PALAEONTOLOGY OF NEW YORK. 
272 
ORTHOCERATA OF THE HAMILTON GROUP. 
In the eastern part of the State, where the strata above the Marcellus shales 
are composed of coarse and finer shales without calcareous matter, the Ortho- 
ceratites are very rarely seen. It is only where these shales, in their western 
extension, become in some degree calcareous, that we find these fossils in any 
considerable number; and as the group becomes attenuated in the same direc¬ 
tion, and the formation consists almost entirely of soft, calcareous shaly beds, 
the Orthoceratites diminish in number, and become extremely rare, except in 
a few irregular or concretionary calcareous layers, where some specimens have 
been found as far west as the shore of Lake Erie. 
The species in their geographical distribution are essentially limited to the 
zone mentioned, which has a considerable breadth in its gradation from the 
coarser materials on the east, to the finer calcareous shaly beds on the west. 
Owing to this condition, we have by far the larger proportion of specimens 
coming from localities in the central portion of the State. 
In the same horizon or zone we have also the greatest development among 
the Gasteropoda, and few species only of both classes are found in the more 
westerly extension of the formation, and fewer still in the coarse beds of the 
extreme eastern portions of the group. 
This partial limitation of the species to a certain zone is probably not alone 
due to the physical or mineral composition of the sediments, but in some 
degree to the depth of water required for their development; for while the 
deposits on the east are clearly littoral in their character, those of the west 
are off-shore or deeper sea deposits, and carry a much larger proportion of 
Brachiopoda than of any other class of fossils. At the same time, the periodical 
oscillations to which this original shore-line and sea-bed have been subjected, 
have served to give a wider horizontal distribution than would have occurred 
in a permanently quiet sea-bottom. 
It is an interesting fact, however, that while the distribution of the Ortho¬ 
ceratites and some other forms is so clearly affected by the causes mentioned, 
the Nautilus is known from almost the most easterly outcrops of the formation, 
