352 GEOLOGY OF THE BLACK HILLS. 



convex ; hinge line nearly straight, varying from a little more than one- 

 third to nearly one-half as long as the body of the shell below ; posterior 

 ear very short, usually not more than half as long as the anterior one, the 

 extremity very obtusely angular, the posterior margin sloping rapidly 

 backward to the junction with the body of the shell ; anterior ear on the 

 left valve flattened, of moderate size, the margin very slightly obtuse, and 

 more rapidly sloping near the body of the valve. That of the right valve 

 is separated from the body of the shell by a deep triangular notch or sinus, 

 extending fully one-halt the length of the wing ; surface of the auriculation 

 marked by a single strong, flattened, and gradually widening rib, at a dis- 

 tance below the hinge line equal to its own width, and crossed by strong, 

 somewhat lamellose lines parallel to the outer margin. Surface of the 

 shell marked by very fine radiating lines or striae, which diverge from the 

 median line and curve rapidly in their direction toward the margin, and 

 on the upper part of the shell are strongly recurved, so as to have an 

 upward direction along the margin below the beak. The striae are rapidly 

 increased both by bifurcation and implantation, and are crossed by a few 

 moderately strong concentric lines of growth, and by finer, regular, con- 

 centric striae, only strongly marked on the interspaces of the radii, where 

 they form transverse puncta along the depressions. 



On the interior of the right valve, the space on the hinge line above 

 the strong rib marking the anterior ear is crossed by rather strong vertical 

 lines, which seems to be a character pertaining to the articulation of the 

 valves, and on the posterior side the border is elevated, forming a distinct 

 ridge similar to those on the hinge of Pleuronectia Swainson. 



The shells of this species vary considerable in outline and degree of con- 

 vexity; many of the right valves are somewhat squarish on the upper side 

 below the auriculations, as in the interior of the right valve figured, in 

 which case they are usually unsymmetrically curved along the basal bor- 

 der, while most are regularly orbicular, and occasionally they are slightly 

 elongated. The degree of strength of the surface striae is alse a point in 

 which they differ in different localities; those from the Big Horn Mountains, 

 which are usually of large size, are the most finely marked, and many 

 smaller specimens from the Black Hills region having the striae twice as 



