of the Bones of the Sternum of Chelom'ans. 163 



a broad rhombic vacant space not quite half the width of the 

 sternum ; and in a specimen 2^ inches long, this vacant space 

 is reduced in size so as not to be a third of the diameter of the 

 disk. But I think it is very probable that in the very young 

 of this species the separate bones of which the sternum is com- 

 posed are all more dilated than in the other species, although 

 I have no sj)ecimens by which I can indicate this fact. 



Freshwater Tortoises or Terrapins. 



The sternum of the young Freshwater Tortoises is composed 

 of the same number of bones (which are united together into a 

 bony disk in the adult animal) as that of the Land-Tortoises ; 

 but these bones are very different in shape and disposition from 

 those of the Land-Tortoises, and differ in the various groups, 

 offering a curious subject of study. 



The sterna of the adult animals are naturally divided into 

 three groups : — 



1st. The sternum solid, continuous, and firmly connected 

 with the marginal and other bones of the dorsal disk by an 

 ascending lateral process from each of the central lateral pair 

 of bones. 



2nd. The sternum is only connected with the marginal 

 plates of the dorsal disk by a cartilaginous suture or an adhe- 

 sion between the bones of the sternum and the marginal 

 plates. These are called Box Tortoises. 



The Box Tortoises present two forms : — 



In the true Box Tortoises the sternum is divided transversely 

 into two portions, which close down on the cavity of the upper 

 shell before and behind. The sutm'C is about the middle, 

 between the two lateral bones — the front lobe consisting of 

 the anterior and anterior lateral bones, and covered externally 

 by the gular, pregular, and pectoral plates, the hinder por- 

 tion consisting of the hinder lateral and the hinder sternal 

 bones. It is covered externally by the abdominal, preanal, 

 and anal shields. 



In the Trap Tortoises the sternum is divided into three 

 portions by two transverse sutures. The middle one, which 

 consists of the anterior and posterior lateral bones (which form 

 a square central portion), is attached by a cartilaginous or 

 more or less bony suture to the margin of the dorsal shield, and 

 does not send any ribs up to the inner part of it. It is covered 

 by two large abdominal shields. The front flap consists of the 

 frontal pair of bones and the odd bone ; the front pair are very 

 much larger than usual. It is covered by the gular plates (which 

 are generally soldered into one) and the intergular and pectoral 

 plates (which are triangular). The hinder flap only consists 



11* 



