8 FOSSIL CHELONIAN REPTILES OF THE 



on the right side, overlap the ends of the costal plates. The entire length of the 

 carapace of this specimen seems to have been about 17 inches; the breadth about 

 15^ inches. 



Pleurosternon ovatum. Owen. Tab. VII. 



The most beautiful and perfect example of the depressed Emydians, with the 

 complex plastron, from the fresh-water limestone of Purbeck, is that from the 

 collection of Charles Willcox, Esq., M.R.C.S., which, by the liberality of its possessor, 

 has been figured, in Tab. VII, for the present Monograph. 



The entire series of marginal plates is preserved with scarcely any dislocation or 

 fracture, in natural connection with the costal plates : they show the carapace to have 

 been nearly elliptical in figure, but a little more pointed, or less obtusely rounded 

 behind than before ; it is not emarginate at the anterior border, and was only very 

 slightly so, if at all, at the posterior border. The Pleurosternon concinnum resembles 

 the Pleurosternon ovatum in the absence of the anterior emargination of the carapace, 

 which distinguishes the Pleurosternon emarginatuni. The first vertebral scute, i> l, is, 

 however, as in that species, 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 1, 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, py, 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 the dermal marginal plates, the series of which it completes posteriorly. 

 The costal plates, j)i. i to j)i. 8, ofi'er no modification worthy of notice. There are eleven 

 marginal plates, i, i', to lo, on each side of the carapace, in addition to the nuchal, ch> 

 and pygal, j) y, plates ; they increase in breadth after the sixth ; the first bears the im- 

 pression of the triradiate line which marks the division between the first, m l, and 

 second, m 2, marginal scutes, and the first, v 1, vertebral scute. 



There was no nuchal scute. The second, third, and fourth margii'xdl plates were 

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

 parison with the transverse breadth, is greater in the costal scutes of thet Pleurosternon 



