352 



(1922) from the acid waters of the Big Muddy. The distribution of 

 Campeloma siibsoUdum in upper Peoria Lake in 1922, was confined 

 to two stations in the far east wide-waters a mile and a half above 

 Spring Bay. Bad odors and abundant bubbling were noted at both of 

 these stations, and the bottom dissolved oxygen was under three parts 

 per million in July, and considerably lower in August. 



While MnscuUum trtmcatum did not appear at all in 1920 collec- 

 tions from any part of Peoria Lake, in August 1922 we found it at 

 seven out of the nine stations in the Rome cross-section, including the 

 channel, and at one station in the upper portion of the middle lake. 

 Numbers in the Rome cross-section ran as high as TOO per square 

 yard, and the species was associated with Tubificidae at stations where 

 they exceeded forty thousand. This species was found in the Illinois 

 River channel in 1919 as far north as De Pue and Spring Valley, and 

 seems to possess not far from as high a degree of tolerance as its 

 more prolific coingener, M. transversiim. It seems, in fact, to differ 

 from it mainly at the present time in Peoria Lake as it did in the past 

 in the cleaner waters of the Illinois, in a lesser aptitude for multiply- 

 ing and occupying the capacity of its range. 



Of the three species of Pisidium taken in the upper lake in 1922, 

 P. compressum had much the widest distribution, occurring at six out 

 of the total of 23 stations, compared with no records at all in 1920. 

 It was present at three stations in the Rome cross-section, including 

 the mid-channel station, and was associated there with Tubificidae and 

 other annelid worms whose combined numbers exceeded 45.000. per 

 square yard. This little shell also occurred in the upper lake in 1922 

 at three stations toward the west end of the Spring Bay cross-section, 

 and extended southward into the middle lake and the upper jwrtion of 

 the lower. In 1912 it was taken in the channel of the Illinois at Spring 

 Valley, De Pue, Henr\-, and Chillicothe. 



Pisidium paupcrculuni, var. crystalciisc Sterki occurred in the 

 upper lake in 1922 at only two stations, both west of the main channel 

 in the cross-section a mile and half above Spring Bay. At one of 

 these two hauls unvisually bad odors were noted and Tubificidae and 

 other small annelids passed 5,000 per square yard. ' The species was 

 not taken in the Illinois River above Henry in the summer of 1912, 

 and is apparently to be graded as somewhat less tolerant than P. com- 

 press uni. 



