216 DOUGLAS WILSON JOHNSON 



streams, could have taken place, because the intervening streams 

 should, at the same time, have become populated by such a process, 

 two answers may properly be made: (i) Such absence in the inter- 

 vening streams is no more remarkable than that shown repeatedly 

 in the distribution of the species of Pleurobema in the southern rivers, 

 and of other forms elsewhere. (2) The migration along the coast 

 may have taken place near the close of the Tertiary, at which time 

 the rivers and portions of rivers referred to were not in existence. 

 In this connection it is suggestive to note that, although Hayes and 

 Campbell consider that the lower portion of the Tennessee River 

 was diverted northward to the Ohio in recent time, not a trace of the 

 Tennessee shells has been found to mark the supposed former south- 

 westward course of the river through Mississippi. 



The evidence recorded by Mr. Adams is much the same as that 

 introduced by Mr. Simpson, except that Mr. Adams emphasizes 

 more especially the differences existing between members of the 

 same group found in the two river systems, and seeks to explain the 

 differences by the proposed -capture, whereas Mr. Simpson seeks to 

 prove the capture by the similarity of these forms. It is believed 

 that all of the phenomena noted are easily explicable independently 

 of the theory of capture, and in this connection it is well to note that 

 the presence of a longitudinal open valley across the low divide 

 between the two basins is peculiarly favorable for the operation of 

 some of the means of dispersal referred to above. The northward 

 and southward migrations of birds along certain valleys are known, 

 and where a low divide in a prominent valley alone separates the 

 waters of two river systems it is to be expected that more or less 

 mingling of forms will very likely take place. 



In closing our consideration of this line of evidence, it is of interest 

 to recall Mr. Simpson's statements regarding the dispersal of these 

 shells, which appear in his paper on the Distribution of North Ameri- 

 can Unionidce, published seven years prior to his Tennessee paper. 

 In a footnote (p. 354) he observes: 



In many cases the Unionidae seem to have had no difficulty in migrating 

 across the country from river to river; an example of this being the Mississippi 

 Valley species which now inhabit all the rivers of Texas, and some of those of 

 eastern Mexico; while, on the other hand, species of South America extend up 



