CHELONIA. 63 



restrict the term Emys* by the presence of a thirteenth scute — the intergular one 

 (iff. T. XXI) upon the plastron ; from the genera Cistudo and Kinosternon it differs by 

 the absence of any moveable joint between the parts of the plastron ; from the Tetronyx 

 by the rounded anterior border of the plastron, and the greater number of scutes that 

 have left their impressions upon it : it resembles the genus Platysternon in the flatness 

 of the plastron and the horizontality of its lateral prolongations ; but it differs from the 

 only known species of that genus in the contour of the sternum, which is elliptical and 

 rounded in front, and has the lateral prolongations one third the length of the entire 

 sternum. It has also the intergular scute, which is absent in the Platysternon, as in the 

 Emydes of Dumeril and Bibron. The presence of this scute, so plainly indicated at iff 

 in the petrified plastron from Sheppy, together with the impressions of six pairs of the 

 more constant scutes of the plastron, indicate that the depressed form of the probably 

 estuary terrapene to which that plastron belonged, has appertained to the section which 

 the eminent French Erpetologists above cited have called Pleuroderes, or those that 

 could retract their neck beneath the side only of the anterior aperture of their thoracic 

 abdominal case. 



From the genus Peltocephalus the fossil under comparison differs by the marginal 

 position of both gular (gu) and intergular (ig) scutes, and by the slight narrow 

 emargination of its posterior extremity (xs). An outline of the natural size of this 

 emargination is added in the plate. 



It more nearly resembles the Podocnemys expansa in the forms and proportions of 

 the plastron scutes ; but the three anterior ones (gu, gu, and ig), are not wedged in 

 (enclavees) between the humeral scutes (hu), but are on a plane anterior to them. 



The form and proportions of the plastron in certain species of the Platemys, 

 Dumeril and Bibron, and the number and relative position of the scutes which covered 

 it, offer the nearest resemblance to those of the present fossil, and, with the 

 results of the foregoing comparisons, have determined my reference of the specimen 

 in question to that genus. 



Like the Platemys Spixii (Emys depressa of Spix), Platemys radiolata, Platemys gihha, 

 and some others of the genus, the sternum is rounded at its anterior border, and notched 

 at its posterior and narrower extremity. 



The intergular scute (ig) which crosses the median suture of the episternals (es) 

 is sub-pentangular and larger than either of the gular pair ; its point encroaches a 

 little upon the entosternal bone (s). The gular scutes (gu) are triangular, and, with 

 the intergular one, cover the anterior border of the plastron. 



The humeral or brachial scutes (hi) are inequilateral quadrate plates ; the pectoral 

 scutes (pe) and the abdominal scutes (ab) are transversely oblong and quadrate. The 

 femoral scutes are inequilaterally quadrate, the posterior external angles being prolonged 

 and rounded off. The anal scutes would be sub-rhomboidal were the posterior 



* ErpStologie Generate, 8vo, 1835, torn, ii, p. 232. 



