ORTMANN: THE CRAWFISHES OF THE STATE OF PENNSYLVANIA 509 
with them. This is preeminently the case with the river forms, C. limosus, C. 
obseurus, and C. propinquus. 
Here we have an instance in which at a given locality two species may be found 
side by side. This, however, is due to secondary processes. Originally each of the 
two species had a different center of radiation, and thus we again see the action of 
isolation. The center of C. bartoni lies in the mountains of the Appalachian sys- 
tem; the common center of C. limosus and the propinquus-group is in the central 
basin of the Mississippi, and the special center of C. limosus in the coastal plain, 
and that of the propinquwus-group in the Erigan and Lower Ohio drainage. 
Nevertheless these species came together (see Ortmann, 1896, p. 186), but the 
migration was in different directions, the river species coming up the rivers, while 
C. bartoni migrated down stream. Although living side by side there is no danger 
of hybridisation, since their morphological differences are such that kyesame- 
chania exists. he different shape of the sexual organs of C. barton: from that in 
the subgenus Faxonius precludes any idea of their being able to cross. Such cases 
do not offer anything remarkable, since the occupation of and the association at the 
same locality of different forms coming from different directions, and not being 
closely allied, is the general rule in any ecological community (biocenosis). 
Conditions are slightly different in the cases where C. barton is found in close 
proximity to the chimney-builders. Here there is closer affinity, but also it seems 
here that these species are so far separated morphologically that kyesamechania 
exists, although the shape of the copulatory organs is similar. Moreover, wherever 
C. bartoni comes into contact with the burrowing species it generally occupies situa- 
tions slightly different from those preferred by the chimney-builders. It favors 
running water in open streams, while the burrowers are found in holes at a certain 
distance from the streams. Nevertheless, C. bartoni is sometimes found in burrows 
and in springs close to the one or the other of the burrowers (it is even found in the 
holes of the latter, see p. 414), but in such cases we have again the same conditions 
as above: different species coming from different centers occupy the same 
locality. 
Yet as a rule C. bartoni occupies a different habitat from the burrowers, even if 
found elose to the latter. A fine illustration of this is in Nine-Mile Run, near 
Pittsburgh. Here three species, C. bartomi, C. monongalensis, and C. diogenes., are 
found together upon a space hardly more than twenty feet square. The locality is 
a pile of talus swept down into the valley of Nine-Mile Run by a small stream. 
The stream comes through an insignificant ravine, and spreads out over the talus, 
forming a kind of a delta, rendering the lower parts of the pile of talus rather 
