234 Geological Society, 



divided into two groups ; the lower consisting of two short and 

 strong teeth projecting inwards, while the rest extend forwards along 

 the inner side of the episternals. The same character may be ob- 

 served in the corresponding processes of the hyposternals, but the 

 external process is relatively much narrower than in C. breviceps. 

 The following differences are stated to distinguish the sternum of 

 C. convexa from that of C. Mydas. The median margin of the hyo- 

 sternals forms a gentle curve, not an angle : that of the hyposternals 

 is likewise curved, but with a slight notch. The longitudinal ridge 

 on the external surface, and near the median margin of the hyo- and 

 hyposternals, is less marked in the Sheppey fossil ; especially in the 

 hyposternals, which are characterized by a smooth concavity in their 

 middle. 



The suture between the hyo- and hyposternals is nearer to the 

 external transverse radiated process of the hyposternals. The me- 

 dian vacuity of the sternal apparatus is elliptical in the Chel. con- 

 vexa, but square in the Chel. Mydas. 



The characteristic lanceolate form of the episternal bone in the 

 genus Chelone is well seen in the present fossil. 



The true marine character of the present Sheppey Chelonite is 

 likewise satisfactorily shown in the small relative size of the entire 

 femur which is preserved on the left side, attached by the matrix to 

 the left xiphisternal. It presents the usual form, a slight sigmoid 

 flexure, characteristic of the Chelones ; it measures one inch in 

 length. In an Emys of the same size, the femur, besides its greater 

 bend, is 1^ inch in length. 



5. Chelone subcristata. — The fifth species of Chelone from Sheppey, 

 distinguishable by the characters of its carapace, approaches more 

 nearly to the Chelone Mydas in the form of the vertebral scutes, 

 which are narrow in proportion to their length, than in any of the 

 previously described species ; but is more conspicuously distinct by 

 the form of the 6th and 8th vertebral plates, which support a short, 

 sharp, longitudinal crest. The middle and posterior part of the first 

 vertebral plate is raised into a convexity, as in the Chel. longiceps, 

 but not into a crest. 



The keeled structure of the sixth and eighth plates is more marked 

 than in the fourth and sixth plates of Chelone Mydas, which are 

 raised into a longitudinal ridge. 



The characters of the carapace are then minutely described. 



Sufficient of the sternum is exposed in the present fossil to show, 

 by its narrow elongated xiphisternals, and the wide and deep notch 

 in the outer margin of the conjoined hyo- and hyposternals, that it 

 belongs to the marine Chelones. 



The xiphisternals are articulated to the hyposternals by the usual 

 notch or gomphosis ; they are straighter and more approximated 

 than in the Chel. Mydas ; the external emargination of the plastron 

 differs from that of the Chel. Mydas in being semicircular instead 

 of angular, the Chel. subcristata approaching, in this respect, to the 

 Chel. breviceps. 



The shortest antero-posterior diameter of the conjoined hyo- and 

 hyposternals is two inches seven lines. The length of the xiphi- 



