STROPHODONT^ OF THE HAMILTON GROUP. 107 



In the interior of the ventral valve, the occlusor muscular impressions 

 occupy a narrow subquadrangular elevated space just beneath the apex ; 

 while the divaricator muscular imprints occupy a short broad space on 

 each side, and are limited by nearly vertical or slightly curving ridges 

 which have in some degree the appearance of dental lamellae : within 

 the limits of these ridges, the muscular imprints are not strongly marked. 

 In the dorsal valve, the posterior occlusor imprints are broad and extend- 

 ing far towards the cardinal line, and often limited externally by a low 

 pustulose ridge : the anterior impressions are small and narrow, sepa- 

 rated by a narrow mesial ridge and margined by diverging elevated 

 ridges, which, above the impression, are united in the mesial ridge from 

 which proceeds the bifurcating cardinal process : the divisions of this 

 process are broad and somewhat flattened vertically or a little obliquely, 

 and sometimes grooved on the inner side and distinctly bilobed at the 

 extremities. 



The condition of the muscular imprints is subject to considerable 

 variation; for in some specimens those of the anterior occlusors are 

 raised in two prominent processes to a height greater than the enclosing 

 ridges, and sometimes the imprints remain depressed, and the enclosing 

 ridges are extremely elevated, arching over and nearly enclosing the 

 muscular area. Just without the muscular areas, in both valves, the inte- 

 rior surface is rather strongly pustulose, and beyond this it is finely 

 pustulose in lines corresponding to the external striae; while the dorsal 

 valve, more often than the ventral, is marked by strong vascular 

 impressions. 



This species bears such a close resemblance to the figures of Prof. Phillips 

 cited above, that in my Report of the Fourth Geological District of New-York I 

 regarded the two as identical. I am now disposed to doubt the propriety of this 

 reference. It varies much in the convexity of the ventral valve ; being in some 

 individuals extremely gibbous, and in others very moderately convex. The area 

 is subject to some variations in width, perhaps .owing to compression ; and in 

 some specimens, the area of the dorsal valve is nearly as wide as the ventral area. 



The general surface character of the convex valve is much like that of good 

 specimens of Strophomena alternata of the Trenton limestone, but it does not 



