L. Agassiz on Animals in Geological Tiines. 313 
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 vol. of the Paleontology of New York, the fossils of the Me- 
dina Sandstone, Clinton group, Niagara and Onondaga Salt 
groups, amount to 341. Medina and Clinton groups 123 spe- 
cies. tagara and Onondaga Salt group, 218 species.” 
“The Medina Sandstone and arenaceous beds of the Clinton 
New York and the western localities. Of all that is yet known 
in these limestones besides Carals aud Bryozoa, it would be un- 
safe for me to estimate more than 100 species.” e) 
“From the Hamilton, Portage and Chemung groups I antici- 
Pate 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- 
Mate. I hope this general statement may meet your present 
fequirement, but I regret that I cannot now give you more defi- 
Hite information, particularly regarding the Upper Helderberg. I 
8ive you from this and the higher groups an estimate based on 
the species known to me at the present time; but my final inves- 
ligations always reveal a greater number than I anticipate.” 
hese statements of Professor Hall place already each of the 
+ 
Pringipal group of rocks of the state of New York in the cate- 
as many local faunse of the present period, for we may repeat that 
the fauna of the Sechelles contains only 258 species, and that of 
