456 MEMOIRS OF THE CARNEGIE MUSEUM 
north of the Allegheny in the region of Claremont. At the same place there is 
another terrace north of the river, only 150 feet high, and consequently belonging 
to a later period, so that here the final shifting of the river to the south took place 
later than in the other cases. 
These facts, if they at all influenced the distribution of C. monongalensis, and I 
believe they did, give us a hint as to the time of the immigration of this species. 
The shifting of the rivers must have taken place sometime during the Glacial 
Period, for we know that during this time a considerable amount of erosion was 
accomplished, the 900 feet level belonging to the beginning of the Glacial time 
(Lake Monongahela stage). 
This leads us to the conclusion that C. monongalensis must have been present in 
this region during the Glacial Period, at least during a part of it, and shortly before 
these channels were changed.” It is quite possible that this species had its preserve 
in Glacial time not far from the edge of the ice in southern Pennsylvania and 
northern West Virginia, and that it began to advance as soon as the ice of the Wis- 
consin stage began to recede. This would fully explain the fact that this species 
was able to cross first the Youghiogheny and Monongahela by the help of the west- 
ward shifting of these rivers, thus opening a way into eastern Allegheny and West- 
moreland Counties, and that it later crossed the Alleghany River at Claremont, 
when its channel was changed to the present more southern position. 
The question remains, why C. monongalensis, having crossed the Alleghany, 
did not advance further north. It is found at Claremont (near Squaw Run), in 
a comparatively restricted locality, which is not altogether favorable, being at a 
rather low elevation. It has not been able to reach more favorable locations at 
higher levels, the ascent being more or less difficult on account of the very precipi- 
tous hillsides, and moreover it may not prosper here because of the presence of the 
competing species, C. diogenes, which is quite abundant in this region. C. monon- 
galensis is here, so to speak, cornered, and surrounded by unfavorable physical, 
ecological, and biocenotic conditions. 
If this species existed in this region during Glacial times its Preglacial origin 
®? The rivers were cut down even deeper than they are at present, but the valleys were filled up again, 100 feet or 
more (see Jillson, 1. c.). According to Foshay (1880, p. 402), the chief erosion falls into the end of the Tertiary ; but 
the presence of glacial material in the old river channels, 900 feet high (East Liberty, Pittsburgh), places the deepen- 
ing of the valleys at a later period. Possibly it was connected with and subsequent to the draining off of Lake Monon- 
gahela ( Wnite, 1896, p. 375), which happened sometime during the Glacial Period. Hice (1903, p. ~29) places this 
cutting down below the present channels between the Kansan and the Wisconsin stages. 
The same conclusion is reached when we consider that this species cannot have immigrated before the present 
Ohio was formed, that is to say, shortly before the Wisconsin stage (see last footnote). Otherwise the Ohio would not 
form its western boundary. 
