MARYLAND GEOLOGICAL SURVEY 285 



Description-. " Shell subrhomboidal in outline, with the basal margin 

 rounded ; hinge-line much longer than the body of the shell ; anterior 

 wing extended into a rather long, acute point, when perfect, forming 

 nearly one-third of the length of the hinge, measured from the point of 

 the beak ; posterior wing large, rather obtusely pointed, and extending as 

 far as the body of the shell below ; body of the shell oblique, a line drawn 

 from the beak to the center of the base forming an angle with the 

 posterior hinge-line of about 65 or 70 degrees; posterior margin of the 

 shell broadly and roundly, but not deeply, excavated between the posterior 

 wing and the postero-basal extremity of the shell; basal margin rather 

 sharply rounded; anterior margin obliquely sloping from the hinge-line, 

 being nearly parallel with the body of the shell; very slightly excavated 

 below the anterior alation. Left valve strongly convex when not com- 

 pressed, prominent and rounded in the center, but flattened and slightly 

 concave toward the alations; beak small, extending but little above the 

 hinge-line; flattened or depressed convex on the umbo. Right valve 

 concave, the concavity not exceeding one-half of the convexity of the 

 opposite valve, and usually somewhat shorter on the basal portion. 



" Surface of the' convex valve marked by regular, concentric, lamellose 

 lines, the edges of which are sharply elevated when well preserved, giving 

 an exceedingly roughened character to the surface. In the degree of this 

 latter feature, as also in the relative distance of the lines, there is con- 

 siderable variation in different individuals. Surface of the concave valve 

 distinctly lamellose, but the precise features have not been very clearly 

 determined, as no very good specimens of this valve have been examined." 

 -Hall and Whitfield, 1875. 



A detailed description of this species, particularly of the shell structure, 

 was given by Foerste in 1914, but the above-quoted description applies 

 better to the specimens as found in the Cumberland Valley of Maryland 

 and Pennsylvania. 



'Occurrence. MARTIXSBURG SHALE (Pairview division). Tuscarora 

 Mountain, one and one-half miles southeast of McConnellsburg, Pennsyl- 

 vania. Not uncommon in the Maysville and Richmond groups of the 

 Ohio Valley, New York, and Canada. 



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



