WEALDEN CLAYS AND PURBECK LIMESTONES. 9 



ovatum than in those of the Pleurosternon emarginatum. The number of marginal scutes 

 was twenty-four, twelve on each side, m i 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 es, in Tab. VIL 



The length of the carapace of the specimen of Pleurosternon ovatum 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. 



This beautiful specimen is in the Collection of Charles Willcox, Esq., M.R.C.S. 



Pleurosternon latiscutatum. Otven. Tab. L 



The species represented by the specimen of mutilated carapace, Tab. I, 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\, and by the great relative size, 

 more especially the superior breadth, of the three succeeding vertebral scutes, v2,v3,vA. 

 The boundary lines, indicating the forms and disposition of the horny scutes, are 

 proportionally larger and deeper than in the other species of Pleurosternon described 

 in the present Monograph. 



The sutures uniting together the different elements of the carapace are more 

 dentated or wavy than those in the other species, more especially the suture uniting 

 the nuchal plate with the first neural plate and first pair of costal plates. The neural 

 plates, from the first to the seventh inclusive, are similar in form, six-sided, with the 

 antero-lateral sides the shortest ; the eighth neural plate is the smallest, is four-sided, 

 and broadest behind ; the ninth and tenth neural plates are remarkable for their great 

 breadth. 



The transverse extent or length of the costal plates is considerable, in accordance 

 with the great breadth of the carapace : the eighth costal plate, in this respect, differs 

 considerably from its homologue in the other species of Pleurosternon. The second 

 marginal scute is not produced backwards between the first vertebral and first costal 

 scute, but, like the first and third marginal scutes, has its antero-posterior diameter 

 much less than the diameter in the direction of the periphery of the carapace. The 

 first, c\, and fourth, c4, costal scutes differ considerably in their forms and proportions 

 from those in Tables II, V, and VII. 



The outer surface of the osseous parts of the carapace of Pleurosternon latiscutatum 

 is minutely punctated and rugose, except near the sutural borders of the several 

 pieces, where it is impressed by rather coarse parallel striae, directed at right angles 

 to those borders. 



Of this well-marked species of Pleurosternon., I have seen portions of the fossil 

 carapace of several individuals, from the Purbeck limestone ; the most complete of 



2 



