^•8 PAL5:0NT0L0GT 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 

 tentricosa and Platyceras nodosum, while Strophostylus erpansus 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 gebhardi, 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 



