428 MEMOIRS OF THE CARNEGIE MUSEUM 
Mountain from the Anthracite basin, are largely charged with mine-water, and in 
this section of the state (Berks and Bucks counties) we see that C. limosus does not 
so closely approach the Blue Mountain, while in Cumberland and Franklin Coun- 
ties, where the streams are clear, it goes to the very foot of the mountain. 
Of course we cannot any longer ascertain what the original conditions were, and 
thus it is hardly profitable to enter into any further speculations. It is probable 
that the original range of this species has been reduced on the one hand by pollu- 
tion of the streams, and has been extended on the other hand by modern river im- 
provements. How far this holds good in detail, remains doubtful. 
General origin of the distribution of C. limosus. 
Aside from the more recent dispersal of this species just discussed, we are 
prompted to inquire how this species was able originally to reach the parts where 
it is now found. 
As the writer has pointed out in a former paper (1905b, p. 108, 111, 114, 127) 
C. limosus stands rather isolated geographically as well as morphologically. It 
belongs to an ancient group of the subgenus Faxonius, probably the most ancient, 
which consists of five species. The other four species are entirely removed geograph- 
ically from C. limosus, and are found in the central basin of the United States, in 
Kentucky, Indiana, and Missouri, that is to say, about four hundred miles to the 
west of the range of C. limosus, with the Appalachian System between them. We 
have to deal here with a marked case of discontinuity of distribution in the /imosus- 
group. Since, as has been shown by the writer in the paper referred to, we locate 
the center of the subgenus Faxonius in the central part of the Mississippi drainage, 
C. limosus must have reached its present home by migration, and there are several 
ways by which it may have gone. 
The most direct route is across the Alleghany Mountains. We may suppose that 
the /imosus-group once extended in the Ohio drainage up into western Pennsylvania 
and West Virginia, and that it was able by some means to cross the divide into the 
Atlantic drainage. This does not appear impossible, inasmuch as in the mountains 
stream-piracy has taken place on a large scale during all ages (Davis, 1889). In 
fact all of the larger rivers now running into the Atlantic have captured large tracts 
originally belonging to the interior drainage, and the divide has been continuously 
shifted westward. 
On the other hand, considering the ecological peculiarities of C. limosus, this as- 
sumption does not appear very likely. The habit of living in larger streams in 
rather quiet water would not favor a migration across the mountains, and if this 
