80 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
ing the geography of wide areas of surface. This was indicated in 
the paper written in 1892." 
The list made by Dr. Kindle from the typical Ithaca formation 
contains 84 species, specifically identified. Of these, 47 are not 
recorded for the eastern Hamilton stations reported by Prosser, and 
2 only of these 47 species are in the Cleland list of Cayuga Lake 
Hamilton, or in the Eighteenmile Creek Hamilton faunal list given by 
Grabau. Thus there are 45 species, or more than half of the species 
listed, which are specifically distinct from the species of the 
Tropidoleptus fauna. 
The other half of the Ithaca faunal list is composed of species 
belonging to the Tropidoleptus carinatus fauna of the Hamilton for- 
mation of the general region. About half of the peculiar species is 
represented by closely related species in the Tropidoleptus fauna, 
and therefore it may be assumed that three-quarters of the fauna of 
the Ithaca formation is derived by evolution directly from the Tropi- 
doleptus fauna. The other quarter may be derived by migration 
from a more distanl source. 
In both cases of origin, however, it will be noted that varietal modi- 
fications have taken place. Enough mutation occurred to furnish 
a list of over 40 species to characterize the Ithaca formation, as it 
occurs in the column of central New York. 
Of the species peculiar to the fauna of the Ithaca formation, only 13 
are reported in the eastern counties at any horizon, from the Hamil- 
ton proper up to the departure of the marine species with the sedi- 
ments of the red Catskill shales and sandstones. 
It will be noted also, b} T examination of the lists already given that 
5 out of the 1<> most dominant species of the Ithaca list are Hamilton 
species — i. e., they belong to the Tropidoleptus fauna, and 10 of the 
most abundant 18 species are Hamilton, all of which are recorded 
from 13 to 24 times among the 54 lots analyzed. 
It is evident from this last observation that the old fauna which 
had spread over the Ithaca region during the sedimentation of the 
Ithaca formation has a preponderance of species belonging to the 
Tropidoleptus fauna, both in the number of species and in domi- 
nance of the species in the fauna. If it were far enough removed from 
the Hamilton formation to make correlation by stratigraphical evi- 
dence impossible, the faunal characteristics would lead to its associa- 
tion with the Hamilton, as a stratigraphically equivalent formation 
whose fauna was modified by change of conditions of environment, 
whereas the facts now before us leave no doubt as to its actual suc- 
cession above the other formation. 
The comparison of the Ithaca fauna with the fauna belonging to 
the eastern extension of the same formation shows that the Tropido- 
«The scope of paleontology and its value to geologists: Proc. Am. Assoc. Adv. Set, Vol. XLI, 
pp. 149-170. 
