WHITE] ANCESTRAL ORIGIN OF UNIONID fd 
While the species and genera of the Unionide are different in 
different regions, the family is so distinct from all other mollus- 
can families except the Mutelide that naturalists, reasoning from 
present physical conditions and biological data only, have gen- 
erally assumed for it a common genetic origin in some one region, 
its subsequent differentiation, and its final distribution to other 
regions. Because of the fresh-water requirements of those mol- 
lusks and the separation by marine waters of the regions which 
they occupy, and also because there are many cases of intraconti- 
nental restriction of regional areas of distribution, the question, how 
the great distribution of the family could have occurred has been a 
most perplexing one. The marine waters of the earth cover the 
larger part of its surface and they are everywhere continuous and of 
essentially the same character. It is therefore easy to understand 
how any family of marine animals might gain a universal distribu- 
tion by successive migrations ; but the case is very different with the 
Unionidz. Since every member of that family dies quickly if 
placed in sea water or upon land they are confined to rivers 
and brooks, lakes and ponds and they cannot by their own act pass 
from one congenial habitat to another, either overland or through 
marine waters. Voluntary migration from one region to another 
being out of the question, some method of distribution by agential 
transportation has been generally advocated. Local dispersion by 
the shifting of drainage lines has been suggested; and even the inde- 
pendent origination of members of the family in each region has 
been assumed. It is desirable to give some account of the views 
which have been held concerning the geographical distribution of the 
Unionidz, both for completeness of statement and for the purpose 
of comparing those views with the hypothesis concerning the geo- 
logical origin of the North American species which I shall propose. 
Those who have suggested the distribution of the family by trans- 
portation have attributed it to the agency of birds and fishes respec- 
tively. For all such cases it has been assumed that it was the eggs, 
or the newly hatched fry, technically called the glochidium, or the 
byssus-bearing fry, and not the adult mollusks, that have been trans- 
ported. Immediately after the fry have been hatched from the 
eggs, and while they are exceedingly minute, some, if not all of the 
species develop hooklets upon the temporary shell by which each can 
attach itself to other objects. Ata later stage the fry attaches itself 
to other objects by a slender, thread-like byssus.t It has been 
thought that these larval mollusks may become attached to, or 
‘I make this statement from personal observation many years ago. 
