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 



