22 FOSSIL REPTILIA OF THE LONDON CLAY. 



well as with the first and third plates, as in the Chel. longiceps (Tab. V, fig. 1). The 

 tooth-like extremity of the connate rib is preserved on the right side. The fourth 

 costal plate {ph) is two inches four lines in length, nine lines in breadth ; the angle at 

 which the expanded part contracts to the extremity of the connate rib is well shown 

 on the right side. The third to the eighth neural plates expand anteriorly, and have 

 the anterior angles cut off to articulate with the costal plates in advance ; they diminish 

 in size very gradually, and the antero-lateral borders, formed by the above-named 

 truncated angles, do not increase in length as in the corresponding plates in the 

 Chelone longiceps. 



The vertebral scutes (^2, vz, ^4) resemble more in form those of the Ckel. longiceps 

 than of Chel. ireviceps ; but, notwithstanding that the whole carapace is narrower than 

 in Chel. longiceps, the vertebral scutes are broader ; and the lines which converge to the 

 lateral angle have a more marked sigmoid curvature. 



t!hel. convexa. Chel. longiceps. 



Inclies. Lines Inches. Lines. 



The length of the second vertebral scute is 1 8 18 



Breadth 2 6 2 2 



The two succeeding scutes (v3 and va) more rapidly diminish in size than in either 

 the Chel. breviceps or longiceps, and the transverse impression between the third and 

 fourth vertebral scute crosses the lower third of the fifth neural plate, as in Chelone 

 latiscutata. All the scutes have left deeper and rather wider impressions than in the 

 preceding species. 



The second to the fifth costal plates inclusive, are more equal in length than in 

 the existing Chelone mydas or Chel. caouanna, and in this character the present species 

 more resembles the Chel. imhricata. 



The distinction of the present from the previously described fossils, already 

 manifested in the structure of the carapace and the form of the vertebral scutes, is more 

 strikingly established in that of the plastron (Tab. VII, fig. 2), which, in its defective 

 ossification, resembles the same part in the existing species of Chelone. 



All the bones, but especially the xiphisternals [xs), are more convex on their outer 

 surface than in other turtles, recent or fossil. The central vacuity is greater than in 

 any of the above-described fossil species. The internal rays of the hyosternals come 

 off from the anterior half of their inner border, and are divided into two groups : 

 the lower consisting of two short and strong teeth, projecting inwards towards the 

 extremity of the entosternal {s) ; while the rest extend forwards along the inner side of 

 episternals ies). The same character may be observed in the corresponding processes 

 of the hyposternals {ps), which are limited to the posterior half of their inner border. The 

 external radiated process of the hyosternals {hs) arises from a larger proportion of the 

 outer margin, than in the Chel. mydas ; but from a somewhat less proportion than in 

 Chel. breviceps. 



