CHELONIA. 15 



extremity ; flattened, and gradually expanded from its humeral third, to its sternal 



end, which is relatively somewhat broader than in the Chelone mydas and Chelone 



caouanna. 



Inch. Lines. 

 Its length is ..... 1 6 



Breadth of sternal end ... 7 



The characters thus afforded by the cranium, carapace, plastron, and by one of the 

 bones of the anterior 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. 



On account of the shortness of the skull, especially of the facial part and of that 

 which intervenes between the orbit and ear, compared with the breadth of the skull 

 across the mastoids, I have proposed to name this extinct species, Chelone breviceps.* 



By the characteristic shape of the median extremities of the costal plates of 

 the carapace, I have been able to determine some fragmentary Chelonites which 

 have afforded better ideas of the size of the species represented by Mr. Bowerbank's 

 more complete but immature specimen' of Chelone breviceps. 



A portion of the carapace of the Chelone breviceps, including the fourth, fifth, sixth, 

 and part of the third and seventh neural plates, with a considerable proportion of the 

 third, fourth, fifth, and sixth costal plates, is preserved in the museum of Mr. Robertson, 

 of Chatham. The characters of the rugous surface of these bones, and of the equal- 

 sided angles by which the costal plates articulate with the neural plates, do both, 

 and especially the latter, point out the species to which the present fragment belongs. 

 It has formed part of an individual double the size of the specimen above described, 

 and figured from Mr. Bowerbank's collection, and therefore it had a carapace sixteen 

 inches in length. 



Although the costal plates have been continued further along the ribs than in the 

 younger example, the more complete state of the sixth rib, in Mr. Robertson's 

 specimen, shows that they retained their longitudinally-striated, tooth-like extremities, 

 which, in the sixth rib, is two thirds of an inch in length ; the length of the expanded 

 part being four inches, and its breadth one inch nine lines. The internally prominent 

 part of the rib is much less developed than in Chelone planimentmn, and Chelone 

 crassicostata, afterwards to be described. The right hyosternals and hyposternals are 

 present, and they likewise preserve the character of the Chel. breviceps in their rugous 

 surface and minor breadth, as compared with those parts in the Chelone longiceps, the 

 extinct species next to be described. 



Besides the specimens above described, on which the present extinct species of turtle 



* Proceedings of Geological Society, December 1, 1841, p. 570. Report on British Fossil Reptiles, 

 Trans. Brit. Association, 1841, p. 178. 



