WEALDEN CLAYS AND PURBECK LIMESTONES. 



Pleurosternon concinnum. Owen. Tab"" II and III. 



The first specimen of Pleurosternon which will be described in the present 

 Monograph is one from the fresh-water limestone of Purbeck, which was kindly 

 transmitted to me by Wm. Cunnington, Esq., for that purpose. It consists of a nearly 

 entire carapace and plastron. 



The carapace, Tab. II, includes the nuchal plate, ch\ the eight neural plates, 5 1 — 5 8, 

 which are connate with the neural spines of the vertebrae of the carapace ; and the 

 corresponding eight pairs of costal plates, except the eighth on the right side, p/. 1—8. 

 The hindmost neural plates, and all the marginal plates, save the first of the left side 

 connected with the nuchal plate, are wanting. 



The length of the carapace, from the anterior margin of the nuchal plate to the 

 posterior one of the eighth neural plate, is 13 inches ; the breadth of the carapace, 

 across the third costal plates, is 11 inches. The outer surface of the carapace is very 

 slightly convex. 



The nuchal plate, ch, is six-sided ; the anterior and antero-lateral borders are of 

 equal length, and are the longest of the six ; the hind border is the shortest : the 

 latter is angularly notched for the reception of the first neural plate, 5 1 , The front 

 border is slightly convex, with a feeble median concavity. The greatest breadth of 

 the nuchal plate, which is across the angles between the antero-lateral and postero- 

 lateral borders, is 3 inches 4 lines ; the length of the nuchal plate is 2 inches 3 lines. 

 The outer surface of the nuchal plate is impressed by a triradiate groove, indicative 

 of the junction of the two nuchal scutes with each other and with the first vertebral 

 scute, v\. The portion of the median series of bony plates answering to the first 

 neural plate in ordinary Chelonia is divided by a transverse suture into two plates, — 

 a circumstance which corroborates the homology of the neural plates with the median 

 dermal bones of the Crocodilia, and opposes their interpretation as the vertebral 

 spinous processes unwontedly expanded. The indented boundary between the first, v 1, 

 and second, v% vertebral scutes crosses the first neural plate, *i, immediately in 

 advance of the dividing suture in question. 



The second, 52, to the eighth, «8, nuchal plates inclusive are six-sided, with the 

 antero-lateral sides or borders the shortest, and the postero-lateral ones the longest ; 

 the third, fifth, and eighth are crossed by the boundary impressions between the 

 vertebral scutes. They progressively diminish in length to the seventh ; the eighth 

 resuming the normal length, unless the indentation between the fourth, u4, and fifth, 

 vb, vertebral scutes conceal, as I suspect, a suture dividing the plate, 58. 



The first pair of costal plates, 'pi. 1, is impressed by the boundary lines dividing 

 the second marginal scute, the first vertebral scute, v 1, the second vertebral scute, i- 2, 

 and the first costal scute, c 1 ; it unites with the nuchal, ch, and first and second mar- 



