284 BULLETIN OF THE UNITED STATES FISH COMMISSION. 
(lie if removed from it, wliile those which inhabit streams or ponds that often dry up 
will live a long time out of watei’. It is well known that the streams of Australia 
often go dry. and the smaller ones of the Western States are quite liable to do so. 
Some species in rocky streams remain in the crevices of the rocks. In the Big 
Vermilion River, in Lasalle County, Illinois, a swift rocky stream, I have found living- 
mussels which had been so washed about that nearly all the epidermis was destroyed. 
The shells in such streams are usually heavier than those from more quiet water. 
There are two great and very distinct groups of pearly mussels in the fresh 
waters of the United States, and several smaller somewhat intermediate ones. In 
the first the shells of the male and female are essentially alike, and the embrjms, 
before being thrown into the water, fill either the entire outer gills, or, in some cases, 
all four of them. At this time the marsupium, as the transformed gills may be called, 
is smooth and pad like. In the second great group the shell of the female shows a 
decided swelling on the base Just behind the middle, and this is absent iu the male 
shell. In the species of this group the embryos are only found iu the hinder part of 
the outer gill, and this part is rounded below, and the ovisacs containing them are 
separated from each other by a furrow and rounded on the lower end. The latter 
group contains the more highly organ- 
ized of our fresh-water mussels. 
From all the observations that have 
been made, it seems that in a majority 
of cases the species of the first group 
become impregnated in the fall, carry- 
ing the eggs in the ovaries through the 
winter. In the late winter or early 
spring they pass into the gills, develop 
a shell, and are thrown out into the 
water along up to June. Those of the 
second group, where the males and 
females are so different, probably be- 
come gravid in the spring or early summer, and have the hinder part of the outer 
gill full of eggs in the summer or early fall. These rules do not hold absolutely, and 
some of the species, or even many of them, especially of the more highly organized 
groups, may breed oftener than once a year under favorable circumstances. 
Little is known as to the winter habits of these mussels. It is quite certain that 
they sometimes burrow into the mud and become almost if not quite dormant, while at 
other times they are active during the winter. Certain species may be found gravid 
at different periods of the year; others seem rarely so. I have never seen or heard of 
a gravid TJnio pUcaius (a common, thick, plicate form of the Mississipid Valley), and 
I have examined many specimens of Mar gar it ana margaritifera taken all through the 
year, and have never seen any with embryos either in the ovaries or gills. 
The number of young contained in the female at a single time is often very great. 
In the species which carry them only iu the hinder part of the outer gills they are far 
less numerous than in those of the other group. Most of these probably contain from 
oOO to 1,000 at a time, if fully adult. The species which have the whole outer gills 
transformed into a marsupium have a far greater number, while those which have all 
four gills filled contain the most. The common swan mussel of Europe, Anodonta 
cygnea, has been estimated to carry 200,000. A Glabaris from Lake Nicaragua, which 
'Fia.G.—LampsilislutenlusJjam. Male. Shell Bot produced 
at posterior base. 
