186 Systematic Paleontology — Middle Devon iax 



vai'iable. Ventral valve obliquely subpyramidal ; beak very prominent, 

 straight or incurved, frequently distorted or turned to one side; mesial 

 sinus wide and sharply defined with rounded or subangular bottom; 

 cardinal area high, triangular, generally arcuate with angular margins; 

 delthyrium narrow, covered by an elongate, convex pseudo-deltidium 

 which is perforated Ijclow the apex by an elongate foramen; the internal 

 impressions show a slit in the bottom of the sinus extending from the 

 beak toward the front of the valve for one-half or more of its length which 

 represents a longitudinal median septum. Dorsal valve slightly convex; 

 broad, prominent mesial fold with sometimes a slight groove along its 

 center which is bounded by broader furrows than those between the plica- 

 tions ; beak scarcely rising above the hinge-line ; cardinal area narrow and 

 linear. Surface marked by six to eight simple, rounded plications on 

 each side of the fold and sinus, crossed by fine concentric lines of growth 

 which are infrequently shown on the impressions of the ]\Iaryland speci- 

 mens; surface minutely granulose or papillose; shell stnicture distinctly 

 punctate. 



This species is not abundant in Maryland and not as common in occur- 

 rence as in the Hamilton formation of l^ew York. The Maryland speci- 

 mens are mainly impressions found in the rather arenaceous shales or thin 

 bedded sandstones of the Eomney formation ; but in this condition they 

 agree very closely with specimens from similar lithologic deposits of the 

 New York Hamilton. This species is readily distinguished by the very 

 convex ventral valve with high generally arcuate cardinal area; slightly 

 convex dorsal valve; small number of plications; granulose surface mark- 

 ing and punctate shell structure. 



Length, 7-10 mm.; width, 10-15 mm. 



Occurrence. — Romney Formation, Oxoxdaga Member ( ?) . Hanging 

 Eock, W. Va. Hamilton Member. East bank Evitts Creek below 

 Wolfe Mill ; on Oldtown Eoad east of Maryland Ave., Cumberland ; on the 

 Hancock-Harrisonville Eoad about 2 miles north of Hancock; ]\IcCoys 

 Ferry; on National Road northeast of Cumberland; in run at Hancock 

 east of Catholic church; J mile north of Green Spring Furnace; west of 

 iron bridge over Town Creek northeast of Oldtown; on road east of Pine 



