142 



Order CHELONIA. 



Tortoises and Turtles. 



Reptiles having the trunk relatively short and broad, with the upper and 

 the lower walls forming two disks, which are united on the sides between 

 the fore and hind limbs. The disks strengthened by bony deposits, which 

 usually, but not always, involve the endo-skeleton. The bony dorsal 

 shield (carapace) usually formed of the expanded and suturally united 

 ribs and vertebral spinous processes. The lower shield (plastron) com- 

 posed of the clavicles and a few dermal bones. No true sternum. Trunk 

 rigid ; only the neck and tail flexible. Jaws without teeth and covered 

 with horny sheaths. Eyes with lids and a nictitating membrane. Tym- 

 panic membrane external, sometimes hidden by the skin. Tongue thick 

 and fleshy. Limbs four; developed for walking, except in the marine 

 turtles, in which they are formed for rapid swimming. Reproduction by 

 means of eggs, spherical or elliptical in shape and protected by a calcar- 

 eous shell. 



Upper and lower disks without horny epidermal plates. A soft skin. 



Trionychoidea, p. 142. 



Shell covered with large, symmetrical horny epidermal plates. 



Testudinata. 



Suborder TRIONYCHOIDEA. 



The dorsal vertebrae and the expanded ribs involved in the carapace. 

 Plastron composed of 9 bones, which enclose a fontanelle. The dorsal 

 disk rarely strengthened by marginal bones, its border therefore flexible. 

 The horny sheaths of the jaws hidden by fleshy lips. Fourth digit with 

 4 or more phalanges. 



A single family living in the rivers of North America, Asia and Africa. 



Family VII. TRIONYCHID^. 



Body broad and much depressed ; the margins of the carapace thin 

 and leathery, in rare cases having marginal bones. No epidermic scutes. 

 Snout much produced, leathery, with the nostrils at the tip. Ear hidden. 

 Only the three inner digits furnished with claws. Head and neck com- 

 pletely retractile. 



Of this family 6 genera are recognized, only one of which lives in 

 America. 



