214 DOUGLAS WILSON JOHNSON 



as undoubtedly important agents in the dispersion of fresh-water 

 shells. Woodworth catalogues a number of agencies recorded as 

 aiding in this dispersion, in addition to those mentioned above (pp. 

 214-17). It appears, then, that other means besides river- capture 

 for the passing of fresh-water shells from one stream to another are 

 not lacking. That these means are efficient is proved by the dis- 

 tribution of these shells. Ponds are sometimes made by excavating 

 a place where no water stands ordinarily, lining the excavation with 

 concrete and allowing the rain to fill it. These ponds, for a time 

 devoid of life, gradually become populated with mollusks and other 

 shells, proving, as Beddard says, the capacity for active or passive 

 migration on the part of the Mollusca. Careful and successive 

 observations have proved in some instances the actual time in which 

 a given pond may become populated. R. Ellsworth Call records the 

 presence of a western species of Unio in a small isolated eastern lake, 

 which was located down between high hills, fed by a mountain brook, 

 and absolutely foreign to any stream through which the species might 

 have been introduced. 



But the most conclusive objection to Mr. Simpson's argument in 

 favor of the theory of capture is found in the actual distribution of 

 the very shells upon which he bases his argument. Mr. Simpson 

 finds the genus Pleurobema in both the Tennessee and Coosa- Alabama 

 basins. He says that in no case are the species in the two basins 

 identical, but only similar. The basis of the argument, then, is 

 similarity of forms; but if mere similarity of form proves former 

 river connection, certainly identity of form should prove it with double 

 force. Accordingly, we should not expect to find the same species 

 of Pleurobema in any two rivers of this section whose location is such 

 as to render practically impossible a former connection with each 

 other, either directly or by way of intervening streams. An exami- 

 nation of Mr. Simpson's later monograph on the pearly fresh-water 

 mussels (pp. 745-65) is of value in this connection. As will be seen, 

 he records Pleurobema similans Lea, from Black Warrior and Cahawba 

 Rivers, Alabama ; and Pine Barren Creek, Escambia County, Florida. 

 So far as can be judged from available maps, previous fresh-water 

 connection between the former and the latter is extremely improb- 

 able. Pleurobema strodeana Wright is recorded from Escambia 



