2434 



TORTOISES, TURTLES, AND PLESIOSAURS 



THE MUD TERRAPINS AND THEIR ALLIES 

 Families ClNOSTERNIDsE and DERMATEMTDID^ 



The mud terrapins (Cinosternum) bring us to the first of two nearly-related 

 families confined to the New World, all of which differ from those previously 

 noticed by the circumstance that the nuchal bone of the carapace gives off from 

 each of its hinder angles a long rib- like process which underlies the marginal bones. 

 From the second family, the mud terrapins, of which there are eleven species inhab- 

 iting America north of the Equator, are broadly distinguished (as indeed they are 



PENNSYI,VANIAN MUD TERRAPIN. 



( One-half natural size. ) 



from all other members of the order) by the fact that there are but eight bones in 

 the plastron, owing to the absence of the unpaired entoplastral bone. As regards 

 their other characteristics, the mud tortoises resemble the Testudinida in the con- 

 formation of the vertebrae of the tail, and in the absence of a roof to the temporal 

 fossa of the skull, as well as in the extreme shortness of the tail. The carapace is 

 more or less depressed, and is articulated by a bony, suture with the plastron, the 

 latter having the gular shields fused into one, or wanting, and its fore- and hind- 

 lobes more or less movable. The toes are fully webbed, and with the exception of 

 the fifth in the hind-foot, strongly clawed. The best-known representative of the 



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