1893.] Distribution of the North American Unionide. 35 
v3 
ON THE RELATIONSHIPS AND DISTRIBUTION OF 
THE NORTH AMERICAN UNIONIDAE, WITH 
NOTES ON THE WEST COAST SPECIES. 
By Cuas. T. Simpson. 
For some years past, it has been generally acknowledged by 
students of the Unionidæ that a more natural system of classi- 
fication was needed for that family than any that has hitherto 
been offered. 
The genera or sub-genera of Rafinesque, Swainson, Agassiz 
and Conrad are of little value, for while certain typical species 
may be fairly referred to their divisions, in many others the 
characters fade out, or so blend together that it is impossible 
to place them, and sub-generic lines cannot be drawn with any 
degree of accuracy. Lea’s arrangement, in which he divided 
the genera into two great groups, characterized by the presence 
or absence of a dorsal wing on the shell, and then into smaller 
divisions based on sculpture and form, was, like the Linnæan 
classification in botany, almost entirely artificial, consequently 
in many cases it brought together side by side forms not at all 
nearly related, and in others it widely separated those having 
close affinities, or even sometimes individuals of the same 
species. 
I believe that the best and simplest system that can be, 
adopted at the present, at least, is to place those species, which 
by the characters of the shell, by what we know of those of the 
animal, and by the facts of geographical distribution are most 
nearly related, in groups, each founded on and taking its name 
from some common, characteristic, and widely distributed 
form. Thus Unio gibbosus, which is nearly everywhere found 
throughout the Mississippi drainage area, is well known to 
every collector who has only a few species of Unios, and, with 
its elliptic oval and sometimes arcuate outline, thick epider- 
mis, heavy cardinal and solid club-shaped lateral teeth, fairly 
typifies an assemblage of forms found from Canada to Central 
America, and from the Missouri river to Florida. Unio liga- 
24 
