STROPHODONTiE OF THE HAMILTON GROUP. 
103 
The interior of the ventral valve, and casts of the same, show a large 
flabelliform divaricator muscular impression, which is somewhat widely 
separated in front, and each division distinctly lobed. The occlusor 
muscular impressions occupy a semielliptical space on each side of a 
narrow central depression, the marking on either side being double in 
well-preserved specimens. The upper extremities of this impression are 
close under the arch of the umbo, and separated by a smooth space from 
the divaricator impressions. 
Beyond the muscular impressions, the interior surface is minutely 
pustulose, the points being more prominent just without their limits; 
beyond which, the course of the vascular impressions can be distinctly 
traced. 
In the dorsal valve, the anterior and posterior occlusor muscular 
impressions are very conspicuous and deeply marked, and often limited 
by an elevated ridge, a narrow longitudinal ridge dividing the two pairs. 
On each side, and below the muscular impressions, the surface is marked 
by small pustules or tubercles; and beyond these the surface is minutely 
pustulose, the vascular impressions becoming distinct towards the 
margin. The cardinal process is divided from the base, the divisions 
strongly diverging. 
The varieties of form and gradations of size usually observed in this species 
are illustrated on Plate xvii. In the greater proportion of New-York specimens, 
the hinge-line is usually extended very little, if at all, beyond the width of the 
shell below ; but in the western specimens, figs. 2 h and 2 i, the extremities are 
much produced. In these specimens the striaa do not diminish towards the margin, 
as in those from more eastern localities. 
In figure 2 r the area is purposely broken away to show the extent of the 
occlusor muscular impressions. The absence of foramen, or callosity marking its 
place, is shown in several of the figures. 
Beginning its existence in the Schoharie grit, this species there attains its full 
size ; but its muscular markings are rarely well developed in the specimens from 
that rock, probably owing to the tenuity of the shells. 
In the Corniferous limestone, the specimens (as far as they have come under 
my observation) do not reach the full dimensions of the species, but are thick 
and robust, having a high degree of convexity. 
