1st PALEONTOLOGY 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 th^n 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. Vol. 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, 

 80 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 

 palaeozoic 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 



