17 



Formation and locality. In reddish and buff colored argillaceous 



limestones of Jurassic age, at T Spring, west of the Black Hills 



of Dakota. 



PSEUDOMONOTIS (EUMICROTIS) ORBICULATA, 11. sp. 

 Plate 3, figs. 17-19. 



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

 subdiscoid. 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 com. 

 pressed, 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 stride of growth, which often give a knotty or roughened sur- 

 face to the radii where crossing them, especially toward the posterior 

 border and near the outer margin. Eight 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 an- 

 terior side of the hinge. The wing is separated from the body of the 

 shell below, as in others of this division of the group, by a narrow and 

 rather deep groove, but not by a distinct sinus. Posterior wing longer 

 and compressed, but not extending to more than half the length of the 

 body of the shell below. Surface of the valve marked, in all cases ob- 

 served, by fainter radii than the left valve, but otherwise of similar 

 character. 



This species is closely related to P. (E.} curta, Hall, but is much larger, 

 several of the individuals measuring seven-eighths of an inch in length, 

 and one specimen measures nearly an inch ; while those of that species 

 seldom attain to more than two-thirds of that size. The shell is also 

 much less oblique; less convex; the radii sharper and not usually so 

 distant, although in this latter feature they both differ very materially 

 among the different individuals, and the right valve is also distinctly 

 marked by the radii, which is not the case in that one. 



Formation and locality. In calcareous clays of Jurassic age, ranging 

 about 20 feet above the red beds, at Red Water Valley, Black Hills, 

 Dakota. 



2 B H 



