of the Remains of Marine Turtles from the Londo?i Clay. 373 



The bony palate presents in an exaggerated degree its great extent 

 from the intermaxillary bones to the posterior nasal aperture, and it 

 is not perforated, as in the Trionyxes, by an anterior palatal fora- 

 men. 



The extent of the bony palate is relatively greater than in the 

 Chelone Mydas ; the trenchant alveolar ridge is less developed than 

 in the Chel. Mydas ; the groove for the reception of that of the lower 

 jaw is shallower than in the existing Cheloniae, or the extinct Chel. 

 breviceps, arising from the absence of the internal alveolar ridge. 



The present species is distinguished by the narrowness of the 

 sphenoid at the base of the skull, and by the form and groove of the 

 pterygoid bones, from the existing Cheloniae, and <i fortiori from the 

 Trionyxes ; to which, however, it approaches in the elongated and 

 pointed form of the muzzle, and the trenchant character of the alve- 

 olar margin of the jaws. 



The general characters of the carapace are next given, and a spe- 

 cimen from Mr. Bowerbank's collection is more particularly described. 



This carapace, as compared with that of the C. breviceps in the 

 same collection, presents the following differences : it is much broader 

 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 C. 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. 



