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 Calymene, 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 manuscript. 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 manuscript, 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 Neolenus]21
[Historical]21
[Neolenus serratus (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 appendages to marking on dorsal surface of glabella]43
[Anal plate]44
[The appendages of Ptychoparia]45
[Ptychoparia striata (Emmrich])45
[Ptychoparia cordilleræ (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 branchiæ]48
[Ventral membrane]50
[Appendifers]51
[Calymene senaria Conrad]52
[Cephalic appendages]52
[Thoracic appendages]53
[Pygidial appendages]54
[Relation of hypostoma to cephalon in Calymene]55[12]
[Restoration of Calymene]56
[Calymene 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 Acidaspis trentonensis Walcott]61
[The appendages of Cryptolithus] (see also [Part IV])61
[Cryptolithus tessellatus 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
[Calymene senaria]80
[Cryptolithus goldfussi]80
[Summary]81
[Gastric glands]82
[Summary]84
[Heart]85
[Illænus]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
[Panderian organs]90[13]
[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
[Burgessia bella Walcott]108
[Waptia fieldensis Walcott]108
[Yohoia tenuis Walcott]109
[Opabina regalis Walcott]109
[Summary]109
[Copepoda]110
[Archicopepoda]111
[Ostracoda]112
[Cirripedia]113
[Malacostraca]113
[Phyllocarida]113
[Syncarida]114
[Isopoda]114
[Marrella splendens Walcott]115
[Restoration of Marrella]116
[Arachnida]117
[Trilobites not Arachnida]117
[Merostomata]119
[Sidneyia inexpectans Walcott]119
[Emeraldella brocki Walcott]119
[Molaria and Habelia]120
[Araneæ]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
[Form of the simplest protaspis]132[14]
[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 employed20
[2]Neolenus serratus (Rominger). Two thoracic appendages24
[3]The same. An exopodite26
[4]The same. A so-called "epipodite"26
[5]The same. The so-called "exites"29
[6]The same. A cephalic limb29
[7]The same. Restoration of a transverse section30
[8]The same. Restoration of the ventral surface31
[9]Isotelus. Restoration of the ventral surface38
[10]Triarthrus becki Green. Restoration of the ventral surface41
[11]The same. Median appendage44
[12]Ceraurus pleurexanthemus Green. Slice showing an exopodite49
[13]Calymene senaria Conrad. Slice showing cephalic coxopodites53
[14]The same. Another similar slice53
[15]The same. Slice showing method of articulation of the appendages53
[16]The same. Restoration of the ventral surface55
[17]Ceraurus pleurexanthemus Green. Slice showing the method of articulation of the appendages58
[18]The same. Slice showing an exopodite above an endopodite58
[19]The same. Restoration of a transverse section60
[20]Cryptolithus tessellatus Green. Restoration of the ventral surface63
[21]Ceraurus pleurexanthemus Green. Slice showing the abdominal sheath79
[22]The same. Slice showing the large alimentary canal79
[23]Calymene senaria Green. Slice showing the large alimentary canal79[15]
[24]Ceraurus pleurexanthemus Green. Restoration of a longitudinal section81
[25]Cryptolithus tessellatus Green. Cheek showing the genal cæca84
[26]Illænus. Volborth's figure of the heart85
[27]Heart of Apus85
[28]Isotelus gigas Dekay. The Panderian organs91
[29]Ceraurus pleurexanthemus Green. Restoration, showing heart, alimentary canal, and extensor muscles93
[30]The same. Longitudinal section of cephalon95
[31]Nileus armadillo Dalman. Moberg's figure of the muscle-scars95
[32]Marrella splendens Walcott. Restoration of the ventral surface116
[33]Triarthrus becki Green. Appendage of the anterior part of the thorax126
[34]Apus. Appendage from the anterior part of the trunk127
[35]Weymouthia nobilis (Ford)138
[36]Naraoia compacta Walcott145
[37]Pagetia clytia Walcott145
[38]Asaphiscus wheeleri Meek145
[39]Pædeumias robsonensis Burling145
[40]Robergia sp.145
[41]Diagram showing possible lines of descent of the Arthropoda150
[42]Triarthrus becki Green. Thoracic appendages155
[43]The same. Pygidial appendages157
[44]The same. Pygidial appendages158
[45]Cryptolithus tessellatus Green. Drawing of the best single specimen159
[46]The same. Part of the thorax and pygidium, with appendages162

[Frontispiece.] Charles Emerson Beecher, 1896.