TERTIARY HISTORY OF THE TENNESSEE RIVER 215 



River, Florida, and Flint River, Georgia. Any former connection 

 in this case seems improbable. Pleurobema harperi Wright is recorded 

 from Altamaha and Flint Rivers, Georgia; and Suwanee River, 

 Florida. Here, again, connection between either of the former and 

 the later seems out of the question. Other cases might be added — 

 indefinitely, if we continue with other genera than Pleurobema. If 

 we find it impossible to hold river-capture responsible for the distri- 

 bution of identical species in all these cases, then mere similarity of 

 forms in the Tennessee and Coosa- Alabama basins cannot be regarded 

 as a valid argument in favor of river-capture near Chattanooga. 



If we carry this line of argument to its logical conclusion, the 

 objections to it become even more apparent. Mr. Simpson records 

 one group of Unios as occurring everywhere in the streams draining 

 into the Atlantic from Labrador to Georgia (1900, p. 134). If the 

 occurrence of the Pleurobema in the Tennessee and Coosa-Alabama 

 Rivers proves river-capture in that case, then the distribution just 

 referred to must prove a great succession of river-captures from 

 Labrador to Georgia. Nor is this all. The same species is in one 

 instance found in Europe, northern Asia, Japan, northern North 

 America, and Iceland (p. 677). According to the argument advanced, 

 this means an inconceivable series of river-captures. Such violent 

 hypotheses compel the conclusion that some other means than river- 

 capture is commonly operative in effecting the distribution of fresh- 

 water shells. 



Certain features of Unionidae distribution have led some to believe 

 that their dispersal may take place by migration along the seacoast 

 from one river system to another, as well as by the means already 

 considered. Mr. Simpson evidently regards this as a possibility in 

 the case of another instance of supposed river-capture (1900, p. 135), 

 but does not consider it in connection with the more important prob- 

 lem of the Tennessee, unless certain statements near the first of his 

 article are meant to imply such a consideration indirectly. He states 

 that none of the Pleurobema are found in the lower 300 miles of the 

 Mississippi River, in the Pearl or Pascagoula Rivers, or any of the 

 small rivers in Mississippi or Louisiana flowing into the Gulf (p. 134). 

 If it is meant to imply by this that no migration down the Tennessee 

 to the Gulf, thence along the coast and up the Alabama and other 



