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