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 V 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. 
AUigator- The alligator-terrapin, or snapping turtle (Chelydra serpentina ) 
Terrapin. i s a giant among river-tortoises, and takes its name from a fancied 
