STROPHODONT^ OF THE HAMILTON GROUP. J 09 



Surface marked by distant slender elevated radiating striae, which are 



increased by interstitial additions ; and the interspaces are marked by 



five, six or more extremely fine striae, which are cancellated by finer 



concentric striae. There is apparently no essential difierence in the 



surface markings of the two valves. 



In the ventral valve, the occlusor muscular impressions occupy very 

 narrow spaces just beneath the beak : the divaricator muscular impres- 

 sions are large flabelliform, each one divided into several lobes, and 

 margined on each side for a part of their extent by a pustulose ridge ; 

 while the anterior limits are not usually well defined. In the dorsal 

 valve, the posterior occlusor imprints are large semioval and concave, 

 defined by a slight thickening of the shell, and separated by a rounded 

 mesial ridge which divides below ; the marks of the anterior occlu- 

 sors being at the lower extremities of this divided ridge. The cardinal pro- 

 cess is bifurcate from its origin, and comparatively slender. 



The interior surface, beyond the muscular impressions, is marked by 

 somewhat distant minute elongate papillae which are arranged in lines 

 in the direction of the striae. The interior of a single dorsal valve shows 

 also prominent radiating folds ; but this feature may not be constant. 



This species may be readily distinguished by its nearly flat form and large 

 size. The surface striae are similar to those of S. inequistriata, and to those of the 

 dorsal valve of S. concava and of S. inequiradiaia. The muscular impressions of the 

 ventral valve are similar to those of the S.perplana; but the interior of the dorsal 

 valve of that species exhibits a shorter cardinal process, and a diflerent disposi- 

 tion of the muscular impressions. The latter are very similar to those of the 

 dorsal valve of S. inequistriata, as is shown in figures 2 A and d of Plate xviii ; 

 while there is a great contrast in the ventral valves of the two species, as shown 

 in the cast of S. texhlis, figure 4, and the interior of S. inequistriata, figure 2 & of 

 the same plate. 



Figures 3 a, b, c, of Plate xvin illustrate the usual forms of the species ; while 

 figure 4 is unusually extended on the hinge-line. 



Some farther illustrations of the interior are given upon Plate xix a. 



Geological formations and localities. This species is known in the Hamilton group 

 on the shores of Seneca and Cayuga lakes ; Mud creek and West-BIoomfield, 

 Ontario county ; York and Moscow, Livingston county ; Bethany, Covington and 

 Darien, Genesee county ; and the shore of Lake Erie. It occurs also in the Tully 

 limestone ; and a single dorsal valve has been seen from the Corniferous limestone. 



