14 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



and provided with a shorter area, on each side of which its margins are 

 sometimes slightly crenulated. 



The muscular impressions of both valves are obscure, and the surface is 

 nearly smooth, or only marked by fine, indistinct lines of growth. 



Locality and position. — At numerous places along the Missouri between 

 the Big Sioux and the Great Bend ; also, on Little Blue River, near the 

 Kansas and Nebraska line, and near the Black Hills, on Cheyenne River, 

 as well as on the North Platte ; in the Niobrara group, or formation No. 3, 

 where it is usually found attached to fragments of a large Inoceramus. It 

 likewise occurs at several localities in New Mexico and Colorado, probably 

 in the same position. 



Oslrea inornata, M. & H. 



Plate 10, fig. 4. 

 Ostrea inornata, Meek aud Hayden (May, 1860), Proceed. Acad. Nat. Sci. Philad., 181. 



Shell small, narrow-subovate, rather thin, attached by the whole under 

 surface of the lower valve; beaks pointed and more or less laterally arcuate, 

 usually turned to the left side ; under valve conforming to the irregularities 

 of the surface to which it is attached, moderately concave, area small and 

 narrow ; upper valve rather convex, having its beak less pointed than that of 

 the other valve ; surface smooth, or only marked by very obscure lines of 

 growth, with sometimes a few very small, irregular, nearly obsolete, radiating 

 markings near the lower border. 



Length, about 1.40 inches ; breadth, 0.87 inch. 



This seems to be a rather rare species, since we have seen but three or 

 four specimens of it. Only one of these (an upper valve) shows the mus- 

 cular impression, which is comparatively large, of an irregular subcircular 

 form, and placed nearer the left side. One individual shows that the inner 

 margin of the under valve, near the beaks, was obscurely crenulate. It is 

 quite distinct from any other species obtained in the northwestern formations. 



In most of its characters, the upper valve of this species resembles that 

 of O. plumosa, Morton, excepting that its radiating markings are more obscure. 

 Its lower valve, however, in the only example of it yet seen, differs remark- 

 ably in being attached by its whole under side. 



Locality and position, — Great Bend of the Missouri, below Fort Pierre; 

 lower part of the Fort Pierre group, or No. 4 of the Upper Missouri Creta- 

 ceous series. 



