Osteology of the Genus Platjsternum. 463 



next ampliicoelous, then follow a series of nineteen opistho- 

 coelous ; the last ten are again procoelous. 



Pelvis. — The pelvis, which isjattached to strong sacral ribs, 

 is intermediate between that of typical Eraydoids, in which 

 the pubis and ischium are in contact on the median line, 

 limiting two obturator foramens, and that of Chelydra, in 

 which the two bones diverge and are connected by ligament. 

 In Platy sternum the symphysial branches of the pubis and 

 ischium are parallel, but yet connected only by ligament. I 

 must remark here that the former type of pelvis, i. e. with 

 two obturator foramens separated by tlie union on the median 

 line of the symphysial branches of the pubis and ischium, 

 occurs in all Testudinidge (land and freshwater) which I have 

 examined, with the single exception of Dermatemys, which 

 belongs, in this respect, to the Ohelydroid type ; also in the 

 Cinosternidffi, but not in the Staurotypidge, which belong to 

 the latter type. 



The pectoral arch does not differ from that of Emys. The 

 tarsus contains seven bones — two in the proximal row, five in 

 the distal. 



This investigation into the bony structure of Platy sternum 

 leads to the result that that genus can satisfactorily be placed 

 neither with the Testudinidee (inch Emydidte) nor with the 

 Chelydridae, which latter family may be characterized by the 

 presence and great development of costiform processes of the 

 nuchal plate, and the opisthocoelous nature of most of the 

 caudal centra. It should therefore, in my opinion, rank as a 

 distinct family, Platysternidae, already established by Gray 

 on different grounds, the position of which is between the 

 Testudinidge and the Chelydridae. 



EXPLANATION OF THE PLATES. 



Plate XVI. 



Carapace, upper and lower view, c, costal plates; «i, margiuals; ne, 

 neurals ; 7iu, nuchal ; py, pygals. 



Plate XVII. 



A. Plastron, outer view, e^ij), entoplastron ; ejjp, epiplastra; hyop, 



hyoplastra ; hypp, hypoplastra ; xyp, xyphiplastra. 



B. Skull, upper vieAV. f . 

 C ,, lower view. |- 



D. ,, side view, f . 



E. Mandible, lower view. |. 



bo, basioccipital ; bs, basisplienoid ; exo, exoccipital ; /, fron- 

 tal ; j, jugal; m, maxillary; p, parietal; pal, palatino ; pm, 

 praemaxillary ; prf, prajfrontal ; pt, pterygoid ; ptf, postfrontal ; 

 q, quadrate; qj, quadratojugal ; so, supraoccipital ; A'y, squa- 

 mosal ; i', vomer. 



