182 BRITISH FOSSIL REPTILES. 



The entosternal element («) 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 

 contoui' is semi-circular ; the length of the entosternal is 2 inches 4 lines. The 

 episternal (e «) is bounded behind by two nearly straight lines, which meet at an 

 open angle. The hyosternal is remarkable, as in other species of Plenrosternon, 

 for the excess of its transverse over its antero-posterior diameter, as compared 

 ■with the same element of the plastron in other Paludinosa : the median sutural 

 border is irregularly wavy : the lateral border united 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 naiTOw 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 {h s) 

 and hyposternals (p s) 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 (a b) in Plate 54, 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 impressed by the line dividing the pectoral (p e) from the 

 abdominal (a h) scutes ; and by that dividing both these from the submarginal 

 scutes. The hyposternals {p s) present nearly the same proportions as the hyo- 

 sternals {h 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 inguinal scutes. The xiphi- 

 sternals {x s) present the form of an inequilateral triangle, and are impressed by 

 the line dividing tlie femoral (/e) from the anal {a n) scutes. The forms and 

 proportions of the perishable horny scutes that covered the bony plastron are 

 indicated 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 {h u) covered the rest of the entosternal («) part of the episternal 

 (e s) and the anterior half of the hyosternal {h s) bones. The pectoral scutes {p e) 

 were transversely elongate, quadrate, and covered the posterior half of the hyo- 

 sternals and the anterior third of the meso-sternals. The abdominal scutes (« b) 

 presented a similar form,, and covered the rest of the mesosternals and less than 



