8 ILLINOIS BIOLOGICAL MONOGRAPHS [106 
Ortmann has made a study of the distribution of the Naiades in the 
streams of Tennessee from this standpoint, using material collected by 
Adams (Ortmann, 1918); and Wilson and Clark (1912) have added greatly 
to our knowledge of the comparative distribution of the mussel faunas 
of the Kankakee and Maumee rivers. 
Studies of this kind bring out the fact that certain species are charac- 
teristic of the upper reaches of a stream while others are found only in the 
lower part. Barriers, such as falls and polluted water, are also seen to have 
a large influence on the distribution of mollusks, especially river mussels. 
The same species may also vary in size or shape in different parts of the 
stream, as noted by Ortmann (1920). In fact, a true picture of the charac- 
teristics of the fauna of a river or smaller stream can be gained only by this 
method, which the present paper clearly shows. 
The Salt Fork of the Big Vermilion is a striking example of the ill 
effect of sewage and other pollution on the fauna of a stream. It was 
carefully studied from this standpoint in order to determine accurately 
the distance that the polluted stream must flow before a normal fauna can 
establish itself, and to ascertain the relative effect of pollution on different 
species of mussels as well as on other aquatic animals. Mussels and cray- 
fishes are considered good indicators of the measure of pollution in a stream 
and the absence of both of these groups of animals from the upper part of 
the Salt Fork is ample evidence of the septic condition of this stream. 
The work was largely carried on during the months of August, Septem- 
ber, and October, when the water was low. Collecting was done by wading 
in the stream as deep as hip rubber boots would permit. In the shallow 
water (up to two feet in depth) the entire bottom was gone over with the 
hands, and the mussels and other mollusks thus picked from the bottom 
of mud, gravel, and sand. Samples of all mollusks, as well as associated 
animals of the other groups, have been preserved in the Natural History 
Museum of the University of Illinois. 
The different areas of the region studied are covered by three maps of 
the United States Geological Survey, the Mahomet, Champaign, and Dan- 
ville Folios, and on these the stations recorded in this paper may easily 
be located. 
ACKNOWLEDGMENTS 
The writer is greatly indebted to Professor Frank Smith, not only for 
invaluable assistance in collecting much of the material upon which the 
paper is based, but also for many notes on the distribution of the Naiades 
and other mollusks in the two river systems. Professor Smith has studied 
the fauna of the Salt Fork and the Sangamon streams for many years, 
collecting from them at different times of the year. His classes in zoology 
have visited Homer Park yearly for a long period and thus the mollusk 
