Maryland Geological Survey 135 



convexity above the middle of its length; cardinal area narrow, vertically 

 striated and crenulated on the inner margin; apex scarcely higher than tlie 

 hinge-line. Dorsal valve slightly concave and frequently nearly flat. 

 Surface marked by fine suboqual striae, those of the ventral valve the 

 finer, sharp and sometimes gently undulating, increasing both by bifur- 

 cation and intercalation, and crossed by fine, close, even concentric striae ; 

 near the apex are frequently a few obscure concentric wrinkles which 

 occur occasionally upon the body of the shell. The interior of the ventral 

 valve marked by large flabelliform diductor muscular impressions, extend- 

 ing more than half the length of the shell, with small adductors between 

 and near the beak; the muscular impressions of the dorsal valve are not 

 strongly marked and are separated above by a median ridge which divides 

 in the bifurcating cardinal process and from each side a rounded ridge 

 curves forward and outward; the interior strongly pustulose. 



Some of the Maryland specimens are quite well preserved and clearly 

 show the fine several times bifurcating striae, as well as the flat form 

 of the shell, concentric wrinkles and slightly mucronate cardinal extremi- 

 ties. Internal impressions of the ventral valve are not uncommon which 

 show well the flabelliform diductor muscular impressions. A few speci- 

 mens have a median ridge or cicatrix extending from the vicinity of the 

 beak or central part of the valve to its front. On the median part of the 

 shell the radiating striae converge toward this ridge and the concentric 

 striae in crossing it cuiwe toward the umbo. The ridge was probably pro- 

 duced by an injury to the shell and the type specimen represented by fig. 

 Ic on pi. 17, vol. iv. Pal. IST. Y., has a similar ridge along the middle 

 part of the opposite valve. The species is readily distinguished by its 

 outline, nearly flat form, fine, distinct, and nearly equal bifurcating striae, 

 while the muscular markings and pustulose surface of the interior are 

 characteristic. 



Length, 25-33 mm.; with, 30-50 mm. 



Occurrence. — RoMxVey Formation, Hamilton Member. Williams 

 Eoad, 3| miles southeast of Cumberland; east bank Evitts Creek below 

 Wolfe Mill ; Williams Eoad f mile east of Queen City Hotel, Cumberland ; 



