428 MEMOIRS OF THE CAKNEGIE MUSEUM 



Mountain from the Anthracite basin, are largel}^ charged with mine-water, and in 

 this section of the state (Berks and Bucks counties) we see that C. liviosus does not 

 so closely approach the Blue Mountain, wdiile 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 an_v furtlier speculations. It is probable 

 that tlie original range of this species has been reduced t>n the one band by pollu- 

 tion of the streams, and has been extended on the other band by modern river im- 

 provements. How far this holds good in detail, remains doubU'ul. 



General orhj'ni of tlt< ilidrihiil khi of <_'. hinosas. 



Aside fi-om the more recent dispersal of this species just discussed, we are 

 prompted to incjuire how this species was able originally to reach the parts where 

 it is now found. 



As the writer lias pointed out in a former paper ( l!)()5/>, p. 108, 111, 114, 127) 

 C. /r;»o.s«.s stands rather isolated geographically as well as morphologically. It 

 belongs to an ancient group of the sul)genus Fa.rouiuf<, probabl}' the most ancient, 

 which consists of five species. The other four species are entirely removed geograph- 

 ically from C lirnosiis, 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 G. Irmosu.^, with the Appalachian System between them. We 

 have to deal here with a marked case of discontinuity of distribution in the /r//u«».s'- 

 gruup. Since, as has been shown by the writer in the paper referred to, we locate 

 the center of the suligenus Faxoii'm^ in the central ])art of the ^Mississippi ch-ainage, 

 G. Vnuoi^iix must have reached its i)resent liome by migration, and there are several 

 ways Ijy which it may have gone. 



The most direct i-oute is acro.ss the Alleghany Mountains. AVe may suppose that 

 the //'/((o.svfi'-group oncte extended in the Ohio drainage uj)into western I*ennsylvania 

 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 mcjuntains 

 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 l)elonging to the interior drainage. an<l the divide has Iteen continuously 

 shifted westward. 



On the other hand, considering the ecological peculiarities of G. linw^Ks, this as- 

 sumption does iKjt 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 



