146 PALEONTOLOGY OP KENTUCKY. 



Ventral valve moderately convex, becoming more gibbous in its center por- 

 tion from umbo to front ; greatest convexity a little above the middle of the 

 valve, from where it slopes in a regular curve to the front or base, and to the 

 basal half of the lateral margins ; to cardinal line and to the rear half of the 

 lateral margins it slopes more rapidly ; the cardinal angles are reflected, by 

 which a shallow concavity is formed between them and the umbo ; the beak is 

 little elevated above the hinge-line and slightly incurved ; the cardinal area is 

 narrow, extending to the extremities, and striate vertically. 



Dorsal valve concave, differing greatly in its depth in different individuals ; 

 but usually its concavity corresponds in general form with the convexity of the 

 ventral valve ; hinge-area very narrow, almost linear, and of not more than 

 half the height of the ventral one ; no beak is observed in this valve. 



Surface is ornamented by radii of different size and character, as may be seen 

 in figure 11, plate 17. Some of these striae are more prominent, most of which 

 extend to the beak, but some are added by intercalation ; the interspaces be- 

 tween these stronger striae are filled by finer ones, which are scarcely visible to 

 the naked eye ; all the radii are crossed by fine concentric striae. The general 

 surface character of the convex valve is much like that of good specimens of 

 Strophomena alternata, of the Trenton limestone, but it does not appear to be 

 subject to such extreme variations in respect to the striae. It also resembles 

 greatly in its surf ace- markings Strophodonta textilis, but it is always smaller, 

 has a more extended hinge-line, and is never so flat as that species. It is often 

 confounded with Strophodonta inequiradiata, which is a much larger species, 

 with different internal characters. Its surface striae are more delicate than any 

 of the other species. 



Formation and Locality. Occurs in the Corniferous limestone at the Palls of the Ohio, in Ken- 

 tucky and Indiana. 



Strophodonta nacrea. HALL. 



Strophodonta nacrea, Hall. Tenth Kep. on St. Cab., p. 144 1857 

 Strophomena lepida, Hall. Geol. Kep. of Iowa, Vol. 1, pt. 2, p. 4931858. 

 Strophodonta nacrea, Hall. Pal. N. Y., Vol. 4, p. 1041867. 



Shell below medium size ; semi-elliptical, having a brilliant nacreous luster ; 

 hinge-line crenulated, equalling, or a little less, than the greatest width of shell 

 below ; cardinal extremities angular. Ventral valve regularly and moderately 

 convex ; depressed or slightly concave within the cardinal angles, caused by a 

 slight deflection of the cardinal extremities ; umbo depressed and the beak 

 very small, scarcely elevated above the hinge-line, and not incurved ; cardinal 

 area very small and without foramen. 



Dorsal valve concave, corresponding in its depression with the form of tbe 



