STREPTOftHYNCHUS. 69 
process between the two branches of the cardinal process; and below 
the hinge-line, the flabelliform muscular impression is divided by a low 
rounded mesial longitudinal ridge or obsolete septum. 
The ordinary size of specimens, in the Corniferous limestone, is about 
one inch in length by one and a quarter inches in width ; while others 
measure one inch and an eighth in length, by an inch and a half in width. 
The largest individual observed is a cast in Schoharie grit, which mea¬ 
sures nearly two inches in width. 
I have referred this species to the S. pandora of Billings, with which it cor¬ 
responds in general form and character; though the author remarks that “Our 
“ shell closely resembles S. crenistria in external form, but differs in not having 
“ the radiating striae crenulated, and further in the form of the occlusor muscular 
“ impressions in the dorsal valve. According to Davidson’s figure, there is a small 
“ process between the two branches of the cardinal process, which does not exist 
“ in ours.” 
The radiating striae, in all this group of shells that have fallen under my ob¬ 
servation, are crenulated by concentric striae; and in well preserved specimens of 
this species, these crenulating striae are very distinct. The median process is well 
preserved in our specimens, and forms one of the slight distinguishing features 
between it and S. woolworthana; though I am not disposed to regard this as a 
constant character, or one of specific importance. 
Externally there is very little difference between the S. woolworthana of the 
Lower Helderberg, and the species under consideration in its symmetrical forms. 
Among the few differences observable are the slightly depressed apex of the ven¬ 
tral valve, giving the greatest elevation a little below that point in the Lower 
Helderberg species; while the area is narrower on that valve, and a little wider 
on the dorsal valve. The striae are perhaps a little finer in S. pandora , but these 
characters are subject to much variation. In the interior, the muscular impression 
of the ventral valve of S. woolworthana is proportionally a little larger, and that 
of the dorsal valve perhaps not so distinctly defined; but this feature may vary 
from difference in the thickness of the shells. In the dorsal valve of S. pandora , the 
socket-plates are stronger than in the Lower Helderberg species, and are thickened 
about the base of the cardinal process, projecting beyond the line of the area, 
while the processes are not (usually) each doubly grooved as in S. woolworthana; 
and while in that one there is no median process, the Upper Helderberg species 
preserves that feature very distinctly, often reaching as high as the lateral divisions 
of the cardinal process. 
