STROPHODONT^ OF THE HAMILTON GROUP. 109 



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 papilla3. IThe 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 speciea 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 ]JIr. 

 Billings as having been found in the Hamilton shales in the town of Bosanquet in 

 Canada Wgst. It also occurs in the Comiferous limestone at East- Victor and at 

 Lapham's mills in Ontario county, New- York. 



[ Paljeontology IV. I l4 



