28 



ILUNOIS 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 wou'ld 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 weU 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 

 Illinois Mollusca (1906). These are indicated by an asterisk. 



Table VIII. Distkibution of Unionidae in Three River Systems 



Length of river in miles 



Quadrula cylindrica 



" metanevra 



" metanevra wardii. . . 



" pustulosa 



" lackrymosa 



" ebena 



Trilogonia tuber culala 



Amblema undttUta 



" peruviana (plicata). 



Vermilion 

 90 



z 

 z 

 z 

 z 

 z 



Kankakee 

 300 



Sangamon 

 150 



