CHELONIA. 31 



greater breadth, and have not their antero-lateral borders increased in length, as in the 

 Chelone longiceps. 



The dechnation of the ribs from the neural plates, gives a greater degree of 

 steepness to the sides of the carapace than in the Chelone convexa, and the 

 impressions of the scutes have equal depth and breadth. The chief difference 

 indicative of specific distinctions, lies in the form of those impressions ; and the question 

 is, whether, in the progress of growth which makes the longitudinal extent of two of 

 the vertebral scutes in one specimen nearly equal to three, in another, so great a change 

 could be effected in their shape as is shown in the specimen of Chelone convexa ; in which 

 it will be seen that the second vertebral scute (PI. 14, fig. 1), though one third shorter 

 than in Chel. declivis (PI. 23, V2), is of the same breadth as that in the larger specimen, 

 and that the rest differ in the same remarkable degree. 



Fig. 3 shows the characteristic declivity of the sides of the carapace in the present 

 species. 



Chelone trigoniceps. Oicen. Plate 25, figs. 4, 5, and 6. 



More than one of the old tertiary turtles {Chelone) are remarkable for the 

 longitudinal extent or depth of the symphysis of the lower jaw. 



The turtles from the Eocene clay at Harwich have this character so strongly 

 developed and the under surface of the symphysis so flattened, especially in one of the 

 species (PI. 18), as to have suggested the " nomen triv'mle" planmentmn for it. The 

 Chelone longiceps (PI. 12), if we may judge by the length of the upper jaw and bony 

 palate, must have had a corresponding extent of the symphysis of the under jaw ; and 

 we may infer the same peculiarity from the straight alveolar borders of the maxillaries 

 and their acute convergence towards the premaxillary bones in an allied species, 

 Chelone trigoniceps, which I have described and figured in the Appendix to Mr. Dixon's 

 work on the 'Fossils of Sussex,' from a specimen which is in the collection of 

 G. A. Coombe, Esq., and which was obtained from the Eocene clay at Bracklesham. 



Amongst the Chelonites which Mr. Dixon has obtained from the same formation 

 and locality, ai'e portions of the fore part of the lower jaw of four individuals of the 

 genus Chelone, all exhibiting the characters of the pointed form and great depth of 

 the symphysis. 



One of these specimens (PI. 18, figs. 5 and 6) agrees so closely in size and shape 

 with the fore part of the upper jaw of the Chelone trif/oniceps (fig. 4) — fits, in fact, so 

 exactly within the alveolar border, and so closely resembles that specimen in texture 

 and colour, that, coming from the same formation and locality, and being obtained 

 by the same collectors, I strongly suspect it to belong to the same species of Chelone, 

 if not to the same individual. 



The known recent Chelones differ among themselves in the shape and extent of the 

 bony symphysis of the lower jaw. Both the Chelone imbricafa, and Chelone caouanna 



