ORISKANY SANDSTONE. 403 



Among the Lamellibranchiata we have a few aviculoid forms, which, 

 except in their size, form no marked contrast with those which preceded 

 or those which follow. 



In the Gasteropoda, I have already shown* that the Genus Platyostoma, 

 in its typical species, occurs both in the Lower Helderberg rocks and in 

 the Oriskany sandstone, while it is also known in the higher formations. 

 The Strophostylus is equally characteristic of the Lower Helderberg 

 limestones and of the Oriskany sandstone, though at present not well 

 determined in the higher formations. The Platyceras, in several formsj 

 is quite abundant in the Oriskany sandstone ; two or more of the species 

 being common to this rock and to the Lower Helderberg limestones. We 

 have likewise the casts of two spiniferous species in this rock, showing 

 their affinities with those of the Upper Helderberg limestones. 



It is not possible, therefore, to point out any changes in the fauna of 

 this period sufficient to indicate the commencement of a new system, 

 and its relations with the formations below are as intimate as with those 

 above ; while in the Northern and Middle States, the Oriskany sandstone 

 bears in its fauna a closer relation to the lower than to the overlying 

 formations. It is, moreover, in the State of New- York, separated from 

 the succeeding fossiliferous rocks of the Upper Helderberg group by the 

 non-fossiliferous or almost non-fossiliferous belt of the Cauda-galli grit, 

 which often attains a thickness of one hundred to one hundred and fifty 

 feet. 



The order is as follows : 



Schoharie orit : Lowest member of the Upper Helderberg groUp. 



Cauda-galli orit : Non-fossiliferous. 



Oriskant sandstone. 



Upper Pentamerus limestone : Highest member of the Lower Helderberg group. 



It may be regarded that the physical conditions which inaugurated the 

 Oriskany sandstone prevailed through the period of the Cauda-galli grit, 

 disappearing in the Schoharie grit and succeeding limestones. It is not* 



* See page 308 of this volume. 



