of the Remains of Marine Turtles from the London Clay. 371 



straight- suture to the post-frontals along three-fourths of its extent, 

 and for the remaining fourth with the temporal or zygomatic ele- 

 ment. 



These proportions are reversed in the Emys expansa, in which the 

 similarly expanded plate of the parietals is chiefly united laterally 

 with the temporal hones. In other freshwater tortoises the parietal 

 plate in question does not exist. 



The same evidence of the affinity of the Sheppey Chelonite in 

 question to the marine turtles is afforded by the base of the skull : — 

 the basi-occipital is deeply excavated ; the processes of the pterygoids 

 which extend to the tympanic pedicles are hollowed out lengthwise ; 

 the palatal processes of the superior maxillary and palatine bones are 

 continued backwards to the extent which characterizes the existing 

 Cheloniae ; and the posterior or internal opening of the nasal passages 

 is, in a proportional degree, carried further back in the mouth. The 

 lower opening of the zygomatic spaces is wider in the Sheppey Che- 

 lonite than in the Emys expansa. 



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 Cheloniae, 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 



