STR0PH0D0NT2E OF THE HAMILTON GROUP. 
S 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 difference 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 strise .are similar to those of S. inequistriata , and to those of the 
dorsal valve of S. concava and of S. inequiradiata. 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 different 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 h and d of Plate xvm; 
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 k of 
the same plate. 
Figures 3 a, b, c, of Plate xvm 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-Bloomfield, 
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. 
