1893.] Distribution of the North American Unionide. 355 
resemble some of those of the Clavus Group, but the species of 
the latter assemblage are confined, with one or two exceptions, 
to the streams of the Mississippi Basin, while those of the 
former belong entirely in waters falling into the Atlantic or 
the Gulf. Another small group of rounded or oval inflated 
forms typified by U. irrasus likewise approaches in a few of its 
members some of the species of the clavus Group, but it is con- 
fined, so far as my knowledge goes, to the waters of the Gulf 
drainage; the Etowah, Coosa, Chattahoochee, and a few neigh- 
boring streams, and nota specimen has, I believe, ever come 
from Tennessee or the rivers emptying into the Atlantic, 
though several of them rise near the Chattahoochee. 
In Mexico and Central America, a totally different Unio fauna 
is found. Some of the Anodons extend into South America, 
and others belong to groups that have their metropolis on that 
Continent, while at the North there is a greater or less ming- 
ling of the species with those of the Mississippi Valley. 
I have been greatly puzzled to account for the origin and 
relations of the few species found on the Pacific slope of North 
America. ‘Two Unios, one Margaritana, and some half dozen 
Anodons, are all that have been hitherto credited to this 
immense region. Of the latter, Anodonta wahlametensis, A. 
nuttalliana, A. oregonensis, and A. californiensis, are believed by 
Dr. Stearns’ and other competent conchologists to be merely 
variations of one species, although they often differ greatly in 
forms and general appearance. A. wahlametensis is provided 
witha wide dorsal wing, which gradually fades out through 
the other forms in the order I have given them, to A. califor- 
niensis, which is nearly or quite destitute of this appendage. 
This very abundant species is found from the Rocky Mount- 
ains on the east—in all the waters draining into the Pacific— 
to the coast, and from British Columbia on the north, well 
into Mexico on the south. Six shells in the collection of the 
United States National Museum (Mus. No. 117951), which 
were identified by Dr. Stearns as Anodonia californiensis, were 
collected by Mr. Duges in the Province of Oajacay. 
3 } Jistory and Distribution of the Fresh Water Mussels, and 
rotate ae fikies by R. E. C. Stearns, Proc. Cal. Acad. Sci., Nov. 
20th, 1882. 
