3G2 



one, a variety of Chirouomus plutnosus Linnaeus or a species very 

 closely allied to it, occurred at fourteen of the thirty stations covered 

 in the middle lake in \'.)'2'2. It was, however, absent in all collections 

 in the upper or Mossville cross-section, and reached its highest aver- 

 age numbers in the cross-section opposite the foot of Horshor Island, 

 or through almost the exact center of the lake. 



Chironomids of more uncertain status were represented in the 

 middle lake hauls of the summer of 1!)22 b)' not less than five species. 

 Of these, Chirouomus digitatus jMalloch has not before to our knowl- 

 edge been recorded from any but ordinarily clean bottom. The others 

 included a species very near C. matiiriis Johannsen, which was taken 

 in foul muds in the lower lake both in 1922 and 1920 ; and an uncertain 

 number of unidentified kinds of Chironomus, Tanypus, and Procladius. 



UNUSUALLY TOLER.ANT SN'.MLS 



While the snails were easily led in abundance in the middle lake 

 as in the upper in the summer of 1922 by the vmusually tolerant Miiscu- 

 liutn transvcrsum Say, the figures for that species were small compared 

 with the numbers reached by it the same season in the upper lake. The 

 largest all-station average of a single cross-section amounted only to 

 a little over 2,500 per square yard, or about a fifteenth of the average 

 of the nine 1922 stations in the best upper lake cross-section (opposite 

 Rome) ; and the smallest to only one sixtieth of the nine-station aver- 

 age of the same cross-section. Though so small compared with upper 

 lake figures, those of the middle lake represented valuation equivalents 

 between 200 and nearly 500 pounds per acre and made up the largest 

 single item by weight of the whole 1922 middle lake bottom fauna. 



LESS toli:r.\nt sn.mls 



Ten other kinds of snails, all presumed to be of lesser tolerance 

 than MuscitHum transvcrsum but more tolerant than several species 

 of the old fauna that have not appeared in any part of the open waters 

 of Peoria Lake since before 1920, and all but one belonging to the 

 Sphaeriidae, were taken in the middle lake in the summer of 1922. 

 Numbers were small in all cases, and all recorded occurrences were 

 at stations well away from the channel and toward the east or west 

 margins of the lake, or, if wiilely distributed, in its lower portion. 

 This number of species is just double that of the less tolerant groi^> 

 of snails listed from the ujiper lake in 1922, and compares with a 



