312 L. Agassiz on Animals in Geological Times. 
a few representatives in the tertiaries, and so with other families. 
The shells found in the deposits of the new red sandstone period, 
of the coal period, and of the still earlier ages, are perhaps less 
numerous on the whole, though they can hardly be said to be 
less diversified; for, the extinct forms which occur among them, 
are quite an equivalent to the variety of their families which have 
lived during more recent periods; and the daily increase of the 
species found in the different paleozoic deposits shows that, even 
in point of numbers those ancient faunze may, even in the pres- 
ent state of our knowledge, be compared with local faune of sim- 
ilar extent at the present day. 
Desirous of making the most accurate comparison possible be- 
tween the subdivisions of the paleozoic formations of the state 
of New York with local faune of similar extent in the present 
seas, I have requested Professor J. Hall to furnish me with sum- 
mary indications respecting the results of his extensive investiga- 
tions in this field, and I have obtained from him the following 
statement : 
“Tregard the Potsdam and Calciferous Sandstone as discon- 
nected with the groups above, forming of themselves with their 
fauna (not yet well known in this country) a distinct geological 
period. The entire number of species thus far known in these 
rocks, admitting all of Owen’s species, is however only twenty- 
Six. 
“The Chazy limestone has 45 species restricted to itself, and 
one other species which is also known in the Black River Lime- 
stone. The Birdseye limestone has 19 species restricted to itself 
and two others which pass upwards. ‘The Black River lime 
stone has 13 species restricted to itself, and one common to It, 
and the Chazy limestone, one common to it and Birdseye, and 
one common to it and the Trenton, and one other which is com- 
mon to the beds below and above, extending into the Hudson 
covered since the publication of the Ist volume of the Palaontol- 
ogy of New York, and which would make the restricted species 
about 200.” 
“The Hudson River group, including Utica slate, has abovt 
- persica sgt besides those which are common to it an 
he rocks below, making altogether about 100 species. 
“You will observe that the development of life at the Trenton 
period, has been far the most marked, though it is true this 
