ie4 PALEONTOLOGY OF NEW-YORK. 



It is common and even abundant in many localitieB of the Hamilton group, 

 vhere it reaches a perfection and degree of development not elsewhere observed. 

 In the eastern part of New- York, in the arenaceous portions of the group, it is 

 usually seen in the condition of casts of the interior ; and its more perfect deve- 

 lopment is in the calcareous shales beyond the Genesee valley. In Illinois and 

 Iowa, in the calcareous shale of the same horizon, it is abundant. It occurs in the 

 Chemung group at several localities in Western New-York, but is by no means a 

 conspicuous fossil. 



Geological formations and localities. This species is found (as already observed ) 

 in the Schoharie grit, Corniferous limestone, Hamilton and Chemung groups. In 

 the Hamilton group, some of the principal localities are Darien in Genesee county, 

 and Eighteen-mile creek in Erie county ; likewise on Seneca and Cayuga lakes, 

 and at some more eastern localities. 



It has been found in the same horizon in Pennsylvania, Maryland and Virginia ; 

 at Rock island ( Illinois), New-Buffalo and Pine creek ( Iowa). ^ 



Stropliodonta nacrea. 



PLA.TE XVIII. 



Strophomena (Strophodonta) nacrea : Haxl, in Tenth Regents' Report on State Cabinet, p. 144. 1857. 

 Strophotnena lepida : Haix, Geol. Report Iowa, Vol.i, part 2, pa. 498, pi. 8, f. 3 a, b, c. 1858. 



Shell small, semielliptical, having a brilliant nacreous lustre* : hinge- 

 line crenulated, equalling or a little less than the greatest width of 

 the shell below, and sometimes terminating in more or less distinct 

 angles. Dorsal valve concave. Ventral valve sometimes regularly con- 

 vex, flattened at the margins, often depressed-convex in the umbonal 

 region and abruptly arched towards the front : beak very small and 

 depressed : area very narrow, linear, without foramen. The area of the 

 dorsal valve is often nearly as wide as that of the ventral valve. 



SoEFACB apparently smooth, but under a lens showing very faint con- 

 centric lines of growth, with sometimes obscure traces of radiating 

 lines. 

 The muscular impressions in the ventral valve extend more than half 



the length of the shell. The divaricator muscular imprints are deeply 



• The shell bas often a brownish or coppery metallic lustre, probably owing to the staining of the 

 Mrface from the decomposition of iron pyrites. 



