WEALDEN CLAYS AND PURBECK LIMESTONES. 5 



with a gentle sigmoid marginal curve to the hinder apex, x s, which is emarginate by 

 a notch with convex borders. The middle third of each lateral border of the plastron 

 is connected, through the medium of three marginal plates, with the carapace. The 

 length of the plastron, as far as it is entire, is 13 inches; its breadth, at the fore part 

 of the lateral walls, is 6 inches 6 lines. 



The entosternal element, s, is as broad as it is long ; its anterior half is defined by 

 two nearly straight borders, which converge at an angle of 45° ; its posterior contour 

 is nearly semi-circular ; the length of the entosternal is 2 inches 4 lines. The episternal, 

 e s, is bounded behind by two nearly straight lines, which meet at an open angle. The 

 hyosternal is remarkable, as in other species of Pleurosfernon, for the excess of its 

 transverse over its antero-posterior diameter, as compared with the same element of 

 the plastron in other Faludinosa .• the median sutural border is irregularly wavy : the 

 lateral border unites by suture with the fifth and part of the sixth marginal plates, the 

 anterior border is united by suture at its median half to the entosternal and episternal 

 bones ; its lateral half is free, smoothly rounded, and indented by a deep and narrow 

 notch. The outer surface of the bone is impressed by the line dividing the humeral 

 from the pectoral scute, which line is crossed at right angles by the line dividing both 

 the above scutes from the axillary and submarginal scutes. 



The supplementary sternal elements, intercalated between the hyosternals, A «, and 

 hyposternals, j3s, and which, from their constancy in the present genus, I propose to 

 denominate, for the convenience of description, the " mesosternals," and which bear 

 the letters p e and « 6 in Tab. Ill, are transversely elongated, quadrate plates of bone, 

 resembling in form the costal plates above, and being their correlatives in the plastron. 

 They are not quite symmetrical in the present specimen, the left one having a greater 

 antero-posterior breadth, and encroaching a little way beyond the median line to the 

 right side of the plastron to join its fellow : at the outer end they articulate with part 

 of the sixth and part of the seventh marginal plates. The mesosternal element is im- 

 pressed by the line dividing the pectoral, p e, from the abdominal, a b, scutes ; and by 

 that dividing both these from the submarginal scutes. The hyposternals, ^ «, present 

 nearly the same proportions as the hyosternals, A s ; they unite externally with part of 

 the seventh marginal plates ; they are impressed by the straight transverse line dividing 

 the abdominal from the femoral scutes, and by that dividing these from the in- 

 guinal scute. The xiphisternal, xs, present the form of an inequilateral triangle, and 

 are impressed by the line dividing the femoral, /e, from the anal, an, scutes. The forms 

 and proportions of the perishable horny scutes that covered the bony plastron are in- 

 dicated by the narrow, well-defined impressions of their boundary lines. The line 

 dividing the intergular from the humeral scutes curves across the entosternal at about 

 one third of the length of that bone from its anterior border. The humeral scutes, hn, 

 covered the rest of the entosternal, «, part of the episternal, es, and the anterior half of 

 the hyosternal, hs, bones. The pectoral scutes, ^je, were transversely elongate, quadrate. 



