FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 205 



Beaks nearly anterior, of moderate size, and somewhat elevated above the 

 hinge-line; both incurved, and the right one below the other. The umbonal 

 ridge extends from the beaks to the produced lower part of the posterior ex- 

 tremity. It is most convex at about the middle of the shell, from where it 

 slopes in a gentle curve to its posterior extremity ; towards the hinge it in- 

 creases in gibbosity. On its dorsal side the umbonal ridge has a sharp, angular 

 limitation, separating what may be called the dorsal incline from the lateral 

 area of the valves. This umbonal line is acute- angular, and runs be.ow the 

 summit of the ridge, on its dorsal side, up to the middle of shell to the point of 

 greatest convexity ; from here it occupies the summit and becomes more or less 

 rounded, losing its distinction entirely on the posterior third of the shell. 

 The dorsal incline is strongly inflected at and near the beaks, but becomes 

 convex for the balance of its extent. On the basal side of the umbonal ridge 

 there is a shallow, broad depression, extending from the beaks obliquely across 

 the valves to the middle of basal margin ; this concavity is more pronounced 

 on the left valve than on the right. 



The cardinal line is straight, and measures about two-thirds the length of the 

 shell. The surface is marked by several strong concentric lines of growth. 

 This shell resembles C. indenta, but differs from it by its shape and surface- 

 markings. 



Formation and Locality. Found in the Corniferous limestone at the Falls of the Ohio, on the 

 Indiana shore. 



Cypricardinia cylindrica. HALL AND WHITFIELD. 



Plate IV., figures 13, 14 and 15. 



Cypricardinia cylindrica, H. and W. 24th Reg. Rep., p. 1901872. 

 Cypricardinia cylindrica, H. and W. 27th Reg. Rep., pi. 11 1875. 



Shell rather small, cylindrical ; extremities rounded ; height little more than 

 the depth, and rather more than twice as long as high ; beaks nearly ter- 

 minal, rounded and incurved ; left valve scarcely less convex than the oppo- 

 site ; umbonal slope slightly angular. Surface marked by faint, distant, 

 concentric, lamellose lines of growth. 



This species is more elongate and cylindrical, less arcuate, and more equi- 

 valve than C. intiata. The lamellose striae have never been so strong, and are 

 more distant. 



Formation and Locality Occurs in the cherty layers superimposed upon the hydraulic limestone 

 of the Devonian formation in Jeffersen 'county, Ky., and in Clark county, Ind. It is a very rare shell, is 

 always silicified, and there are only very few specimens of this species known. 



