274 



THE VERTEBRATE SKELETON. 



jaw bones. The upper jaw of Sphenodon and other Rhyn- 

 chocephalia is provided with two parallel rows of teeth, one 

 borne on the maxillae and one on the palatines, the man- 

 dibular teeth bite in a groove between these two rows. The 

 bone of the jaws in Sphenodon is so hard that when the teeth 

 get worn away, it can act as a substitute. In the young 

 Sphenodon the vomers bear teeth, as they do also in Protero- 

 saurus, 



There is generally a continuous succession of teeth through- 

 out life, the new tooth coming up below, or partly at the side 

 of the one in use, and causing the absorption of part of its 

 wall or base. In this way the new tooth conies to lie in the 

 pulp cavity of the old one. This method of succession is well 

 seen in the Crocodilia. 



FIG. 50. PBEPABATION OP PART OF THE BIGHT MANDIBULAB BAMUS or 

 Crocodilus palustris x \. (Brit. Mus.) 



1. tooth in use. 



2. fairly old germ of future 



tooth. 



3. symphysial surface of the 

 mandible. 



Teeth have been detected in embryos of Trionyx, but 

 otherwise no teeth are known to occur in Chelonia, or in 

 Pteranodon (Pterosauria), while the anterior part of the jaw 

 is edentulous in Iguanodon, Polyonax and some other Dino- 

 saurs, and in Rliamphorhynchus. 



