354 M. L. Agassiz on the Primitive Diversity and 



period has been far the most marked, though it is true that this 

 formation is much thicker than either of the preceding lime- 

 stones, the Chazy being the thickest, and the Black River the 

 thinnest of the three below the Trenton. 



" In that portion of the upper Silurian period included in the 

 second volume of the ' Palaeontology of New York,^ the fossils of 

 the Medina sandstone, Clinton group, Niagara and Onondaga 

 salt groups, amount to 341. Medina and Clinton groups, 123 

 species. Niagara and Onondaga salt group, 218 species. 



" The Medina sandstone and arenaceous beds of the Clinton 

 group contain 50 species, leaving for the calcareous beds of the 

 Clinton group 73 species, which, added to the 218 species of the 

 Niagara and Onondaga salt groups, give 291 species as the total 

 number of species of the calcareous beds of these groups. The 

 Niagara is here the more important period, and though not 

 thicker than either of the others, contains about 200 species 

 restricted to itself. Of the Niagara group 67 species are Corals 

 and Bryozoa. Of the 73 species from the calcareous beds of the 

 Clinton group, 19 are Corals and Bryozoa. 



" In the lower Helderberg group, including the Water lime, 

 Pentamerus limestone, Delthyris shaly limestone, and upper 

 Pentamerus limestone, I expect to describe about 200 species, 

 exclusive of Corals and Bryozoa, of which I know already about 

 50 species. 



"The Oriskany sandstone may contain about 60 species of 

 fossils altogether, perhaps less. 



"In the upper Helderberg group, which is the next great 

 calcareous formation, I anticipate a less number of species except 

 Corals and Bryozoa, of which there are more than 100 species 

 in New York and the western localities. Of all that is yet known 

 in these limestones besides Corals and Bryozoa, it would be un- 

 safe for me to estimate more than 100 species. 



"From the Hamilton, Portage, and Chemung groups I anti- 

 cipate at least 300 species within New York, and I shall not be 

 surprised if more complete investigations produce double that 

 number in New York and the West. 



" The number of species given here I regard as only approxi- 

 mative. I hope this general statement may meet your present 

 requirements, but I regret that I cannot now give you more 

 definite information, particularly regarding the upper Helderberg. 

 I give you from this and the higher groups an estimate based on 

 the species known to me at the present time, but my final inves- 

 tigations always reveal a greater number than I anticipate.^' 



These statements of Professor Hall place already each of the 

 principal groups of rocks of the State of New York in the cate- 

 gory of distinct independent successive faunae, each equivalent 



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