CHELONIA. 65 



comparison, might at first be led to suspect that the transverse impressions of the 

 second (pectoral) or third (abdominal) pairs of scutes had here been mistaken for a 

 suture ; but due care was observed to avoid this error ; the scutes of the plastron 

 have left obvious impressions at pe, fe, which prove that they were in the same 

 number as in the Platemydians generally, and were quite distinct from the sutures in 

 question. 



Thus the intergular scute (iff) is in the form of an ancient shield ; the gular scutes 

 (c/u) are small inequilateral triangles, with their posterior border parallel with that of 

 the succeeding pair of scutes. The posterior transverse boundary of these, — the 

 humeral scutes (hu) — crosses the plastron four inches and a half from its anterior 

 margin ; that of the pectoral pair of scutes crosses at seven inches and a half from the 

 anterior border, and between the two transverse sutures ; that of the abdominal pair 

 (ad) at ten inches distant from the anterior margin, and about one inch and a quarter 

 behind the second transverse suture ; passing straight across the plastron between the 

 posterior concave margins of the lateral wall. The posterior boundary of the fifth or 

 femoral pair of scutes (fe) inclines obliquely backwards from the median line, as 

 usual ; it is three inches behind the preceding transverse impression. 



It is in the interspace of these impressions that traces of the transverse suture 

 between the hyposternals and xiphisternals are obvious, about four inches from the 

 posterior extremity of the plastron. If these traces were not so obvious, it might be 

 supposed that the xiphisternals were of unusual length, entering into the formation of 

 the lateral wall, and extending backwards from the second transverse suture to the 

 end of the plastron ; but this disproportion would be hardly less anomalous than the 

 existence of the additional pair of bones intercalated between the hyo- and hypo- 

 sternals which the present fossil evidently displays. 



In most of the existing large Emydes and Platemydes, the median transverse 

 suture traverses the plastron a little behind the third pair of scutes, and so crosses 

 the fourth or abdominal pair (ad, ad); and according to this analogy, the second 

 transverse suture in the fossil agrees with the single one ordinarily present, and has 

 most right to be regarded as the normal boundary between the hyo- and hypo- 

 sternals. One of the most distinctive characters of the present extinct Platemys is, 

 therefore, the division of each hyosternal into two, the plastron consisting of eleven 

 instead of nine pieces ; if the very interesting anomaly which it displays be not an 

 accidental or individual variety. Viewed in the latter light, its explanation is sug- 

 gested by that homology of the hyosternals and hyposternals which determines them to 

 be connate and expanded abdominal ribs (hsemapophyses), and thus we may view the 

 oldest of the known Platemydians as exhibiting, like many other extinct forms, a nearer 

 approach to the more typical condition of the abdominal ribs, as they are shown, e. g. 

 in the Plesiosaurus. Whereas, on Geoffroy's hypothesis, that the plastron is the 

 homologue of the sternum of the bird, it would be a further deviation from that type. 



9 



