io6 ANIMAL LIFE PAST AND PRESENT. 



size. In all Chelonians, on the contrary, teeth are 

 totally wanting, and their function is consequently 

 performed by the margins of the jaws, which form 

 sharp cutting-edges, and are ensheathed with a coating 

 of thick horn. This total absence of teeth is well 

 shown in Fig. 27, and still better in Fig. 31, where the 

 bones of the skull are represented with the flesh and 

 skin removed. Another remarkable peculiarity of the 

 Chelonian skull is found in the circumstance that the 

 two sides of the lower jaw are firmly united together 

 by bone at the chin, instead of being more or less 

 completely separated, as in all other living reptiles. 

 A third peculiarity is the development of the hinder 

 extremity of the skull into a long spine-like process 

 (Fig. 31, 0), which gives a very characteristic contour 

 to this part. The whole of the skull is covered over 

 with horny shields (Fig. 27) in the living animal ; so 

 that in this respect the structure of the skull cor- 

 responds exactly with that of the shell. 



In the entire absence of teeth, coupled with the 

 horny sheathing of the jaw, and the solid union of the 

 two bones of the lower jaw at the chin, tortoises 

 resemble birds. Many, or perhaps all, of the birds 

 of the Secondary geological epoch were, however, pro- 

 vided with a complete set of teeth, and it is there- 

 fore highly probable that we shall some day find the 

 fossil remains of extinct tortoises which were also 

 furnished with these useful implements, since there 

 appears to have been a tendency in many groups of 

 animals, and more especially in birds and their near 



