CIIELONIA. 31 



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

 Chelone longiceps. 



The declination 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 (T. VII, V2), though more than one third 

 shorter than in Chel. declwis (T. XIV, v2), is of the same breadth as that in the larger 

 specimen, and that the rest differ in the same remarkable degree. 



Chelone trigoniceps. Owen. 



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, as to have suggested the " nomen triviale" planimentum for it. The Chelone 

 longiceps, 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, are 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 agrees so closely in size and shape with the fore part 

 of the upper jaw of the Chelone trigoniceps — 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 imbricata, and Chelone caouanna 

 have this part deeper and more pointed than the Chel, mydas, but neither species has 



