64 FOSSIL REPTILIA OF THE LONDON CLAY. 



end of the plastron entire. There are impressions of three scutes — the axillary, the 

 inguinal, and a supplementary one, — upon each lateral prolongation of the plastron, 

 covering the suture between this and the marginal plates of the carapace iaa), in which 

 the present fossil resembles the Platysternon or large-headed Emys of China ; but the 

 lateral walls are relatively longer, being equal in antero-posterior extent to one third 

 the same diameter of the entire plastron ; whilst in the subgenus Plcdystemon they are 

 less than one fourth. The general form of the plastron is also very different ; in the 

 Platysternon megacepJialwm. e. g. the plastron has an oblong quadrilateral figure, with an 

 open-angled notch behind. 



Retaining, then, the present species in the genus Platemys, as defined by Dumeril 

 and Bibron, we find that it enters into that small minority of the group in which the 

 plastron is rounded instead of being truncate anteriorly. 



In the present remarkable fossil the plastron forms almost a long ellipse, the hinder, 

 division being very little narrower, but tending to an apex, which is cut off by a 

 shallow emargination. The lateral walls, of the length above defined, extend outwards 

 almost parallel with the plane of the sternum, and expand to join by a wavy or rather 

 zigzag suture the marginal plates ; sLx of these [a a a a a a) are preserved on each 

 side ; their lower sides form a very open angle with the lateral walls : but the 

 fractures of these parts indicate that their horizontality may be in part due to acci- 

 dental pressure. 



The anterior part of the entosternal (s) is bounded by two nearly straight lines, 

 converging forwards at an angle of 65°, with the apex rounded off; the posterior 

 contour of this bone is nearly semicircular. The length of the entosternal is two 

 inches ten fines ; its breadth three inches seven lines ; the forms and relative positions 

 of the other elements of the plastron are sufficiently illustrated by Tab. XXI : es, es 

 marks the extent of the left episternal; hs,hs are the hyosternals ; ps the hyposternals; 

 xs the xiphisternals. 



The chief peculiarity of this plastron is the intercalation of a supernumerary piece 

 of bone, bearing the letters pe and ab between the hyosternal and hyposternal 

 elements on each side ; so that the middle third of the plastron is crossed by two 

 transverse sutures instead of one ; each suture being similarly interrupted in the 

 middle by an angular deflection from the right, half an inch back, to the left side. 



The extremities of the transverse sutures terminate each at the apex formed by the 

 inner or lower border of the parallel marginal plates. The first or anterior of these 

 sutures is distant from the anterior margin of the plastron six inches five lines ; 

 the second suture is distant from the same margin eight inches nine lines ; the right 

 half of the suture, which is a few lines in advance of the left, is the part from which 

 these measurements are taken. 



Since this deviation is rare, it having been noticed for the first time in the original 

 description of the present specimen, a naturalist, not having the specimen at hand for 



