HIGH SCHOOL ZOOLOGY. 



105 



both the edge of the carapace, and the whole of the plastron are of 

 leathery consistence. These forms lie buried in mud and may remain 

 for hours under water ; their respiration is then efTected by water 

 taken in and rejected through the nostrils, in such a way that the 

 mucf)us memlirane of the pharynx, which is provided with vascular 

 pa[)ill;o arranged on the arches of the visceral skeleton, is constantly 

 bathed with fresh water. This is an interesting point of contact with 

 the Uro<lela. 



9. The feet of the Trionychid«? are broadly webbed, but not converted 

 into flippers as they are in the marine turtles, the Chelonldse. In these 

 the anterior flippers are largest, and the claws are much reduced. One 

 of the genera, Dermatochelys, has a leathery skin in place of the horny 

 shields which are present in the other genera, the green or edible turtle 

 (Chdonia mydas), and the Tortoise-shell turtle (Eretmochelys imhrkatn), 

 (Fig. 7-t), in which latter form the horny shields overlap each other. 



10. Proceeding from Chelydra towards the J;urtles of more terrestrial 

 habit, in all of which the plastron is much more complete than it is in 

 that genus, we come first to the Cinostemidse in which the carapace 

 is more vaulted, although the feet are still webbed, the creatures living 

 for the most part in muddy ponds. The most northerly American form 

 is the Musk turtle {Aromochdys odoratus), the secretion from the 

 cutaneous glands of which has a somewhat offensive musky odour. 

 Closely allied are certain more southerly mud-turtles, which are able to 

 close the shells. 



11. Most of our species 

 of turtles, however, be- 

 long to the Emydidse, all 

 of which are afjuatic 

 when young, some like 

 the painted turtle ( Cliry- 

 si'inys pkta), and the 

 spotted turtle {Nanemys 

 guttata) throughout life, 

 while others like the 

 ^V*ood turtle {Chelopus 

 insculj^tus) are found in 

 dry jjlaces away from 

 water. The most ter- 

 restrial of the family is the common Box-turtle (Cistudo carhmta), in 

 which the plastron can be shut up over the retracted extremities. It 

 lives in sandy hills, and forms burrows into which it retreats during rain. 



Fig. 75.— European Land-Tortoise. 

 (After Brehm). 



Testudo gracca. {. 



