126 G. B. Wi eland — Upper Cretaceous Turtles. 



not so good a swimmer as any of the existing marine turtles. 

 In any ease it is very evident that Osteopygis and the follow- 

 ing closely related JPropleura are among the most interesting 

 of all known forms on the border line between littoral and 

 marine Chelonians. 



The 7th cervical is coelo-bicyrtean. the double convexity of 

 the posterior end not being very marked. It is very broad 

 in front, mainly because of heavy somewhat downwardly 

 directed transverse processes, and had a heavy median keel 

 running along its full length. The base of the neural arch is 

 rather slender. Regarding the skull of Osteopygis we are as 

 yet uncertain. It may prove to be like that of the prior genus 

 Euclastes, as was thought possible by Cope or has since been 

 assumed on ground not known to me. This remains to be 

 carefully proven or disproven. 



Systematic Position. — The type of the genus Osteopygis is 

 the species 0. emarginatus, and the present species appears to 

 be more nearly related to it than to any of the several subse- 

 quently described forms. The type specimen of Osteopygis 

 Gribbi diifers from 0. emarginatus in having a convex (not 

 slightly concave*) outer surface of the 1st marginal, and in 

 the conformation of the horn shields of the posterior mar- 

 ginals, several being distinctly smaller. Also the marginals 

 are not notched. The lack of figures, and the fragmentary 

 condition of the type, prevent further comparison. 



In the case of Osteopygis pjlatylomas. which has well marked 

 pleuro-marginal fontanelles, the differences are clear, and it is 

 not entirely certain that this species falls within the genus 

 Osteopygis, which as hitherto constituted is quite certainly too 

 far extended. 



There is no living form to which Osteopygis can be at all so 

 nearly compared as can be Adocus to Derma.ternys. But if 

 the carapace only be considered, there is a certain resemblance 

 to such forms as Kachuga and HardeUa (Burma and Pegu). 

 On the other hand, there is marked similarity between the 

 plastron of Osteopygis and that of Stan rot yp>us of Central 

 America. Moreover the carapace of this latter genus pre- 

 sents certain features that seeminglv could have been derived 

 from some ancestral Osteopygid form by fusion of parts, espe- 

 cially in the pygal region. It is therefore with the existing 

 Central American genus Stau.rotypus that we may best com- 

 pare Osteopygis. the differences however being of full family 

 value. It is also verv interesting how the dorsal character- 

 of such a pleurodiran as Plesioehelys solodurensis (Upper 

 Jura. Kimmeridgian) are strikingly like those of Osteopygis. 

 while the plastron is almost identically like that of Adocus. 



* Proc. of the Phil. Acad.. May 12. 1868. p. 147. 



