230 Geological Society. 



The external surface of the cranial bones in the fossil is broken by 

 small irregular ridges, depressions, and vascular foramina, which give 

 it a rough shagreen-like character. 



The lower jaw, which is preserved in the present fossil, likewise 

 exhibits two characters of the marine turtles ; the dentary piece, e. g., 

 forms a larger proportion of the lower jaw than in land or fresh- 

 water tortoises. The under part of the symphysis, which is not 

 larger than in Chelone My das, is slightly excavated in the fossil. 



In the rich collection of Sheppey fossils belonging to Mr. Bower- 

 bank, there is a beautiful Chelonite, including the carapace, plastron, 

 and the cranium, which is bent down upon the forepart of the plas- 

 tron ; and which, though mutilated, displays sufficient characters to 

 establish its specific identity with the skull of the Chelone breviceps 

 just described. The outer surface of the carapace and plastron has 

 the same finely rugous character as that of the cranium, in which 

 we may perhaps perceive a slight indication of the affinity with the 

 genus Trionyx. 



The carapace is long, narrow, ovate, widest in front, and tapering 

 towards a point posteriorly ; it is not regularly convex, but slopes 

 away, like the roof of a house, from the median line, resembling in 

 this respect, and its general depression, the carapace of the turtle. 

 There are preserved eleven of the vertebral plates, the two last alone 

 being wanting. The eight pairs of expanded ribs are also present, 

 with sufficient of the narrower tooth-like extremities of the six an- 

 terior pairs to determine the marine character of the fossil, which is 

 indicated by its general form. Other minute characters are detailed ; 

 and a comparison with the Chelonite from the tertiary beds near 

 Brussels, figured by Cuvier, is instituted. 



The sternum of the Chelone breviceps, although more ossified than 

 in existing Chelonise, yet presents all the essential characters of that 

 genus. There is a central vacuity left between the hyosternals and 

 hyposternals ; but these bones differ from those of the young Emys 

 in the long pointed processes which radiate from the two anterior 

 angles of the hyosternals, and the two posterior angles of the hy- 

 posternals. 



The xiphisternals have the slender elongated form and oblique 

 union by reciprocal gomphosis with the hyposternals, which is cha- 

 racteristic of the genus Chelone. 



The posterior extremity of the right episternal presents the equally 

 characteristic slender pointed form. 



With these proofs of the sternum of the present fossil being modi- 

 fied according to the peculiar type of the marine Chelones, there is 

 evidence, however, that it differs from the known existing species in 

 the more extensive ossification of the component pieces : thus, the 

 pointed rays of bone extend from a greater proportion of the margins 

 of the hyo- and hyposternals, and the intervening margins do not 

 present the straight line at right angles to the radiated processes. 



In the Chelone My das, for example, one half of the external margin 

 of the hyo- and hyposternals, where they are contiguous, are straight, 

 and intervene between the radiated processes, which are developed 

 from the remaining halves ; while in the Chelone breviceps about a 



