MUD-TERRAPINS. 



77 



THE MUD-TERRAPINS AND THEIR ALLIES. 

 Families ClNOSTERNID^ and 



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-process which underlies the marginal bones. 

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

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



PENNSYLVANIAN MUD-TEREAPIN (J nat. size). 



are 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 characters, the mud-tortoises resemble the Testudinidce in the 

 conformation 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 

 genus is the Pennsylvanian mud-terrapin (C. pennsylvanicum), which attains a 

 length of about 4J inches, and inhabits eastern North America from New York 

 to the Gulf of Mexico. In colour, the shell is brown or brownish above, and either 

 yellow or brown beneath, the lines of junction between all the shields being dark 



