Maryland Geological Survey 233 



Description. — Shell of medium size, or larger, trigonal, short; length 

 greater than the height; basal margin regularly curving, sometimes 

 straight on the posterior portion, abruptly rounded at both extremities; 

 posterior margin obliquely truncate; anterior end short, rounded; cardinal 

 margin arcuate. Valves convex, gibbous in the middle and above; beaks 

 at the anterior third or fourth, very prominent, incurved, arching over the 

 hinge ; umbo prominent and gibbous ; umbonal ridge distinct, subangular, 

 extending from the beak to the post-inferior extremity; post-cardinal 

 slope short, descending abruptly from the angular umbonal ridge to the 

 obliquely truncate posterior margin. Test thin, marked by very fine 

 concentric striae; hinge comparatively short, furnished with more than 

 twenty small teeth, which are continued in a row under the beaks without 

 interruption ; muscular scars faintly marked ; clavicular ridge very strong, 

 sharply defined and ciirved. 



This species is common in the bluish somewhat arenaceous Hamilton 

 shales of Maryland and West Virginia in which the various forms have 

 been obtained. It was reported by Hall from the Hamilton group " at 

 Pattersons Creek, [W.] Virginia" (Pal. IST. Y., vol. v, pt. i, Lamellibran- 

 chiata ii, p. 327) and on pi. xlvii a cast of the left valve is figured from 

 the Hamilton group at Cumberland, Md. This is a very clearly defined 

 species and is readily recognized from its short trigonal outline; obliquely 

 truncate posterior margin and short abrupt post-cardinal slope; sub- 

 angular, distinct umbonal ridge; and by ihe strong, sharply-defined and 

 curved clavicular ridge which in the internal impressions is represented 

 by the conspicuous, deep and curved furrow. 



Length, 9^-16 mm. ; height, 8-14 mm. ; depth, 4-11 mm. 



Occurrence. — Eomney Formation, Hamilton Member. East bank 

 Evitts Creek below Wolfe Mill; W. Va. Cent. E. E. cut at 21st Bridge; 

 B. & 0. E. E. cut at 21st Bridge; W^illiams Eoad 3^/2 miles southeast of 

 Cumberland; McCoys Ferry; southwest of McCoys Ferry; along Flint- 

 stone Creek in Gilpin ; W. Va. side Potomac Eiver 3 and 4 miles south of 

 Cumberland ; hill about 3 miles south of Cumberland. 



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

 American Museum of Natural History. 



