56 FOSSIL REPTILIA OF THE LONDON CLAY. 



pair, in the Tri. incrassatus, differs from those of the three other species of Trionyx here 

 described, in having only the antero-internal angles truncated, instead of both these 

 and the postero-internal ones. 



In the form and general proportions of the first pair of costal plates, the Tri. mar- 

 ginatus shows an intermediate character between the Tri. Henrici and Tri. Barbara ; 

 in the great breadth of the peripheral end of the seventh pair of costal plates it 

 differs in a well-marked degree from both species, and especially from the Tri. Henrici, 

 which it most resembles in its general contour. In the Tri. incrassatus the seventh 

 costal plates maintain nearly an uniform breadth from end to end. 



The antero-posterior diameter of each of the triangular plates of the last costal 

 pair exceeds the transverse diameter, whilst these proportions are reversed in Tri. 

 Henrici ; the difference in part depending on the different form of the posterior border 

 of the carapace in the Tri. marginatus, which is truncated ; the free borders of the last 

 costal plates forming a straight transverse line. The marginal pattern of the costal 

 plates may be traced in a slighter degree round the neural plates. 



The reticular sculpturing is better defined, and of a coarser pattern in the Tri. mar 

 ginatus than in any of the previously defined species. 



The middle line of the carapace is slightly depressed, as in the Tri. incrassatus. 

 The general degree of convexity of the carapace, which is less than that in the Tri. 

 Henrici and Tri. Barbara, agrees also with that of the Tri. incrassatus. 



The length of the carapace from the fore part of the first neural plate is eleven 

 inches ; its greatest breadth, across the suture between the third and fourth neural 

 plate, is twelve inches. 



This species is from the eocene deposit at Hordwell Cliff, Hampshire : it was 

 discovered by the Marchioness of Hastings, and is preserved in her ladyship's 

 collection at Efford House. 



Trionyx rtvosus. Oiven. Tab. XVllIJ. 



This beautiful species of Trionyx, also discovered by the Marchioness of Hastings 

 in the Eocene beds at Hordwell Cliff, has fortunately a characteristic pattern of 

 sculpturing, which, like that in the Tri. marginatus, would serve for the determination 

 of detached portions of the carapace. An}' of the costal plates, for example, of the 

 posterior half of the carapace, figured in Tab. XVllIJ, might be distinguished by the 

 sub-parallel longitudinal, and more or less wavy ridges, superadded to the more 

 common reticulate sculpturing from the homologous parts of the carapace of any of 

 the preceding fossil species of Trionyx, and, so far as I have yet seen, from any of the 

 recent species. 



The ridges in question, it will be understood, are longitudinal in respect of the 



