1893.] Distribution of the North American Unionidæ. 357 
IfI am right in this view of the case, it is probable that the 
group of Anodonta cygnea is confined in North America to the 
Pacific slope, and that its connection with the rest of the 
species in the Old World was made over a land passage 
between Asia and North America. Dr. Theodore Gill informs 
me that the boreal fresh-water fishes of the Old World and 
those of the western slope of our continent have a similar dis- 
tribution to the cygnea group of Anodons as I have outlined it. 
One of the most remarkable of the Unionidæ, Anodonta 
angulata, is an inhabitant of the waters of Oregon, Washing- 
ton, and California. The anterior end of this species in ordi- 
nary specimens is much narrowed ; the posterior is wide and 
inflated, and, running from the beaks to the posterior ventral 
regions is an extraordinarily developed sharp-edged ridge or 
keel. Nothing in the least resembling this peculiar form has 
hitherto been found. But the National Museum possesses 
specimens in which the anterior end is wider, and the keel is 
far less developed, and others in which it is almost entirely 
wanting, and that indefatigable collector, Mr. Henry Hemp- 
hill, has sent me a photograph of a specimen in his collection 
which has no vestige of a ridge, but is furnished with a slight 
groove running down the posterior slope. Recently, Messrs 
Mearns and Holzner, of the International Boundary Commis- 
sion, have sent to the National Museum a large number of 
specimens of what is apparently a new species of Anodonta, 
from San Bernardino Ranch, Arizona, which, externally, very 
much resembles some of the varieties of Unio complanatus, 
but has a very different texture, and, on comparing these 
shells with Mr. Hemphill’s photograph, and the more com- 
pressed specimens of A. angulata, I was convinced at once that 
they group together. They have the same outline, the same 
beak sculpture, and, when viewed from the dorsal or ventral 
region, have a much greater diameter through the posterior 
area than elsewhere, and nearly all exhibit the slight groove 
I have mentioned. 
The type of Unio oregonensis was presented to Mr. Lea by 
Mr. C. M. Wheatley, and the former mentions that the latter 
reserved a specimen for his own collection. Lea’s shell is said 
to come from the Columbia River, and is in rather bad condi- 
