232 Geological Society. 



and flatter ; the vertebral plates are relatively broader ; the lateral 

 angle, from which the intercostal suture is continued, is much nearer 

 the anterior margin of the plate ; the C. longiceps in this respect re- 

 sembling the existing species : the expanded portions of the ribs are 

 relatively longer ; they are slightly concave transversely to their axis 

 on their upper surface, while in C. breviceps they are flat. The ex- 

 ternal surface of the whole carapace is smoother, and although as 

 depressed as in most turtles, it is more regularly convex, and sloping 

 away by two nearly plane surfaces from the median longitudinal ridge 

 of the carapace. 



Among the minor differences of the two Sheppey fossils the author 

 states, that the first vertebral plate of C. longiceps is more convex at 

 its middle part, and sends backwards a short process to join the 

 second vertebral plate, in which it resembles the 0. Mydas. The 

 second plate is six-sided, the two posterior lateral short sides being 

 attached to the second pair of ribs, in which the present species differs 

 from both C. Mydas and C. breviceps. The third vertebral plate is 

 quadrangular instead of the second, as in C. breviceps and C. Mydas. 

 The impressions of the epidermal scutes are deeper, and the lines 

 which bound the sides of the vertebral scutes meet at a more open 

 angle than in the C. breviceps, in which the vertebral scutes have 

 the more regular hexagonal form of those of the C. Mydas. 



The plastron is more remarkable than that of the C. breviceps for 

 the extent of its ossification, the central cartilaginous space being 

 reduced to an elliptical fissure. The four large middle pieces, called 

 hyosternals and hyposternals, have their transverse extent relatively 

 much greater, as compared with their antero-posterior extent, than 

 in C. breviceps. The median margins of the hyosternals are deve- 

 loped in short toothed processes along their anterior two-thirds ; and 

 the median margins of the hyposternals have the same structure 

 along their posterior halves. 



The xiphisternals are relatively broader than in C. breviceps or in 

 any of the existing species, and are united together by the whole of 

 their median margins. The entosternal piece is flat on its under 

 surface. 



Each half of the plastron is more regularly convex than in C. My- 

 das. The breadth of the sternum along the median suture, uniting 

 the hyosternals and hyposternals, is five inches ; and the breadth at 

 the junction of the xiphisternals with the hyposternals is two inches. 



The posterior part of the cranium is preserved in this fossil, with- 

 drawn beneath the anterior part of the carapace ; the fracture shows 

 the osseous shield covering the temporal fossae ; and the pterygoids 

 remain, exhibiting the wide and deep groove which runs along their 

 under part. 



It has been most satisfactory, the author says, to find that the two 

 distinct species of the genus Chelone, first determined by the skulls 

 only, should thus have been established by the subsequent observa- 

 tion of their bony cuirasses ; and that the specific differences mani- 

 fested by the cuirasses should be proved by good evidence to be cha- 

 racteristic of the two species founded on the skulls. 



Thus the portion of the skull preserved with the carapace first 



