52 BraeMojwda of the Cincinnati Group. 



lateral margins more or less geniculated at the margin of the visceral 

 cavity ; whole surface more or less irregularly furrowed and granulated. 



Ventral valve more or less convex at the umbo and over the visceral 

 cavity; lateral and anterior margins more or less curved downward ; 

 area of moderate height, flat, and directed obliquely backward, forming 

 with the area of the other valve an acute angle ; beak very small, 

 scarcely distinct from the margin of the area and minutely perforated ; 

 foramen broadly triangular, and arched over above by the pseudo- 

 deltidium, which is deeply sinuous on its inner edge ; the sinus being 

 nearly or quite closed by the dental process and pseudo-deltidium of 

 the other valve. Interior with hinge teeth moderately prominent, 

 remote and widely divergent ; dental ridges obscure, and extending 

 obliquely outward and forward ; scars of adductor muscles narrow, 

 long, and closely approximated or almost in contact; those of cardinal 

 muscles on each side very large, fan-shaped, but shallow, separated 

 sometimes by a small ridge in advance of the adductor scars, and 

 marked by radiating furrows and ridges ; anterior and lateral margins 

 usually marked by striae and scattering granulations. 



Surface of both valves ornamented by numerous radiating stride, that 

 increase on the ventral valve mainly by intercalation of smaller striae, 

 and on the doi'sal valve by division, thus making the stride more uni- 

 form in size. Subimbricating marks of growth are usually seen near 

 the free margins of large shells and concentric strise may be detected- 

 by the aid of a magnifier. 



This species, with its varieties, has a range co-extensive with the 

 Cincinnati Group, and is remarkably abundant. It differs so much in 

 size and convexity, and in its proportions, that the dimensions of one 

 shell would throw but little light upon the species. The form usually 

 found, from low water-mark to two hundred feet above, is thin and 

 frail, about one and a half inches, or a little more, in length and 

 breadth, which are about equal ; greatest width of the shell a little 

 more than the length of the hinge line, and quite regularly concave on 

 the dorsal and convex on the ventral side. ' The forms which come in 

 at and about 300 feet above low water-mark, many of which continue 

 to the Upper Silurian rocks, are quite numerous ; some of them are 

 quite flat and thin, with acute lateral extremities and no imbricating 

 lines ; others are strongly convex, thick, having many imbricating 

 lines of growth around the margins, from the line of the visceral cavity 

 to the edge, with lateral extremities somewhat truncated. Between 

 these two forms there are no two shells exactly alike, and they differ in 

 dimensions from three fifths of an inch in length to two and three fifths 

 inches, and quite as much in width, and proportionally as much in 



