60 FOSSIL REPTILIA OF THE LONDON CLAY. 



times cut down vertically, or nearly so, as in the thickened border of Tri. incrassatus ; 

 sometimes with a marginal modification of the external sculpturing before the edge 

 was formed, as in Tri. marginatus ; sometimes without any such border-pattern, as in 

 Tri. rivosus. But whatever character the border of a carapace has presented, has 

 been constant in the same species, in which it is modified only at the fore part of 

 the border formed by the nuchal plate, and at the back part formed by the shoi't 

 and small eighth pair of costal plates. 



From this, therefore, it is to be inferred that the peculiar modification presented 

 by the free border of the fourth costal plate, T. XlXi?, fig. 3, was repeated in all the 

 other costal plates, excepting, perhaps, the last pair ; and consequently that the carapace 

 was almost entirely surrounded by a thick, vertical border, deeply grooved, — a character 

 which is expressed by the specific name circumsulcatus, selected to denote the Eocene 

 Trionyx, represented by the fragment of the carapace here described. 



This fragment, which consists as before said of the fourth costal plate of the left 

 side, presents the common reticulate pattern of its external sculptured surface, but 

 with some, modifications not presented by the before-described species ; the meshes 

 are smaller near the ends of the plate than at its middle part, and the network is finest 

 near the peripheral end. In the Tri. marginatus more particularly, and in a minor 

 degree in Tri. incrassatus, the Tri. Henrici, and Tri. Barbara, we observe the raised 

 parts of the network assuming a linear arrangement, more or less concentric, with the 

 circumference of the carapace ; but there is nothing of the kind observable in the 

 Tri. circumsulcatus. In this species also the outer surface of the costal plate presents 

 a distinct though slight double curvature ; the usual convexity being changed into a 

 concavity near the peripheral border : and, as the inner surface presents the usual uniform 

 concavity, the peripheral part of the plate suddenly augments in thickness as it 

 approaches the grooved border. (See fig. 2.) The character which distinguishes the 

 Tri. incrassatus from Tri. Henrici, Tri. Barbara, and Tri. rivosus, is exaggerated in 

 Tri. circumsulcatus, and there is added to it the groove, of which there is no trace in 

 Tri. incrassatus, and but a feeble one in the fifth and sixth plates of Tri. marginatus. 



The connate rib is almost wholly sunk into the substance of the superincumbent 

 costal plate in the Tri. circumsulcatus ; it is less prominent than in any of the foregoing 

 species, especially at its distal part, which is also less expanded than in the Tri. 

 incrassatus. The free extremity of the rib is entire, and is very short, as is shown in 

 figure 1 . 



Trionyx pustulatus. Tab. XlXi?, figs. 7, 8, 9- 



The contrast which the fragment above referred to, of apparently the homologous 

 costal plate to the one last described, presents in the character of its peripheral 



