STR0PH0D0NTJ3 OF THE HAMILTON GROUP. 
105 
depressed, elongate-ovate, narrowing above and somewhat widely sepa¬ 
rated below; while the occlusor impressions are narrow-ovate spots on 
the thickened portion of the shell, and below these extends a narrow ele¬ 
vated band separating the other impressions. In the dorsal valve, the ante¬ 
rior and posterior occlusor muscular impressions are small and not 
strongly defined. The cardinal process is slender, bifurcating, with a 
narrow rounded ridge extending from its base down the middle of the 
valve. The interior of the shell, excepting the muscular impressions, is 
studded with prominent scattered granules or papillae. The crenulations 
on the hinge-line are rather distant, but conspicuous. 
Even in fragments this species may be always distinguished from its 
associates by the peculiar lustre. 
This species bears some resemblance to Orthis lepis of Bronn, but is much less 
distinctly striated than well-preserved specimens of that shell, and the crenula¬ 
tions of the hinge-line are only about half as many as in the European species. 
The generally smooth surface and strong lustre of the two, in specimens before 
me, give a similarity in external appearance, which is not confirmed by a compa¬ 
rison of the details. Observations of a larger number of specimens has shown 
that there are no constant characters by which -the New-York and Iowa forms 
can be separated from each other, and the S. lepida therefore becomes a synonym. 
Geological formations and localities. In the shales of the Hamilton group on the 
shore of Lake Erie, at Darien, York, and on the shore of Canandaigua lake. It 
occurs in the same formation -at Rock island, in Illinois ; and is cited by Mr. 
Billings, as having been found in the Hamilton shales in the town of Bosanquet in 
Canada West. It also occurs in the Corniferous limestone at East-Yictor and at 
Lapham’s mills in Ontario county, New-York. 
[ Palaeontology IV. ] 
14 
