274 ZOOLOGY 



external temperature compensates in a way for the insuf- 

 ficiency of the mechanism for maintaining from the inside 

 a high blood temperature. Turtles are abundant in 

 Africa, and are much commoner in North America than 

 in Europe. Three families of Chelonia may be distin- 

 guished. A short account of each follows. 



The family Chelonidae includes certain marine turtles. 

 Turtles live in all oceans and may acquire a weight of as 





Fi<;. 2")ii. Hawkbill-turtle. Much reduced. From Brehra. 



I 



much as one thousand pounds. The green-turtle, used in 

 making soup, occurs on the Atlantic coast as far north as 

 Long Island. From the hawkbill-turtle (Fig. 256) comes 

 the tortoise shell used in certain ornaments. In the 

 leather-back the shields are incompletely ossified. 



The family Trionychidae l includes our soft-shelled tur- 

 tles, which live in rivers or ponds of the Mississippi valley 

 and the Gulf drainage basin (Fig. 257). 



1 rpt's, tliriec ; 6w, claw. 



