Geological Society. 231 



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 fossse 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. 

 The bony palate presents in an exaggerated degree its great extent 

 from the intermaxillary bones to the posterior nasal aperture, and it 

 is not perforated as in the Trionyxes, by an anterior palatal fora- 

 men. 



The extent of the bony palate is relatively greater than in the 

 Chelone My das ; the trenchant alveolar ridge is less developed than 

 in the Chel. My das ; the groove for the reception of that of the lower 

 jaw is shallower than in the existing Chelonise, or the extinct Chel. 

 breviceps, arising from the absence of the internal alveolar ridge. 



The present species is distinguished by the narrowness of the 

 sphenoid at the base of the skull, and by the form and groove of the 

 pterygoid bones, from the existing Chelonise, and & fortiori from the 

 Trionyxes ; to which, however, it approaches in the elongated and 

 pointed form of the muzzle, and the trenchant character of the alve- 

 olar margin of the jaws. 



The general characters of the carapace are next given, and a spe- 

 cimen from Mr. Bowerbank's collection is more particularly described. 



This carapace, as compared with that of the C. breviceps in the 

 same collection, presents the following differences : it is much broader 



