186 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



young, or less developed, and slightly more compressed individuals ; though 

 it is barely possible that they may be distinct. 



Locality and position. — The typical smaller specimens are from the 

 upper part of the Fort Pierre group on Cheyenne and Moreau Rivers ; while 

 the larger specimens, upon which the foregoing description is mainly based, 

 came from Deer Creek, on the North Platte River, where they occur in the 

 Fox Hills group. 



Call is ta (Dosiniopsis!) orbiculata, H. & M. 



Plate 5, figs. 2, a, h, c. 



Cytherea orbicnlala, Hall and Meek (1854), Mem. Am. Acad. Arts and Sci., V (n. s.), pi. 1, tig. 7. 

 Merelrix orbiculata, Meek and Hayden (May, 18(>0), Proceed. Acad. Nat. Sci. Philad., XII, 185. 

 Dione orbiculata, Meek (1804), Smithsonian Cbeck-List Cret. N. Am. Fossils, 13. 



" Shell thick, suborbicular; beak moderately elevated, and near the 

 anterior side ; posterior margin regularly rounded ; surface marked by fine, 

 equal, concentric lines.' 



"Length, 1.08 inches; height, 1 inch; width, 0.66 inch." 



Our specimens of this species are merely internal casts, with portions of 

 the shell attached. Like that first figured by Professor Hall and the writer, 

 they present no characters by which we can determine definitely its generic 

 relations. Some of the specimens show the muscular impressions to be 

 shallow and ovate in form ; the posterior impression being, as usual, broader 

 than the anterior. The pallial line is provided with a triangular sinus, which 

 extends a little obliquely upward and forward, about two-thirds of the dis- 

 tance from its base toward the middle of the valves, its sides converging at 

 an angle of about 125° to 130°. 



Although some of our specimens present exactly the form of the figure 

 above cited, the majority of them differ considerably, being more nearly 

 transversely-ovate in outline, in consequence of the posterior side being more 

 extended and less broadly rounded. These may belong to a different species ; 

 but as they vary in this respect, and their muscular and pallial impressions, 

 as well as the thickness of the shell, are exactly as in the more orbicular 

 individuals, I do not think it advisable to separate them without being satis- 

 fied, from the comparison of better specimens, that they are distinct. 



Should these more nearly ovate specimens be identical with C. orbiculata, 

 as believed, the specimen first figured in the Memoirs of the Am. Acad. Ails 



