WEALDEN CLAYS AND PURBECK LIMESTONES. 7 



with a truncated apex and a broad rounded base, which articulates with the pygal 

 and adjoining marginal plates. 



The costal plates offer nothing particularly worthy of note, in comparison with 

 those of Pleurosternon concinnum. 



The second and third marginal plates bear not only the impressions of the lines 

 dividing the corresponding marginal scutes from each other, but those dividing the 

 marginal from the first costal scutes. The succeeding costal scutes do not encroach 

 on the marginal plates, which consequently only show the impressions dividing the 

 marginal scutes from each other. Some of the marginal scutes are slightly dislocated, 

 and the posterior ones, from the ninth to the pygal scute inclusive, have their free 

 borders mutilated. 



The first vertebral scute, » i, is narrower than the second and third vertebral 

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

 scute, v2, is proportionally broader behind than is its homologue in PI. concinnum. 

 The fifth vertebral scute, vb, 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 which the inner surface is 

 exposed in the specimen figured in Tab. IV, 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, 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, hu, from the pectoral, ^9 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, fe, and anal, an, scutes is wavy, instead of being simply convex, as in 

 \Pl. concinnum. The impressions of the three accessory (submarginal) scutes, between 

 the axillary and original scutes, on the right side of the plastron, Tab. VI, are 

 well shown ; they have not encroached so far upon the marginal plates as in the 

 PI. concinnum. 



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

 in Tab. IV, is 2 1 inches 9 lines ; the breadth of the carapace is 20 inches. In the 

 specimen figured in Tab"" V and VI, the breadth seems to have been less ; but allowance 

 must be made for the partial dislocation inwards of the broad marginal plates, which. 



