184 BRITISH FOSSIL REPTILES. 



The first vertebral scute (v l) is narrower than the second and third vertebral 

 scutes, instead of being broader, as in Pleurosternon concinmmi. The second 

 vertebral scute (« 2) is proportionally broader behind than is its homologue in PL 

 concinnum. The fifth vertebral scute {v 5) has the three angles of its hinder 

 border sharply produced in the interspaces between the last marginal scutes. 



The character of the outer surface of the carapacial pieces resembles that in 

 the Pleurosternon concinnum. 



The more entire posterior border of the carapace of a second specimen from 

 the Purbecks shows it to be slightly emarginate at the middle of that border ; and 

 there is sufficient of the anterior border of the same carapace preserved to show 

 that it is more widely and deeply emarginate at the middle of that end. 



With regard to the plastron (Plate 56), the lateral borders of the anterior 

 freely-projecting portion are straighter, and those of the posterior portion more 

 uniformly convex, than in the Pleurosternon concinnum; the terminal notch has its 

 sides concave instead of convex. The impression of the line dividing the humeral 

 {h u) from the pectoral (p e) scutes advances at the median plane so as almost to 

 touch the entosternal (s). The mesosternals differ from those of the PI. concinnum 

 by the right extending a little to the left of the median line, but not more than 

 may be expected from the admitted extent of variety in different individuals of 

 the same species. The line between the femoral (/«) and anal {a n) scutes is wavy, 

 instead of being simply convex, as in PL concinnum. The impressions of the 

 three accessory (submargmal) scutes, between the axillary and original scutes, on 

 the right side of the plastron, are well shown ; they have not encroached so far 

 upon the marginal plates as in the PL concinnum. 



The length of the carapace of the Pleurosternon emarginatum, in the specimen 

 figured in Plate 55, is 21 inches 9 lines; the breadth of the carapace is 20 inches. 

 The entire length of the carapace is about 17 inches; the breadth about 15|- 

 inches. 



Pleueosteenum ovatdm, Oiven. Plate 57. 



The most perfect example of the depressed Emydians, with the complex 

 plastron, from the fresh-water Limestone of Purbeck, is that figured in Plate 57. 



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 Plerosternon con- 

 cinnum resembles the Pleurosternon ovatum in the absence of the anterior emargi- 



