STROPHODONTiE OF THE HAMILTON GROUP. 
im 
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 p 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 alternatu of the Trenton limestone, .but it does not 
