WEALDEN CHELONIANS. 1S5 



nation of the carapace, whicli distinguishes the Pleurosternon emarginatum. The 

 first vertebral scute (« i, PI. 57) is narrower than the second, instead of being of 

 equal breadth, as in the PI. concinnum : it covers, also, a larger proportion of the 

 first neural plate {s l), which, moreover, is not divided into two, as in the two 

 previously described species. The place of the fourth neural plate is occupied by 

 the conjoined median ends of the fourth pair of costal plates, ossification having 

 extended continuously from them into the dermal matrix overlying the subjacent 

 neural spine, instead of commencing from that spine or from a separate centre; 

 but this may be an individual variety. It leads, however, to a modification of 

 form of the fifth neural plate (« 5), which is pentagonal, instead of being six-sided, as 

 is usual, and as is the case with the two succeeding neural plates. The eighth neural 

 plate expands posteriorly, and the expansion in this direction is progressive in the 

 ninth and tenth neural plates; the eleventh or pygal plate {p^) is narrower than 

 the back part of the tenth neural plate, is quadrate, and shows, both by its shape, 

 size, and median impression, that it belongs rather to the category of dermal 

 marginal plates, the series of which it completes posteriorly. The costal plates 

 (.pl 1 to pi. 8) offer no modification worthy of notice. There are eleven marginal 

 plates (l, 1', to 10) on each side of the carapace, in addition to the nuchal (c h) and 

 pygal (|>y) plates; they increase in breadth after the sixth; the first bears the 

 impression of the triradiate line which mai'ks the division between the first (m i) 

 and second {m 2) marginal scutes, and the first (« i) vertebral scute. 



There is no nuchal scute. The second, third, and fourth marginal plates are 

 slightly overlapped by the first costal scute (c l). The antero-posterior breadth, 

 in comparison with the transverse breadth, is greater in the costal scutes of the 

 Pleurosternon ovatum than in those of the Pleurosternon emarginatum. The 

 number of marginal scutes is twenty-four, twelve on each side (m 1 to m 12). 



The fore part of the plastron appears to have projected in advance of the 

 carapace, as is indicated by the plate of bone marked (e s) in Plate 57. 



The length of the carapace of the specimen of Pleurosternon oratmn here 

 described is 19 inches 6 lines; its breadth is 14 inches 6 lines. It is very slightly 

 convex, with the margins a little raised. The feeble sculpturing of the outer 

 surface of the carapace resembles in general character that of the other species of 

 Pleurosternon. 



Pleueosternon LATrscuTATDii, Oicen. Plate 58. 



The species represented by the specimen of mutilated carapace, here figured, 

 differs from all the other recognised species of the genus by its distinct nuchal 

 scute {ch), by the small relative size of the first vertebral scute {v l), and by the 

 24, 



