130 



THE EXTINCT BATRACHIA, REPTILIA 



bono. The hyosternals are prolonged forwards, extensively embracing the mosostcrnum; 

 the latter piece is snbcircular, and truncate behind. The episternals and mesosternal 

 are lost. The extreme anterior lip of the hyosternals is crossed by a groove, apparently 

 the suture of the gular plate; it has reached the external margin a half inch in front of 

 the posterior margin of the episternal. Tins would leave sufficient width for an intergular 

 plate, which would refer the genius to the Hydraspididae. As however the other pieces 

 of the sternum have been found in England, it is certain that the genus is Cryptodire. 



PLEUROSTERIS'UM PEOTORALB, Cope ep. nov. 



The plastron of this species is massive, and three times as thick in the middle as at the sides. The posterior 

 byosteraa] suture has been immoveable, and its rugae are minute. The anterior or axillary buttresses have risen 

 higher on the costal plates, and are directed obliquely forwards. The axillary outline is deeply concave. Tho 

 external surface is without sculpture. Behind the truncate axillary plate is one long hexagonal inner marginal, 

 while the anterior third of a second is truncated by the posterior suture. The posterior humeral dermal suture, 

 approaches the hyosternal suture towards the median line but slightly. 



Width of a hyosternal, 



Depth at middle, on hyost. suture, 



" outer end " " 



Length median suture, 

 From posterior suture to axilla, 



Anlero-posterior extent hyosternal (cxclus. sutural process), 

 Width inguinal scute interiorly, 



" " " externally, 



The inferior surface of the plastron is convex, tho interior nearly plane. 



The species was about the size of the rtyeluinys rugosa Ag. of the Delaware. It is distinguished from the 

 Emys pravu s of Leidy by the transverse hyosternal suture, the greater transverse extent, and thickness of hyos- 

 ternals. In A. firm us Leidy the humcro-pootoial dermal suture Is anterior, and the mesosternal is transversely 



truncate behind. 



CHELYDRINAE, 



The extinct species of this group indicate a suocessional relation of forms such as the 

 theory of evolution would anticipate. 



Those of the Miocene and Eocene periods in America are not known, but so far as 

 they are in Europe, they resemble those of the present. In the cretaceous, the; genus 

 Propleura appears, which approximates the genus Chelone in the probable more natatory 

 character of the fore limb than in Chelydra; the humerus has a more flattened shaft, 

 and is not quite so much curved as in the latter; its proximal condyle and crests arc 

 those of Chelydra, and this point bus chiefly decided me in referring the genus to the 

 neighborhood of the latter, rather than to the Cheloniidac. The independence of the 

 Cheloniidae rests entirely on the structure of the fore limb, for its other peculiarities are 

 repeated by genera in various other families. The natatory character has a strong ex- 



In. 



Lin. 



3 







10. 





2.8 



1 



0.4 



1 



4.0 



2 



2,5 





4. 





8.5 



