338 GEOLOCIY OF THE BLACK HILLS. 



or coucavo outlines, lietiks more or less produced, that of the hirger valve 

 attenuate and acutely pohited. Surface marked by rather obscure con- 

 centric stria>, which are sometimes crossed by faint radiating lines, usually 

 most apparent on exfoliated surfaces, and generally quite distinct near the 

 front, on the interior surface, and on internal casts." 



Although there are a number of fragmentary individuals of this spe- 

 cies in the collection, there are none in a condition to figure or describe; 

 we have, therefore, cof)ied the original description, and also the figures 

 from the Paleontology of the Upper Missouri, in order to give as perfect an 

 exposition as possible of the fauna of this period, represented in this 

 interesting region. The species differs from L. j)innaformis Owen, as it 

 occurs at the Falls of the Saint Croix, and also from the same locality in 

 its straight sides or cardinal slopes ; in the more broadly rounded front, 

 and in the somewhat more regularly convex valves. 



Formation and locality. — In brown, somewhat feiTuginous and friable 

 sandstones of the Potsdam formation, at Castle Creek, Black Hills, Da- 

 kota. 



OBOLID^. 



Genus OBOLUS Eichwald. 



OBOLUS? PECTENOIDES. 



Plate 2, figs. 18, 19. 



Obohis pectenoides Wliitf., Eept. on a Keconnaissance of the Black Hills of Dakota, by 

 Capt. W. Ludlow, 1875, p. 103, PI. — , figs. 1-3. 



Shell rather small, transversely oval or broadly ovate, the length and 

 breadth being subequal ; apex of the larger valve very obtusely pointed ; 

 cardinal margins sloping from the beak, and inclosing an angle of from 

 110° to 120° in most cases, but often a much greater angle, or in many 

 specimens the cardinal margin forms a segment of an elliptical curve ; 

 sides and base somewhat regularly rounded, the latter often being broadly 

 rounded. Dorsal valve shorter than the ventral and proportionally more 

 transverse. Exterior surface of the shell strongly lamellose ; substance 

 comparatively thick. 



