MARYLAND GEOLOGICAL SURVEY 263 



extend a little further forward than the mesial ridge. The mesial ridge 

 first gives place to a flat, unmarked interval, when it again rises more 

 conspicuously, but narrower and sharper, extending nearly to the sinus 

 separating the lobes of the outer muscular scar. The cardinal area of tho 

 convex valve slopes from the hinge-line obliquely backward, instead 01 

 being in plane with the lateral edges, thus differing -from R. alternata. 

 From three to five undulations of the shell transverse to the cardinal line 

 are seen often between the umbo and the cardinal angles, the heavier ones 

 being near the cardinal angles. The cardinal process is bifid and promi- 

 nent, the two parts being short, smooth, dentate protruberances that 

 stand prominently exposed about parallel with the plane of the cardinal 

 area. 



" The interior of the dorsal valve is very different from that of the 

 dorsal valve of R. alternata. The general visceral disc is nearly flat, sur- 

 rounded by a suddenly flexed margin, inside of which is a shallow im- 

 pressed broad line, most evident round the front; inside the cardinal 

 angles are a few scattered, radiately interrupted, short ridges or elevations 

 (genital markings), but these do not prevail along the side nor in front, 

 the surface there being smooth or finely granulated instead ; in the center 

 of the valve are five smooth, abrupt, digitately spreading ridges, the 

 middle one of which is a little larger and longer than the others ; these 

 rise more abruptly at their anterior extremities than behind, but none 

 of them reach the beak, or even the umbonal region, though the exterior 

 pair of lateral ones are placed further back than the others, converging 

 at an angle of about 700 (and often pass through the large pair of adductor 

 scars). Socket (crural) ridges very short and widely divergent; behind 

 them are small, doubly grooved sockets. The beak of the ventral valve is 

 often perforated by a minute, circular, pedicle opening." Winchell and 

 Schuchert, 1893. 



Occurrence. CHAMBERSBURG LIMESTONE (Echinospherites bed). Wil- 

 son and Pinesburg Station, Maryland. A characteristic Black River 

 species of the Mississippi Valley. 



Collections. Maryland Geological Survey, TJ. S. National Museum. 



