CHELONrA. 563 



sented by the Galapagos lizard, which swims out among the 

 sea-weed ; by some estuarine crocodilians ; by some turtles 

 which live far out at sea, and only seek the shores to lay 

 their eggs ; and by the sea-snakes (Hydrophidse), which 

 never leave the water. 



•r 



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, within the shelter of which the head and neck, tail 

 and lifnbs, can be more or less retracted. 



The dorsal carapace is usually formed from — (a) the 



Fig. 241. — External appearance of tortoise. 



flattened neural spines (plus dermal bones); (b) expanded and 

 more or less coalesced ribs {plus dermal bones ; (c) a series of 

 dermal marginal bones around the outer edge. In the Athecce 

 the dorsal vertebra and ribs are not fused to the dermal plates 

 which form the carapace. The ventral shield or plastron is 

 formed of nine or so dermal bones. There is no sternum. 



Overlapping, but in no way corresponding to the bony 

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

 though very hard, are not without sensitiveness, numerous 

 nerves ending upon them. 



The quadrate is immovably united with the skull. 



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

 teeth, though hints of these have been seen in some embryos. 



The average life of Chelonians is sluggish. Perhaps this is 

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



