90 ILLINOIS BIOLOGICAL MONOGRAPHS [188 
SUMMARY AND CONCLUSIONS 
The mussel fauna of the Big Vermilion River consists of at least 35 
species and varieties. Ten species of small pelecypods and 15 species 
of gastropods also occur, making a total molluscan fauna of 60 species 
and varieties occurring in a distance of upwards of 50 miles. The mussel 
fauna of this stream compares well with that of two other Illinois streams 
of comparable size, the Sangamon, 150 miles long, with 25 species, and the 
Kankakee, 300 miles long, with 48 species. The total length of the Big 
Vermilion River is 90 miles, with 35 species. Seventeen species of Naiades 
occur in the Big Vermilion River that have not been found in the Sangamon 
River, while five species have been collected from the Sangamon that have 
not yet been detected in the Big Vermilion. The former stream belongs 
to the Mississippi drainage while the latter is a part of the Wabash drain- 
age. 
In the Big Vermilion it was noted that there was a progressive increase 
in number of species as the distance down stream increased, the head- 
waters containing but few species, which are usually smaller than the 
same species from lower down in the stream. It was also observed that 
the headwater inhabitants, as well as many naiads farther down the stream, 
were more compressed and of greater comparative length than the same 
species as it occurred in the larger part of the river. 
The dam at Homer Park, 27 miles below Urbana, appears to form a 
barrier between the fauna above and below this obstruction. Above the 
dam, 19 species occur, while below the dam, 33 species have been observed. 
It is noteworthy that immediately above the dam the largest number of 
species found at one habitat was 14, while below the dam, at Homer 
Park, 28 species have been collected. It is probable that the fall of water 
over the dam aerates the stream and provides an especially favorable 
environment for the mussels. 
UNFAVORABLE INFLUENCES 
Sewage pollution has killed all clean water life for a distance of fourteen 
miles below Urbana and has made the stream an unfavorable environ- 
ment for a distance of twenty miles. Below this point the fauna is normal 
and is not affected by sewage conditions. In the desert area between St. 
Joseph and Urbana, slime worms and septic Protozoa were observed 
among the animals and foul water algae among the plants. A few beetles, 
breathing free air, were observed in the stream near St. Joseph and for 
some distance above this locality. Many of the old pond-like bodies of 
