Number of Animals in Geological Times. 353 



down to a few representatives in the tertiaries, and so with other 

 families. The shells found in the deposits of the new red sand- 

 stone 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 families 

 which have lived during more recent periods; and the daily 

 increase of the species found in the different palaeozoic deposits 

 shows that, even in point of numbers, those ancient faunae may, 

 even in the present state of our knowledge, be compared with 

 local faunae of similar extent at the present day. 



Desirous of making the most accurate comparison possible 

 between the subdivisions of the palaeozoic formations of the State 

 of New York with local fauncB 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 investi- 

 gations in this field, and I have obtained from him the following 

 statement : — 



" I regard 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 

 common to the beds below and above, extending into the Hud- 

 son River group,^' making together 81 species for these three 

 sets of beds. 



"The Trenton limestone has 188 species restricted to itself, 

 and 30 species passing upwards into the Hudson River group. 

 The entire number of species known as occurring in the Trenton 

 limestone, including those which occur in rocks above and below, 

 is about 230. This statement includes some species discovered 

 since the publication of the first volume of the ' Palaeontology 

 of New York,' and which would make the restricted species 

 about 200. 



"The Hudson River group, including Utica slate, has about 

 60 restricted species, besides those which are common to it and 

 the rocks below, making altogether about 100 species. 



" You will observe that the development of life at the Trenton 

 Ann. ^' Mag. N. Hist. Ser. 2. Vol. xiv. 23 



