Maryland Geological Survey 137 



surface slightly concave toward the cardinal angles and sometimes a 

 slightly raised median ridge crosses the valve. Dorsal valve moderately 

 concave, rarely following the convexity of the opposite valve; sometimes 

 an undefined median depression extends from beneath the beak to the 

 front of the shell. Surface marked by mimerous striae, about nine 

 or ten of which are much stronger and more elevated on the umbo of the 

 ventral valve, with finer striae appearing in the middle of the inter- 

 vening space and on either side of the coarser ones; the striae frequently 

 increasing by intercalation and bifurcation, until they become very num- 

 erous and much finer at the margin ; the striae of the dorsal valve similar 

 to those of the ventral ; in well preserved specimens fine concentric striae 

 cover the surface, and there are frequently heavy concentric lines of 

 growth. The interior of the ventral valve, and impressions of the same, 

 show a large flabelliform diductor muscular impression, separated towards 

 the front and distinctly lobed, with small adductors between them, sepa- 

 rated from each other by a depression ; in the dorsal valve the adductor 

 impressions are conspicuous, divided longitudinally by a narrow ridge, 

 and often limited in front by elevated ridges; beyond the muscular im- 

 pressions the interior surface of both valves is minutely pustulose. 



Specimens from Maryland are almost identical in form and markings 

 with some of those figured by Hall from the Hamilton formation of New 

 York, in particular see figs. 2d and 2g, pi. 17, vol. iv, Pala?ontology New 

 York, which are stated to be ventral and dorsal valves of the ordinary form. 

 This species is readily distinguished from 8. perplana (Con.) bv its 

 thicker shells, greater convexity and much coarser striae which are 

 slightly undulating and variable in strength so that the external appear- 

 ance of the two species is quite different. 



Length, 23 mm.; width, 33 mm. 



Occurrence. — Eomney Formation, Hamilton Member. East bank 

 Evitts Creek below Wolfe Mill; in Jennings Eun, ^ mile west of Corrigan- 

 ville; on Hancock-Harrisonville Eoad about 2 miles north of Hancock. 



Collections. — -Maryland Geological Survey; New York State Museum; 

 American Museum of Natural Historv. 



