78 SMITHSONIAN MISCELLANEOUS COLLECTIONS [voL. 48 
entangled upon, the feet of aquatic birds and carried by them in 
their flight from the fresh waters of one region to those of other 
regions and there set free. This suggestion seems to be plausible 
but no known migratory range of aquatic birds will connect any con- 
siderable part of the regions of the earth which the Unionide are 
known to inhabit. Besides this all the species, and even the genera, 
of those mollusks which live in certain of the widely separated 
regions are different from those of other regions, and those differ- 
ences are not accounted for in the supposition that the one region 
was stocked from the other. Moreover, very little interchange of 
species seems to have occurred under primeval conditions in cer- 
tain of the intracontinental regions which are constantly visited and 
revisited by aquatic birds. For example, the Mississippi and St. 
Lawrence river systems closely approach each other by some of their 
head waters, and yet each system originally contained a very differ- 
ent Unione fauna from that of the other, although the annual migra- 
tory range of millions of aquatic birds has for centuries traversed 
both regions. It is true that a small number of species are now 
known to inhabit both of those river systems, most of which prob- 
ably owe their double habitat to the agency of man. It is also 
likely that the traffic-canals which are now constructed or projected 
will increase the number of emigrants from each fauna. 
The suggestion that fishes have been instrumental in the distri- 
bution of the Unionidee refers to the glochidia, or minute fry, before 
mentioned. It is well known that these minute larval mollusks attach 
themselves to fishes which live in the same waters, and that thev 
burrow into their skin, where they become hermetically encysted for 
further development. Migratory fishes coming from the sea into 
fresh waters to spawn may thus become infested if the spawning 
season of the mollusks and fishes should be approximately coinci- 
dent. If such fishes should return to the sea bearing in their skins 
the hermetically encysted parasites, and then enter another river 
with them, it has been thought that the young mollusks might 
escape into the new congenial waters and stock them with their kind. 
Unfortunately for this suggestion the encysted term for the young 
mollusk is only about seventy days, while the fishes would not nat- 
urally return from the sea to fresh waters before the spawning 
season of another year. Even then they would be much more likely 
to return to"the same river again than to enter any other. Long 
before that time the young mollusks would have dropped from their 
cysts and died in the salt water. 
The suggestion that the dispersion of the Unionide has been 
