54 BracMopoda of the Cincinnati Group. 



Strophomena frada — (Meek , 1873). 



Shell semi-elliptical, thin and fragile, longer than wide, hinge not 

 extended, lateral extremities rectangular ; lateral margins straight and 

 rounding forward to the front in a semi-circular outline. 



Dorsal valve flattened in the umboual and cardinal regions, more or 

 less concave in the central and anterior regions, and curved very 

 moderately upward around the anterior and latei'al margins ; beak 

 scarcely projecting beyond the edge of the almost linear area. Interior 

 with flattened, bifid, cardinal process directed obliquely forward, and 

 longitudinally striated on the posterior faces ; sockets for the teeth of 

 the other valve, and socket ridges moderately distinct ; muscular 

 scars well defined and deep, situated nearly one eighth of an inch 

 anterior to the cardinal process, and separated by a well defined 

 mesial ridge ; muscular scars furrowed longitudinally, but the balance 

 of the surface of the visceral region is free from lines, furrows or 

 granulations. 



Ventral valve a little convex at the umbo, compressed over the 

 visceral region, and more or less curved downward at the anterior and 

 lateral margins ; beak scarcely distinct from the margin of the area, 

 and minutely jjerforated ; area flat and directed backward on a plane 

 with the anterior margin of the shell ; foramen broadly triangular and 

 arched above by the pseudo-deltidium. Interior with hinge teeth • 

 moderately prominent, remote and widely divergent ; dental ridges 

 oljscure, and extending obliquely outward and forward; mesial ridge 

 separating an ovate visceral region, moderately distmct ; visceral region 

 marked with irregular radiating furrows and ridges ; anterior and 

 lateral margins granulated. 



Surface of both valves ornamented with coarse radiating striae, and 

 distant imbricating laminae of growth. 



Length of a medium specimen, 1.30 inches ; breadth, 1.20 inches; 

 convexity, 0.32 inch. 



Found at Cincinnati in aggregated masses, about 400 feet above 

 low water-mark ; its range seems to be confined to a few feet in thick- 

 ness. While the strata at some places is literally a mass of these shells, 

 yet, on account of their thin and frail character, it is not easy to obtain 

 good specimens. 



It is readily distinguished externally from the S. alternata, by its 

 thin, fragile character, distinct imbricating lamellre, and acute angle 

 produced at the junction of the area of the two valves ; and on the 

 Ulterior by the flattened, elongated cardinal process, anteriorly situated 



