williams.] FAUNAL DISSECTION OF THE DEVONIAN. 81 
leptus fauna is dominant to a greater degree in the eastern counties 
than at Ithaca, not only for the particular part of the column in 
which the Ithaca fauna is abundant, hut all the way upward so long- 
as a marine fauna is present in the rocks of the region. On the other 
hand, very few species characteristic of the Ithaca formation (though 
enough to mark the horizon), reach into the extreme eastern part of 
the New York area. Following the strata farther westward it is 
found that in the Genesee River Valley the fauna so abundant in the 
Ithaca formation is entirely wanting, and is there replaced by the 
sparse Cardiola fauna of the Portage formation of that region. 
MUTATION AND CORRELATION OF THE FAUNAS. 
This critical examination of the typical fauna of the Ithaca forma- 
tion at Ithaca and its representatives at corresponding horizons east 
of Ithaca demonstrates some important facts regarding the mutation 
and correlation of fossil faunas. 
(1) The Tropidoleptus fauna, belonging, typically, to the Hamilton 
formation, and in western New York known to cease entirely with the 
Genesee shale or at a corresponding horizon, appears in eastern New 
York with its dominant species still prominent at a horizon much 
higher stratigraphically. 
(2) Above the Genesee shale, in the meridian of Cayuga Lake, a 
fauna (the Productella speciosa fauna) appears with many of the 
dominant species of the Tropidoleptus fauna, but with other species 
characteristic of the Ithaca formation. 
(3) Eastward from Cayuga Lake, at the stratigraphical place in the 
sections corresponding to the Ithaca formation, the characteristic 
species of the Productella speciosa fauna become more infrequent, 
while at the same time the Tropidoleptus fauna increases in domi- 
nance. 
(4) Westward from Ithaca the Productella speciosa fauna is trace- 
able a few miles only, and disappears before reaching the Genesee 
Valley, where it is replaced by the Cardiola fauna of the Portage. 
This series of facts demonstrates another general law of the history 
of organisms, as expressed by the range of species, viz: 
(5) The stratigraphical horizon of the incursion of new species into 
a region may be sharply recognized long before the common fauna of 
the region is dispersed or dies out. 
((5) The characteristic species of the Productella speciosa fauna of 
the Ithaca formation as it occurs at Ithaca arc present and dominanl 
in these eastern counties of the State, although the Tropidoleptus fauna 
still constitutes 75 per cent of the fauna and is represented by all its 
most characteristic species. 
(7) If the composition of the faunules still higher up in the eastern 
counties be examined, it will be found that this same Tropidoleptus 
Bull. 210—03 6 
