464 MEMOIRS OF THE CARNEGIE MUSEUM 
ceptional cases, however, do not represent, in my opinion, original conditions. This 
is most evidentin the case of C. obscwrusin Wills Creek, where no other explanation 
is possible except that of artificial introduction. With reference to the occurrence 
of C. limosus in the middle Susquehanna and the Juniata, within the Alleghany 
Mountains, I think in this case also a recent immigration took place, favored by 
artificial means (canals). This is, however, somewhat doubtful, since it is impos- 
sible at present to ascertain the normal and original conditions prevailing in eastern 
Pennsylvania before modern improvements were introduced.” 
The region containing only C. bartoni goes beyond the boundary of the physio- 
graphical division of the Alleghany Mountains. In the northwest the Susquehanna 
has captured a large part of the drainage of the Alleghany Plateau, and the whole 
basin of the West Branch of the Susquehanna is included in the section containing 
only C. bartom. But again a physiographic boundary is formed by the divide 
between the Susquehanna and Alleghany river-systems. 
All the rest of the state belongs to the Alleghany Plateaw. Here much more 
varied conditions prevail with regard to the distribution of crawfishes, and this is 
chiefly due to the fact that this region was open to Preglacial and Postglacial immi- 
gration from the south, southwest, west, and northwest. Aside from (© bartoni, 
which is found everywhere, the following five species immigrated into this region : 
C. propinquus, C. obscurus, C. carolinus, C. monongalensis, and C. diogenes. Since 
each of these species had a different center of radiation, a different geological history, 
different ecological habits, and since, consequently, different barriers existed to the 
dispersal, no two species possess the same range. 
C. propinquus came from the west and northwest in Postglacial times. [It belongs 
to the Erie basin, and is restricted by the divides of the latter. Thus it is confined 
in Pennsylvania to the Lake Erie drainage in Erie and Crawford Counties. 
C. obscurus belongs to the Preglacial Spencer River or Old Monongahela, and to 
Lake Monongahela of early Glacial age. The Postglacial dispersal includes first of 
all the Ohio-Monongahela-Alleghany drainage, and thus generally the divides of 
this basin constitute its boundaries, with a few exceptions. In the region of the 
upper Youghiogheny and the Conemaugh the roughness of the streams has pre- 
vented it from reaching the plateau to the east of the Chestnut Ridge. In Crawford 
and Erie Counties it has crossed over into the Erie drainage (due to stream-piracy 
or to artificial means), and in Potter County it has found a way into the Genessee 
system (due to stream-piracy). 
U. carolinus came from the south, along the high level plains of early Tertiary 
5a The green color on the map, Pl. XLIII, should be continued up the Juniata to Bedford County. 
