2436 



TORTOISES, TURTLES, AND PLESIOSAURS 



THE SNAPPERS AND ALLIGATOR TERRAPINS 

 Family ClfELl'DRIDsE 



Resembling the big-headed tortoise in the great relative size of their hook- 

 beaked heads, and their elongated scaly tails, the snapper and alligator terrapins of 

 North and Central America constitute a well-marked family by themselves. In the 

 first place, they differ from the species named in that the nuchal bones give off 

 rib-like processes underlying the marginals; while the temporal region of the skull 

 is but partially covered with a bony roof. The American forms are further charac- 



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ALLIGATOR TERRAPIN. 

 (One-twelfth natural size.) 



erized by the relatively-small size of the carapace, of which the hinder border is 

 strongly serrated; while the cruciform plastron is likewise small, and but loosely ar- 

 ticulated with the upper shell by a very narrow bridge. Moreover, both the upper 

 and lower shells are not completely ossified till very late in life, vacuities remaining 

 for a long time between the costal and marginal bones in the former, and in the 

 middle line of the latter. Then, again, the plastron is peculiar in that the abdom- 

 inal shields, which are separated from the marginals by an inframarginal series, do 



