571 
of the deep emargination which separates the superior maxillary from 
the tympanic bone in freshwater tortoises, especially the Hmys 
expansa. 
In general form the skull resembles that of the Chelone Mydas, but 
it is relatively broader, the anterior frontals are less sloping, and the 
anterior part of the head is more vertically truncate: the median 
frontals also enter into the formation of the orbits in rather a larger 
proportion than in C. Mydas. In Chelone imbricata they are wholly 
excluded from the orbits. 
The trefoil shape of the occipital tubercle is well marked; the 
laterally expanded spinous plate of the parietal bones is united by a 
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 bones. In other freshwater tortoises the facta 
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 pedicies are hollowed out lengthwise ; 
the palatal processes of the superior maxillary and palatine bones are 
continued backwards to the extent which characterizes the existing 
oo. and the posterior or internal opening of the nasal passages 
,ina proportional degree, carried further back in the mouth. The 
er opening of the zygomatic spaces is wider in the Sheppey Che- 
lonite than in the Hiys 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, ¢. 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 Mydas, 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 
Wwe 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 aloae 
