STROPHODONTJE OF THE HAMILTON CROUP. 
101 
The figures of Plates xi & xn have already been referred to, pages 81 & 87. 
Figures la-ld Plate xvii are of the ordinary forms of the species from the Hamilton 
group of New-York. 
Figure 1 c shows an enlargement of the area. 
Figure If represents the surface characters of the Illinois and Iowa specimens. 
" Figures l<g and 1A are interiors of the ventral valve. 
Figure \l is a cast of the ventral valve of a large specimen. 
The figures Ik, 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. We 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. 
Strophodonta demissa. 
PLATE XVII. 
Strophomena demissa : Conrad, Journal Acad. Nat. Sciences, Philadelphia, Vol. viii, pa. 258, pi. 14, 
f. 14. 1842. 
Strophomena ( Strophodonta ) demissa : Hall in Tenth Report on State Cabinet, p. 137. 1847. 
Strophodonta demissa : Hall, Report Geological Survey of Iowa, pa. 495, pi. 3, f. 5. 1859. 
Strophomena demissa : Billings, Canadian Journal, 1861. 
“ « « Geology of Canada, p. 367. 1863. 
Shell semielleptical, usually 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. 
