144 [ Senate 



This shell may be recognized by its excessive tenuity and the extremely 

 narrow space between the two valves, which seems scarcely greater than 

 the thickness of the shell. There is no appearance of a foramen in the 

 ventral valve, or of the closing of one by a deltidium ; but in place of it 

 are two prominent dental processes on the inner side. Numerous separate 

 ventral valves have been seen, but, up to the present time, we do not know 

 the interior of the dorsal valve. From the character of the area, absence 

 of foramen, and form of vascular impressions, it seems strictly referable 

 to the Genus Strophodonta. 



Geological position and locality. In the shales of the Hamilton group 

 in Central and Western New-York ; and also in considerable numbers in 

 the shales of the same age at Rock island, Illinois, and at New-Buffalo in 

 Iowa. 



Strophomena (Strophodonta) nacrea (n. s.). 



Shell small, semicircular, having a brilliant metallic (or coppery- 

 nacreous ) lustre ; hinge crenulated, equalling the greatest 

 breadth, and terminating in more or less distinct angles : dorsal 

 valve concave : ventral valve convex, flattened at the extremi- 

 ties, depressed - convex in the umbonal region, and abruptly 

 arched towards the front ; beak very small and depressed ; 

 area rather narrow, having no foramen. Surface apparently 

 smooth, but showing under a lens very faint concentric lines of 

 growth, with sometimes obscure traces of radiating lines : whole 

 interior, excepting the muscular impressions, studded with 

 prominent scattering granules or papillte. The crenulations of 

 the hinge-line are rather distant, but quite conspicuous. 

 Fragments, even, of this species may be always distinguished from its 

 associates, by the peculiar metallic 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 crenulations of the hinge-line are less numerous, as well as the 

 hinge-line less proportionally extended. The generally smooth surface and 

 strong lustre of the two, in specimens before me, give a similar external 

 appearance, which is not confirmed by a comparison of the details. 



Geological position and locality. Hamilton group : Town of Darien, 

 Genesee county. 



