TRIONYCHID^. — LXX. 167 



forward; head and neck very large; jaws strong-lr 



hooked, and exceedingly powerful; tail long, stroiig, 



with a crest of horny, compressed tubercles; plastron 



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



very narrow. 



I>arge turtles of great strength and voracity, chiefly 



aquatic; two of the three species are American, the 



third (^Plati/stermmi) 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 scak's beneath; ridges of carapace disappearing with 

 age; jaws moderately hooked. . . . Ciielydra, 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, 3. 



/. CHELYDRA, 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. M. lacertina, (Schw.) Mississippi Snapper. Gulf 

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

 for their size, the strongest of reptiles." 



FAMILY LXX. — TRIONYCHID^. 



{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 



