388 Annals of the Carnegie Museum. 



quus and rusticus are really identical, belonging to one and the same 

 species, which, however, is not to be called by either name, but is 

 Cavibarus obsci/n/s. Consequently, Williamson's list of the crawfishes 

 of Allegheny county really comprises only four species : Cambarus 

 bartoni, Cambarus diogenes, Cam barns nova species (^monongalensis) , 

 Cambarus obsciirus, and these four species are all that were known from 

 western Pennsylvania up to the present time. 



Extended collecting excursions undertaken by the present writer 

 during the summer of 1904 have confirmed the presence of these four 

 species in this region (or part of it), and have added two more species : 

 Cambarus propinquus , from Erie and Crawford counties, and Camba- 

 nis carolinus, from Westmoreland, Fayette, and Somerset counties. 



In studying the crawfishes of this region it was the special object of 

 the writer to ascertain the exact boundaries of the distribution of each 

 species, and, if possible, to correlate these boundaries with physical 

 features of the country. The results obtained, although not yet com- 

 plete in every respect, have proved to be highly interesting and apt 

 to throw light upon the postglacial immigration of the freshwater 

 fauna into this part of the state.. At the same time, numerous obser- 

 vations on ecology, habits, and life-history of the different species have 

 been made, which shall be set forth in a larger paper comprising the 

 crawfish fauna of the whole state, since, at present, they are too frag- 

 mentary to be presented. 



The present paper is to be regarded only as a preliminary account 

 of the work done in the western portion of the state. This portion is 

 sharply separated in its fauna from the central and eastern portions and 

 comprises, generally speaking, the drainage of the Ohio River (Ohio, 

 Monongahela, Allegheny), and consequently belongs to the Alissis- 

 sippi system. Only the northwestern corner of the state (parts of Erie 

 and Crawford counties) does not belong here, draining into Lake Erie 

 (St. Lawrence system), but it is included on account of its geographic 

 situation, and the remarkable faunal conditions presented by it. Thus 

 we may say that western Pennsylvania, as understood in the following 

 pages, means that part of Pennsylvania that lies west of the divide 

 between the waters that run to the Atlantic ocean (Delaware, Susque- 

 hanna and Potomac), and the waters that drain through the Ohio to 

 the Mississippi and the Gulf of Mexico. This dividing line runs, 

 roughly speaking, through Potter, McKean, Elk, Clearfield, Indiana, 

 and Cambria counties, and thence along the main chain of the Alle- 



