356 GEOLOGY OF THE BLACK HILLS. 



between the anterior ear of the right valve and the body of the shell below. 

 In this species there is no true notch, at least no sinus, the depression or 

 groove separating the wing from the body of the shell being simply a fold 

 of the shell projecting on the interior surface in form of a strong fold below 

 the hinge area, just as it does in some specimens of the right valve of the 

 the genus Meleagrina Lam., to which shell we are inclined to think them 

 closely related. If the genus Pseudoinonotis Bronn, must be retained for 

 the M. speluncaria group, it seems to us it might be well to retain the name 

 Eumicrotis, proposed by Meek and Hayden for that same group for this 

 section, as it is equally applicable to these shells. 



Formation and localities. — In rocks of Jurassic age, two miles south of 

 the Belle Fourche River, near Bear Lodge Butte; on Beaver Creek; on Red 

 Canon Creek, near the Cheyenne; and in Redwater Valley, Black Hills. 



PSEUDOMONOTIS (EUMIOEOTIS) OEBIOULATA. 



Plate 3, figs. 17-19. 



Pseudomonotis (Uumicrotis) orbiculata Whitf., Prelim. Eept. Pal. Black Hills, 1877, p. 17. 



Shell of moderate size, orbicular in outline, nearly equilateral, and sub- 

 discoid. Left valve depressed convex, most rotund just below and anterior 

 to the beak; beak small, full, slightly incurved, and projecting somewhat 

 above the cardinal border. Anterior wing very short, almost obsolete, the 

 anterior end regularly rounded from its extremity to and along the basal 

 margin; posterior wing of moderate size and compressed, shorter than the 

 shell below, the posterior margin rounding backward from its extremity to 

 near the middle of the length of the valve, thence somewhat regularly 

 rounded to the base. Surface of the valve marked by numerous unequal, 

 slender, and slightly elevated radii, which are separated by wider flattened 

 interspaces, also by irregular concentric strise of growth, which often give 

 a knotty or roughened surface to the radii where crossing them, especially 

 toward the posterior border and near the outer margin. Right valve less 

 convex than the opposite one, with a smaller inconspicuous beak, which 

 does not project beyond the cardinal line. Anterior side of the hinge line 

 characterized by a small and very obscure wing, which is reduced to a mere 

 point, reaching to not more than one-third of the length of the very short 



