372 Geological Society: Prof. Owen's Description 



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 

 sixth part only of the corresponding external margins are similarly 

 free, and there form the bottom, not of an angular, but a semicircular 

 interspace. 



The radiated processes from the inner margins of the hyo- and hy- 

 posternals are characterized in the Chelone breviceps by similar mo- 

 difications, but their origin is rather less extensive ; they terminate 

 in eight or nine rays, shorter and with intervening angles more equal 

 than in existing Chelones. The xiphisternal piece receives in a notch 

 the outermost ray or spine of the inner radiated process of the hy- 

 posternal, as in the Chelones, and is not joined by a transverse 

 suture, as in the Emydes, whether young or old. 



The characters thus afforded by the cranium, carapace, plastron, 

 and some of the bones of the extremity, prove the present Sheppey 

 fossil to belong to a true sea-turtle ; and at the same time most 

 clearly establish its distinction from the known existing species of 

 Chelone ; from the shortness of the skull, especially of the facial part 

 as compared with its breadth, the author proposes to name this extinct 

 species Chelone breviceps. 



2. Chelone longiceps. — The second species of Sheppey turtle, called 

 Chelone longiceps, is founded upon the characters of* the cranium, ca- 

 rapace, and plastron. The cranium differs more from those of exist- 

 ing species, by its regular tapering into a prolonged pointed muzzle, 

 than does that of the Chelone breviceps by its short and truncated jaws. 



The surface of the cranial bones is smoother ; and their other mo- 

 difications prove the marine character of the fossil as strongly as in 

 the Chelone breviceps. 



The orbits are large, the temporal fossae are covered principally 

 by the posterior frontals, and the exterior osseous shield completely 

 overhangs the tympanic and ex-occipital bones. The compressed 

 spine of the occiput is the only part that projects further backwards. 



The palatal and nasal regions of the skull afford further evidence 

 of the affinities of the present Sheppey Chelonite to the Turtles. 



