SNAPPERS. 



79 



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 majority of the 

 vertebrae of the tail have the articular cup behind, and the ball in front ; while the 

 temporal region of the skull is but partially covered with a bony roof. The 

 American forms are further characterised 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 articulated 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 



ALLIGATOR-TERRAPIN ( T ^ nat. size). 



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 abdominal shields, which 

 are separated from the marginals by an inframarginal series, do not meet one 

 another in the middle line, although they may be connected by some small, 

 irregular, unpaired, additional shields. Further, the enormous head cannot be 

 completely retracted within the carapace, of which the anterior margin is deeply 

 excavated in order to afford it room ; and the chin is provided with one or more 

 pairs of pendent wattles. With the exception of the fifth in the hind-limb, the 

 toes are furnished with claws ; and the long tail is crested above. 



Alligator- The alligator-terrapin, or snapping turtle (Chelydra serpentina) 



Terrapin. j s a gi an t among river-tortoises, and takes its name from a fancied 



