THE NAUTILUS. 



The characteristic Goniobasis of the upper Wabash river is 

 livesc&ns Menke, a species which, with the possible exception 

 of virginica Gmel., is the most adaptive of all members of the 

 genus. It appears as far down the river as Logansport. But 

 somewhere below that point the conditions become inimical. 

 It does not occur in the extensive collections made by Daniels 

 in the Wabash at Lafayette. A small depauperate form was 

 taken by Hinkley under stones at the "Chains" in Posey 

 county, and he reports it also from Mt. Carniel, higher up the 

 river. Its relationship is with livescens. We have here the 

 case of a fairly robust species that has been isolated by river 

 conditions but which, by reason of its adaptiveness, has been 

 able to plant struggling colonies in an unfavorable environ- 

 ment, the colonies developing subspecific characters. 



This isolating effect of river conditions is compactly illus- 

 trated at Big Stone Gap, Virginia. In a collection from the 

 south fork of Powell river at this point - - made without dis- 

 criminating among species Pleurocera unciale Hald., a river 

 form, was exceedingly abundant; G. simplex Say, a race of 

 the creeks, was rare. In the north fork of the Powell, a 

 smaller stream about a mile away, nearly 16 percent of the 

 Pleuroccridce were simplex, the rest unciale. In a brook tribu- 

 tary to the south fork, 78 percent of the specimens taken 

 were simplex, 22 percent unciale. Near Arthur, Claiborne 

 county, Teiin., the Powell seemed to contain no Goniobases. 

 Conditions were suitable in the brook at Big Stone Gap. The 

 genus survived, but under difficulties in the north fork of the 

 Powell. The struggle was all but over in the south fork and, 

 farther down stream, the isolation had been made complete. 



The inhospitable nature of the true river to most species of 

 Goniobasis may again be indicated by a quotation from a 

 letter from Herbert H. Smith to Dr. Walker in November, 

 1909, writing while collecting at the Muscle (Mussel) Shoals 

 of the Tennessee. " It is remarkable, ' ' Mr. Smith says, ' ' that 

 we have found no Goniobasis in the river except a few creek 

 forms evidently washed in. The predominating genus is 

 Pleurocera. ' ' 



A great many forms of Gomobasis occur in east Tennessee. 



