ORTMANN: THE CRAWFISHES OF THE STATE OF PENNSYLVANIA 455 
species. Living underground near springs, and positively avoiding even the smaller 
streams, it is clear that a large river does not offer congenial conditions, and that it 
eyen may become dangerous to single individuals when they are accidentally swept 
into such a stream, they then being unable to get out and reach more favorable 
locations. 
The restriction of this species to a comparatively small area in southwestern 
Pennsylvania is thus easily explained. ‘The northward expansion was stopped by 
the first large river flowing from east to west in this region. 
A few additional points need discussion. Coming from the south, this species 
migrated largely in the direction of the great tributary of the Ohio, the Mononga- 
hela, and this river did not offer a barrier. It is different with the Youghiogheny. 
The latter comes through the Chestnut Ridge, and should form a barrier to the east, 
preventing it from entering Westmoreland County and eastern Allegheny County, 
On the other hand we see that this species has in one instance crossed the Alleghany 
River. I do not think that this is due to direct and actual crossing of the rivers, 
but to a shifting of their courses, of which we have many evidences. The geological 
history of the rivers of this region is as follows. The highest elevations of the 
country between Chestnut Ridge and the Ohio River are very uniform, rarely going 
beyond 1,200 or 1,500 feet. This seems to represent an old base-level, belonging to 
Old Tertiary times, according to Campbell (1903, p. 292 ff.). This was again cut into 
by a drainage system belonging to the Old Monongahela or Spencer River, which, 
at the end of the Tertiary, was running again at base-level (White, 1896, p. 377), at 
an elevation of about 900 feet (in the region of Pittsburgh), having eroded its valley 
about 300 feet below the Old Tertiary base-level. This river was rather sluggish 
and frequently formed ox-bows. The most important old channels, having regard 
to the matters in hand, are in the first place those which are marked by a terrace 
about 225 feet above the present river (at Pittsburgh), both along the Youghiogheny 
and Monongahela, at McKeesport, Allegheny County, to the east of the present 
rivers, which, consequently, have been shifted to the west (Jillson, 1893, p. 12, pl. 1). 
East of Pittsburgh we have an old ox-bow of the Monongahela in the ‘ Wilkinsburg 
Valley” at about the same level (Jillson, ibid., p. 8 ff). Here also the river has been 
shifted to the west. ‘These instances are sufficient to show that repeatedly and at 
various places opportunities were offered to C. monongalensis to passively cross the 
Youghiogheny and the Monongahela Rivers on account of the shifting of the latter, 
and the same seems to be the case with reference to the Allegheny River in the 
region of Squaw Run. According to Jillson (1. ¢., p. 10), there is a terrace 250 feet 
high belonging to the same general level as those mentioned above, one to two miles 
