TORTOISES AND TURTLES. 563 



Class Chelonia. Tortoises and Turtles. 



General Characters. — The body is compact and broad 

 in the region of the trunk. There is a dorsal and a ventral 

 shield, ivithin the shelter of which the head and neck, tail 

 and limbs, ca?i be more or less retracted. 



The dorsal shield or carapace is formed from the Pleural 

 spines of the vertebrce, from the expanded ribs, and from a 

 series of marginal plates around the outer edge. 



The ventral shield or plastron consists of nine or so 

 dermal plates. There is no sternum. 



Overlapping, but in no way corresponding to the bony 

 plates, are epidermic horny plates of " tortoise shell" which, 



Fig. 190. — External appearance of Tortoise. 



though very hard, are not without sensitiveness, numerous 

 nerves ending upon them. 



The quadrate is inunovably fixed. 



The jaws are covered by a horny sheath, and are without 

 teeth, though rudiments of these have been seen in some 

 embryos. 



The average life of Chelonians is sluggish. Perhaps this is 

 in part due to the tvay in which the ribs are lost in the 

 carapace, for this must tend to make respiration less active. 



All are oviparous. The eggs have firm usually calcareous 

 shells. 



Some Peculiarities in the Skeleton of Chelonia. 



The dorsal vertebrae seem to be without transverse processes, and 

 along with the ribs are for the most part immovably fused in the 

 carapace. The tail and the neck are the only flexible regions. 



