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 Palaeontology 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 palseozoic 
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 gradually become attenuated or 
altogether disappear, giving place to fine calcareous shales, which are not favor¬ 
able to the development of this class of organisms. 
