ON BRITISH FOSSIL REPTILES. 16? 



of three inches and a half from it. The line bounding the lower part of the 

 marginal scutes, which in Tortoises is either parallel with or a little above the 

 suture uniting the lateral wall of the plastron with the marginal plates, here 

 intersects the marginal wall of the plastron at a distance of from two-thirds 

 of an inch to one inch and a half from that margin : impressions of four of 

 the marginal scutes may thus be traced upon this part of the sternum, a struc- 

 ture in which the present fossil differs most remarkably from all known ex- 

 isting Tortoises. This difference it is the more necessary to bear in mind, 

 since, in the antero-posterior extent as well as the transverse extent of the 

 lateral wall of the sternum, and in the form and extent of the emarginations 

 which bound the anterior and posterior pai't of this wall, the present fossil 

 exhibits a closer resemblance with the Land-tortoises than with the ordinary 

 Emydes. But the external surface of the plastron, instead of being slightly 

 concave, as in most tortoises, is slightly convex ; and where the plastron is 

 convex externally in existing tortoises, namely, at the outer margin of the 

 lateral wall, the fossil exhibits a slight concavity. In short, the character of 

 the surface is such as Avould lead one having in his mind the plastron of a 

 Tortoise as the ground of comparison, to suppose, at the first sight of the fossil, 

 that he was looking on the inner side of the plastron ; but the distinct and 

 well-marked impressions of the epidermal scutes proves that it is actually the 

 outer surface of the plastron which is here exposed. The anterior margin of 

 the plastron is truncated, as in most Platemydians. 



The osseous basis of the present plastron is half an inch in thickness ; the 

 structure of the bone is compact at the surface, including a coarsely spongy 

 diploe, as in the Chelonians generally. 



Portions of ribs of the Tretosternon pu7ictatum*, which from their specific 

 punctation and sculpturing of the outer surface have been referred to the 

 genus Trionyx, have been discovered by Dr. Mantell in the Wealden of Til- 

 gate. 



Amongst recent Emydians an approach to the Trionyces is made by the 

 subgenus Cryptopus {Emyda of Gray), inasmuch as the marginal plates, espe- 

 cially of the posterior free margin of the carapace, exist in a rudimental or 

 abortive state, as small granulated ossicles, suspended in the integument cover- 

 ing that border. 



The subgenus Chelydra manifests its affinity to Trionyx by another modi- 

 fication of its osseous structure, viz. the absence of the lateral osseous walls, 

 or alaj joining the plastron to the carapace, which are united only by flexible 

 cartilage, throughout life. 



No known existing species of Emydian has a free unossified central space 

 in the sternum in the full-grown state; but this is an immature character 

 common to all Chelonians, and is persistent in marine Turtles and Trionyces. 



In the present highly interesting extinct genus, Tretosternon, it would ap- 

 pear that the absence of marginal plates, and a cartilaginous union of the 

 plastron with the carapace, Avere associated likewise with a small vacuity in the 

 middle of the sternum of the mature animal. The evidently feebly-developed 

 scutes, and the sculpturing of the external surface of the flattened carapace, 

 complete the last step in the transition from the Elodite to the Potamite fami- 

 lies of Chelonians in the system of MM. Dumeril and Bibron. 



Platemys Mantelli, Emys de Sussex, Cuv., Emys Mantelli, Gray The 



fossils discovered by Dr. Mantell in the Wealden strata of Tilgate Forest, and 

 the resemblance of which to the flat species of Emydian discovered by M. 

 Hugi in the Jura limestone at Soleure has been pointed out by Cuvier, are 

 referable to the Pleuroderal section of the Emydian family, as arranged by 

 * Illustrations of the Geology of Sussex, 4to, pi. vi. figs. 1, 3 & 5. 



