170 Dr. J. E. Gray on the Development and Cohesion 



and forming a more or less bony disk protecting the 

 greater part of the sternum. 



* The sternal bones in the very young expanded and forming 

 two groups : — the front, of the two anterior pairs of bones 

 and the odd bone ; the hinder, of the two hinder pairs of 

 bones, leaving a space in the middle of the sides. Tylo- 

 jpoda or Land-Tortoises : Testudo &c. 



** The sternal bones of the very young united into a disk or 

 marginal ring. Steganopoda or Terrapins, as Mala- 

 clemmys, Pelomedusa, Chelydra, and Staurotypus. 



It is to be observed that it is among the latter genera of the 

 family Chelydrad^ that the sternum of these animals is smaller 

 and less developed compared with the size of the animal than 

 in any other Chelonians. 



h. The bones of the sternum in the young animal slender, and 

 merely forming a ring round the circumference of the 

 sternum, leaving the centre part vacant, to be filled up 

 by the development of the bones. 

 In the most developed form of this group the bones form a 

 simple external ring, leaving the centre of the disk vacant, as 

 in the genus Rhiywclemmys among the Terrapins with a con- 

 tinuous sternum, and Cyclemys among the Box Tortoises (which 

 have the sternum divided into two parts by a central suture) ; 

 and the structure seems to be similar in the genus Kinosternon 

 (PI. V. fig. 6), which have the sternum divided into three 

 parts by two cross sutures, and have been called Flap-Tortoises. 

 Some of the tortoises that have the sternum in the very 

 young state supported by a ring of bones send forth bony lobes 

 from the inner side of the three pairs of lateral bones, which 

 divide the vacant central space into four parts ; this has only 

 been observed in the genus Kachuga among the Asiatic 

 Batagurs. This group is intermediate between the two sections 

 a and h ; and the sternum of the young has considerable affinity 

 to the sternum of the adult turtles. 



II. The bones of the sternum in the adult animal remaining 

 separate, and only forming a ring of bones round the 

 centre part of the disk. 



In the marine Turtles the marginal bones are only slightly 

 developed ; and in the freshwater Mud-Tortoises the marginal 

 bones are not developed at all, or only deposited on part of the 

 margin when the animal arrives at the adult age. These may 

 be divided into : — 



The Mud-Tortoises (Trionychidae). The front pair of ster- 

 nal bones separate, slender, bent at a right angle in the middle, 



