FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 145 



angular, sometimes salient and auriculate ; length smaller than the width, 

 their proportion about as four to five. The largest specimens measure about 

 two inches in length, by two inches and a half in width. Ventral valve is very 

 gibbous ; its depth is sometimes equal to half of its length. The point Of 

 greatest convexity below the middle of the length, from which it slopes rapidly 

 or rather abruptly to the front, and to the basal half of the lateral margins, 

 while it curves gently to the cardinal lines. 



The umbo is more or less elevated, and the beak incurved. The cardinal 

 area is narrow, and its margin crenulated. 



Dorsal valve shallow, concave in its larger portion, and only more abruptly 

 deflected in a marginal strip surrounding basal half of the valve, which strip 

 becomes sometimes even geniculated. 



Surface is ornamented by fine radiating striae, which are somewhat unequal 

 on the upper and middle portions of the ventral valve, but are fine and regular 

 towards the margins. In some specimens of the dorsal valve the striae show 

 a tendency to the alternation of three or four finer ones with a distinctly 

 stronger one between ; but this feature is not prevalent ; most shells have the 

 striae or their dorsal valve fine, close, and nearly equal in strength. Fine con- 

 centric lines cross the radii in well preserved specimens, but they are usually 

 obliterated. Four or five marked wrinkles extend obliquely from the cardinal 

 extremities into the body of the valve, but reach only to the base of the umbo. 

 This species diifers from St. inequiradiata by its larger size and greater gib- 

 bosity ; while, according to Prof. Hall' s statement, it is so nearly allied to 

 Stroph. concava, that it probably belongs with that shell in one and the same 

 species. 



Formation and Locality. Occurs in the Corniferous limestone of the Devonian formation in Jeffer- 

 son county, Ky., and in Clark county, Indiana. The specimens for the figures 7, 8 and 9, on plate 18, 

 seem to be an internal cast of Strophodonta hemispherica. 



Strophodonta mequistriata. CONRAD. 



Plate XVII., figures 10 and 11. 



Strophomena inequistriata, Conrad. Jour. Acad. Nat. Sci. Phil. 1842. 

 Strophomena inequistriata, Hall. Geol. Kep. 4th Dist., p. 290 1843. 

 Strophomena (Strophodonta) inequistriata, Hall. 10th Keg. Kep., p. 142 1857. 

 Strophodonta inequistriata, Hall. Pal. of N. Y., Vol. 4, p. 1061857. 



Shell semi-oval or somewhat semi elliptical or semi-circular in outline ; hinge- 

 line extended beyond width of shell below ; cardinal extremities acute, some- 

 times mucronate. In the specimen before me, which is illustrated on plate 17, 

 figure 10, the lateral margins below the mucronate extremities, and the basal 

 margin, form a regular curve, 



GEOL. SUE. 19 



