40 
4. Percentage, by weight, of average total hauls 
contributed by snails 
Channel 4-7-ft. zone 1-3-ft. zone 
1915 1920 | 1915 1920 | 1915 1920 
Upper lake 99.4 W316 4/9 be! 6.6 | 85.3 91.1 
Middle lake 93.2 0 91.1 1E8) |) 8925 12.5 
Lower lake 98.8 bball 98.8 15.3 95.3 0 
The changes in composition and abundance of the chironomid pop- 
ulation of the bottom muds in this section of the river since 1915 are 
perhaps even more interesting and significant than those of the small 
Mollusca, including, as they do, the appearance of several new and more 
or less pollutional species, the disappearance of a number of formerly 
common or abundant cleaner-water forms, and an almost sixty-fold 
multiplication in weight of the average chironomid contribution to hauls 
since 1915. 
Unusually tolerant or pollutional Species that have come into Peoria 
Lake since 1915 seem to be several in number, and include Chironomus 
plumosus Linn., C. maturus Joh., C. frequens Joh., and a variety of the 
first-named species. Chironomus plumosus L. has before been recorded 
by writers in Europe as a species common in foul muds. It was taken 
by us in 1920 in large numbers all the way from Chillicothe to the foot of 
Peoria Lake, both in the channel and in the shallower zones, and also 
occurred in moderate numbers in the gassy river-muds eight miles above 
Havana. It was not found by Malloch in 1912 or 1913 collections from 
either above or below Peoria, but appears to have been present in the 
badly fouled bed of the Des Plaines River at Lockport in the autumn of 
1911, as indicated by a provisional determination by Mr. Chas. A. Hart. 
Chironomus maturus Joh. and C. frequens Joh. are here listed as species 
capable of maintaining themselves in polluted muds because of their oc- 
currence in 1920 in abundance in the Illinois River opposite Peoria, and 
because of the absence of definite records of them from clean muds else- 
where—either in the Illinois River or other waters. 
Three more or less tolerant or pollutional species, Tanypus dyari 
Coq., Chironomus decorus Joh., and C. crassicaudatus Mall., may be listed 
as hold-overs from 1913—1915, when, as in 1920, they were common in 
collections from some stations opposite or abové Peoria. We note that 
C. decorus was the commonest species of this family found by Weston 
and Turner in the contaminated bottom of the Coweeset River in 1916, 
just below the drains of the city of Brockton, Mass. Tanypus dyari Coq. 
was found by Malloch some years ago to frequent a badly polluted local 
