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 
b. The same. 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 scnaria 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 
