34 POSSIL REPTILTA OF THE LONDON CLAY. 



condyle characteristic of the Chelonia. The paroccipitals (4) project backwards to a 

 Httle beyond the posterior plane of the condyle, indicating an affinity to the Trionycidce. 

 The inferior surface of the part of the tympanic to which they unite is concave. The 

 parietals (fig. 2, 7) form together a large semielliptic, almost flattened, platform, 

 relatively broader than in Chel. mydas, not convex, as in Chel. caouatrna ; not in- 

 dented by the mastoids, as in the Chel. lonyiceps, and not forming an angle between 

 the frontals and postfrontals, as in the Chel. breviceps. The frontals (11) together form 

 a pentagon, with the longest margin joining the parietals, the next in length con- 

 verging to a point between the prefrontals, and the shortest borders joining the post- 

 frontals. The postfrontals (12) and prefrontals (14) almost meet above the orbits, 

 and exclude the frontals from entering into the formation of its superior border. 

 The Chel. mydas comes nearest to the Chel. cuneiceps in this particular ; whilst in the 

 Chel. imbricata the frontals enter as largely into the formation of the upper border 

 of the orbit as they do in the Chel. brevicejhs, Chel. longiceps, and Chel. convexa. 



The precise form of the termination of the prefrontonasals, the maxillaries, and 

 premaxillaries cannot be determined in the present specimen ; fortunately, the fracture 

 of the anterior extremity of the skull has not extended to that of the bony palate. If 

 this be bounded by a transverse line behind, drawn across the anterior border of the 

 temporal fossae, the space included forms a right-angled triangle, and includes the 

 Avhole of the posterior nostrils. In the Chel. longiceps the similarly defined space has 

 the base shorter than the converging sides, and the posterior nasal aperture is behind 

 the transverse line. The bony palate, also, of Chel. acneiceps, instead of being pretty 

 uniformly concave and even, as in Chel. longiceps and Chel. caouanna, is raised on each 

 side between the middle line and the marginal alveolar plate into two convexities, 

 as in Chel. mydas and Chel. imbricata ; but the most prominent part of the palatal 

 convexities (figs. 3 and 4, 21) is obtuse in Chel. cuneiceps, not sharp or angular, as in 

 Chel. mydas and Chel. imbricata. 



The palatal part of the vomer (13) forms the median longitudinal groove dividing 

 the convexities, which are formed by the palatal processes of the maxillary bones. 

 The small part of the alveolar border of the maxillary which is entire terminates in a 

 sharp edge, extending about four and a half lines below the level of the palate. 



The ridge of the palatines, which forms the anterior boundary of the posterior 

 nostril, is not produced or bent below the level of the bony palate, as in Chel. caouanna, 

 and as it is, although in a minor degree, in Chel. mydas ; and there is not that concavity 

 between it and the oblique palatal tuberosity which exists in the Chel. mydas and 

 Chel. imbricata. 



The pterygoids are more deeply (semicircularly) emarginate laterally than in any 

 of the existing species of Chelones, and they are shorter in proportion to their breadth ; 

 they bound internally the lower apertures of the temporal fossae, which are broader 

 than they are long ; in all the existing Chelones the opposite proportions prevail. 



