to any
group now known. The exterior form is, however, Limulus-\\\x, and since it is known from
ontogeny that the ancestor of that genus was an animal with free body segments, there
is still a temptation to try to see in the Aglaspidae the progenitors of the limulids.

The oldest known Lintnliis-like animal other than the Aglaspidae is Ncolimulns falcatus
Woodward (Geol. Mag., dec. i, vol. 5, 1868, p. i, pi. i, fig. i). The structure of the head of
this animal is typically limuloid, with simple and compound eyes and even the ophthalmic
ridges. Yet, curiously enough, it shows what in a trilobite would be considered the posterior
half of the facial suture, running from the eye to the genal angle. The body is composed
of eight free segments with the posterior end missing. Bclinurus, from the Mississippian
and Pennsylvania!!, has a sort of pygidium, the posterior three segments being fused together,
and Prcstwichia of the Pennsylvanian has all the segments of the abdomen fused together.
So far as form goes, a very good series of stages can be selected, from the Aglaspidse of
the Cambrian through Neolimulus to the Belinuridie of the late Palaeozoic and the Limu-
lidre of the Mesozoic to recent. Without much more knowledge of the appendages than is
now available, it would be quite impossible to defend such a line. It is, however, sug-
gestive.

EVOLUTION OF THE "TRACHEATA/'

The trilobites were such abundant and highly variable animals, adapting themselves to
various methods of life in the sea, that it appears highly probably that some of them may
have become adapted to life on the land. The ancestors of the Chilopoda, Diplopoda, and
Insecta appear to have been air-breathing animals as early as the Cambrian, or at latest,
the Ordovician. Since absolutely nothing is yet known of the land or even of the fresh-
water life of those periods, nothing can now be proved.

In discussing the relationship of the trilobites to the various tracheate animals, I have
pointed out such paheontologic evidence as I have been able to gather. Studies in the field
of comparative morphology do not fall within my province. I only hope to have made the
structure of the trilobite a little more accessible to the student of phylogenies.

SUMMARY ON LINES OF DESCENT.

In order to put into graphic and concise form the suggestions made above, it is neces-
sary to define and give names to some of the groups outlined. The hypothetical ancestor

148 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

need not be included in the classification and for reasons of convenience may be referred
to merely as the Protostracean.

The group of free-swimming trilobites without thoracic segments was probably a large
one, and within it there were doubtless considerable variations and numerous adaptations.
While the only known animal which could possibly be referred to this group, Naraoia, is
blind, it is entirely possible that other species had eyes, and that the cephala and pygidia
were variously modified. For this reason and because of the lack of all thoracic segments,
it seems better to erect a new order rather than merely a family for the group, and Nek-
taspia (swimming shields) may be suggested. The only known family is Naraoidre Wal-
cott, which must be redefined.

Marrella and Habclia are types of Crustacea which can neither be placed in the Trilo-
bita nor in any of the established subclasses of the Eucrustacea. They represent a transi-
tional group, the members of which are, so far as known, adapted to the crawling mode of
life, though it may prove that there are also swimmers which can be classified with them.
To this subclass the name Haplopoda may be applied, the feet being simple.

The two known families, Marrellidze Walcott and Aglaspidse Clarke, belong to differ-
ent orders, the second having already the name Aglaspina Walcott. The name Marrellina
may therefore be used for the other.

For Sidneyia, Walcott proposed the new subordinal name Limulava, placing it under
the Eurypterida. While Sidneyia, Emeraldclla, and Amiclla may belong to the group that
gave rise to the Eurypterida, they are themselves Crustacea, and a place must be found for
them in that group. The possession of only one pair of antenme prevents their reception
by the Haplopoda, and allies them to the Trilobita, but the modifications of the trunk and
its appendages keep them out of that subclass, and a new one has to be erected for them.
This may be known as the Xenopoda, in allusion to the strange appendages of Sidneyia.

Synopsis.
Class Crustacea.
Subclass Trilobita Walch.
Crustacea with one pair of uniramous antenna?, and possessing facial sutures.

Order Nektaspia nov.
Trilobita without thoracic segments. Cephala and pygidia simple.

Family Naraoidse Walcott.

Cephalon and pygidium large, both shields nearly smooth. Eyes absent. A single species:
Naraoia compacta Walcott, Middle Cambrian, British Columbia.

Subclass Haplopoda nov,.

Crustacea with trilobate form, two. pairs of uniramous antenna;, no facial sutures, sessile
compound eyes present or absent, pygidium and pleural lobes generally reduced, large
labrum present, appendages of the trunk biramous.

Order. Marrellina nov.

Form trilobite-like, pleural lobes reduced, endobases absent from coxopodites of body, pygid-
ium a small .plate.

SUMMARY. 149

Family Marrellidje Walcott.

Cephalon with long genal and nuchal spines. Eyes marginal. A single species : Marrella
splendcns Walcott, Middle Cambrian, British Columbia.

Order Aglaspina Walcott.

Body trilobite-like, with few thoracic segments, and a spine-like telson. Appendages bira-
mous.

Family Aglaspidse Clarke.

Cephalon trilobate, with or without compound eyes, seven or eight segments in the thorax.

Genus Aglaspis Hall.
Compound eyes present, seven segments in thorax. Upper Cambrian, Wisconsin.

Genus Molaria Walcott.
Compound eyes absent, eight segments in thorax. Middle Cambrian, British Columbia.

Genus Habelia Walcott.
Compound eyes absent. Not yet fully described. Middle Cambrian, British Columbia.

Subclass Xenopoda nov.

Crustacea with more or less eurypterid-like form, one pair of uniramous antennae, biramous
appendages on anterior part of trunk, modified endopodites on cephalon.

Order Limulava Walcott.

Cephalon with lateral or marginal eyes and large epistoma. Body with eleven free seg-
ments and a telson. Cephalic appendages grouped about the mouth.

Family Sidneyidae Walcott.

Trunk probably with exopodites only, and without appendages on the last two segments.

Telson with a pair of lateral swimmerets.

•

Genus Sidneyia Walcott.

Third cephalic appendage a large compound claw. Gnathobases forming strong jaws. Mid-
dle Cambrian, British Columbia.

Genus Amiclla Walcott.
Middle Cambrian, British Columbia.

Family Emeraldellidae nov.

Trunk with biramous appendages in anterior part, and appendages on all segments except
possibly the spine-like telson.

Genus Emcraldella Walcott.

Cephalic appendages simple spiniferous endopodites. Fyes unknown. Middle Cambrian,
British Columbia.

150

THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

Recent

Fig. 41. — A diagram showing possible lines of descent of the other Arthropoda from the Trilobita. The
three recognized orders of the latter are shown separately. The known geological range is indicated in solid
black, the hypothetical range and connections stippled. The short branch beside the Opisthoparia represents
the range of the Haplopoda. The term Arachnida is used for all arachnids other than Merostomata, merely
as a convenient inclusive name for the groups not especially studied.

SUMMARY. I 5 I

FINAL SUMMARY.

It is generally believed that the Arthropoda constitute a natural, monophyletic group.
The data assembled in the preceding pages indicate that the other Arthropoda were
derived directly or indirectly from the Trilobita because :

1 I ) the trilobites are the oldest known arthropods ;

(2) the trilobites of all formations show great variation in the number of trunk seg-
ments, but with a tendency for the number to become fixed in each genus;

(3) the trilobites have a constant number of segments in the head;

(4) the position of the mouth is variable, so that either the Crustacea or the Arach-
nida could be derived from the trilobites;

(5) the trilobite type of appendage is found, in vestigial form at least, throughout the
Arthropoda ;

(6) the appendages of all other Arthropoda are of forms which could have been derived
from those of trilobites;

(7) the appendages of trilobites are the simplest known among the Arthropoda;

(8) the trilobites show practically all known kinds of sessile arthropodan eyes, simple,
compound, and aggregate;

(9) the apparent specializations of trilobites, large pleural lobes and pygidia, are primi-
tive, and both suffer reduction within the group.

The ancestor of the trilobite is believed to have been a soft-bodied, free-swimming, flat,
blind or nearly blind animal of few segments, because :

(a) the form of both adult and embryo is of a type more adapted for floating than
crawling ;

(b) the large pygidium is shown by ontogeny to be primitive, and the elongate worm-
like form secondary;

(c) the history of the trilobites shows a considerable increase in the average number of
segments in successive periods from the Cambrian to the Permian;

(d) the simplest trilobites are nearly or quite blind.

PART IV.

DESCRIPTION OF THE APPENDAGES OF INDIVIDUAL SPECIMENS.

TRIARTHRUS BECKI GREEN.

In order to make easily available the evidence on which the present knowledge of the
appendages of Triarthrus and Cryptolithus rests, it has seemed wise to publish brief descrip-
tions and photographic figures of some of the better specimens preserved in the Yale Uni-
versity Museum. These specimens are pyritic replacements, and while they do not as yet
show any signs of decomposition, it should be realized that it is only a matter of time when
either they will be self-destroyed through oxidation, or else embedded for safe keeping in
such a fashion that they will not be readily available for study. It is therefore essential to
keep a photographic record of the more important individuals.

Specimen No. 220 (pi. 3, fig. 2).

Illustrated: Amer. Geol., vol. 15, 1895, pi. 4 (drawing); Amer. Jour. Sci., vol. 13, 1902, pi. 3
(photograph).

This is one of the largest specimens showing appendages, and is developed from the ven-
tral side. It shows some appendages on all parts of the body, but its special features are
the exhibition of the shafts on the proximal ends of the antennules, the rather well pre-
served appendages of the cephalon and anterior part of the thorax, and the preservation of
the anal opening. In the drawing in the American Geologist, the right and left sides are
reversed as in a mirror, a point which should be borne Mn mind when comparing that figure
with a photograph or description.

The shaft of the left antennule is best preserved and is short, cylindrical, somewhat
enlarged and ball-shaped at the proximal end. It is 1.5 mm. long. The posterior part of
the hypostoma is present, but crushed, and the metastoma is not visible, the pieces so indi-
cated in Beecher's figure being the rim of the hypostoma. Back of the hypostoma may be
seen four (not three as in Beecher's figure) pairs of gnathites, the first three pairs broad
and greatly overlapping, the fourth pair more slender, but poorly preserved. The inner
edges of the gnathites on the right side are distinctly nodulose, and roughened for mastication.

The outer ends of one endopodite and three exopodites project beyond the margin on
the right side. The dactylopodite of the endopodite is especially well preserved. It is cylin-
drical, the end rounded but not enlarged or pointed, and bears three small sharp spines, all
in a horizontal plane, one anterior, one central, and one posterior. The outer ends of the
exopodites show about ten segments each (in 2.5 mm.) beyond the margin of the test, and
from three to five setae attached to the posterior side of each segment. These hairs are
attached in a groove, well shown in this specimen. On the anterior margin of the exopo-
dite there is a minute spine at each joint.

Measurements: Length, 38 mm. ; width at back of cephalon, 19 mm.

Specimen No. 210 (pi. 2, fig. 3).

Illustrated: Amer. Jour. Sci., vol. 46, 1893, p. 469, fig. I (head and right side); Amer. Geol., vol. 13,
1894, pi. 3, fig. 7 (same figure as the last); Amer. Jour. Sci., vol. 13, 1902, pi. 2, fig. I (photograph).

This individual supplied the main basis for Professor Beecher's first figure showing the
appendages of the thorax, the head and appendages of the right side having been taken

SPECIMENS OF TRIARTHRUS. I S3

from it, and the appendages of the left side from No. 206. Such of the endopodites as are
well preserved show from three to four segments projecting beyond the test, and the dacty-
lopodites have one or two terminal spines. The antennules are unusually well preserved and
have about forty segments each in front of the cephalon, or an average of five to one
millimeter.

Specimens 209 and 210 are on a slab about 7X5.5 inches, and with them are twelve
other more or less well preserved individuals, all but one of which are smaller than these.
Two of the fourteen are ventral side up on the slab, which means dorsal side up in the rock.
Nine are oriented in one direction, two at exactly right angles to this, and three at an angle
of 45° with the others. If the majority of the specimens are considered to be headed north-
ward, then seven are so oriented, two northeast, one east, two south, one southwest, and
one west.

Nine of the specimens show antennules. Five of these are specimens headed north, and
in all of them the antennules are in or very near the normal position. The antennules of
two, one headed east and the other west, are imperfectly preserved, but the parts remain-
ing diverge much more than do the antennules of those in the normal position. The indi-
vidual headed southwest has one antennule broken off, while the other is curved back so that
its tip is directed northward. Another one, headed south, has the antennules in the normal
position. These observations indicate that the specimens were oriented by currents of water,
rather than in life attitudes, and that the distal portions of the antennules were relatively
flexible.

Measurements: The specimen (No. 210) is 20 mm. long, 9.5 mm. wide at the back of
the cephalon, and the antennules project 8 mm. in front of the head. The smallest specimen
on the slab is 6.5 mm. long. A specimen 7.5 mm. long has antennules which project 2.5 mm.
in front of the cephalon.

Specimen No. 201 (pi. 2, fig. i; pi. 3, fig. 4).
Illustrated: Amer. Jour. Sci., vol. 46, 1893, p. 469, figs. 2, 3; Amer. Geol., vol. 13, 1894, pi. 3, figs. 8, 9.

An entire specimen 17 mm. long, exposed from the dorsal side. It shows only traces
of the appendages of the head, but displays well those of the anterior part of the thorax, and
a number of appendages emerge from under the abdominal shield. This specimen is of par-
ticular interest as it is the subject of the first of Professor Beecher's papers on appendages
of trilobites. On the right side the pleura have been removed, so as to expose the appen-
dages of the second, third, and fourth segments from above. The first two of the appen-
dages on the right are best preserved, and these are the ones figured. They belong to the
second and third segments. The endopodites of each are ahead of the exopodites, and the
proximal portion of each exopodite overlies portions of the first two segments (second and
third) of the corresponding endopodite. The coxopodites are not visible, but very nearly
the full length of the first segment of the endopodite (the basipodite) is exposed. The first
two visible segments (the first and second) extend just to the margin of the pleural lobe,
while the other four extend beyond the dorsal cover. The segments decrease in length out-
ward, but not regularly, the meropodite being generally longer than the ischiopodite or the
carpopodite. The terminal segment (dactylopodite) is short and bears short sharp hair-like
spines which articulate in sockets at the distal end. On this specimen the anterior limb on
the right side shows one terminal spine, the second endopodite on that side has two, and two
of the endopodites on the left-hand side preserve two each. The segments of the limbs

I 54 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

are nearly cylindrical, but the ischiopodites and meropodites of several of the endopodites
show rather deep longitudinal grooves which appear to be rather the result of the shrinkage
of the thin test than natural conformations.

The endopodites on the left-hand side have a number of short, sharp, movable, hair-
like spines, and cup-shaped depressions which are the points of insertion of others. On
the distal end of the carpopoclite of the first thoracic segment there seems to have been a
spine, whose place is now shown by a pit. This same endopodite shows, rather indistinctly,
three pits in the groove of the carpopodite, and the propodite has two. On the endopodite
of the second appendage on this side, both the carpopodite and propodite possess a fine hair-
like articulated spine at the distal end, that of the propodite arising on the dorsal and
that of the carpopodite on the posterior side. On the dorsal side of the carpopodite there
are three pits for the articulation of spines, and on the propodite, one.

The exopodites belonging to the thoracic segments are of equal length with the endopo-
dites, and while the proximal portion of each is stouter than that of the corresponding
endopodite, the exopodites taper to a hair-like termination, while the endopodites remain
fairly stout to the distal segment. Most of the setae of the exopodites have been removed,
so that each remains as a curving, many-segmented organ, transversely striated, with a con-
tinuous groove along the posterior side. The setae appear to be set in this groove, one for
each of the transverse ridges on the shaft.

A good deal of the test has been cut away on the left-hand side from the thorax and
pygidium, and the appendages exposed from above. Enough of the dorsal shell has been
cut away so that the anal opening is exposed, and directly behind the pygidium, on the
median line, is a bilaterally symmetrical plate with serrated edges which appears to be the
appendage of the anal segment. (See pi. 3, fig. 4.)

Measurements: The specimen is 17 mm. long, and 8 mm. in greatest width (at the back
of the cephalon). From the median tubercle to the outer edge of the pleuron of the second
thoracic segment the distance is 3.7 mm. From the point of articulation to the distal end
of the spines on the dactylopodite of the second endopodite on the right-hand side is 4.3 mm.
The basipodite of this appendage is 1.5 mm. long, the ischiopodite i mm. long, the meropo-
dite 1.2 mm. Jong, the carpopodite 0.5 mm. long, the propodite 0.35 long, and the dactylopo-
dite 0.15 mm. long. On the left-hand side the endopodite of the first segment projects
3 mm. beyond the pleuron, the second, 3.2 mm. At the back the appendages extend a
maximum distance of 2.5 mm. behind the pygidium. The median spinose process of the
anal segment extends 0.75 mm. behind the pygidium, and is 1.6 mm. in greatest width.

Specimen No. 204 (pi. 3, fig. i ; pi. 4, fig. 6; text fig. 42).
Illustrated: Amer. Jour. Sci., vol. 13, 1902, pi. 2, figs. 4, 5 (reproduced from photographs).

This specimen, which is developed from the dorsal surface, shows especially well nine
appendages of the left side. The first represent the last segment of the cephalon; the re-
mainder belong to the thorax. As is usual, the exopodites of these appendages overlie and
curve behind the endopodites. All the exopodites have lost their setae and the segments of
the endopodites are flattened by crushing. The endopodites, while retaining only one or two
of the movable spines, have the cup-like bases of from two to four on each of the visible
segments, namely, the meropodite, carpopodite, propodite, and, in one case, the dactylopo-
dite. These appendages, although really marvellous in preservation, are of such small size
and react so badly to light that their study is very difficult, and Professor Beecher, who had

SPECIMENS OF TRIARTHRUS.

'55

observed hundreds of specimens through all stages of the laborious process of cleaning the
matrix from them, undoubtedly was much better equipped to interpret them than any other
person.

The drawing is made on the assumption that the appendages are displaced and all
moved uniformly outward so that the distal ends of the coxopodites emerge from under the
pleural lobe, whereas these ends would normally be under the dorsal furrow, and the distal
end of the ischiopodite should reach the margin of the pleural lobe. While it seems very
remarkable that it should happen, that all the appendages should be so moved that they
would lie symmetrically a few millimeters from their normal position, nevertheless it is
found on measuring that they bear the same proportion to the length and width that the

Fig. 42. — Triarthrus becki Green. Ap-
pendages of specimen 204. Inked in by
Miss Wood from the original tracing.
Xio.

appendages of other specimens do, thus indicating that Professor Beecher's interpretation of
them was correct. I am unable, however, to see the coxopodites which he has drawn as
articulating with the two branches of the limb.

This individual shows, better than any other, the connection of the exopodite with the
endopodite. Even though the coxopodites are gone, the two branches of each appendage re-
main together, showing that the basipodite as well as the coxopodite is involved in the artic-
ulation with the exopodite. Just what the connection is can not be observed, but there
seems to be a firm union between the upper surface of the basipodite and the lower side of
the proximal end of the exopodite, as indicated diagrammatically in text figure 33.

Measurements: The specimen is 20 mm. long and 9 mm. wide at the back of the cepha-
lon. From the tubercle on the middle of the first segment of the thorax to the tip of the
corresponding appendage the distance is 8 mm. The entire length of the exopodite of the
first thoracic segment is 4.6 mm. The exopodite of the appendage belonging to the seventh
segment is only 3.5 mm. long. The pleural lobe is 2.5 mm. wide at the front of the thorax.

Specimen No. 205 (pi. 2, fig. 4).
Illustrated: Amer. Jour. Sci., vol. 13, 1902, pi. 5, figs. 2, 3 (photographs).

This is a small imperfect specimen, developed from the ventral side. It retains the best
preserved metastoma in the collection, but was used by Professor Beecher especially to illus-

156 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

trate the convergent ridges on the inside of the ventral membrane in the axial region of the
thorax. These ridges are very low, and on each segment of the thorax there is a central one,
outside of which is a pair which are convergent forward, making angles of 35° to 45°
with the axis.

The metastoma is shaped much like the hypostoma of an Illanus. It is convex, nearly
semicircular, with the straight side forward, and there is a continuous raised border around
the curved sides and back. This border is separated from the central convex body by a deep
linear depression.

The hypostoma is also rather well preserved and has a narrow, slightly elevated border
at the sides and back.

Measurements: The incomplete specimen, from which only a very small portion of the
length is missing, is 9 mm. long. The metastoma is 0.45 mm. long and 0.58 mm. wide.

Specimen No. 214 (pi. i, fig. 2; pi. 3, fig. 6).

This is a large specimen, developed from the ventral side. It shows the antennules and
some other appendages of the head, but derives its special interest from the excellent pre-
servation of a few of the exopodites, which are turned back parallel to the axis of the body
and lie within the axial lobe.

The shaft of the exopodite is made up of numerous short segments which at their an-
terior outer angles are produced into spines, and which also bear movable spines along the
anterior border. As shown in several other specimens, the exopodite ends in a more or less
long spoon-shaped segment bearing on its lower surface a broad groove. No set;e appear
to be attached to this, but both anterior and posterior margins bear numerous small, appar-
ently movable spines. From the groove along the ventral side of the remainder of the
exopodite arise numerous long slender filaments which become progressively shorter toward
the tip. This specimen shows that they are not cylindrical, but are flattened along opposite
faces, at least at their distal ends. While no connection can be seen between adjacent setae,
they seem to stay together like the barbs on a feather.

Measurements: Length, 33 mm., width at back of cephalon, 16 mm.; from front of
cephalon to back of hypostoma, 6 mm.

Specimen No. 219 (pi. 2, fig. 6; pi. 4, fig. 4).
Illustrated: Amer. Jour. Sci., vol. 13, 1902, pi. 4, fig. i, pi. 5, fig. 4 (photograph and drawing) >

The endopodites of most of the appendages of the thorax are well shown, and occasional
portions of exopodites. The coxopodites are long, flattened, and do not taper much. The
anterior and posterior edges of the basipodites of the endopodites of the first two segments
are approximately parallel, but on the succeeding endopodites the basipodites and ischiopo-
dites are triangular in form, with the apex backward. In successive endopodites toward
the posterior end, the angle made by the backward-directed sides of the basipodites becomes
increasingly acute, so that in some of the posterior appendages this segment is wider than
long. The ischiopodite shows a similar increase of width and angularity on successive seg-
ments, and the meropodites and carpopodites also become wider on the posterior segments,
and even triangular in outline toward the back of the thorax and on the pygidium.

Along the median portion of the axial lobe the specimen has been cleaned until the inner
side of the ventral membrane was reached. Here the test shows on the inner surface at each
segment of the thorax a series of low ridges which are roughly parallel to the axial line, but

SPECIMENS OF TRIARTHRUS. 157

which really converge in an anterior direction. Between the ridges are shallow canoe-shaped
depressions, which have the appearance of areas for the insertion of muscles.

Measurements: Length, 31 mm. ; width at back of head, 15 mm. ; distance, in a straight
line, from point of insertion of the right antennule to its tip, 14.25 mm.; it projects 12 mm.
beyond the cephalon.

Specimen No. 218 (pi. 6, fig. 3; text fig. 43).

This specimen is a large one, developed from the lower side, but retains only the endopo-
dites of a few appendages. The cephalon and anterior portion of the thorax are missing.

Professor Beecher had a drawing made to show the appendages on the right-hand side
of the last two segments of the thorax, seen of course from the ventral side. This drawing
shows well the broadening of the basipodite, ischiopodite, and meropodite, while the coxopo-
dite is thick and heavy, and the inner end of the gnathobase somewhat rugose. Almost

Fig. 43. — Triarthrus
bccki Green. Drawing
to represent the writer's
interpretation of the ap-
pendages of specimen
218. Drawn by Miss
Wood. X 10.

every segment of the endopodites has one or more pits for insertion of spines, these being
along the anterior or posterior margins. The exopodites lack the setae, but show no unusual
features.

Specimen No. 222 (pi. 4, fig. 5).
Illustrated: Amer. Jour. Sci., vol. 47, 1894, pi. 7, fig. 3 (drawing).

A small specimen, developed from the lower side, and used by Professor Beecher to
illustrate the form of the segments of the endopodites of the pygidium. In addition to this,
it shows very well the form of the endopodites of the thorax. All of the appendages on
the specimen are shifted to the left of their normal position. This specimen differs from
most of the others in that the segments of the endopodites do not lie with their greatest
width in the horizontal plane, but were embedded vertically, with the posterior edge down-
ward. From this circumstance they retain their natural shape, and it is seen that they are
naturally flattened, with about the same thickness in proportion to length and width as in
some of the modern isopods (Serolis, for instance). In even the most anterior of these
endopodites (that of the second segment) the ischiopodite, meropodite, and carpopodite are

I58

THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

triangular in shape, with the point backward, but in all the endopodites at the anterior end
of the thorax, the triangle has a very obtuse angle at the apex, and the base is much longer
than the perpendicular. On the other hand, those of the pygidium, which were figured by
Beecher, have a number of short wide segments, all wider than long, and, excepting the
dactylopodites, triangular in form.

Measurements: Length, 8.75 mm. ; width at back of cephalon, about 4 mm.

Specimen No. 230 (pi. 5, fig. 3; text fig. 44).
Illustrated: Amer. Jour. Sci., vol. 47, 1894, pi. 7, fig. 2 (drawing); Ibid., vol. 13, 1902, pi. 2, fig. 2.

An entire specimen of medium size, developed from the ventral side. It seems to have
been the first one to yield to Professor Beecher any satisfactory knowledge of the appen-

Fig. 44. — Triarthrus becki Green. Appendages of the posterior
part of the thorax and pygidium of specimen 230. Inked by Miss
Wood from a tracing made under the direction of Professor Beecher.

dages of the pygidium. There are five endopodites, all on one side, which appear to belong
here. The segments in this region are characterized by their short, wide, triangular form.
At the apex of each is a small tuft of spines or short hairs, and the ventral surfaces of some
of the endopodites show pits for the insertion of spines.

Measurements: Length, 21 mm.; width at back of cephalon, 10 mm.

CRYPTOLITHUS TESSELLATUS GREEN.
Specimen No. 233 (pi. 7, fig. i; text fig. 45).

This is the best preserved entire specimen. It is developed from the lower side, and
shows the hypostoma, antennules, and a few fragmentary appendages of the cephalon, the
outer portions of the exopodites of thorax and pygidium on both sides, and the endopodites
on the left side.

The hypostoma is imperfectly preserved and is turned completely around, so that the
anterior margin is directed backward, and the posterior one is so much in the shadow that
it does not show well in any of the photographs. The form is, however, essentially like

SPECIMENS OF CRYPTOL1THUS.

159

that of Trinucleoides rcussi (Barrande), the only other trinucleicl of which the hypostoma
is known, except that the border does not extend so far forward along the sides, and it is
much smaller.

The antennules are not inserted close to the hypostoma, as in Triarthrus, but at some
distance from it, and, as nearly as can be determined, directly beneath the antennal pits
which are seen near the front of the glabella in many species of trinucleids.

Fig. 45. — Cryptolithus tessellatus Green. Drawing of specimen 233, made by Professor
Beecher. X 9. Below are parts of two of the endopodites of specimen 236, showing the
interarticular membranes. X 41.

The antennules are long, and are composed of far fewer and longer segments than
those of Triarthrus. In this specimen they converge backward, cross each other and at the
distal end are more or less intertwined.

As is shown in the drawing and photograph, very little can be learned from this indi-
vidual about the other appendages of the cephalon. A few fragments of exopodites on
either side suggest that these members pointed forward and were much like those in Triar-
thrus, but nothing conclusive is shown.

i6o

THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

The exopodites and endopodites of the left side of the thorax are best preserved. The
exopodites are above the endopodites, and only that portion exposed from the ventral side
which projects beyond the line at which the endopodites bend backward. The endopodite
on the left side of the first thoracic segment is the best preserved. It shows seven segments,
the outer ones best. The coxopodite is short and narrow, the basipodite somewhat heavier
and longer, while the carpopodite and propodite are the widest and strongest segments. The
propodite is triangular and flattened, like the segments on the middle and posterior part of
the thorax of Triarthrus. At the inner end of the ischiopodite and meropodite are tufts of
spines pointing inward and backward. These are not shown on any of the photographs,
but may be seen with the light striking the specimen at the proper angle.

It is not possible to count the exact number of limbs, but one gets the impression that
on the left side of this specimen there are twenty-one sets of appendages, six of which of
course belong to the thorax. On the thorax and anterior part of the pygidium, successive
endopodites show the propodites and dactylopodites becoming progressively more slender and
shorter, while the ischiopodites, meropodites and carpopodites become shorter and more tri-
angular, and with increasingly large numbers of short spines on their posterior borders. Back
of the fourth endopodite on the pygidium it is not possible to make out the detail, but the
appearance is of an endopodite consisting of short broad segments fringed at the back with
short spines, the ones at the very posterior end appearing to be exceedingly short and rudi-
mentary.

The exopodites are not so well shown as in some others but the setae are flattened and
blade-shaped, and often bear numerous small spines.

Measurements: Length (lacking most of the fringe), 10.5 mm. Width of thorax,
10.5 mm. Length of hypostome, 1.41 mm., width at front, 1.46 mm. The distance from
back of fringe to end of antennules is 5.4 mm. If straightened out, the left antennule
would be about 6.1 mm. long. In the first 3.1 mm., there are only ten segments, so that the
average length of a segment is 0.31 mm. The distance from the inner end of the endo-
base of the first segment of the thorax to the outer end of the meropodite is 2,43 mm., and
from that point to the end of the dactylopodite 2.47 mm. making the total length 4.90 mm.
These measurements are taken from the photograph. Measurements taken from Professor
Beecher's drawing, which was made with the camera lucida, give a total length of 4.57 mm.,
the distance to the outer end of the meropodite being 2.3 mm. and thence to the tip of the
dactylopodite 2.27 mm. Detailed measurements of the segments, on the photograph, are as
follows: coxopodite, 0.321 mm. ; basipodite, 0.78 mm. ; ischiopodite, 0.68 mm. ; meropodite,
0.642 mm.; carpopodite, 0.642 mm.; propodite, i.oi mm., dactylopodite, 0.825 mm.

Illustrated

, . . , . ., , .

Specimen No. 235 (pi. 7, fig. 2; pi. 8, fig. 3; pi. 9, figs, i, 2).
: Amer. Jour. Sci., vol. 49, 1895, pi. 3, figs. 5, 6.

Specimens 235 and 236 were originally parts of an entire Cryptolithus, but, as
Professor Beecher has explained, the specimen was cut in two longitudinally on the median
line, and the halves transversely just back of the cephalon, so that each now represents one
half of a thorax and pygidium. Both halves have been cleaned from both upper and lower
side, a perfectly marvelous piece of work, for the thickness is no greater than that of a
thin sheet of paper, and the soft shale of the matrix has a very slight cohesive power.

Both sides of specimen 235 were figured, but the dorsal side was apparently then some-

SPECIMENS OF CKYPTOLITHUS. l6l

what less fully developed than at present. On plate 9 are two figures in which specimens
235 and 236 are brought together again, and both dorsal and ventral sides illustrated.

On the dorsal side, specimen 235 shows portions of three exopodites which lie in a
direction roughly parallel to the outer portions of the endopodites on the lower side, that is,
their direction if projected would reach the axis in an acute angle back of the end of the
pygidium. The seta? stand at right angles to the shaft, and on a portion of it 0.5 mm. long
there are seven of them. This is a fragment of an exopodite near the front of the thorax,
and the setae, which are flattened, are about 1.63 mm. long.

On the ventral side this same specimen shows incomplete endopodites and exopodites of
about seventeen segments, six of which would belong to the thorax and the remainder to
the pygidium. The greater part of the appendages belonging to the pygidium are exceedingly
small (about 0.15 mm. long) and so incompletely exposed that the structure can not be
definitely made out.

The endopodites of the thoracic segments all lack the greater part of their proximal
segments and are all of practically the same form. They turn abruptly backward at the
outer end of the meropodite, and the carpopodite of each is greatly widened, projects inward
and is armed with tufts of spines. The propodite and dactylopodite are wide, flattened,
and taper but slightly outward, the dactylopodite bearing on its distal end a tuft of spines.
On several of the endopodites, the meropodites are visible and they bear on their inner ends
fringes of spines pointing inward. Behind these well preserved appendages the proximal
segments of several endopodites are visible, and a regular succession of flattened, oval bodies
armed with numerous forward-pointing spines. These latter bodies Professor Beecher took
to be leaf-like exopodites, which they certainly resemble, and as they lie beyond the line of
endopodites they probably do belong to the outer halves of the appendages.

The exopodites under the thorax are long, the shaft shows numerous short segments,
and is in each case bent backward, though not through a right angle. They extend consid-
erably beyond the endopodites. The seta? do not diverge from the shaft at a right angle
as on the dorsal side of this same specimen, but at an acute angle, indicating that they were
not rigid. The individual hairs are broad and blade-shaped, frequently with a linear depres-
sion along the median line, perhaps due to collapse of the internal tube.

Measurements: The greatest length of the fragment in its present state is 5 mm. The
dactylopodite of the second endopodite (without terminal spines) is 0.18 mm. long, the propo-
dite 0.23 mm. long and 0.15 mm. wide; the carpopodite is 0.24 mm. long and 0.38 mm. wide.
All measurements were made on the photographs.

' Specimen No. 236 (pi. 7, figs. 3-5; pi. 9, figs, i, 2); (text fig. 45).

The right half of the same thorax and pygidium as specimen No. 235.

The specimen is cleaned from both upper and lower sides and, the dorsal test being re-
moved, reveals the long blade-like seta? of the exopodites, each blade being concave along
its median line. They are long on the exopodites of the thoracic segments, but become shorter,
without, however, any visible change of form on the pygidium. Although the posterior end
is not well preserved, one gets no suggestion from a study of this side of the specimens that
the exopodites of the posterior end are in any striking way different from those of seg-
ments further forward. The tips of some of the seta? show minute spines, one to each
blade.

I 62

THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

On the ventral side are a number of endopodites, but they are more fragmentary than
those of the other half of the specimen. Some of the exopodites are well shown, the blades
being in all cases broken from the shaft. Two of the endopodites of this specimen are of
especial interest, as they have interarticular membranes between the last three segments.
Professor Beecher made a drawing of one of these which he placed under his pen drawing
(text fig. 45).

Measurements: The specimen is 5 mm. long from the front of the second thoracic
segment to the end of the pygidium. The setae on the exopodites of the anterior thoracic
segments are 1.7 mm. long, as exposed from the dorsal side. Some of those on the posterior
part of the pygidium, only incompletely exposed, are 0.31 mm. long.

The dactylopodite of the first endopodite showing the articular membranes is 0.23 mm.

'

r±~::::r:._ L
'. >'!

1

— ~ ;t

- — •'" Y

v/

!

-7} —

\

.: ; ->• /

..'--" >• ~"s

\

Fig. 46. — Crypolithus tessellatus Green. A part of a thorax and pygidium,
showing appendages. Drawn by Professor Beecher. Specimen 238. X 10.

long and 0.13 mm. wide. The propodite is of the same length and 0.17 mm. wide. The
interarticular membrane between them is 0.066 mm. thick. The spines on the dactylopodite
of this appendage are 0.15 mm. long. All measurements were made on photographs.

Specimen No. 238 (pi. 8, fig. 4; text fig. 46).

A triangular specimen consisting of the greater part of a pygidium and parts of all the
thoracic segments. Under the thorax the specimen has been so cleaned that the outer por-
tions of the endopodites are well shown, while under the pygidium the greater part of the
endopodites seem to have been removed, disclosing the set£e of the exopodites. As in other
specimens, the endopodites of the thorax turn backward at the distal end of the carpopo-
dite, which is broad and curved, and bears a tuft of spines on the posterior margin. The
dactylopodites seem to preserve their natural shape, and are very nearly cylindrical in form.
Under the pygidium are several sets of overlapping fringes of setae of exopodites, and along
the edge of the dorsal furrow, a number of fragments of segments of what may be coxopo-
dites while with them are a number of fragmentary shaft of exopodites.

Measurements: The pygidium is 3.3 mm. long, the thorax 3 mm.

BIBLIOGRAPHY.
AGASSIZ, L.

1873.— Discovery of the basal joint of legs of trilobites. Amer. Nat., vol. 7, pp. 741-742.

ANGELINA N. P.

1854. — Palasontologia Scandinavica, pars i, Crustacea formationis transitionis.

AUDOUIN, J. V.

1821. — Recherches sur les rapports naturels qui existent entre les trilobites et les animaux

articules. Ann. Gen. Sci. Phys. Nat. Bruxelles, vol. 8, p. 233, pi. 26.
1822. — Isis (Encycl. Zeitung), Oken., vol. 10, p. 87, pi. i, No. 4, figs. 1-5.

BARRANDE, J.

1852,. — Systeme Silurien du centre de la Boheme, vol. i, pp. 226-230, and 629, pi. 30,

figs- 38, 39-
1872. — Ibid., vol. i, Suppl., p. 180, pi. 4.

BARTH, HERMANN VON.

1875. — Die Stellung der Trilobiten in zoologischen Systeme. Das Ausland, 26. Jahrg.,

P- 25-
BEECHER, C. E.

1893. — On the thoracic legs of Triarthrus. Amer. Jour. Sci. (3), vol. 46, pp. 367-370,

467-470, text figs. 1-3.

1894 A.- — On the mode of occurrence, and the structure and development of Triarthrus
becki. Amer. Geol., vol. 13, pp. 38-43, pi. 3.

1894 B. — The appendages of the pygidium of Triarthrus. Amer. Jour. Sci. (3), vol. 47,

pp. 298-300, pi. 7, text fig, i.

1895 A. — Further observations on the ventral structure of Triarthrus. Amer. Geol., vol.

15, pp. 91-100, pis. 4-5.

1895 B. — Structure and appendages of Trinucleus. Amer. Jour. Sci. (3), vol. 49, pp.
307-311, pi. 3.

1895 G — The larval stages of trilobites. Amer. Geol., vol. 16, pp. 166-197, pis. 8-10.

1896 A. — The morphology of Triarthrus. Amer. Jour. Sci. (4), vol. i, pp. 251-256, pi.

8; Geol. Mag., dec. 4, vol. 3, pp. 193-197, pi. 9.

1896 B. — On a supposed discovery of the antennas of trilobites by Linnaeus in 1759. Amer.

Geol., vol. 17, pp. 303-306, text figs. 1-3.

1897 A. — Outline of a natural classification of trilobites. Amer. Jour. Sci. (4), vol. 3, pp.

89-106, 181-207, pi. 3.
1897 B. — Remarks on Kingsley's "Systematic position of the trilobites." Amer. Geol.,

vol. 20, pp. 38-40.
1900. — Trilobita. Eastman-Zittel Text-book of Paleontology, vol. i, pp. 607-638, text

figs. 1261-1331; ed. 2, 1913, p. 700. London.
1901. — Structure and development of trilobites. In "Studies in Evolution," pp. 109-225.

New York and London.
1902. — The ventral integument of trilobites. Amer. Jour. Sci. (4), vol. 13, pp. 165-174,

pis. 2-5, text fig. i; Geol. Mag., dec. 4, vol. 9, pp. 152-162, pis. 9-11, text

figs. 1-8.

164 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

BERNARD, H. M.

1892. — The Apodidae.

1893. — Trilobites with antennae at last! Nature, vol. 48, p. 582.

1894. — The systematic position of the trilobites. Quart. Jour. Geol. Soc., London, vol. 50,

pp. 411-434, text figs. 1-17.

1895 A. — The zoological position of the trilobites. Science Prog., vol. 4, pp. 33-49.
1895 B. — Supplementary notes on the systematic position of the trilobites. Quart. Jour.
Geol. Soc., London, vol. 51, pp. 352-360, figs. A-C.

BEYRICH, E.

1846. — Untersuchungen ueber Trilobiten. 2. Stuck, p. 30, pl. 4, fig. ic.

BILLINGS, E.

1870. — Notes on some specimens of Lower Silurian trilobites. Quart. Jour. Geol. Soc.,
London, vol. 26, pp. 479-486, pis. 31-32. Abstract in Geol. Mag., vol. 7, p
291, and Nature, vol. 2, p. 94.

BRONGNIART, A.

1822. — Histoire naturelle des crustaces fossiles. Paris.

BRUNNICH, F. E.

1781. — Beskrivelse over trilobiten, en dyreslaegt og dens arter, med en ney arts aftegning.
Nye Samlig of det Kong. Danske Vidensk. Selskabs. Skriften, Copenhagen.

BURLING, L. D.

1916. — Ptedeumias and the Mesonacidae, with description of a new species, having at least
44 segments, from the Lower Cambrian of British Columbia. Ottawa Nat.,
vol. 30, pp. 53-58, pl. i.
1917. — Was the lower Cambrian trilobite supreme? Ibid., vol. 31, pp. 77-79, text figs. 1-2.

BURMEISTER, H.

1843. — Die Organisation der Trilobiten. Berlin.

1846. — The organization of trilobites, deduced from their living affinities. Eng. transla-
tion, Ray Society, London.

CALMAN, W. T.

1909. — Crustacea, in "A treatise on zoology," edited by Sir Ray Lankester. London.
1919. — Dr. C. D. Walcott's researches on the appendages of trilobites. Geol. Mag., dec.
6, vol. 6, pp. 359-363, P^ 8, text fig. i.

CARPENTER, G. H.

1903.- — On the relationships between the classes of Arthropoda. Proc. Roy. Irish Acad.,
vol. 24, pp. 320-360, pl. 6.

CASTELNAU, F. DE.

1843. — Systeme Silurien de 1'Amerique Septentrionale, p. 15, pl. 2, figs, i, 4.

CLARKE, J. M.

1888. — The structure and development of the visual area in the trilobite, Phacops rana
Green. Jour. Morph., vol. 2, pp. 253-270, pl. i.

CRAMPTON, G. C.

1916. — The phylogenetic origin and the nature of the wings of insects, according to the
paranotal theory. Jour. New York Entomol. Soc., vol. 24, pp. 1-39, pis. I, 2.

BIBLIOGRAPHY. 165

1919. — The evolution of the arthropods and their relatives, with especial reference to
insects. Amer. Nat, vol. 53, pp. 143-179.

DALMAN, J. W.

1826. — Om Pakeaderna eller de sa kallade Trilobiterna. Stockholm, Acad. Handl., pp.

113-152, 226-294.
1828. — Ueber die Palaeaden, oder die sogennanten Trilobiten. Nuremberg.

DANA, J. D.

1871. — On the supposed legs of the trilobite, Asaphus platyccphalus. Amer. Jour. Sci.
(3), vol. i, pp. 320-321, 386; Ibid. (3), vol. 3, 1872, pp. 221-222. Also
printed in Ann. Mag. Nat. Hist, vol. 7, 1871, pp. 366, 451.
DEKAY, J. E.

1824. — Observations on the structure of trilobites, and description of an apparently new
genus. Ann. Lye. Nat. Hist. New York, vol. i, p. 174, 2 pis. ; Isis (Encycl.
Zeit. ), Oken, 1825 and 1832.
DOLLO, L.

1910. — La paleontologie ethologique. Bull. Soc. Beige de Geol., Pal., et d'Hydrol., vol.
23, PP- 377-421, figs. 1-13, pis. 7-1 1.

ElCHWALD, E. VON.

1825. — Geognostico-zoologioe per Ingriam Marisque Baltici Provincias nee non de trilo-

bitis observationes. Section 45.
1858. — Beitrage zur geographischen Verbreitung der fossilen Thiere Russlands. Bull. Soc.

Imp. des Natural, de Moscou, vol. 30, 1855-1857, p. 204.
1860. — Lethaea Rossica, pi. 21.
1863. — Beitrage zur nahern Kenntniss der in meiner Lethrea Rossica beschriebenen Ilkenen.

Bull. Soc. Imp. des Natural, de Moscou, vol. 36, p. 408.

EMMRICH, H. F.

1839. — De trilobitis dissertatio petrefactologica, etc. Berlin.

EXNER, S.

1891. — Die Physiologic der facettirten Augen von Krebsen und Insecten. Leipzig and

Vienna. Pp. 33-35, pi. 2, figs. 18-19.
FINCH, G. E.

1904. — Notes on the position of the individuals in a group of Nilcus mgilans found at
.Elgin, Iowa. Proc. Iowa Acad. Sci. for 1903, vol. n, pp. 179-181, pi. 14.

GEGENEAUR, C.

1878. — Elements of comparative anatomy. Eng. ed. (Bell and Lankester). London.

GOLDFUSS, A.

1828. — Observation sur le place qu'occupent les trilobites dans le regne animal. Ann. Sci.

Nat., Zoologie, vol. 15, p. 83, pi. 2, figs. 5, 7, 9, 10.
GREEN, J.

1839 A. — The inferior surface of the trilobite discovered. The Friend, Philadelphia,

March 16.
1839 B. — The inferior surface of the trilobite discovered. Illustrated, with colored models.

Philadelphia.
1839 C. — Remarks on the trilobites. Amer. Jour. Sci. (i), vol. 37, p. 25 et scq.

l66 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES.

1840. — An additional fact, illustrating the inferior surface of Calymenc bufo. Ibid., vol. 38,
p. 410.

HANDLIRSCH, A.

1906. — Ueber Phylogenie der Arthropoden. Verhandl. d. k. k. zool.-bot. Gesell., Vienna,

Jahrg. 1906, pp. 88-103.

1907. — Functionswechsel einiger Organe bei Arthropoden. Ibid., Jahrg. 1907, pp. 153-158.
1908. — Die fossilen Insekten. Leipzig.
1914. — Eine interessante Crustaceenform aus der Trias der Vogesen. Verhandl. d. k. k.

zool.-bot. Gesell., Vienna, Jahrg. 1914, pp. 1-7, pis. i, 2.

HAWLE, I., and CORDA, A. J. C.

1847.- — Prodrom einer Monographic der boehmischen Trilobiten, pp. 9, 24, 56, pi. 2, fig.
10; pi. 3, fig. 15; pi. 4, fig. 33b-g.

JAEKEL, O.

1901. — Beitrage zur Beurtheilung der Trilobiten, Theil I. Zeits. d. deutsch. geol. Gesell.,
Bd. 53, pp. 133-171. Pis. 4-6, text figs. 1-30.

KlNGSLEY, J. S.

1897.— The systematic position of the trilobites. Amer. Geol., vol. 20, pp. 33-38.

KOENEN, A. VON.

1872. — Ueber die Organisation der Trilobiten. Verhandl. d. naturhist. Ver. d. preuss.

Rheinl. u. Westphalen, vol. 29, C, pp. 93-95.
1880. — Ueber die Unterseite der Trilobiten. Neues Jahrb. f. Min.., Geol., u. Pal,, Bd. i,

pp. 43°-432> pl- 8.
LANG, A.

1891. — Text-book of comparative anatomy, Eng. ed. (Bernard). London.

LANKESTER, E. R.

1 88 1. — Observations and reflections on the appendages and on the nervous system of
A pus cancriformis. Quart. Jour. Micros. Soc., vol. 21, pp. 343-376.

LAURIE, M.

1911. — A reconstructed trilobite. Nature, vol. 88, p. 26.

LlNDSTROEM, G.

1901.- — Researches on the visual organs of the trilobites. K. svenska Vet-Akad. Handl.,
new ser., vol. 34, pp. 1-86, pis. 1-6.

LINNE, K.

1759. — Petrificatet entomolithus paradoxus sadant, som det finnes uti Hans Excellence Riks.
Radets Hogoalborne Herr Grefve C. G. Tessins Samling. K. svenska Vet.-
Akad. Handl., vol. 20, pp. 21, 22, pl. i, fig. i.

McCoY, F.

1846. — A synopsis of the Silurian fossils of Ireland, p. 42.

MATTHEW, W. D.

1893. — On antenn;e and other appendages of Triarthrus becki. Amer. Jour. Sci. (3),
vol. 46, pp. 121-125, pl. i; Trans. New York Acad. Sci., vol. 12, pp. 237-
241, pl. a

MlCKLEBOROUGH, J.

1883. — Locomotory appendages of trilobites. Jour. Cincinnati Soc. Nat. Hist., vol. 6, pp.

BIBLIOGRAPHY. 167

200-204; Geol. Mag., dec. 3, vol. i, 1884, pp. 80-84; Amer. Jour. Sci. (3),
vol. 27, 1884, p. 409. Reviewed by Dames, Neues Jahrb. f. Min., Geol., u.
Pal., Bd. i, 1885, p. 477.

MILLER, S. A.

1880. — Silurian ichnolites, with definitions of new genera and species. Jour. Cincinnati
Soc. Nat. Hist, vol. 2, pp. 217-218, fig.

MILNE-EDWARDS, H.

1 88 1. — Compte rendu des nouvelles recherches de M. Walcott relatives a la structure des
trilobites, suivi de quelques considerations sur 1'interpretation des faits ainsi
constates. Ann. Sci. Nat, Zoologie, ser. 6, vol. 12, pp. 1-33, pis. 10-12.
Paris.

MOBERG, J. C.

1902. — Bidrag till Kannedomen om trilobiternas byggnad. Geol. Foren Forhandl., Bd.
24, pp. 295-302; pi. 3, text fig. i.

1907. — Om ett gatfultt fossil fran sveriges olenidskiffer samt en kort ofversigt af vikti-
gase data rorande trilobiternas ventrala skelettdelar. Ibid., Bd. 29, Heft 5,
pp. 265-272, pi. 4, fig. 2; pi. 5, fig. i,

CEHLERT, D. P.

1896. — Resume des derniers travaux sur 1'organisation et le developpement des trilobites.
Bull. Soc. Geol. France, ser. 3, vol. 24, pp. 97-116, text figs. 1-34.

PACKARD, A. H.

1872. — On the development of Limulus polyphemus. Mem. Boston Soc. Nat. Hist., vol. 2,

pp. 155-202, pis. 3-5.

1880. — The structure of the eye of trilobites. Amer. Nat., vol. 14, pp. 503-508.
1882. — On the homologies of the crustacean limb. Ibid., vol. 16, pp. 785-799, figs, u, 12.

PANDER, C.

1830. — Beitrage zur Geognosie des russischen Reiches. St. Petersburg.

PEACH, B. N.

1882. — On some fossil myriopods from the Lower Old Red Sandstone of Forfarshire.

Proc. Roy. Physical Soc., Edinburgh, vol. 7, pp. 177-187, pi.. 2.
1899. — O° some new myriopods from the Pakeozoic rocks of Scotland. Ibid., vol. 14,

pp. 113-126, pi. 4.

QUENSTEDT, A.

1837. — Beitrag zur Kenntniss der Trilobiteri, mit besonderer Rucksicht auf ihre bestimmte
Gliederzahl. Archiv f. Naturg., Berlin, 3. Jahrg., I Bd., pp. 337-352.

RAYMOND, P. E.

1910. — On two new trilobites from the Chazy near Ottawa, Ontario. Ottawa Nat., vol.

24, pp. 129-134, pi. 2.
1917. — Beecher's classification of trilobites, after twenty years. Amer. Jour. Sci. (4), vol.

43, pp. 196-210, text figs. 1-3.

RAYMOND, P. E., and BARTON, D. C.

1913. — A revision of the American species of Ceraurus. Bull. Mus. Comp. Zool., vol. 54,
PP- 52S-543. Pis- J. 2, 3 text fi§s-

1 68 THE APPENDAGES, ANATOMY, AND RELATIONS OF TKILOBITES.

REED, F. R. C.

1916. — The genus Trinuclcus. Pt. 4. Geol. Mag., dec. 6, vol. 3, pp. 121, 122.

RICHTER, R.

1848. — Bitrag zur Pakeontologie des Thiiringer Waldes. Dresden and Leipzig.

RlNGUEBERG, E. N. S.

1886. — A trilobite track illustrating one mode of progression of the trilobites. Proc.
Amer. Assoc. Adv. Scu, vol. 35, p. 228 (abstract only).

RUEDEMANN, R.

1916. — The presence of a median eye in trilobites. Bull. New York State Mus., No. 189.
pp. 127-143, pis. 34-36.

SCHLOTHEIM, E. F. VON.

1823. — Nachtrage zur Petrefactenkunde, II. Gotha.

Six, ACHILLE.

!884. — Les appendices des trilobites d'apres M. Ch. D. Walcott. Ann. Soc. Geol. du
Nord, vol. ii, pp. 228-236.

SPENCER, W. K.

1903. — The hypostomic eyes of trilobites. Geol. Mag., dec. 4, vol. 10, pp. 489-492.

STAFF, HANS v., and RECK, HANS.

1911. — Ueber die Lebensweise der Trilobiten. Eine entwicklungsmechanische Stuclie.
Gesell. naturforsch. Freunde, Sitzb., pp. 130-146, figs. 1-20.

STERNBERG, K. M.

Ig3o. — Ueber die Gliederung und die Fiisse der Trilobiten. Isis (Encycl. Zeitung), Oken,

p. 516, pi. 5, figs. 1-3.
STOKES, C.

1823. — On a trilobite from Lake Huron. Trans. Geol. Soc., London, ser. 2, vol. i, p.
208, pi. 27.

SWINNERTON, H. H.

1919. — The facial suture of the trilobite. Geol. Mag., dec. 6, vol. 6, pp. 103-110.

TORNQUIST, S. L.

1896 A. — On the appendages of trilobites. Ibid., dec. 4, vol. 3, p. 142.
1896 B. — Linn;eus on the appendages of trilobites. Ibid., pp. 567-569.

TOTHILL, J. D.

1916.— The ancestry of insects, with particular reference to chilopods and trilobites. Amer.
Jour. Sci. (4), vol. 42, pp. 373-383. text ngs- I~8-

TROEDSSON, G. T.

1918. — Om skanes Brachiopodskiffer. Lunds Universitets Arsskrift, n. f., Avd. 2, Bd. 15,

Nr- 3- PP- 57-67, Pi- i, ngs- 19~24-
VALIANT, W. S.

1901. — Appendaged trilobites. The Mineral Collector, vol. 8, No. 7, pp. 105-112.

VOLBORTII, A. VON.

1858. — Ueber die Bewegungs-Organe der Trilobiten. Verhandl. russ. k. mineral. Gesell.

zu St Petersburg, 1857-1858, p. 168.
1863. — Ueber die mit glatten Rumpfgliedern versehenen russischen Trilobiten, nebst einem

BIBLIOGRAPHY. 169

Anhange ueber die Bewegungs-organe und ueber das Herz derselben. Mem.
Acad. Imp. Sci. St. Petersburg, sen 7, vdL 6, No. 2, pp. 44-47, pi. i, fig. 12.
1866. — Ueber Herrn von Eichwald's Beitrag zu naheren Kenntniss der Illsenen. Bull.
Soc. Imp. des Natural, de Moscou, vol. 39, p. 40.

WAHLENBERG, G.

1821. — Petrificata telluris Suecana examinata a Georgio Wahlenberg. Nova Acta Reg.
Soc. Scient. Upsala, vol. 8.

WALCOTT, C. D.

1875. — Description of the interior surface of the dorsal shell of Ceraurus pleurexanthemus

Green. Ann. Lye. Nat. Hist. New York, vol. n, pp. 159-162, pi. u.
1876. — Preliminary notice of the discovery of the natatory and branchial appendages

of trilobites. 28th Rept. New York State Mus. Nat. Hist., adv. sheets, pp.

89-92; published as full report in 1879.
1877. — -Notes on some sections of trilobites. 3ist Rept. New York State Mus. Nat. Hist.,

adv. sheets, pp. 61-63, pi. i ; published as full report in 1879. Reviewed by

Dames, Neues Jahrk f. Min., Geol., u. Pal., Bd. i, 1880, p. 428.
1879. — Notes upon the legs of triiobites. 3ist Rept. New York State Mus. Nat. Hist.,

adv. sheets, p. 64.
1 88 1. — The trilobite : New and old evidence relating to its organization. Bull. Mus. Comp.

Zool., vol. 8, pp. 192-224, pis. 1-6.
1884. — The appendages of the trilobite. Science, vol. 3, pp. 276-279, figs. 1-3. Reviewed

by Dames, Neues Jahrb. f. Min., Geol., u. Pal.., Bd. i, 1885, Referate, p. 102.
1894. — Note of some appendages of the trilobites. Proc. Biol. Soc. Washington, vol. 9,

pp. 89-97, pi. i; Geol. Mag., dec. 4, vol. i, pp. 246-251, pi. 8.
1911. — Middle Cambrian Merostomata. Smithson. Misc. Coll., vol. 57, No. 2, pp. 17-40,

pis. 2-7.
1912 A. — Middle Cambrian Branchiopoda, Malacostraca, Trilobita, and Merostomata.

Ibid., No. 6, pp. 145-228, pis. 24-34, text figs. 8-10.

19126. — New York Potsdam-Hoyt fauna. Ibid., No. 9, pp. 251-304, pis. 37-49.
1913. — Eastman-Zittel Text-book of Paleontology, ed. 2, vol. i, figs. 1343, 1376, 1377.
1916. — Ann Rept., Secretary Smithsonian Inst, for 1915, pi. 9.
1918. — Appendages of trilobites. Smithsonian Misc. Coll., vol. 67, No. 4, pp. 115-226,

pis. 14-42.
WATASE, S.

1890. — On the morphology of the compound eyes of arthropods. Johns Hopkins Univ.,

Studies from Biol. Lab., vol. 4, no. 6, p. 290 (footnote).

WOODWARD, II.

1870. — Note on the palpus and other appendages of Asaphus, from the Trenton limestone,

in the British Museum. Quart. Jour. Geol. Soc., London, vol. 26, pp. 486-488,

fig. i. Abstract in Geol. Mag., dec. i, vol. 7, p. 292, also in Nature, vol. 2,

p. 94.

1871. — On the structure of trilobites. Geol. Mag., dec. i, vol. 8, pp. 289-294, pi. 8.
1884. — Notes on the appendages of trilobites. Geol. Mag., dec. 3, vol. i, pp. 162-165, 2

text figs.
1895. — Some points in the life history of the Crustacea in early Palseozoic times. Quart.

Jour. Geol. Soc., London, vol. 51, pp. Ixx-lxxxviii, i pi.

PLATE i.
Photographs of Triarthrus bccki, made by C. E. Beecher.

Fig. i. — Specimen 213. The dorsal test has been removed from the glabella, reveal-
ing the outline of the posterior end of the hypostoma, the proximal ends of the antennules,
the gnathites, and incomplete endopodites of some appendages, x 5.43.

Fig. 2. — Specimen 214. The head of a complete large specimen. Part of the thorax
is shown on pi. 3, fig. 6. Note especially the form of the segments of the endopodites and
of the anterior coxopodite on the right side, x 7.33.

Fig. 3.— Specimen 217,. This specimen shows better than any other the form of the
gnathites of the cephalon. Note also the setae of the exopodites under the cheek at the right.
The appearance of a hook on the posterior gnathite on the right may be accidental, but it
does not show broken edges, x 6.85.

Fig. 4. — Specimen 215. The ventral side of the cephalon of a small entire specimen.
Shows well the form of some of the gnathites and a few of the endopodites. Note the
unusual position of the antennules. x 7.63.

Fig. 5. — Specimen 226. This specimen did not photograph well, but is important as
showing the exopodites and endopodites emerging from under the cephalon. x about 6.

MEM. CONN. ACAD., VOL. VII.

PLATE I.

HEUOTYPE CO. BOSTON

PLATE 2.
Photographs of Triarthrus becki, made by C. E. Beecher.

Fig. i. — Specimen 201. The entire specimen, details of which are shown in pi. 3,
fig. 4 and pi. 4, figs, i, 2. The dorsal test has been removed from the anterior segments
on the right side. X4.I2.

Fig. 2. — Specimen 206. A small individual with the endopodites, and the exopodites
minus their setae; well preserved on the left side. Note the position of the antennnles. The
course of the facial suture is unusually well shown, x 10.

Fig. 3.— Specimen 210. The specimen which served as the main basis for Professor
Beecher's first figure of the appendages of the thorax, specimen 206 (fig. 2, this plate) hav-
ing supplemented it. Note the "normal" position of the antennules and the extension of
the appendages from beneath the pleural lobe. Specimens with the antennules in this posi-
tion may possibly be males, x 4.

Fig. 4. — Specimen 205. A small specimen with some of the appendages preserved,
especially toward the posterior end, but particularly valuable for the unusually well pre-
served metastoma. xn.

Fig. 5. — Specimen 211. A small cephalon, cleaned from the ventral side, and show-
ing well the gnathites which approach each other unusually closely on the median line,
x 10.5.

Fig. 6. — Specimen 219. An entire specimen of medium size, developed from the ven-
tral side. It shows particularly well the "normal" curvature of the antennules, the change
in form of the segments of the endopodites from cephalon to pygidium, and, along the axial
lobe, the apodemes of the ventral integument. See also pi. 4, fig. 4. x 3.6.

MEM. CONN. AC AD., VOL. VII.

PLATE II.

PLATE 3.
Photographs of Triarthrus becki, made by C. E. Beecher.

Fig. i. — Specimen 204. See also text fig. 42 and pi. 4, fig. 6. The exopodites and
endopodites of the first few segments of this specimen are better preserved than those of
any other revealing them from the dorsal side, x 9.5.

Fig. 2. — Specimen 220. A large individual exposed from the lower side. It shows
well the endopodites and part of the exopodites, and, rather better than any other specimen,
the endobases of the coxopodites. x 2.4.

Fig. 3. — Specimen 216. A small entire specimen showing considerable of the detail of
the appendages of the cephalon, and some of those of the remainder of the body, x 7.4.

Fig. 4. — Specimen 201. This figure shows the details of the appendages of the left
side and of the pygidium. Note the plate on the median line back of the pygidium, the
sockets for spines, and the terminal spines on the anterior endopodites. See also pi. 2, fig.
i and pi. 4, figs, i, 2. xf.i.

Fig. 5. — Specimen 207. One half of the posterior part of the thorax and pygidium,
showing exopodites and endopodites as seen from the dorsal side, x 7.6.

Fig. 6. — Specimen 214. The exopodites have been turned back nearly parallel to the
axis of the shell. Notice particularly the long flattened sete and the spinose spatula-shaped
terminal portion of each shaft. See also pi. i, fig. 2. x 7.

MEM. CONN. ACAD., VOL. VII.

PLATE HI.

3

HEUOTYPE CO. BOSTON

PLATE 4.
Photographs of Triarthrus becki, made by C. E. Beecher.

Fig. i. — Specimen 201. Another photograph, similar to fig. 4, pi. 3, but showing more
clearly some details of spines on the endopodites. x 12.66.

Fig. 2. — Specimen 201. Three appendages on the right side of the thorax. See also
pi. 2, fig. i and pi. 3, fig. 4. x 12.66.

Fig. 3. — Specimen 223. A small crushed specimen which nevertheless shows well the
appendages of the right side of the thorax, developed from the ventral side. Note coxopo-
dites, exopodites, and endopodites, and that all appendages are moved equally laterally from
their original position, x 11.4.

Fig. 4. — Specimen 219. Another photograph, with different lighting, of the individual
shown in pi. 2, fig. 6. This print brings out better the coxopodites and the folds of the
ventral membrane, x 3.23.

Fig. 5. — Specimen 222. This specimen is interesting, because it shows the endopodites
in what is probably their natural position, that is, in a plane nearly vertical to the plane
of the body, instead of being flattened down, as is usually the case. The appendages under
the pygidium are unusually well preserved, x 12.

Fig. 6.— Specimen 204. Photograph of the entire specimen of which a part is shown
in text fig. 42 and pi. 3, fig. i. X4_5.

MEM. CONN. AC AD., VOL. VII.

PLATE IV.

PLATE 5.
Photographs of Triarthrus bccki, made by C. E. Beecher.

Fig. i. — Specimen 209. Photograph of the pygidium shown in pi. 6, fig. 2. This
specimen shows especially well the way in which the exopodites of the pygidium decrease
in length backward, x 11.5.

Fig. 2. — Specimen 229. The under side of the posterior end of a medium-sized speci-
men, showing the appendages, especially the endopodites. On and among the limbs are
scattered numerous minute spheres of pyrite, of the kind usually known as "trilobite eggs."
They do not show very well in the photograph, but can be made out much more clearly
with a hand lens, x 12.

Fig. 3. — Specimen 230. A specimen showing the appendages of the posterior part
of the thorax and the pygidium. The same individual is also shown in text fig. 44. Note
particularly the form of the segments of the endopodites, and the spines on them, x 13.

Fig. 4. — Specimen 227. The small doubly curved bodies shown in this figure lie under
the axial portion of the cephalon and anterior part of the thorax. The specimen still has
a very thin coating of matrix between it and the shell. Whether the curved bodies have
anything to do with the trilobite is not known, x about 12.

Fig. 5. — Specimen 221. A small individual which shows well the exopodites of the
posterior part of the thorax. Note the spatulate terminations and the spines of the shaft,
x ii.

Fig. 6. — Specimen 202. Posterior part of the thorax and pygidium, showing endopo-
dites and exopodites projecting under the dorsal test. Note the spiniferous plate on the median
line, and the large opening in the anterior portion of it. x 9-75-

MEM. CONN. ACAD., VOL. VII.

PLATE V.

HELtOTYPE CO. BOSTON

PLATE 6.
All figures except 4 and 5, from photographs by C. E. Beecher.

Fig. i. — Triarthrus bccki. Specimen 203. A well preserved small individual, showing
the appendages of the right side of the thorax, x 11.46.

Fig. 2. — Triarthrus bccki. Specimen 209. A well preserved individual, showing the
antennules and some appendages of thorax and pygidium. For detail of the pygidium, see
pi. 5, fig. i. <4.

Fig. 3. — Triarthrus bccki. Specimen 218. Ventral side of the pygidium and greater
part of the thorax of an individual of medium size. Note especially the relation of exopo-
dites to endopodites of the'last two thoracic segments. A drawing of these appendages is
shown on text fig. 43. x 4,3.

Figs. 4 and 5. — Endopodites, probably from a species of Calyincnc. These specimens,
with several others, are on a small slab of limestone from the Point Pleasant (Trenton) beds

opposite Cincinnati, Ohio. Specimen in the U. S. National Museum. Photographs by R. S.

n

'>:

II

Bassler.

Fig. 6. — Acidaspis trentoncnsis Walcott. Both the specimen, No. 245, and the photo-
graph are poor, but show that in this genus the endopodites are like those of Triarthrus.
x8.5.

Fig. 7. — Cryptolitlius tessellatus Green. Specimen 234. This specimen shows well the
backward directed antennules and also the outer segments of some of the cephalic endopo-
dites. XII.

MEM. CONN. AC AD., VOL. VII.

PLATE VI.

HELIOTYPE CO. BOSTON

PLATE 7.
Photographs of Cryptolithus tessellatus Green, made by C. E. Beecher.

Fig. I.— Specimen 233. The best preserved individual, the one from which Professor
Beecher's drawing (text fig. 45) was made, and which served as the principal basis for the
restoration (text fig. 20). Note the long, backward directed antennules, the abrupt back-
ward turn of the outer portions of the endopodites, the way in which the exopodites extend
beyond the endopodites, and the fact that alt are beneath the cover of the dorsal shield.
The hypostoma is turned entirely around, x 10.9.

Fig. 2. — Specimen 235. Half of the thorax and pygidium, with the appendages re-
vealed from the ventral side. Note the abrupt manner in which the outer portions of the
endopodites are turned backward. See also pi. 8, fig. 3, and pi. 9, fig. i (right half), x 14.45.

Fig. 3. — Specimen 236. Detail from fig. 4, to show the blade-like sete of the exopo-
dites and the numerous terminal spines of the endopodites. x 30.

Fig. 4. — Specimen 236. The appendages of the thorax and pygidium, seen from the
lower side. Specimen 236 is the right half of the same individual from which specimen 235
was obtained. Note the interarticular membranes between the segments of the endopodites
and the blade-like setae of the exopodites. See also pi. 9, fig. i (left side), x 19.

Fig. 5. — Specimen 236. The same specimen, seen from the dorsal side, showing, when
the test is removed, the long blade-like seta; of the exopodites. See also pi. 9, fig. 2 (right
half), x 19.

MEM. CONN. ACAD., VOL. VII.

PLATE VII.

HELIDTYPE CO.

PLATE 8.
Photographs of Cryptolithus tessellatus Green, made by C. E. Beecher.

. L — Specimen 231. A nearly complete individual, cleaned from the ventral side
and showing obscurely the hypostoma and fragments of numerous appendages. Note the
lines of appendifers along the sides of the axial lobe, x 1 1.

Fig. 2. — Specimen 232. Although this is not very well preserved, it shows more of
the cephalic appendages than any other. Even so, only just enough is shown to indicate
that they were similar to those on the thorax, x la.

Fig. 3. — Specimen 235. Dorsal side of the appendages of the thorax and pygidium.
See pi. 7, fig. 2 for the ventral view. On pi. 9, fig. 2 (left side) is a drawing taken from
the same specimen, x 1 1 .

Fig. 4. — Specimen 238. Part of a thorax and pygidium, seen from the ventral side.
The series of heavy segments shown in the upper part do not belong to one appendage, but
are the distal ends of several endopodites. See also text fig. 46 for a drawing of this speci-
men, x 18.

Fig. 5. — Specimen 237. Pygidium and part of the thorax, with some of the appen-
dages. XII.

MEM. CONN. ACAD., VOL. VII.

PLATE VIII.

HELlOTYPi CO. BOSTON

PLATE 9.

Cryptolithus tessellatus Green. Upper drawing by C. E. Beecher ; lower drawing by Miss
F. E. Isham, under the direction of C. E. Beecher.

Fig. i. — Appendages of the thorax and pygidium, seen from the ventral side. These
are not restorations, but drawings from the halved individual numbered 236 (right side of
drawing) and 235. For photographs of these specimens, see pi. 7, figs. 2, 4. x 20.

Fig. 2.— Appendages of the thorax and pygidium, seen from the dorsal side. Same
specimen as in fig. i. For photographs, see pi. 7, fig. 5, and pi. 8, fig. 3. x 20.

MEM. CONN. ACAD., VOL. VII.

PLATE IX.

HEUOTYPE CO. BOSTON

PLATE 10.
From photographs made by C. E. Beecher.

Fig. i. — Isotelusjatus Raymond. Ventral surface of the specimen in the Victoria
Memorial Museum at Ottawa, Canada. Note the large, club-shaped coxopodites and the
more slender endopodites. The first large coxopodite back of the hypostoma belongs to
the last pair of cephalic appendages. The coxopodite of the appendage in front of it is seen
turning in beneath the tip of the hypostoma. x 2.

Fig. 2. — Isotelus maximus Locke. The ventral side of the specimen described by Mickle-
borough and now in the U. S. National Museum. The tips of the hypostoma may be seen
at the front, and the first two pairs of coxopodites behind them belong to the last two pairs
of appendages of the cephalon. Note how much stronger the coxopodites are than the en-
dopodites. The appendages of the pygidium show but poorly, x 1.45.

MEM. CONN. ACAD., VOL. VII.

PLATE X.

HELIOTYPE CO. BOSTON

PLATE ii.

Ccraurus plcurexanthcmns Green. A restoration of the ventral surface and appen-
dages, made by Doctor Elvira Wood, under the supervision of the writer, from data obtained
from the translucent slices prepared and described by Doctor Walcott. x 5.

MEM. CONN. ACAD., VOL. VII.

PLATE XI.

HELIOTVPE CO. BOSTON

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