FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 89 



Mesial depression broad but very shallow, and not extending back beyond the 

 basal third of the valve : in some instances no depression is observable. Dor- 

 sal valve more or less convex, according to age; in young shells nearly flat, in 

 old ones, on the contrary, even gibbous. Mesial fold only faintly indicated in 

 very old specimens; in all others not the slightest trace of it is visible. Surface 

 marked by strong angular plications, which increase towards the margins by 

 bifurcation, and by lamellose, imbricating concentric striae, which give the 

 shell a rugose appearance. 



This species is easily distinguished from its congener, Atrypa reticularis, by 

 its stronger radii, and by its rugose surface ; also by its smaller size ; its largest 

 specimens seldom exceed in length and width one-half of cardinal dimensions 

 of the larger A. reticularis. 



Formation and Locality. Found in the upper strata of the Devonian limestone, in Jefferson county, 

 Ky., and in Clark county, Indiana. The steep banks on the Indiana side of the Ohio river, opposite the 

 Falls, have furnished a considerable number of very fine specimens of this species. It is a peculiar ap- 

 pearance, that in the rotten hornstone of those banks, the ventral valves of this species occur very abund- 

 antly, weathered out and silicified, while very seldom a single dorsal valve. is found. I have, for instance, 

 collected there more than twenty-five single ventral valves, but did not find a single dorsal one. In some 

 of the specimens there found, both valves are perfect and at the same time separate, showing exquisitely 

 the interior arrangement of the shell. 



Atrypa calvini. x. SP. 



Plate XXXII., figures 64, 65 and 66. 



Shell below medium size ; sub-circular or sub-elliptical ; moderately convex ; 

 hinge-line less than greatest width of shell ; cardinal extremities rounded or 

 forming a very obtuse angle ; width greater than length. Surface plicated. 



Ventral valve depressed convex ; greatest convexity below the umbo, from 

 where it slopes very gently toward the cardino-lateral margins ; both margins 

 of the valve, from the cardinal extremities down to the front of the mesial 

 extension, are abruptly and prominently turned downward, forming a flange 

 of considerable size in proportion to the shell, and establishing on the valve 

 two lateral depressions, which are separated from the sinus by its elevated mar- 

 gins ; the sinus begins at the beak, is well defined, curves regularly and gently 

 towards the front, where it deflects abruptly upward, and forms a large quad- 

 rilateral extension, which fits in a corresponding indentation in the base of the 

 dorsal valve ; beak prominent and elevated above the beak of the other valve; 

 only slightly arched, and having a small, round perforation in the apex. Dor- 

 sal valve more convex, almost gibbous ; greatest convexity at the middle of the 

 valve, from where it slopes abruptly to the lateral basal margins ; the mesial 

 fold is well defined from beak to base, becoming prominent at the front ; beak 

 very small and incurved into the fissure of the opposite valve ; umbo almost 

 Depressed. 



OKOL. SUR. 12 



