Chapter VI: 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
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