STROPHODONT^ OF THE HAMILTON GROUP. 1«3 



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 striae 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 haye come under 

 my observation) do not reach the full dimensions of the species, but are thick 

 and robust, having a high degree of convexity. 



