adi NS a aa ss 
and Number of Animals in Geological Times. 277 
“You will observe that the development of life at the 
Trenton period has been far the most marked, though it is 
true that this formation is much thicker than either of the 
preceding limestones, 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 2d volume of the Paleontology of New York, the fossils of 
the Medina Sandstone, Clinton group, Niagara and Onon- 
daga Salt groups, amount to 341. Medina and Clinton 
groups 128 species, Niagara and Onondaga Salt group 218 
species. 
«The Medina Sandstone and arenaceous beds of the Clin- 
ton group contain 50 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 spe- 
cies are Corals and Bryozoa. Of the 73 species from the cal- 
careous beds of the Clinton group, 19 are Corals and Bryozoa. 
“In the lower Helderberg group, including the Water- 
lime, Pentamerous limestone, Delthyris Shaly limestone, and 
upper Pentamerous limestone, I expect to describe about 200 
species, exclusive of Corals and Bryozoa, of which I know 
already about fifty 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 unsafe for me to estimate more than 
100 species. : 
“From the Hamilton, Portage, and Chemung groups, I 
anticipate 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 ap- 
