STREPTORHYNCHUS. 
7S 
I am unable to find any characters by which the specimens from the two localities 
may be separated into distinct species or even marked varieties. In my collections 
from Ohio, the specimens of Streptorhynchus are comparatively less common than 
in similar collections made in the Chemung group of New-York. In the rock still 
farther to the west, the sandstones of the “Knobs” near New-Albany, I find a 
Streptorhynchus of similar form and proportions, varying chiefly in the more 
distinct crenulation of the striae. This feature, however, appears to me oftener due 
to the nature of the matrix in which the fossil is imbedded, than to any original 
difference in the shell itself. 
Whether these forms can be traced into the large species of the upper sand¬ 
stones of the Knobs and of the Keokuk formation (Orthis keokuk, which Mr. 
Davidsojn has identified with Streptorhynchus ( Orthis ) cr'enistria of Europe), re¬ 
mains still undetermined. My collection does not furnish intermediate forms suf¬ 
ficient to justify such a conclusion at the present time. 
The large specimens from the higher part of the sandstone and the concretio. 
nary bands of limestone near New-Albany have always a thin median septum in 
the ventral valve, extending from the apex more than one-third the length of the 
shell. The cardinal process of the dorsal valve has, moreover, at its base, on one 
side, a short vertical ridge; while between these, and immediately below the 
process, the valve is a little thickened on the inner side. The dental sockets are 
nearly filled up, leaving a thin accessory ridge on each side of the cardinal pro¬ 
cess. The shells have been so thin that the casts show very little muscular marking- 
The illustrations of S. crenistria of Europe, which I have seen, do not presenl 
the distinct median septum in the ventral valve, which is seen in the large speci¬ 
mens referred to. » 
Specimens from the Keokuk limestone show a large muscular impression in the 
ventral valve, which is divided by a longitudinal septum reaching to the base of 
the imprint. The same feature exists in specimens of all dimensions from the Coal 
measures. 
