XVI 



INTRODUCTION. 



stone and from the Guelph limestone), sixteen species, which are of Niagara 

 age. 



In the western extension of the Niagara group, the magnesian calcareous beds 

 of the north-west have furnished a considerable addition to the number of species 

 known in eastern localities ; while the more southerly exposures of the shaly 

 calcareous beds of the group in Indiana have furnished others which are quite 

 unlike either the New York, or the Wisconsin and Iowa species. The Niagara 

 of Tennessee has afforded little information in regard to the Lamellibranchiate 

 fauna of that period. 



From the calcareous and shaly measures of the Lower Helderberg group, in 

 New York, there were described, in the PALiEONTOLOOY of New York, vol. Ill, 

 thirty-four species, and five species peculiar to the Oriskany sandstone. The 

 exposures of the same formation in the south-west have added little to our 

 knowledge of this class of fossils : though it is probable that many more species 

 will be found on careful investigation. 



Rocks of the same age as the Lower Helderberg and Oriskany in eastern 

 Canada have afforded a considerable number of additional species, which have 

 been described in the Canadian Geological Reports. 



Succeeding the Oriskany sandstone, which is usually considered as the line 

 of demarkation between the Silurian and Devonian systems, we find the Upper 

 Helderberg group with a moderately developed Lamellibranchiate fauna. 



In the middle Devonian, including the Hamilton, Portage and Chemung 

 groups, this fauna for the first time in the geological history of the palaeozoic 

 formations in the State of New York, forms a conspicuous feature, both in 

 number of species and abundance of individuals. 



In the eastern and central counties of the State, the fossils of this class are 

 distributed through more than one thousand feet in thickness in the sediment- 

 ary strata of the Hamilton group. These forms gradually diminish in number 

 and abundance as the formation becomes more calcareous in its western exten- 

 sion, and the arenaceous sedimentary portions grcodually become attenuated or 

 altogether disappear, giving place to fine calcareous shales, which are not favor- 

 able to the development of this class of organisms. 



