466 Wisconsin Academy of Sciences , Arts, and Letters. 
tween maximum and minimum at the West Bay station is not as 
great, however, as it is at two of the other stations, being less than 
twelvefold. At the Maple Bluff station the difference is only a 
little more than eightfold, while at station II it is slightly more 
than twenty-onefold; the other stations fall between these two ex¬ 
tremes. 
Differences in density ranging from tenfold to twentyfold show 
that these Oligochaeta are not distributed uniformly over the bot¬ 
tom in the deep-water area, but these differences are no greater 
than might reasonably be expected. Indeed, the population of a 
large city, if considered block by block, would probably show as 
large differences as these, perhaps even larger, in the various areas. 
At station II, where the maximum difference was noted, the samples 
were all obtained within a fairly definite area because a buoy was 
anchored at this point. The launch was anchored to this buoy 
each time when samples were taken, so that the particular area in 
the vicinity of this buoy from which a sample was obtained de¬ 
pended upon the direction, and to a less extent upon the strength, 
of the wind. 
No buoys were placed at the four other stations, but by means 
of landmarks on shore and by soundings it was possible to secure 
samples at each of these stations within a reasonably definite area. 
It was not desirable, in fact, to obtain samples from too small an 
area at each station because the main purpose was to secure a gen¬ 
eral idea of the density of the bottom population at the various 
stations. Such results could then be used to estimate the popula¬ 
tion of the whole of the deep-water area. Increasing the number 
of observations and extending them over a considerable period of 
time also made the results more serviceable for this general con¬ 
sideration. 
Table 2 shows the distribution of the Oligochaeta at these five 
stations by months. The results do not indicate a seasonal varia¬ 
tion in numbers, but the monthly averages are higher between 
October, 1917, and May, 1918, than at other periods covered by this 
investigation. The average number of Limnodrilus and Tubifex 
together in the 213 samples amounts to 3,500 individuals per square 
meter. 
Ekman (3) found two species of Limnodrilus and four species 
of Tubifex in the bottom fauna of Lake Yaettern, but the total 
number of individuals of both genera was smaller than that found 
in Lake Mendota. His results also show that the tubificids were 
