ORTMANN: THE CRAWFISHES OF THE STATE OF PENNSYLVANIA 453 
by such minor barriers as an insignificant divide and a deeply eroded system of 
valleys, renders it very probable that these obstructions are only temporary, and 
may be overcome in time, and, on the other hand, that the immigration of this 
species is rather recent, its northward migration being not yet finished, but only 
temporarily stopped. i 
The fact that this species is restricted to a narrow strip within the mountains 
is clearly due to its ecological habits. It prefers a certain altitude and clay bottoms. 
The latter are found in Pennsylvania chiefly on the Old Tertiary base-level, and this 
is represented to a large degree only within the mountains. East of the Alleghany 
Front and west of the Chestnut Ridge only insignificant remnants of this base-level 
are found, and thus this species is missing. 
We do not know anything about the Preglacial history of this species, and the 
facts at hand furnish no evidence with regard to this question. According to the 
morphological characters, and compared with C. diogenes and C. monongalensis, we 
must assume Preglacial age for it. Its immigration into Pennsylvania probably is 
Postglacial, and thus it possibly belongs to Adams’ third wave of migration, starting 
from the sowtheastern center (Adams, 1905, p. 62). However, in analogy to C 
monongalensis it may belong to the second wave, and the northeastern biota (see below 
under C. monongalensis). 
6. Cambarus monongalensis. 
a. Summary of Facts. (See pp. 400-401.) 
Cambarus monongalensis occupies in Pennsylvania (see Pl. X LITT) a continuous 
area in the southwestern part of the state. ‘Toward the east, beginning at the south- 
ern state-line, the limit of the distribution is formed by the Chestnut Ridge as far as 
the point where the Loyalhanna River cuts through this ridge in Westmoreland 
County. From this point the boundary follows the Loyalhanna to the north, and 
continues northwestward along the Kiskiminetas River. From the point where the 
Kiskiminetas empties into the Alleghany, the latter river, and further down the 
Ohio, form the northern boundary of this species, until the Ohio leaves the state in 
Beaver County. 
Within this area this species has been found wherever it has been searched for, 
namely: in Greene, Washington, and southern Beaver Counties; in the northwest- 
ern section of Fayette County ; in the larger part of Westmoreland County, anil in 
southern Allegheny County. It has also been traced beyond the boundaries of the 
state in a western and southern direction: it is found all over the Panhandle of 
West Virginia (Hancock, Brooke, Ohio, and Marshall Counties), and has also been 
