172 Dr. J. E. Gray on the Osteology o/ Spliargis. 



longitudinal ridges of the back, the sides of the sternum, and 

 on the sides of the tail, and are produced above into hard 

 conical elevations or tubercles, which are largest on the ridges 

 of the tail. These tubercles are somewhat like those to be 

 observed on some species of Ostracion and on Lopliius and 

 other fishes. 



The form of the two hinder central bones of the dorsal disk 

 (placed beyond the one that bears the pelvis, and forming the 

 central line of the hinder part of the shell that covers the tail 

 of the animal) is very different in the young and halfgrown 

 specimens of the different kinds of turtles, and affords a very 

 good character to determine the species ; but these bones expand 

 in the more adult state when the dorsal shell becomes solidified 

 by the dilatation and coherence of the ribs, when they lose the 

 distinctness of their form, or at least they become coalesced 

 with the other bones and are not to be observed. 



Thus in the young Caouana the hinder bone is narrow and 

 compressed, with a prominence on its outer side ; in the other 

 turtles this bone is flat and expanded. In the Green Turtle 

 {Mydas) the last bone is lanceolate, ovate, and broad at the 

 base, and slightly contracted at the front edge, and the hinder 

 part is gradually contracted into a point. The last bone of the 

 Hawk's-beak [Caretta) is similar, but broader and more rapidly 

 attenuated behind, and not contracted in front next to the 

 pelvis. 



EXPLANATION OF THE PLATES. 



Plate IV. 



Fig. 1. Testudo tatulata. 

 Fig. 2. Testudo elephantopus. 

 Fig. 3. Testudo platynota. 

 Fig. 4. Testudo stellata. 

 Fig. 5. Notochelys platynota. 



Plate V. 



Fig. 1, JMalaclemmys concentrica. 



Fig. 2. Pelomedusa subnifa. 



Fig. 3. Cyclemys dlior. 



Fig. 4. Rhinoclemmys scabra. 



Fig. 5. Elseya dentata. 



Fig. 6. Kinosteruon pennsylvanicum. 



Plate VL 



Fig. L Kachuga major. 



Fig. 2. Kachuga deutata (outside). 



Fig. 3. Hydromedusa tiavilabris (outside). 



Fig. 4. Chelonia mydas. 



Fig. 5. Sphargis mercurialis. 



