ORTMANN: THE CRAWFISHES OF THE STATE OF PENNSYLVANIA 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 Somerset and parts of Fayette and West- 
moreland counties. The northern boundary is formed by rather insignificant and 
possibly temporary barriers. 
C. monongalensis is a form parallel to C. carolinus. It also came from the south, 
and invaded southwestern Pennsylvania, keeping to locations of less altitude than 
C. carolinus. 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-A lleghany-Ohio. 
C. diogenes seems to be similar, at least in western Pennsylvania, to C. mononga- 
lensis, but it was 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 that 
formed by the northern and eastern boundary of this species. 
Thus western Pennsylvania is divided into several sections characterized by their 
crawfish-fauna, namely : 
1. Area of C. carolinus (containing C. bartoni and carolinus): Somerset and 
southeastern Fayette Counties. 
2. Area of C. diogenes and C. monongalensis (containing C. obscwrus, bartoni, 
monongalensis, and diogenes): Greene, Washington, northwestern Fayette, western 
Westmoreland, southern Allegheny, and southern Beaver Counties. 
3. Area of C. diogenes without monongalensis (containing C. obscwrus, bartoni, 
diogenes): northern Beaver, northern Allegheny, northeastern Westmoreland, west- 
ern Indiana, southern Jefferson, southern Armstrong, southern Butler, Lawrence: 
and southern Mercer Counties. 
4. Area of C. obscwrus (containing C. obscwrus and bartoni): northern Jefferson, 
northern Armstrong, northern Butler, northern Mercer, the largest part of Craw- 
ford, Venango, Clarion, and Forest, western Elk, northwestern Potter, McKean, 
Warren, and southeastern Hrie Counties. 
5. Area of C. propinquus (containing C. propinquus, obscurus, bartont) : northern 
and western Erie and a small part of Crawford Counties. 
These are the chief divisions, but there are a few minor differentiations. ‘The 
ereatest variety prevails in Westmoreland County. Its western part (west of the 
Chestnut Ridge) belongs to two of the above areas (2) and (3), divided by the Loyal- 
hanna River.. But besides the valley between the Chestnut and the Laurel Hill 
Ridges presents three different conditions, namely : 
