354 The American Naturalist. [April, 
mentinus represents another section, Unio complanatus another, 
| mph 2 margaritifera a fourth, Anodonta ovata a fifth, and 
so on.’ 
This plan of grouping by relationships is not a new one, 
having been adopted by R. Ellsworth Call, Wm. A. Marsh, and 
other students of the Unionide in the arrangement of their 
collections, and even Mr. Lea acknowledged some such affini- 
ties, though he did not put his ideas into practice. 
It is found that a common assemblage of Unionide inhabits 
the entire Mississippi drainage basin, and that a considerable 
number of the species have a distribution covering the- greater 
part of this area, as well as the whole of Texas, and even parts 
of eastern Mexico. 
Those streams which fall into the Atlantic are peopled by 
an entirely different set of forms, the Appalachian chain seem- 
ing to act as a sharp barrier between the two regions. Many 
of the Mississippi Valley species have spread into Michigan, a 
few into Canada, into streams in New York that flow into the 
Atlantic, and two or three of these are met with in the Red 
River of the North, and the waters about Hudson Bay; while 
below the southern end of the mountain range, there is another 
mingling of groups.” 
There are some forms which, apparently, may be with equal 
propriety assigned to either of two or three groups, but, in 
such cases, the location of the species with regard to drainage 
often gives us a key as to its relationship. There is a group 
of oval, rather smooth, compressed Unios, which may be fairly 
typified by U.favosus, which is found in the rivers from North 
Carolina to Alabama, several members of which very strongly 
1 With such an arrangement, I believe that the North American Unios would fall 
into something like forty quite natural assemblages, whose group characters agree 
fairly well with their distribution so far as drain areas are concerned. There 
would be some half dozen sections of Margaritana, and ten or a dozen of Anodonta. 
*In many cases, the Unionidz seem to have had no difficulty in migrating across 
the country from river to river; an example of this being the Mississippi Valley species 
which now inhabit all the rivers of Texas, and some of those of Eastern Mexico; 
„in some cases, may be carried by aquatic birds in the manner else- 
where mentioned in this paper; in others, they probably migrate across overflowed 
regions near the sea, in time of floods. 
I 
or aed 
