162 
ORIGIN 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 America 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, C. T., “ Relationship and Distribution of Unionidae,” 
pp. 354—358. 
