July 27, 1900.] 



SCIENCE. 



133 



northwest than they do in the vicinity of 

 Dahlonega. 



Thus the purely physiographic evidence 

 shows that there was a former connection 

 between the upper Tennessee River and 

 the Coosa system by which the molluscan 

 fauna could easily pass from one to the 

 other. It also shows conclusively that a 

 part of the Etowah Eiver has been trans- 

 ferred bodily to the Chattahoochee system. 

 Such a wholesale shifting of divides would 

 result in the transference of such of the 

 Coosa-Tennessee forms as then existed in 

 the headwaters of the Etowah River. 



This infusion of new forms spread 

 throughout the Chattahoochee system, even 

 to its headwaters, but the foreign types 

 presumably constituted only a small pro- 

 portion of the existing fauna. When the 

 Savannah River cut through the divide 

 and captured the upper part of the basin of 

 the Chattahoochee, it carried with it a 

 limited number of forms belonging to the 

 Coosa-Tennessee type. Thus in each suc- 

 cessive transfer the percentage of the orig- 

 inal forms has grown less and less, until in 

 the Savannah River, as reported by Mr. 

 Simpson, they are scarcely recognizable. 



Beyond Savannah, toward the northeast, 

 none of the peculiar Tennessee forms have 

 been found, nor is there any indication in 

 the surface configuration of there having 

 been any drainage changes of consequence 

 in this region. 



In most respects the biological evidence 

 simply corroborates the conclusions based 

 upon a study of the surface features, but in 

 the question of age relations it throws some 

 new light upon the problem. The migra- 

 tion of Coosa-Tennessee fauna from west to 

 east shows conclusively that the changes in 

 drainage must have followed a similar 

 order, hence the diversion at Dahlonega 

 must have preceded that which occurred 

 near Tallulah Falls. This important fact 

 presumably could never have been deter- 



mined from the physiographic evidence 

 alone. 



Throughout the whole region there is a 

 surprisingly close agreement between the 

 biologic and the physiographic evidence 

 which clearly indicates that biology should 

 stand in the same relation to physiography 

 that paleontology does to paleo-physiog- 

 raphy. 



The following brief statement of the evi- 

 dence on which Mr. Simpson bases his con- 

 clusions was prepared at our suggestion for 

 publication in advance of the more detailed 

 report which the author has in preparation. 

 C. W. Hates, 

 M. R. Campbell. 



U. S. Geological Suevey. 



ON THE EVIDENCE OF THE UNIONID^ RE- 

 GARDING THE FORMER COURSES OF 

 THE TENNESSEE AND OTHER 

 SOUTHERN RIVERS. 



Several years ago while studying the 

 life history and distribution of theUnionidse, 

 or Pearly Fresh Water mussels I was struck 

 by the close relationship existing between 

 that part of the mollusk fauna of the Tennes- 

 see River drainage system and that of the 

 Alabama. 



Within the Mississippi drainage basin 

 there is found the richest and most wonder- 

 ful, as well as the most highly developed 

 Unione fauna of any part of the world. Per- 

 haps not less than 400 species, at a most con- 

 servative estimate, are found in this area. 

 The Unione fauna of the Tennessee drainage 

 system (including that of the Cumberland) 

 contains a very large proportion of the spe- 

 cies found throughout the Mississippi area, 

 and in addition to these a gi-eat many 

 peculiar species found nowhere else in the 

 IMississippi system. The genus Pleurobema, 

 as I have defined it, a large group of forms 

 having rather heavy, triangular shells, gen- 

 erally tawny colored, with broken, green 

 rays, has its metropolis in the Tennessee 



