ORTMANN: THE CRAWFISHES OF THE STATE OF PENNSYLVANIA 447 
however, CO. obscuwrus has advanced north in the Genessee basin to a considerable 
distance. 
3. Cambarus barton. 
a. Summary of Facts. (See pp. 381-386.) 
Cambarus bartoni is very uniformly distributed all over the state, being, how- 
ever, rather scarce in the extreme northwest in Erie County, where it is replaced 
by the form ©. bartoni robustus. We shall discuss this later. 
The species extends considerably beyond the limits of this state, chiefly toward 
the southwest and northeast. In these directions it ranges from Tennessee and 
North Carolina to New Brunswick and Quebee. Westward it reaches central Ken- 
tucky and southern Indiana. The Atlantic Coastal Plain is apparently not invaded 
by it to any considerable degree. 
We clearly see that its range follows the main strike of the Appalachian system, 
and knowing that ecologically this species is a form of the rapid and cool waters 
of the uplands and mountains, living preferably in small streams and even springs, 
we understand that the distribution must be entirely different from what we have 
learned with reference to the river-species already discussed. 
In Pennsylvania conditions seem to be favorable for this species everywhere, 
possibly with the exception of a very narrow strip on the eastern border, along the 
Delaware River (coastal plain); but even here it approaches the lowlands very 
closely, the Piedmont Plateau reaching the river at many places. 
In the mountains elevation is no barrier for it, | found it myself at 2,600 feet on 
Laurel Hill Ridge, west of Jennerstown, and at 2,300 feet near Sandpatch, Somerset 
County, and at other places at elevations not much less (Chestnut Ridge in West- 
moreland County ; near Cresson, Cambria County ; Keating Summit, Potter County). 
At Davis, Tucker County, West Virginia, I collected it in Blackwater River at 3,050 
feet, and Faxon, 1898, p. 649, records it from Roan Mountain, North Carolina, 
6,000 feet. 
b. Origin of the distribution of C. bartoni. 
The first point is to ascertain the center of radiation of this species. As I have 
pointed out in a previous paper (1905, p. 121), we must regard the southern section 
of the Appalachian system as the original home of the subgenus Bartonius, to which 
this species belongs, and (/. ¢., p. 122) the advance and dispersal of the subgenus 
took place over the eastern mountains of the United States, the axis of the dispersal 
being directed from southwest to northeast. 
We have reason to believe that the origin of this species falls into Preglacial 
times, it being rather primitive within the subgenus (at least in comparison with the 
