464 MEMOIRS OF THE CARNEGIE MUSEUM 



ceptional cases, however, do not represent, in my opinion, original conditions. This 

 is most evident in the case of G. ohscwrusin Wills Creek, where no other explanation 

 is possible except that of artificial introduction. With reference to the occurrence 

 of C. liviosus 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, liowever, 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 6'. hartoni 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. hartoni. 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 Allcghaiu/ Plateau. Here much more 

 varied conditions prevail with i-egard 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 ('. hartoni, 

 which is found everywhere, the following five species immigrated into this region : 

 C. ■propinqims, C. ohscurus, C. carolinus, C. niowmgalcnsis, and C. diogenes. Since 

 each of these species had a different center of radiation, a different geological history, 

 different ecological habits, and since, consequentl}', different barriers existed to the 

 dispersal, no two species possess the same range. 



(J. 'prcpinquus came from the west and northwest in Postglacial times. It belongs 

 to the Erie basin, and is restricted b}' the divides of the latter. Thus it is confined 

 in Pennsylvania to the Lake Erie drainage in Erie and Crawford Counties. 



C. ohscurus 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 (bounty it has found a way into the Genessee 

 system (due to stream-piracy). 



(J. carolinus came from the south, along the high level plains of eai-ly Tertiary 



''» The green color on the map, PI. XLIII, should be continued up the Juniata to Bedford County. 



