418 GEOLOGY OF THE BLACK HILLS. 



FAMILY ? 

 Genus LEIOPISTHA Meek. 

 Subgenus CYMELLA Meek. 



LEIOPISTHA (OYMELLA) MEEKI. 

 Plate 11, figs. 27, 28. 



Leiopistha (Cymella) MeeU Wkitf., Prelim. Kept. Pal. Black Hills, 1877, p. 35. 



Shell of moderately large size, transversely-oval in outline, about once 

 and a half as long as high. Valves very convex, with large, tumid, incurved 

 beaks, which are located a little more than one-third of the entire length 

 from the anterior end of the shell, and are considerably elevated above the 

 cardinal line ; anterior and posterior ends of the valve broadly and nearly 

 equally rounded, and the basal line between the longest points of the shell 

 forming a very regular and nearly symmetrical semi-oval curve ; cardinal 

 line sloping gradually on both sides of the beak, the anterior side marked 

 by a narrow, lanceolate, lunule-like depression ; and the posterior side by 

 a larger, broader area, bordered by an elevated ridge. Concentric undu- 

 lations of the surface, strong and distinct, continuing their sharpness to the 

 margins of the cardinal depressions, and gradually increasing in strength 

 with the increased size of the shell to below the middle, beyond which point 

 they become flattened and fainter ; about twenty-seven may be counted on 

 the entire surface. The concentric undulations are crossed by comparatively 

 strong, radiating costae, which do not mark the depressions between the 

 undulations, but exist only on their flattened surfaces. The radii are 

 strongest on the central parts of the shell, and become gradually finer and 

 closer toward the lateral extremities, and are visible on the anterior end to 

 the margin of the cardinal depression, but on the posterior end are not 

 seen to exist quite so far. The crests of the radii appear to have been 

 marked by a row of minute puncta, seen only under a magnifier, three and 

 four existing to each undulation. The surface of the shell is also marked 

 by fine, somewhat wrinkled, concentric lines of growth, especially on the 

 larger parts of the valve, but apparently confined to the outer coating of 



