OKTMANN : THE CKAWFISHES OF THE STATE OF PENNSYLVANIA 509 



with them. This is preeminently the case with the river forms, C. limosus, C. 

 obscurus, and C. propinquus. 



Here we have an instance in which at a given locaUty 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 (jf C. barloni lies in the mountains of the Appalachian s\'s- 

 tem ; the common center of C. limosics and the propiufjini.'i-gronp is in the central 

 basin of the Mississippi, and the special center of C. liiiKJim-s in the coastal plain, 

 and that of the propinquus-groni) 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. Althoiigh living side by side there is no danger 

 of hybridisation, since their morphological differences are such that kyesame- 

 chania exists. The different shape of the sexual organs of C. bartoni 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 {bwceno»is). 



Conditions are slightly different in the cases where C. bartoni 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 burnjwing species it generally occupies situa- 

 tions slightly different from those preferred by the chimney-builders. It favore 

 running water in open streams, while the burrowers are fuund in holes at a certain 

 distance from the streams. Nevertheless, C. barloni 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 dillerent centei-s occupy the same 

 locality. 



Yet as a rule C. bartoni occupies a different habitat from the burrowers. even if 

 found close to the latter. A fine illustration of this is in Nine-Mile Run. near 

 Pittsburgh. Here three species, ('.bartoni, C mononijaloi.tin. and C tlioijeur.-i 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 nitlu-r 



