ILLUSTRATIONS 



FIGURR PAGE 



18. — Cross-section of Labyrinthodont Teeth 



following 30 



A. Lobe-finned Devonian fish (Polyplocodus). 

 (After Pander.) 



B. Primitive amphibian of Carboniferous age 

 {Loxomma allmani). (After Embleton and Atthey.) 



19. — Two Critical Stages in the Early Evolu- 

 tion OF THE Skull ..... 33 



A. Generalized reptile (Seymouria), retaining the 

 full complement of amphibian skull elements. The 

 temporal region, covering the upper jaw muscles, is 

 still covered with a shell of bone as in primitive am- 

 phibians and fishes. The otic notch (where the tym- 

 panum, or drum membrane, was attached) is retained. 

 (After data of Broili, Watson, Williston.) 



B. Primitive theromorph reptile (Mycterosaurus) 

 with reduced number of skull elements and perforated 

 temporal roof. The otic notch has disappeared. 

 (After Williston.) 



20. — Skulls of Earlier and Later Mammal- 

 like Reptiles from South Africa . . 35 



A. Scymnognathus, a primitive, more reptile-like 

 member of the therapsid series. 



B. Ictidopsis, a more advanced mammal-like mem- 

 ber of the same series. Specimens in the American 

 Museum of Natural History, with data from Broom, 

 Watson, Haughton. 



21. — Progressive Upgrowth of the Dentary 

 Bone of the Lower Jaw to Form a New 

 Joint with the Skull .... 37 



A. Primitive mammal-like reptile (Scymnogtiathus). 



B. Advanced mammal-like reptile (Ictidopsis). 



C. Primitive mammal (Thylacinus). 



22. — Origin of the Interarticular Disc, or 

 Meniscus, Lying Between the Lower 

 Jaw and Its Socket in the Skull . . 38 



(After Gaupp.) 



xvi 



