MOLLUSCA. 



LAMELLIBRANCHIATA. 



Fam. PTERIID^, Meek. 



Genus PTERIA, Scop. 



Ptkria (Psexjdopteeia) sublevis, n. sp. 



Plate 7, fig. 6. 



Shell small, erect, sub-rhomboid or sub-ovate in outline, left valve 

 depressed-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 imper- 

 ceptibly 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 posterobasal 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 

 specimens is highly nacreous. 



Several individuals of the left valve have been noticed in the collec- 

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

 the same character, and one 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. (Pseudopteria) fibrosa, M. & H. 

 (Paleont. D". S. Geol. Surv. Terrifc., p. 36, pi. 17, fig. 17), but differs en- 

 tirely in the surface features, being entirely destitute of the faint plica- 

 tion of that species. 



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

 Woman's Fork, Black Hills, Dakota. 



Among the collections from the Fort Pierre group there are large 

 numbers of Inocerami, representing several of the species and varieties 

 recognized by Mr. F. B. Meek, in his excellent work on the Upper Mis- 

 souri Cretaceous formations, just issued by the Department of the In- 

 terior. But notwithstanding the careful manner in which these forms 

 .have been described and figured by that author, we find great difiBeulty 



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