434 MEMOIRS OF THE CARNEGIE MUSEUM 
what toward C. obscwrws, and one individual has been found which represents 
typical C. obscurus. In Middle Island Creek near St. Mary’s, Pleasants County, 
West Virginia, which is about twenty-five miles further down the Ohio, the few 
specimens collected seem to be typical C. propinquus sanborni. 
Thus it appears that C. obscwrus goes down the Ohio River to about Mounds- 
ville, West Virginia. All the tributaries of the Ohio in the Panhandle possess this 
species, and very probably it will be found also in Ohio on the opposite side of the 
river. But crossing over the divide between this part of the Ohio and the Muskin- 
gum-Tuscarawas River in Ohio, we again find C. propinquus sanborna in the drain- 
age of the latter. The western boundary of C. obscwrws consequently is formed by 
the divide just mentioned, but this line crosses the Ohio River between Mounds- 
ville and New Martinsville, West Virginia (Pl. XLII, Fig. 2, and Pl. XLITI). 
Further to the south in West Virginia in the drainage of the upper Mononga- 
hela this species has not been traced. It surely goes up the Monongahela beyond 
the southern boundary line of Pennsylvania, but how far has not been ascertained. 
The fact that C. obscwrus is found also in the Potomac drainage, in Wills Creek, 
between Hyndman, Bedford County, Pennsylvania, and Ellerslie, Alleghany County, 
Maryland, deserves special mention, and will be commented upon elsewhere. 
b. Origin of the distribution of C. propinquus, propinquus sanborm, and C. obscurus. 
In order to get a fair understanding of the distribution of these forms, we must 
take notice of the Preglacial physiography of the region in which they are found, 
for, as we shal] see below, we are led to believe that these forms are of Preglacial 
age, and survived during the Glacial Period in the southern parts of the drainage 
systems, which now constitute that of the Ohio. 
First of all, we should bear in mind that at the end of the Tertiary Period before 
the ice pressed down from the north, the Ohio River in its present form did not 
exist. In the whole region, drained now by the middle and upper Ohio, the drain- 
age was at that time not to the west, but to the north, and it was collected by a 
river running in a northeasterly direction toward the present Gulf of St. Lawrence, 
(the Erigan River or Ancient Grand River).” 
Disregarding some smaller streams, for instance the Old Middle and Old Upper 
Alleghany, which do not concern us here, three main rivers, tributary to the Erigan 
river, have been traced with more or less accuracy, and the evidence for their ex- 
istence, although fragmentary, leaves no doubt as to the general correctness of the 
* This is the opinion of Spencer (1881 and 1894). Others believe that this river drained toward the Southwest, 
into the Mississippi; see Grabau, 1901, maps, p. 44 and 45 (Dundas River). 
