DEVONIAN SECTION OF ITHACA, N. Y. ToL 
widely diverse regions of the United States. The attempt is made in 
the present discussion by close analysis of the fauna at the typical 
locality, to ascertain the particular species whose appearance in the 
strata begins at a definite horizon. This analysis has furnished the 
practical basis for drawing the line across the Watkins Glen quad- 
rangle. 
In order to explain the fact of such a definite change in the fauna 
at such a boundary line, it has been necessary to assume an incursion 
of the Chemung fauna from outside into the region at a particular 
point of time. Thereafter, for a considerable length of time, the 
species of fossils represented in the rocks were of the same fauna and 
constitute the typical Chemung fauna. 
However, it must be distinctly remembered that such an assump- 
tion involves the pre-existence of the fauna in full vigor prior to the 
time of its incursion into this region. 
If we were to assume that the species of the Chemung fauna were 
directly evolved from species living in the same region when the under- 
lying rocks were being deposited, the sharpness of the line would 
necessarily be lost and no definite boundary horizon could be drawn 
on the basis of fossil faunules alone; for the reason that the change 
in specific character does not take place at such a rapid rate that the 
differences could be detected. 
It is therefore considered to be a confirmation of the hypothesis 
of incursion of the fauna that the stratigraphic line marking the first 
appearance of the chief diagnostic species of the Chemung fauna 
occurs at a horizon so consistent with the general structure of the 
strata of a thirty-minute quadrangle. Another confirmation of the 
theory comes from the geographical extension of that horizon. On 
passing westward from the Ithaca meridian the line of separation 
of the faunas becomes more sharp and distinct; while on passing 
eastward the fauna below the line contains more and more species 
which are also found above the line. 
In this eastern extension of the line, however the distinctive Che- 
mung species become less dominant and as has been shown in earlier 
papers, the reappearance of species, which in the western New York 
outcrops are confined to horizons far below the boundary, is observed 
to be the rule rather than an occasional exception. 
