162 OKIGIN OF LIFE IN AMERICA 



son,* as a result of which we now recognise sixty -one 

 genera. 



To return to the strictly North American fresh-water 

 mussels, it has been found that a common assemblage 

 inhabits the entire Mississippi drainage basin, and that a 

 considerable number of the species have a distribution cover- 

 ing the greater part of this area, as well as 'the whole of 

 Texas and even parts of eastern Mexico. The streams which 

 fall into the Atlantic are peopled by an entirely different set 

 of forms, the Appalachian chain seeming, according to Dr. 

 Simpson, to act as a sharp barrier between the two regions. 

 In the greater part of Mexico and Central Am'erica a totally 

 different fresh -water mussel fauna is found. Two Unios, 

 one Margaritana and some half-a-dozen Anodons, are all that 

 have hitherto been credited to the immense region on the 

 Pacific slope of North America. One of the Unios, says 

 Dr. Simpson, f has been recorded in error, the other is a 

 form of the most abundant and most widely distributed Unio, 

 viz., U. luteolus. The causes which led to this striking 

 difference between the fresh -water mussel fauna of the 

 Central basin and that of the Pacific slope will be discussed 

 in another chapter. I may only mention that a somewhat 

 similar 'disparity between the two faunas has been observed 

 among the fresh-water fishes. Before dealing with these 

 there is one other matter of importance that I should like 

 to refer to in connection with the geographical distribution 

 of fresh -water mussels. 



The far-reaching results of the study of the geographical 

 distribution of such a group as the fresh-water mussels is 

 exemplified in a striking manner by the following physio- 

 graphic problem. In discussing the origin and recent history 

 of the physical features of the southern Appalachians 

 Messrs. Hayes and Campbell advocated the theory that the 

 upper Tennessee river, now a tributary of the Ohio, formerly 

 flowed into the Gulf of Mexico by way of the existing Coosa 

 and Alabama Rivers. The conclusions were based entirely 



* Simpson, C. T., " Synopsis of the Naiades." 



t Simpson, 0. T., " Belationship and Distribution of Unionidae," 

 pp. 354358. 



