CRETACEOUS FOSSILS. 387 



PTERIA (PSEUDOPTERIA) SUBLEVIS. 

 Plate 7, tig. G. 



Pteria (Pseudopteria) sublevis, Wliitf., Prelim. Rept. Pal. Black Hills, 1877, p. 29. 



Shell small, erect, subrhoinboid or subovate in outline, left valve de- 

 pressed-convex, most ventricose on the umbo and toward the beak, near 

 which it becomes subangular. Hinge line shorter than the length of the 

 shell below, and about two-fifths as long as the height; anterior wing 

 minute, obtusely pointed, posterior wing moderate in size and convex, 

 scarcely separated from the body of the shell by a slight, almost impercepti- 

 bly depressed line; anterior side of the shell gradually receding from 

 below the anterior wing to the basal border, which is rather sharply rounded, 

 and the postero-basal margin a little more sharply rounded than the basal. 



The surface of the valve, in all the specimens seen, is much exfoliated, 

 so that the surface features are not positively known, but it has, appar- 

 ently, been quite smooth; the substance of the shell remaining on the speci- 

 mens is highly nacreous. 



Several individuals of the left valve have been noticed in the collection, 

 but none of the right have been recognized. Those seen all have the same 

 character, and are of about the same size, being a little less than three- 

 fourths of an inch in length by about half an inch in width. It most nearly 

 resembles in form P. (Pseudopterhi) fibrosa M. & H., Paleontology of the 

 United States Geological Survey of Territories, p. 36, Plate 17, Fig. 17), 

 but differs entirely in the surface features, being entirely destitute of the 

 faint plications of that species. 



Formation and locality. — In limestones of Cretaceous age, No. 4, Old 

 Woman Fork, Black Hills. 



Among the collections from the Fort Pierre Group there are large num- 

 bers of Inocerami, representing several of the species and varieties recog- 

 nized by Mr. F. B. Meek, in his excellent work on the Upper Missouri 

 Cretaceous formations, just issued by the Department of the Interior. But 

 notwithstanding the careful manner in which these forms have been de- 

 scribed and figured by that author, we find great difficultv in arriving at 

 any very satisfactory conclusions as to the limits of species when so large 



