•2~-2 PALJEOSTOLOQY OF NEW YORK. 



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 rpiiet 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, 



