PALAEONTOLOGY OF NEW-YORK. 
and might consider whether these apparently similar forms were not due 
to different physical conditions existing in distant parts of the ocean of 
the same period, which had given a fuller development to those species 
which in the Niagara group are always smaller than in the Lower Hel- 
derberg group. 
Nevertheless it should not be overlooked, that, mingled with these 
analogous forms of the same genera, and occurring in materials which 
indicate the same physical conditions, there are other genera in the Lower 
Helderberg group which are quite unknown in the Niagara period. We 
have here appearing for the first time the genera Waldheimia? Meganteris, 
Eatonia, etc.; while Strophodonta, which had scarcely an existence in 
the Niagara period, becomes numerously developed. The Genus Merista, 
which is represented by few species in the Niagara period, becomes 
conspicuous in the successive beds of the Lower Helderberg period. 
I have before (Palaeontology N. Y. Yol. ii, p. 249) spoken of the ap¬ 
parent identity in age of the Niagara group of the United States, with 
the Wenlock formation of Great Britain; and of the incompleteness in 
the palaeozoic analogues, which are only filled by those of the Lower 
Helderberg group. The illustrations of the present volume will therefore 
furnish the means for a comparison not heretofore existing in published 
records. 
I have also shown that in a southwesterly direction the rocks of the 
Niagara and Lower Helderberg periods approach each other more closely, 
so that in Tennessee they appear to be inseparable, and the fossils of the 
two groups are collected within the space of thirty feet in thickness from 
what appear to be the same beds. It is quite probable, therefore, that this 
palmozoic ocean, which, to the north and northwest was invaded by the 
materials forming the Onondaga-salt group, and which is for the most 
part destitute of organic remains, continued undisturbed at the southwest; 
and that the faunas of the Niagara and Lower Helderberg groups, which 
are here separated by the great marl and calcareous deposit of the 
Onondaga-salt group, then succeeded each other upon the same ocean 
