36 
in the tray. Some improvement in odor and less bubbling was noted in 
samples taken well toward the east edge of the upper lake. 
In the middle lake, opposite Mossville, the mud from the channel 
was spongy with gas, and in the channel of this lake as far south as Al 
Fresco Park, a mile and a half above Peoria Narrows, odors were foul 
and strong. Considerably less bubbling and milder odors were noted at 
the shallower-water stations in this section of Peoria Lake. 
In the lower lake, opposite and below the principal sewers of the 
business and residence districts of the city of Peoria, all dredgings were 
made in the center or to the eastward of the steamboat channel and were 
for the most part out of the way of the worst of the local pollution. At 
our stations in the lake, neither in the main channel nor in the shallower 
water eastward were odors so bad as at upper-lake stations, although 
bubbling was abundant in whatever depth the dredge was let down. At 
all stations, both in this and in the upper and middle lake, it was noted 
that all dead shells of snails were badly blackéned or otherwise dis- 
colored. 
The principal changes in the bottom fauna in this 18-mile section 
since 1915 may be summarized under three heads: (1) disappearance of 
most species and genera and of several families of small Mollusca, along 
with important average decrease in numbers of the more tolerant forms 
still surviving; (2) enormous increase in larval midges (Chironomidae), 
with invasion of several more or less distinctly pollutional species, and 
similar or even greater increase in sludge-worms (Tubificidae) ; and 
(3) disappearance throughout Peoria Lake, except immediately along 
shore or in the short, half-mile, stretch of swifter water in Peoria Nar- 
rows, of all “other insects’* (Ephemeridae, Odonata, Phryganeidae, 
Corixidae, etc.), as well as of planarians and leeches, Amphipoda, Isop- 
oda, sponges, and Bryozoa. 
Although in 1918—1915 at least fifteen or sixteen species of snails, 
representing no less than eight families, were readily taken in the lake 
and river channel between Chillicothe and Peoria, but four species, repre- 
senting three families, were found in the same range in July and August, 
1920. The three. principal surviving kinds (Campeloma subsolidum, 
Pleurocera species, and Musculium transversum) were those found by 
us farthest north—and hence most tolerant of foul water and bottom— 
in the upper Illinois in 1912, the first-named species occurring as far 
north that year as Starved Rock, and the last as far north as Mar- 
seilles. Two of the formerly abundant species of Viviparidae (Vivip- 
ara contectoides and Lioplax subcarinata), two species of Valvata, two 
Amnicolas, three or four species of Physidae and Planorbinae, a species 
of Ancylus, and at least three species of Sphaeriidae seem to have dis- 
appeared altogether from the muds between 1915 and 1920. 
* Understood in text and tables following to mean other insects than Chiro- 
nomidae. 
