20 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
identified with forms living within a particular geographical area of 
distribution ; but this is not a sufficient discrimination of a fossil species. 
The life period through which successive generations reproduce the 
same characters is an important part of the paleontological discrimina- 
tion of a species. In order to so discriminate fossil species, their time 
relations must not be obscured by making them coordinate with the 
formations in which the fossils are preserved. The time relations of 
a fauna are so obscured so long as we have, for instance, no means of 
naming the fauna of the Hamilton formation except by calling it the 
Hamilton fauna. So long as we have but a formational name to apply 
to the fauna, an}^ question as to the continuance of the fauna later in 
one region than in another can not be stated, since the presence of the 
fauna is the only certain evidence of the upward extension of the 
formation. 
In order, therefore, to deal with the fauna separately, it must be 
designated by a biological name. 
GEOLOGICAL FAUNAS AND THEIR NOMENCLATURE. 
In order to demonstrate the independence of faunal history from 
the history of formations, as commonly defined, on a lithological basis, 
it has been found necessary to study a fossil fauna as an aggregate 
of species living together, and not as an aggregate of fossil remains 
occurring in and characterizing some particular geological formation. 
As commonly understood and as represented in the collections of 
museums, fossils are tabulated and arranged by formations. What- 
ever specimens have come from rocks classified as the Hamilton for- 
mation, for instance, are put together as constituting the fauna of 
the Hamilton formation, and, as has been previously noted, this 
makes it rarely possible from the lists (or from the collections so 
gathered) to determine with precision the range of the species. Again, 
rarely in the older lists is the abundance or rarity of species of a 
fauna noted, and the collections are often deceptive in this respect, 
since the collector is, for economical reasons alone, apt to neglect 
common forms, while rare forms are selected with great care and 
every trace of a newly discovered species is retained. 
In order, therefore, to exhibit the full time value of fossil faunas, it 
becomes necessary to observe all those relations which the individual 
fossils bear to the environment in which they lived and to each other 
as they were associated as living individuals of a composite fauna. 
In thus analyzing fossil faunas the most conspicuous fact presented 
to the collector is the different degrees of abundance in the general 
distribution of fossils in the rocks. Fossiliferous zones are thus set 
off from unfossilliferous or barren zones. Such zones, distinguished 
on purely paleontological grounds, are entirely distinct from the 
geological formations of our maps and geological reports. A fos- 
siliferous zone may be coextensive with a formation vertically in one 
