ix CHELYDRIDAE DERMATEMYDIDAE 341 



the cross-shaped plastron are subject to much individual variation, 

 small shields being frequently intercalated, or rather retained, 

 between the usual ones, especially between the pectorals and 

 abdominals, in the gular region, and on the narrow bridge, where 

 the inframargiuals number one to three or even more. This species 

 inhabits, broadly speaking, the whole basin of the Mississippi and 

 Missouri rivers. 



This beast is as vicious as the other Snapping Turtle. 

 According to Agassiz it does not withdraw its head and limbs on 

 the approach of danger, but resorts to more active defence. It 

 raises itself upon the legs and tail, highest behind, opens the 

 mouth widely, and throwing out the head quickly as far as the 

 long neck will allow, snaps the jaws forcibly upon the assailant, 

 at the same time throwing the body forward so powerfully as 

 often to come down to the ground when it has missed its object. 



It lives mostly in the water, but makes considerable journeys 

 overland. Both in the water and on dry land the limbs move 

 nearly perpendicularly, and the body is raised high. On dry 

 land a considerable part of the weight of the body is borne by 

 the long, strong tail. 



" They are as ferocious as the wildest beast of prey, but the 

 slowness of their motions, their inability to repeat the attack 

 immediately, their awkwardness in attempting to recover their 

 balance when they have missed their object, their haggard look, 

 and the hideous appearance of their gaping mouth, constitute at 

 such times a picture as ludicrous as it is fearful and revolting. 

 Their strength is truly wonderful. I have seen a large specimen 

 bite off a piece of a plank more than an inch thick. They take 

 hold of a stick with such tenacity that they may be carried for a 

 considerable distance suspended to it free above the ground. 

 Fishes and young ducks are their ordinary prey. They lay from 

 twenty to forty or more round eggs only about the size of a small 

 walnut in holes which they dig in sloping banks not far from the 

 water " (Agassiz). 



Fam. 2. Dermatemydidae. The pectoral shields are widely 

 separated from the marginals by inframarginals, the gular shields 

 are very small or absent, and the tail is extremely short. Only 

 two or three genera, \vith three or four species in Central America. 



The plastron is composed of nine plates. In Dermatemys 

 maivi it is large, firmly joined to the carapace, covered with 



