TKIONYCHIDJE. LXX. 167 



forward; head and neck very large; jaws stronglr 

 hooked, and exceedingly powerful; tail long, strong, 

 with a crest of horny, compressed tubercles; plastron 

 small, cross-shaped, covered with twelve plates; bridge 

 very narrow. 



Large turtles of great strength and voracity, chiefly 

 aquatic; two of the three species are American, the 

 third (Platy sternum) is from China. Their fierceness is 

 well known ; when angry they elevate the body, and, in 

 biting, throw themselves forcibly forward. 



* Head rough, covered with soft skin; tail with two rows of 

 large scales beneath; ridges of carapace disappearing with 

 age; jaws moderately hooked. . . . CHELYDRA, 1. 



** Head very large, covered with smooth, symmetrical plates; 

 tail with many small imbricate scales beneath; carapace very 

 strongly three-keeled; jaws very strongly hooked. 



MACROCHELYS, 2. 



/. CHEL YDRA, Schweigger. SNAPPING TURTLES. 

 1. C. serpentina, (L. ) Schw. COMMON SNAPPING 

 TURTLE. Canada to Equador, every where abundant. 



2. MACROCHELYS, Gray. ALLIGATOR SNAPPERS. 



= Gypochelys, Ag. 



1. Nl. lacertina, (Schw.) MISSISSIPPI SNAPPER. Gulf 

 States, N. to Illinois; &quot;perhaps the most ferocious, and, 

 for their size, the strongest of reptiles.&quot; 



FAMILY LXX. TRIONYCHID^E. 



(The Soft-Shelled Turtles.) 



Body flat, nearly orbicular; carapace not completely 

 ossified, the ribs projecting freely towards the outer 

 extremities ; marginal ossicles rudimentary ; carapace 

 and plastron covered by a thick leathery skin which is 

 flexible at the margins. Head long and pointed with a 



