FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 151 



in width the deflected basal margin ; the height of this basal deflection meas- 

 ures from one- third to one-half of the length of disc. 



The ventral valve is slightly convex at and around the umbo, but flat or even 

 somewhat depressed between the umbo and the geniculation ; its beak is small 

 and only slightly curved. 



Dorsal valve generally corresponding in its concavity with the convexity of 

 the ventral, only differing in the depth of the valve ; but in some shells there 

 is an exception to this rule, inasmuch as the dorsal valve is entirely straight 

 from the cardinal line to the level of the lateral and basal geniculation. 



The surface is ornamented on the flattened portions of the shell by con- 

 centric, undulating, strong elevations, which are parallel with the curve of 

 the geniculation, and deflect outwards at the cardinal margin ; they are most 

 prominent in the portion parallel with the base, decrease in strength in their 

 lateral course, and become almost obsolete in some shells near the hinge-line ; 

 their number is variable, according to size and shape of the shell. The entire 

 surface is covered by radiating thread-like striae. The triangular fissure of tbe 

 ventral valve is partially closed by a deltidium and the apex of the ventral 

 valve. 



Formation and Locality. Occurs in the upper strata of the Devonian rocks in Jefferson county, 

 Ky., and in Clark county, Indiana. It is met with very often, but seldom found in fair, undistorted speci- 

 mens. The individual illustrated is one of the largest found here, though there is little difference in the 

 size of our shells belonging to this species 



Strophomena rhomboidalis. WAHLENBERG. 



This species has been described from the Devonian formation, in which the 

 conditions for the development of the shell appear to have been more favora- 

 able, inasmuch as the animal attained a considerably larger size. Outside of 

 this difference we find no distinction between the shells from both formations, 

 and there are no reasons for a new description here. 



Formation and Locality. Occurs in the Niagara limestone in the quarries east of the city of Louis- 

 ville, where it is, however, rare, and where it attains not even half the size of those found in the Devonian 

 rocks ia Clark county, Indiana. 



Genus Leptocoelia. Han. 



Leptocoelia, Hall. Rep. on the St. Cab. of Nat. Hist. 1856. 



Copied from Hall's Pal. N. Y., Vol. 3, page 4471859. 



Etymology: leptos, minute; koilia, belly, in allusion to the shallow visceral cavity. 



Shell inequivalve ; variable in form, usually semi-oval or sub-circular, 

 transverse or elongate, plano-convex or concavo-convex ; hinge-line sometimes 



