INVERTEBRATE PALEONTOLOGY. 239 



marked, is located immediately behind the projecting tooth, close up under 

 the cardinal margin. The anterior muscular impression is larger, propor- 

 tionally broader, and more shallow than the other; and the sinus in the 

 pallial impression is broad, triangular, and short. Both the anterior and 

 posterior pedal scars are distinct from those of the adductor-muscles. 



Leda fibrosa of Evans and Shumard* (Trans. Acad. Sci. St. Louis, 40, 

 1857) has very nearly the form of this species, but will probably be distin- 

 guished, even where the interior cannot he seen, by its more unequal beaks, 

 and fibrous structure. 



Our shell is also quite similar to the recent Necera cuspidata ; but its 

 rostrate posterior extremity is straighter on the upper border and the hinge 

 thicker; while the posterior tooth is more prominent, and the sinus of the 

 pallial impression a little deeper. 



Locality and position. — Moreau River, Dakota Territory, from the Fox 

 Hills group, or formation No. 5 : also, from the upper part of the Fort Pierre 

 group, at the same locality. 



Mesera Moreauensis, M. & H. 



Plate 17, tigs. 11, a, b, c. 



Corbula Moreauensis, Meek and Hayden (1856), Proceed. Acad. Nat. Sci. Philad., VIII, 83. 

 Neaera Moreauensis, Meek and Hayden (1860), ib., XII, 185. 



Shell small, nearly equi valve, very gibbous or subglobose toward the 

 front; anterior side prominently rounded" below; base deeply rounded in 

 front, and contracted behind : posterior side very narrow, abruptly compressed, 

 and rather distinctly rostrated ; dorsal border concave in outline just back of 

 the beaks, thence extended nearly horizontally behind ; convex and declining 

 abruptly in front; beaks prominent, ventricose, and apparently inclined a 

 little backward, located in advance of the middle. Surface of the gibbous 

 part of the shell ornamented by strong, regular, concentric lines, or small 

 costae, which, in passing upon the contracted posterior side, become fine, 

 closely-crowded striae. 



Length, nearly 0.36 inch; height, 0.19 inch; convexity, 17 inch. 



I have not yet seen the hinge of this species ; but from its form and gen- 

 eral appearance, there can be very little room for doubt in regard to the pro- 

 priety of placing it in the genus Necera. ■ It will be easily distinguished from 



* Possibly a .Yen /•«. * 



