CLASS REPTILIA. 



It would appear that the Purbeck form described as Pleurosternum 

 emarginatum, as well as the Wealden specimens to which the names 

 Chelo?te Belli (costata), and Platemys Dixoni and Mantelli have 

 been applied, are likewise referable to Hylceochelys. It is, moreover, 

 very probable that a Chelonian, from the Upper Greensand of Kent, 

 to which the name Plastretnys has been given without description, is 

 also referable to this genus. 



The genus Idiochelys (Chelonemys) and the allied Hydropelta, of 

 the Lower Kimeridgian of the Continent, would appear to be closely 

 allied to the preceding. Idiochelys resembles Hylceochelys in its ex- 

 tremely wide vertebral shields, and also in its small and diamond- 

 shaped entoplastral bone, but differs in that the number of neural 

 bones is generally much reduced, so that many of the costals meet 

 in the middle line ; while there are also differences in the contour 

 of the lower plastral shields. This genus, which is known only by 

 immature specimens, was indeed regarded by Professor Riitimeyer 

 as essentially Pleurodiran, although there was no absolutely decisive 

 evidence in support of this view. The shell is thinner than in Plesio- 

 chelys ; and the skull has the temporal fossae roofed over by bone, 

 and apparently had a long sutural union between the postfrontal and 

 parietal. 



Mesozoic Chelonians of Uncertain Position. — It will be con- 

 venient to notice in this place several Mesozoic Chelonians mostly 

 known to us only by the skull, of which the serial position cannot 

 at present be determined. A large skull from the Portland Oolite, 

 originally described as Chelone platyceps, but subsequently made the 

 type of the provisional genus Stegochelys, is characterised by the 

 incomplete roofing of the temporal fossae, and the meeting of 

 the prefrontals in the middle line, while it is stated to have distinct 

 nasals. The palate is unknown. The size of this specimen sug- 

 gests that it may perhaps be referable to the Cryptodiran genus 

 Thalassemys of the Kimeridgian. 



In the Wealden of Belgium there occur remains of young Chelon- 

 ians to which the name Chitracephalus has been applied. There is 

 no mesoplastral bone; and the skull is characterised by its elongated 

 form, open temporal fossae, and the approximation of the orbits to 

 the nares. 



In the Upper Cretaceous genus Rhinochelys the shell is only 

 known by fragments. The skull (fig. 1014 bis) has an inferior 

 temporal arcade, the temporal fossae are completely roofed over, 

 the palatines meet in the middle line, the nasals are distinct from 

 the prefrontals, which are separated from one another by the 

 frontals, the pterygoids are comparatively narrow, and laterally 

 emarginate, while the symphysial suture of the mandible is obli- 

 terated. The humerus is of the Pleurodiran type found in Para- 



