ILLUSTRATIONS 



FIGURE PAGS 



C. Tremataspis. (After Patten.) 



D. Pteranpis. (After Powrie and Lankester.) 



E. Cephalaspis. (Composite, mainly after Patten.) 



5. — The Face of the Most Primitive Living 



Shark, Chlamydoselachus anguineus facing 12 



(After Carman.) 



6. — Instruments of Precision in the Head of a 



Shark, Chlamydoselachus anguineus . . 13 



(After Allis.) 



Lateral line canals black, bordered with white; 

 nerves white; muscles streaked; cartilage stippled. 



7. — Cartilaginous Skeleton of Head of Shark 17 



Comprising braincase, primary upper and lower jaws 

 and branchial arches. 



8. — Jaw Muscles of Shark {Chlamydoselachus) . 18 

 Showing the essential similarity of the jaw muscles 

 to the constrictors of the branchial arches. (Com- 

 posite drawing based on the data of Allis and Carman.) 



9. — Cross-section of the Skull of a Fossil Gan- 

 oid Fish, Showing the Bone Cells {Osteo- 

 lepis FROM THE DEVONIAN OF Russia) . facing 20 

 (After Pander.) 



10. — The Wedge-shaped Braincase of a Fish, 

 Acting as a Thrust-Block or Fulcrum 

 FOR THE Backbone ..... 22 



The surface bones of the left cheek region have been 

 removed to show the base of the skull and the elements 

 dependent from it on the right side. (Modified from 

 a drawing of the skull of the Striped Bass by F. A. 

 Lucas.) 



11. — The Facial Armor and Jaws of a Devonian 



Lobe-finned Ganoid Fish (Osteolepis) facing 22 



(After Pander.) 



The skull seen from above. 



xiv 



