468 
PALAEONTOLOGY OF NEW-YORK. 
GASTEROPODA OF THE ORISKANY SANDSTONE. 
In general features, and in generic characters, the Gasteropoda of the 
Oriskany sandstone do not differ from those of the Helderberg limestones. 
The prevailing forms in the sandstone in New-York are Platyostoma 
ventricosa and Platyceras nodosum , while Strophostylus cxpansus is very 
rarely seen. In the same rock in Maryland and Virginia the first of these 
species is comparatively rare, while the two last have not been observed : 
at the same time, these southern localities have furnished three other 
species of Strophostylus. The Platyceras gehhardi, known only in the Hel¬ 
derberg limestones in New-York, is found in the Oriskany sandstone in 
Maryland and Virginia, associated with several other species of the same 
genus, some of which occur in great numbers. 
While in New-York the rock is of such a character that the shells of 
these fossils are not preserved, and we have simply the casts of the in¬ 
terior, in Maryland and Virginia they occur as silicified shells, the silica 
having entirely or almost entirely replaced the calcareous matter; and 
the fragile forms are free from adhering stone, with aperture, peristome, 
and the cavity of the shell as distinctly seen as in living species. This 
unusual of the fossils condition enables us to determine more accurately 
the characters of some of these forms, than could have been done from 
all the other palaeozoic collections. 
The great number of specimens examined has shown, even in the same 
species of Platyceras, a wide variation in certain characters. While in 
some examples the peristome is quite free and the aperture symmetrical, 
in others the peristome is closely joined to the body volution, and even 
sometimes recurved so as to form a columella, leaving a wide umbilicus; 
characters which are incompatible with the genus as originally described. 
At the same time, in several examples observed of Strophostylus, the 
peristome is almost entirely free ; and in some specimens of Platyostoma 
there is a close approach to Platyceras on the one hand, while, in the 
