PALAEONTOLOGY OF NEW-YORK. 
common on the ventral valve. Numerous strong concentric lines of 
growth mark the surface, and are very closely arranged near the front 
in full-grown specimens, giving a strongly imbricate character to that 
part of the shell. 
Interior of the ventral valve marked by a somewhat quadrangular me¬ 
dium sized muscular impression, which is bordered in the upper part 
by strong dental lamellae : teeth prominent, of moderate strength. The 
-fissure is usually partially or entirely covered by a pseudo-deltidium, 
and the upper part of the rostral cavity is often filled and solid, with 
a strong plate or septum closing the bottom of the fissure for one-third 
or one-half its length. In a single individual, the deltidial covering was 
observed to be composed of successive arching and imbricating plates, 
which, in the lower part, apparently consist of a double alternating 
series. The muscular impressions of the dorsal valve are less strong 
than those of the ventral, but often well marked, with a distinct 
median ridge ; teeth-sockets projecting into the cavity of the valve ; 
lateral portions of both valves strongly marked by the plications. 
This shell is subject to many and extreme variations in the extent of hinge-line, 
convexity of the valves, height of area, and incurving of the beak : the plications 
also vary in number and strength. It is an abundant form, coming next in number 
of individuals to the-N. mucronata . The following species, described by me in the 
Tenth Report on the State Cabinet, is probably only a variety of this Species with 
fewer and stronger ribs. In its ordinary or prevailing form and features, it is a 
species readily recognized : it is only since larger collections have been examined 
from numerous and widely-separated localities, that these extreme variations have 
been observed. 
Geological formation and localities. This species is found in casts in the coarser 
shales of Otsego and Madison counties, and becomes common on the shores of 
Cayuga and Seneca lakes and on the outlet of Crooked lake. It is found on the 
