CHELONIAN REPTILES. 4(J3 



not attained that exclusive adaptation to a ter- 

 restrial existence, which we find in the higher 

 orders of the Mammalia. But before proceed- 

 ing to consider these, we have to notice a sin- 

 gular group of animals, whose conformation 

 appears to be exceedingly anomalous, and as if 

 it interrupted the regularity of the ascending 

 series, of which it seems to be a collateral rami- 

 fication. 



§ 5. Chelonia. 



The order of Chelonian Reptiles, which com- 

 prises all the tribes of Tortoises and Turtles, 

 appears to constitute an exception to the general 

 laws of conformation, which prevail among Ver- 

 tebrated Animals : for instead of presenting a 

 skeleton wholly internal, the trunk of the body 

 is found to be enclosed on every side in a bony 

 case, which leaves openings only for the head, 

 the tail, and the fore and hind extremities. 

 That portion of this osseous expansion which 

 covers the back is termed the Carapace; and 

 the flat plate which defends the lower part of 

 the body is termed the plastron. It is a form of 

 structure that reminds us of the defence j^ro- 

 vided for animals very low in the scale of or- 

 ganization, such as the echinus, the Crustacea, 

 and the bivalve moUusca. Yet the substance 



