28 ILLINOIS BIOLOGICAL MONOGRAPHS [126 
stream, these species have not been able to enter the Big Vermilion drain- 
age. 
As has been noted in the Illinois River (Forbes and Richardson, 1919), 
the mussel fauna gradually increases as the distance from the source of 
sewage pollution becomes greater. In the Salt Fork the fauna becomes 
normal at about 20 miles from the source of pollution at Urbana. In the 
Illinois River a normal fauna is not found within 80 miles (Hennepin) of 
the source of pollution indicating that the quantity of sewage is so great 
that the river must flow this distance before purifying itself sufficiently for 
the residence of normal aquatic life. That a normal fauna should be 
found within 20 miles of the source of pollution in the Salt Fork, though a 
much smaller stream carrying a smaller amount of sewage, is quite sur- 
prising when it is remembered that no large tributaries enter the stream 
above Spoon River, and indicates that self purification is active. The 
shallowness of the water (less than a foot on the average in fall and winter) 
probably provides a larger quantity of dissolved oxygen than would be 
possible in waters of a deeper stream. It was especially noted that Am- 
blema undulata and Lasmigona complanata, of the larger species, withstood 
the absence of water better than any of the other comparable species. 
These mussels also resisted polluted conditions better than others and this 
fact is important in connection with mussel propagation for button shells. 
COMPARISONS WITH OTHER RIVER SYSTEMS 
It is of interest and value to compare the mussel fauna of the Big 
Vermilion River with that of some other rivers of comparable size and 
development. The United States Bureau of Fisheries has conducted 
mussel investigations of several of the rivers of Illinois and adjacent states 
and one of these, the Kankakee (Wilson and Clark, 1912), may well be 
compared with the Big Vermilion. The mussel fauna of the Sangamon 
River is also included, the data given being gathered from several sources, 
but principally from personal collections and from collections in the 
Museum of Natural History of the University of Illinois. Some species 
not listed by Wilson and Clark are included from Baker’s Catalog of 
Tllinois Mollusca (1906). These are indicated by an asterisk. 
TaBLe VIII. DistRIBUTION OF UNIONIDAE IN THREE RIVER SYSTEMS 





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