ORTMANN: THE CRAWFISHES OF THE STATE OF PESNSYLVANMA 465 



age, being partial to extensive clay bottoms. Such conditions are found well 

 developed only between that part of the Alleghany Plateau which is enclosed 

 between the Alleghany Front and the Chestnut Ridge. Possibly also elevation has 

 played a part. Thus it is found only in Somei-set and parts of Fayette and West- 

 moreland counties. The noi-thern boundary is formed In- rather insignificant and 

 possibly temporary barriers. 



C mononyalcusis is a form parallel to C. canilinus. It also came from the south, 

 and invaded southwestern Pennsylvania, keeping to locations of less altitude than 

 C caroHnus. Thus its eastern boundary is formed by the Chestnut Ridge. Its 

 northward advance was checked by the first large river flowing east and west, 

 namely, the Loyalhanna-Kiskiminetas-Alleghany-Ohio. 



C. diogenes seems to be similar, at least in western Pennsylvania, to C. mononga- 

 lensis, but it w^as able to cross the rivers northward. The northern boundary is ob- 

 scure, and may be not entirely due to topographical conditions. Attention may be 

 here called to the fact that the isotherms have a curve somewhat similar to tliat 

 formed by the northern and eastern boundary of this species. 



Thus western Pennsyh^ania is divided into several sections characterized by their 

 crawfish-fauna, namely : 



1. Area of C. carolinus (containing C. bartoni and carolimis) : Somerset and 

 southeastern Fayette Counties. 



2. Area of C. diogenes and C. monongaleiisis (containing C. obscurus, bartoni, 

 monongalensis, and diogenes) : Greene, Washington, northwestern Fayette, western 

 Westmoreland, southern Allegheny, and southern Beaver Counties. 



3. Area of C. diogenes iritliont monongalmsis (containing C. obsourus, bartoni, 

 diogene-i) : northern Beaver, northern Allegheny, northeastern Westmoreland, west- 

 ern Indiana, southern Jefferson, southern Armstrong, southern Butler, Lawrence- 

 and southern Mercer Counties. 



4. Area of C. obscurus (containing C. obscur us and bartoni): northern Jetlerson, 

 northern Armstrong, northern Butlei", northern fiercer, the largest part of Craw- 

 ford, Venango. Clarion, :ind Forest, western Klk, northwestern Potter, McKean, 

 Warren, and southeastern Erie Counties. 



5. Area of C. proj)iiiquus (containing C. jirujiliiiiiins. i)bscurn.'<, tnirtoni): northern 

 and western Erie and a small part of Crawford Counties. 



These are the chief divisions, but there are a few minor diflerentiations. The 

 greatest variety prevails in Westmoreland County. Its west4.Tn part (west of the 

 Chestnut Ridge) belongs to two of the above areas (2) and (3). divided by the lA)yal- 

 hanna River. But besides the valley Ijetween the Chestnut and the Laurel Hill 

 Ridges presents three different conditions, namely : 



