protostegid^:. 229 



been stated that marginals are wanting in Protosphargis, but ac- 

 cording to Baur * they were present. If this be so, further obser- 

 vations are required to prove the right of the European form to 

 generic distinction. 



Genus PROTOSTEGA, Cope 2 . 



The type genus. Postaxial border of humerus more or less 

 deeply emarginate, and the lateral process closely approximated to 

 the head. 



The humeri figured by Leidy, ' Cretaceous Eeptilia of the 

 United States ' (Smiths. Contrib. Knowl. vol. xiv. 1865), pi. viii. 

 figs. 1-8, as Mosasaurian, and subsequently referred by Cope (Proc. 

 Amer. Phil. Soc. vol. xii. p. 333, 1872) to Protostega as P. nep- 

 tunia and P. taberosa, appear to be certainly generically distinct 

 from P. gigas, and are much more like the bones of Chelonidce, one 

 of the specimens being very similar to the humerus referred to 

 Lytoloma cantabrigiense (p. 69). These specimens were subsequently 

 referred by Leidy (' Rep. U.S. Geol. Surv. Terrs.' vol. ii. pt. i. 

 p. 342) to Ailantochelys, Agassiz 3 , under the names of A. mortoni and 

 A. nepiimia, the former specimen being apparently the type of that 

 genus. The term AtlantocJielys, as stated by Cope (Proc. Amer. Phil. 

 Soc. vol. xii. p. 433), was, however, unaccompanied by a sufficient 

 description to justify its adoption ; and it would appear, moreover, 

 from the foregoing observations that it is not really identical with 

 Protostega, for which Leidy and, at one time, Baur have proposed 

 to employ it. 



The following specimens are provisionally referred to the present 

 genus, since the humerus of Protosphargis is at present unknown ; 

 but it is quite possible that they may belong to the latter genus, if 

 that be really distinct. 



Protostega anglica, Lydekker, n. sp. 



Yery imperfectly known, but specifically distinguished from the 

 typical P. gigas, Cope 4 , of the Cretaceous of North America, by the 

 less deeply emarginate postaxial border of the humerus. 



Hab. Europe (England). 



49926. Fragment of chalk exhibiting the ventral aspect of a small 

 imperfect right humerus ; from the Chalk of Lewes, 

 Sussex. The proximal portion, which is nearly entire, 



1 Biol. Centralblatt, vol. ix. p. 184 (1889). 



2 Proc. Amer. Phil. Soc. vol. xii. p. 175 (1872). 



3 Proc. Ac. Nat. Sci. Philad. vol. iv. p. 169 (1849). 



4 Loclcit, 



