22 UNITED STATES GEOLOGICAL SUEVEY OF THE TERRITORIES. 



tions, and probably attains its greatest development in the seas of the present 

 period. It has a wide geographical distribution; the recent species occurring 

 in the Mediterranean and European seas, as well as on the coast of North 

 America, India, Australia, &c. 



• Anomia! obliqua, M. & H. 



Plate 9, fig. 2. 

 Anomia obliqua, Meek and Hayden (May, 18G0), Proceed. Acad. Nat. Sci. Philad., 181. 



Shell thin, broad-oval, subcircular, or somewhat irregular, and more or 

 less oblique. Upper valve rather convex ; beak nearly or quite marginal, 

 placed near the anterior side, moderately gibbous. Surface marked concen- 

 trically by fine obscure lines and small wrinkles of growth. 



Length, about 1.33 inches ; breadth, 1.16 inches. 



I have not seen the under valve of this species, nor the inside of the 

 upper one ; and consequently know nothing of the nature of the muscular 

 .impressions. It resembles somewhat the figure of A. argentea of Morton 

 (Synop. Org. Rem., pi. 5, fig. 10) ; but the umbo is less prominent and much 

 more oblique. These differences, and the fact that it occurs in a lower geo- 

 logical formation, are perhaps sufficient reasons for regarding it as a different 

 species. 



It does not show the usual pearly luster of Anomia; and as nothing is 

 known of the nature of the under valve, it is only retained provisionally in 

 that group. The specimen is far from being in as good a condition as might 

 be wished; but it has been thought desirable to call attention to it as one of 

 the forms to be looked for in this part of the Cretaceous series of the North- 

 western Territories. 



Locality and position. — Near the mouth of the Niobrara River, on the 

 Missouri ; in the Niobrara division of the Upper Missouri Cretaceous series. 



Aiioiniii ! snblrigonalis, M. & H. 



Plate Hi, figs. 4, a, 6. 

 Anomia tsiibtrigonalis, Meek and Hayden (May, 1800), Proceed. Acad. Nat. Sci. Philad., 181. 



Shell subtrigonal, approaching subcircular, extremely thin and fragile. 

 Upper valve moderately convex; anterior side subtruncate, with a slightly 

 convex outline, rounding abruptly ;il its junction with the ventral margin; 



