1886.] Zoic Maxima, or Periods of Numerical variations, etc. 1 009 
ZOIC MAXIMA, OR PERIODS OF NUMERICAL 
VARIATIONS IN ANIMALS. 
BY L. P. -GRATACAP, 
N° feature, perhaps, in his geological and field study affords 
the palzeontologist more interesting material for his specula- 
tions on the conditions of the past, in its zoological bearings, 
than the irregular distribution of organic remains in the fossil- 
bearing rocks. Not only in the same geological horizon will he 
find striking variations in the abundance in which the fossils occur, 
as he passes from layer to layer of contiguous and often of the same 
beds, but he soon discovers the important fact that localities are 
distinguished by peculiar fossils, that a limited range circum- 
Scribes the lateral as well as the vertical diffusion of a species, as 
far as regards numerical concentration, and that again points or 
limited areas present, in overflowing numbers, representatives of 
an organism which, generally occurring throughout a wide geo- 
graphical range, are at these points illustrated in crowded and 
exuberant colonies. 
The well-known fish beds, located by Newberry and Worthen in 
the Lower Carboniferous limestone of Illinois, are examples of the 
first case mentioned, the remarkable localization of forms in Wis- 
consin, instanced by Chamberlain and hy him denominated as evi- 
dences of “colonial tendencies,” is an example of the ‘second, as 
_ also to some extent, though these are perhaps in the main in- 
Stances of a different class of facts, the faunal stations of Williams 
_ 80 admirably depicted in the papers on the “ Fossil Faunas of the 
Upper Devonian,” while the interrupted display of the same 
Species in the same line of outcrop in respect to the relative num- 
bers. of specimens to be seen or their local disappearance when 
the area examined has any considerable extent, corresponding to 
a beach line of miles in length, illustrates the third class of facts 
which we refer to, as the diminishment westward from Genesee of . 
Pentamerus in the Clinton rocks of New York. 
= Associated with these familiar facts is the closely related one 
-Of the contrasted size of the same fossil species in different parts 
_ Of the same formation, a difference of size not always explicable 
On the mere assumption of favorable or unfavorable environment, 
9f which perhaps the Spergen Hill fossils afford a very pertinent 
=» Bulletin U.S: Geol. Surv, No, 3. 
| * 
