442 MEMOIRS OF THE CARNEGIE MUSEUM 
in front of this morainic system. he fall of the creeks running northward to Lake 
Erie from this divide is much more considerable than that of those running south- 
ward, and thus it is clear that erosion on the northern slope must have been more 
efficient than on the other side. The consequence is that the tributaries of Lake Erie, 
at least some of them, have worked back through the original divide, and have cap- 
tured parts of the original Postglacial drainage of the Ohio. ‘This is most evident 
(see Pl. XLIII) in the cases of Conneaut and Elk Creeks, and it is just in these 
creeks that I found ©. obscurus associated with C. propinquus,” while in Walnut 
Creek, which has apparently not entirely cut through the original divide, C. ob- 
scurus 1s not found. 
Thus it is possible that the presence of C. obscurus in the Lake Erie drainage is 
due to stream-piracy. Both species, C! obscurus and propinquus, are associated here, 
but it seems that they are antagonistic to each other to a certain degree. In the 
tributaries of Conneaut Creek I found C. propinquus exclusively, while Conneaut 
Creek itself contained both, but C. obscwrus prevailed, and it appears as if the latter 
had driven out the other species, which took refuge in the smaller tributaries. 
We might expect to obtain some light upon the question, whether C. obscwrus 
reached the Lake Erie drainage in consequence of stream-piracy or by the help of 
the canal, by the analogy offered in the Genessee drainage, but conditions seem to 
have been not entirely identical here. The type locality of C. obscwrus (see Pl. 
XLII, Fig. 3) is the Genessee River at Rochester, Monroe County, New York, where 
this species also is found associated with C. propingquus. Mr. W. P. McConnell has 
discovered C. obscwrus in the upper Genessee drainage near Ulysses, Potter County, 
Pennsylvania. The material consists of numerous males of the first and second 
form and of females, and there is not the slightest question that this is the true C. 
obscurus, no trace of C. propinquus being present here. How did this species get 
from the Alleghany drainage into that of the Genessee ? 
The drainage of the Genessee River lying entirely within the glaciated area, ‘Take 
must have happened in Postglacial times. Fairchild (1896, p. 423) has shown that 
during the recession of the ice the Genessee basin was occupied by a lake, which had 
its outlets in different directions successively, draining either to the Susquehanna or 
to the Ohio. He distinguishes ten stages, and the sixth was the last in which the 
water flowed to the Susquehanna; in the seventh and eighth stages Genessee Lake 
became connected with Lake Warren, which drained to the west into the Missis- 
sipp1 basin (but not into the upper Ohio), and finally the St. Lawrence drainage was 
57 The sources of Elk Creek are.in a tamarack-swamp, which also drains to the south, to French Creek, so that some 
kind of a direct connection may be present. I have not yisited this swamp. 
