OHTMANN : 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 

 even 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 

 I'ennsylvania 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 tiie 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 Kidge, 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 tliat this is due to direct and actual crossing of the rival's, 

 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 ver}' uniform, rarely going 

 beyond 1,200 or 1,300 feet. This seems to represent an old base-level, belonging to 

 Old Tertiary times, according to Campbell (1903, p. 292 ff.). Tliis 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, 1<S96, p. 377), at 

 an L'k'vation 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 plaee those which are marked by a terrace 

 about 225 feet above the present river (at Pittsburgh), both along the Youghiogheny 

 and Monongahela, at Mcdveesport, Allegheny County, to the east of the present 

 rivers, which, consequently, have been shifted to the west (Jillson. I<sy3. p. 12, pi. 1). 

 East of Pittsburgh we have an old ox-li«)w of the Monongahela in the '' Wilkinsburg 

 Valley " at about the same level (Jillson, ibid., p. 8 IT.). Here also tiie river has been 

 shifted to the west. These instances are sufficient to show tiiat repeatedly and at 

 various places opportunities were offered to ('. nionongaloi.ii.'^ to passively cross the 

 Youghiogheny and the Monongahela Rivers on aceount t>f 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. c, p. 10), there is a terrace 25<> feet 

 high belonging to the same general level as those mentioned above, one to two miles 



