ORTMANN: THE CRAWBFISHES OF THE STATE OF PENNSYLVANIA 449 
markedly in the eastern section of the state. This fact is significant, in so far as it 
points out that the center of radiation for the state of Pennsylvania is rather on the 
western side of the mountains than on the eastern (see Adams, 1902, p. 122, “fourth 
criterion for the determination of centers of dispersal’). 
4. Cambarus barton robustus. 
a. Summary of Facts. (See pp. 390-391.) 
This form is found in Pennsylvania in a rather continuous area in the extreme 
northwest, in McKean, Warren, Erie, and Crawford Counties, both in the Alleghany 
River and the Lake Erie drainages. It is often associated with the typical C. bartoni, 
but has been found at different localities in Erie County without the latter. In 
addition it is not rare in the northeastern part of Allegheny County in the Alle- 
ghany River, and its tributaries, and has also been found in Chartiers Creek, in 
southwestern Allegheny County. Here it is always associated with the typical form. 
In no other part of the state has this variety been discovered, and it is very im- 
portant to note that no trace of it has been found in southwestern, central, southern, 
and eastern Pennsylvania. Although C bartoni is abundant in these parts, and 
particular attention has been paid to the possible presence of C. bartoni robustus, 
all attempts to find it have failed, and I feel justified in asserting that it is absent here. 
I am not sosure of this with reference to the region between Crawford and Warren 
Counties on the one side, and Allegheny County on the other. I have searched in 
this section for C. bartoni robustus, for instance near Tionesta, Forest County, at Oil 
City, Venango County, in Mercer and northern and central Butler Counties, and near 
Kittanning and Mosgrove, Armstrong County, but did not discover it. However, it 
is possible that it is present along the course of the Alleghany River, in the river itself, 
and some of its tributaries, in Forest, Venango, and Armstrong Counties. In some of 
the places mentioned I did not strike streams which looked very favorable, being gen- 
erally not large enough. Yet in Erie and Crawford Counties I sometimes found this 
species in rather small streams. Conditions in Otter Creek, Mercer County, Slip- 
pery Rock Creek and Thorn Creek, Butler County, were apparently identical with 
those under which it is generally found in Erie County, but this form was not found. 
b. Origin of the distribution of C. bartoni robustus. 
Considering that the true C. bartoni robustus is a northern form, being found 
outside of Pennsylvania in northern Ohio, western New York, and Canada (St. 
Lawrence Basin), its center of distribution seems to be at the northwestern edge of 
the range of C. bartoni, in the St. Lawrence drainage. In Pennsylvania, however, 
