STROPHODONTJE OP THE HAMILTON GROUP. 1«1 



The figures of Plates xi & xii have already been referred to, pages 81 & 87. 



Figures la-Id Plate xvii are of the ordinary forms of the species from the Hamilton 



group of New- York. 

 Figure le shows an enlargement of the area. 



Figure If represents the surface characters of the Illinois and Iowa specimens. 

 Figures Ig and lA are interiors of the ventral valve. 

 Figure It is a cast of the ventral valve of a large specimen. 

 The figures lk,u are of the interior of the dorsal valve enlarged. 

 Figures 1 n-o are enlargements of the striae of New-York and western specimens. 

 On Plate xix, figures 15- 19, are illustrations of the species as it has been observed in 



the Chemung group. 



Geological formations and localities. This species is found in the Schoharie grit 

 of Albany and Schoharie counties, in the condition of casts of the interior with 

 impressions of the exterior surface. It is found in the Comiferous limestone, usually 

 smaller than the medium size of specimens in the Hamilton group, but rarely in 

 larger forms : occurring in the Helderberg mountains ; at Schoharie ; at Cherry- 

 valley, and at Williamsville, N.Y. ; and near Columbus, Ohio ; and at the Falls 

 of the Ohio river. 



In the Hamilton group, it is found at numerous localities throughout the extent 

 of the outcrop. In the eastern part of the State, it usually occurs in the condition 

 of casts of the interior with impressions of the exterior. In the calcareous shales 

 on Cayuga and Seneca lakes, at Moscow, York, Pavilion, Darien, and at Eighteen- 

 mile creek, the shell is preserved in great perfection. Wo find the species in the 

 same horizon at Rock island, Illinois ; and near New-Buffalo, Iowa. 



In the Chemung group, it is found at Chemung-narrows ; along the line of the 

 Blossburg railroad, and at other places. 



Stropliodonta demissa. 



• PLATE XVII. 



Strophomena demUsa : Coskad, Journal Acad. Nat. Sciences, Philadelphia, Vol. viii, pa. 258, pi. 14, 



f. 14. 1842. 

 Slrophomena (Strophod[>nta) demista : Hall in Tenth Keport on State Cabinet, p. 137. 1847. 

 Strophodonta demiita : Hall, Report Geological Survey of Iowa, pa. 495, pi. 3, f. 6. 1859. 

 Strophomena demUta : Billings, Canadian Journal, 1861. 



<• " " Geology of Canada, p. 867. 1868. 



Shell semielleptical, u-sually wider than high, length and breadth often 

 nearly equal : hinge-line equalling or greater or less than the width 

 of the shell below, abruptly contracted beneath the extremities, 

 which are often auriculate ; in some specimens the sides are nearly 

 straight, and parallel for more than half the length of the shell. 



