PALAEONTOLOGY OP NEW-YORK. 
84 
and the divaricator muscular impressions are broad, spreading, and some¬ 
what truncate on their lower margins. The lateral and basal margins are 
usually a little more strongly marked by the impressions of the surface 
striae ; and sometimes there is a partial or entire elevation of the part, 
or a callosity more or less defined. 
Figures 6 & 7, Plate xi, are of the natural, size of the shells. 
Figures 9 & 10 of Plate xi are illustrations of an extreme form of the cast of this shell, 
where the median depression is much wider than usual, and the lateral margins show 
a partial callosity, 
A cast taken from an impression of the dorsal valve, which occurs in the same association 
with the ventral valves, gives the characters shown in figure 8 of Plate xi, differing 
hut slightly from figure 4 : this difference may be owing to the imperfect preservation 
of the parts in a coarse material. 
Among the collections are two or three specimens, which, preserving the shell 
more or less completely, have the size and form of the casts. In one of these, where 
the shell is apparently entire, the surface of the middle and lower part of the shell 
is marked by distant elevated striae with wide interspaces, which do not show 
radiating striae, but are marked by concentric striae. On the umbo, the surface is 
marked by wide ‘radiating bands without distinct striae. 
In another partially exfoliated specimen, the surface near the margin of the 
shell shows minute striae between the coarser ones; and the same characters are 
partially shown in one or two other specimens. 
In the collections before me there are about thirty specimens with the characters 
described, and all these are of nearly the same size; nor can I trace any connexion 
between these and any of the larger forms. Notwithstanding, therefore, that I am 
opposed to creating new species where it can be avoided, and knowing that the 
species of this genus have been unnecessarily multiplied, I must regard this as a 
distinct and well-marked form. In its dimensions, it corresponds with the next 
described species, but differs in its surface markings and form of muscular im¬ 
pressions. It is nearly of the same size as S. nacrea , but differs in the greater 
convexity of the ventral umbo, and in the form of the muscular impressions as 
well as in the surface-markings. 
The illustrations on Plate xv show the form of muscular impression, surface 
striae, etc. 
ue;j! . 
Geological formation and locality. In the Schoharie grit at Clarksville and Knox 
in Albany county. The species is not at present known in the Corniferous limestone. 
