66 TORTOISES AND TURTLES. 



Hinged The third genus of this group (Cyclemys), which is confined to 



Terrapins. India, Malay ana, and the south of China, is represented by some 

 half a dozen species, which, while agreeing with the members of the foregoing 

 genus in the presence of a bony temporal arch to the skull, differ by having a 

 well-marked transverse ligamentous hinge across the middle of the plastron, 

 whereby its hinder lobe is rendered movable, and capable of more or less com- 

 pletely closing the posterior aperture of the shell. None of the species have a 

 shell of more than 8 inches in length. The genus may be subdivided into two 

 groups, each containing three species. In the former, as represented by C. dhor, 

 of Northern India and the Malayan region, the plastron, which is notched behind, 

 cannot completely close the shell; while the hinder margin of the carapace is 

 serrated. In the second group, on the other hand, of which the Amboyna hinged 

 terrapin (G. amboinensis) is a familiar example, the plastron is capable of 

 completely closing the hinder aperture of the shell, while the posterior margin 



CHAIBASSA TERRAPIN ( nat. size). 



of the carapace is not serrated. These species also have the shell keeled on the 

 back in the young state. In the Amboyna species, as also in C. flavomarginata, 

 the hinder end of the plastron is entire, although in a third (C. trifasciata) it 

 is notched. 



Agreeing with the hinged terrapins in the presence of a trans- 

 verse ligamentous hinge across the middle of the plastron, by the aid 

 of which the openings of the shell can be closed, the two North American species 

 of box-tortoises, together with all the remaining members of the family, differ 

 from the former in that the hexagonal neural bony plates of the carapace have 

 the shorter of their two lateral surfaces placed anteriorly, instead of posteriorly, 

 this arrangement being shown when the shell is stripped. The presence of 

 the hinge in the plastron serves to distinguish the box-tortoises from all the 

 members of the second group, with the exception of the pond-tortoises, while from 

 the latter they are separated by the beak being hooked, and the absence of a bony 

 temporal arch to the skull. In the box-tortoises the head is covered with smooth 

 skin above, the toes having only a rudimental web, and the tail is short. The 

 Carolina box-tortoise (Cistudo Carolina) is a somewhat variable species as regards 

 size, the length of the shell ranging from a little over 4 to somewhat more than 

 5 inches. The highly convex carapace is almost hemispherical in shape, and is 



