84 • OOEEELATION OF OEOLOGICAL FAUNAS. [bull. 210. 
but it is reported in the typical Chemung fauna on the following page, 
as found at Chemung Narrows, and is conspicuous in the more char- 
acteristic Chemung faunules of the western part of the State. 
Another basis for estimating the dominant characteristics of the 
fauna of the Chemung formation is found in the statistics published 
in Bulletin 41, U. S. Geological Survey." 
In this bulletin lists of the species were tabulated primarily to 
indicate the composition of the local and temporary faunules. Thirty- 
seven such Chemung faunules are analyzed. The value of clearly 
distinguishing the geographical from the geological modification 
of the faunules was not full}' appreciated when the bulletin was 
written. As the investigations have progressed, however, it has 
become clear that modification of a general fauna, coincident with a 
few miles of separation in space, geographically, may be as great as, 
or even greater than, the modification coincident with passage upward 
stratigraphically through tens or even hundreds of feet of sediments. 
These two kinds of bionic value (geographical and geological) are not 
so sharpl} T distinguished in Bulletin 41 as they might be, but the statis- 
tics there given will serve for estimating the general bionic values of 
the constituent species of the fauna. These values are not generally 
evident in the descriptive reports of the individual species concerned, 
and particular attention to collecting the evidence must be given, 
both in the field and when the collections of fossils are analysed in 
the laboratory, in order to exhibit the bionic values of the species of 
the fauna. 
Difficulties in the way of preparing an exact list of the dominant 
species of the Spirifer disjunctus fauna arise from still another source. 
Many of the species of this fauna are in a variable condition, and the 
separate faunules present strong contrasts in the particular aggrega- 
tion of species making up the faunules, which call for still fuller 
investigation. This elasticity of the fauna is what might be expected 
on the theory of its origin in the New York province by immigration. 
The various elements of the fauna were occupying new territory (or 
aquilory, we might more properly say), and were struggling into a 
new adjustment of equilibrium among themselves and in their new 
environment. The more vigorous the species were the more plastic 
we may suppose them to have been. However the facts may be 
theoretically explained, it is noticeable that many of the species 
of both the Ithaca and Chemung formations are in a remarkably 
variable condition. 
The spirifers, the productellas, the orthids, the rhynchonellas, the 
pterineas, and aviculoids in general, which constitute the larger part 
of any good sample of the Spirifer disjunctus fauna, are so vari- 
able that two authors will almost certainly disagree in naming the 
species of any particular lot of fossils, and even the ablest paleon- 
tologist will differ in his own distribution of the specimens among the 
( On the fossil faunas of the Upper Devonian— the Genesee section, New York. 
