226 WATER REPTILES OF THE PAST AND PRESENT 



time. Only four species are known, three of them from North 

 America, the fourth from New Guinea. The family is one of the 

 most primitive of living turtles, though no members of it are known 

 with certainty from earlier rocks than the Oligocene. In all prob- 

 ability, also, they have retained, more than have any other group 

 of turtles, unless it be some of the fresh-water tortoises, the primi- 

 tive habits of the earlier or earliest turtles, though of course there 

 have been modifications, both in structure and in habits. The 

 three species of the United States include two of the snapping 

 turtles proper and the alligator turtles of the southern states, 

 which sometimes reach a length of three feet. All the species are 

 largely aquatic in habit, powerful and active swimmers, with 

 webbed feet and strong claws, and both on the land and in the 

 water they are bold and fierce. They have a relatively large head 

 and very strong jaws. Agassiz saw one bite off a piece of a plank 

 an inch in thickness, and they can usually be raised from the ground 

 by any object which they seize. The carapace and plastron are 

 much reduced, and are rather loosely united. The shell is not 

 large enough for the complete withdrawal of the head and legs 

 within it, and the tail is unusually large and strong. The common 

 snapping turtle, Chelydra serpentina, is found from Canada to Ecua- 

 dor, and its remains have been found with those of the mammoth 

 and mastodon in Pleistocene deposits; and related species of the 

 same genus have been reported from the Miocene of England. 



FRESH-WATER OR MARSH TORTOISES 



The family of turtles or tortoises (Emydidae) represented at 

 the present time by the common terrapin, painted tortoise, and 

 box tortoise of the United States, and commonly called fresh- 

 water turtles or tortoises, comprises the largest group of living 

 chelonians — nearly a third of all existing members of the order. 

 They are widely distributed over all parts of the earth except 

 Australia, and are of very varied habits. Some are almost exclu- 

 sively aquatic; others, like the painted tortoise, are partially so- 

 while others, especially the common box tortoise, are almost as 

 exclusively terrestrial as the true land tortoises, dying even, if 

 forced to live long in water. 



