waiTK.j CRETACEOUS. 291 



trations of Cretaceous Ostreidce. While a considerable number of the 

 names which are given in the following list are shown to be synonyms, 

 or to represent spurious species, it is probable that some of those which 

 are given as representing true and distinct species, ought really to pass 

 into the list of synonyms. IMuch work needs to be done by a compe- 

 tent student, upon the North American fossil Ostreidse, especially upon 

 those of the Cretaceous period. The utility of treating the fossil Os- 

 treidae upon the basis of definite specific diagnoses is becoming more 

 and more questionable ; and I am convinced that a more general treat- 

 ment of the subject will, in the future, be the more rational. 



Genus Osteea Linnteus. 



Ostrea americana Deshayes. 



See Exogyra costata Say, on a following page, with which it is synony- 

 mous. 



Ostrea anomiceformis Eoemer. 



Professor Eoemer described this form as a species of Ostrea, in Kreide- 

 bildungen von Texas, page 75, Plate IX, Figs. 7, a, b, c, d, e. The in- 

 terior of Professor Ecemer's shell is not known ; but in external charac- 

 ter it is so closely like certain known forms oi Anomia that I believe it 

 to belong to that genus. It is, therefore, not considered in this memoir. 



Ostrea anomioides Meek. 



(Plate XXXIX, Figs. 4, 5.) 



Mr. Meek described this form without illustrations in the annual re- 

 port of the United States Geological Survey of the Territories for 1872. 

 It was afterward illustrated by myself in that series of reports for 1878, 

 Plate XI, Figs. 4 and 4 a. Those figures, as well as those which illus- 

 trate the species on Plate XXXIX, are drawn from Mr. Meek's type 

 specimens. 



Ostrea ajrpressa Gabb. 



(Plate XXXIX, Fig. 9.) 



The form published by Mr. Gabb under this name is probably identi- 

 cal with his 0. idriaensis. This view is suggested, both by the close 

 similarity of the two forms and the fact that both are reported to come 

 from one and the same formation, the Tejou Group of California. The 

 difference between them is certainly no greater than it is between cer- 

 tain of the varieties of the living Ostrea virginica, as may be seen by 

 referring to the figures on Plates LXVII to LXXXII. Some geologists- 

 and paleontologists, notably Professor Heilprin, contend, and with much 

 apparent reason, that the Tejon Group ought to be referred to the Ter- 

 tiary period and not to the Cretaceous. For the present, however, I 

 leave this species with the Cretaceous fauna, where Mr. Gabb placed it 



