35 



tance of the posterior border of the hinge-plate, where it is marked or 

 divided by au impressed pit, which may be accidental. 



Compared with S. obliquata Meek, this shell is more equilateral, the 

 beaks more nearly central, and the shell less oblique. It differs from 

 S. Moreanenris of the same author in having much larger beaks, more 

 strongly enrolled, and the shell is also more transverse. 



Formation and locality. In ferruginous sandstone of Cretaceous Xo.5. 

 On Che Cheyenne River, near Old Woman's Fork, Dakota. 



ZELLINIDJ^. 



GENUS LIOPISTHA, Meek 



SUBMENUS CYMELLA, Meek-. 



LEIOPISTHA (CYMELLA) MEEKI, n. sp. 



Plate 11, figs. 27 and 23. 



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

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

 curved 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 undulations of the surface strong and distinct, con- 

 tinuing 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 cost, 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 ex- 

 tremities, and are visible on the anterior end to the margin of the car- 

 dinal 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, some- 

 what wrinkled, concentric lines of growth, especially on the larger parts 

 of the valve, but apparently confined to the outer coating of the shell, 

 and presenting the appearance of a thick epidermis. 



We were at first inclined to consider this as P. umlulata M. & H., but 

 on strict comparison with Mr. Meek's description in the Pal. of the U. S. 



