•8 PALAEONTOLOGY OF NEW-YORK. 



I liave therefore adopted the name of the locality where the beds are 

 best developed, for designating the entire group. The fossils are in 

 some degree restricted in their vertical range, where the physical 

 characters of the strata present the distinctive varieties peculiar to the ' 

 subdivisions. Where, however, lithological character is more uniform 

 throughout, the fossils are less restricted ; and it would be impossible to 

 indicate limits to any of the species below the Upper Pentamerus beds. 

 This terminal member of the group does not everywhere exist ; but 

 where occurring, it is marked by a species of Pentamerus having nearly 

 the same form as P. galeatus, but always smooth, and possessing other 

 characters by which it can readily be distinguished from that species. 

 Associated with this Pentamerus are several species of Rhynchonella, 

 ^equally restricted in their vertical range ; as well as a single Spirifer, of 

 which only one specimen has been found, in a lower bed. A few species 

 at the base of the formation are likewise very much restricted in their 

 vertical range, as far as the formation has been examined. 



The Oriskany sandstone rests directly upon the upper member of the 

 Lower Helderberg group. This rock is usually highly fossiliferous, being 

 often composed of a mass of shells closely packed together. It is some- 

 times, however, quite non-fossiliferous, and consists of a thin band of 

 dark-colored, compact, siliceous rock, which graduates into the arenaceous 

 shaly mass above. 



Although usually very distinct from the limestones below, there are 

 nevertheless localities where a passage occurs between the two rocks ; 

 and in such instances, some of the fossils, usually restricted to the lower 

 beds, pass into those above. Instances of this kind occur in Maryland ; 

 and from the collections of the Canada Survey, by Sir William E. Looan, 

 we are prepared to find in some parts of the continent an intimate 

 blending of these formations. 



In this volume, the fossils of the Lower Helderberg group, and those 

 of the Oriskany sandstone, are arranged as fossils from separate and 

 successive groups, which is fully justified by their physical condition in 

 the State of New- York. 



