71 
the muds, and in the comparative amount and variety of the old mixed 
aquatic vegetation still surviving. Though the evidence furnished by 
the state of the bottom fauna itself in this lake in 1920 was also in some 
directions indicative of a much more nearly normal condition than in the 
lakes north of Havana at the same time, the disappearance from Stewart 
Lake since 1915 of a large part of its old bottom fauna tells as surely of 
an advance of pollution thus far southward at some time or other since 
1915. The evidence of extinction of almost the whole snail fauna, along 
with the fairly wholesome condition of the bottom muds in 1920, is, I 
think, probably to be taken as proof that the destruction of the ‘snails 
took place a considerable time back, and that a wave of pollution such as 
accomplished it has not carried so far south in all recent flood seasons. 
The effect of the high Spoon River bar and the large natural impounding 
area just north of Havana in favoring sedimentation are probably more 
important than the eighteen or twenty miles of additional distance in 
protecting Stewart Lake from the regular invasions of putrescible set- 
tling suspended matter that have apparently been affecting Liverpool, 
Thompson, and other lakes in the Liverpool—Havana district evéry 
spring in recent years. 
A snail list for Stewart Lake, 1915 to 1920, is not essentially differ- 
ent from one for Thompson Lake, showing the same single species (the 
little sphaeriid Musculiwm transversum) surviving in small numbers; and 
a list of around thirteen species, representing seven families, that have 
dropped out since 1915. While Stewart Lake was not even several years 
ago a heavy bottom-fauna producer, either on the snail side or otherwise, 
as compared with the lakes north of Havana, showing only 66 pounds per 
acre of snails and 74 pounds of all small bottom-animals in 1915, the 1920 
figures of only 2 pounds per acre of snails (one species) and 28 pounds 
of all other bottom-animals look small enough even in comparison with 
the older figures to show that something has happened. 
In the group of “other insects, etc.,’ Stewart Lake last year made 
fully as bad a showing as in small Mollusca, neither Bryozoa, leeches, 
small bottom-Amphipoda or Isopoda, Corixidae, Trichoptera, or 
Ephemeridae—all of which were found commonly in small numbers at 
collecting stations in this lake six or seven years ago—appearing in 1920 
in any bottom collection. 
Perhaps the best biological indication of a wholesomer condition of 
the mud in this lake than in those farther north, next after the more 
thriving condition of the vegetation, is the survival here in good numbers 
of a few of the more sensitive species of Chironomidae which have been 
completely exterminated in the lakes north of Havana except Quiver, 
and the failure of the sludge-worms to show as marked increase as in 
the lakes farther north. The total number of small oligochaete worms 
increased in Stewart Lake between 1915 and 1920 only from three to 
twelve per square yard—a negligible increase as compared with, for ex- 
