Juday—Quantitative Studies of Fauna in Lake Mendota. 467 
more common than the limnodrilids; the reverse was true of Lake 
Mendota. 
There was considerable variation in the size of these two forms, 
so that it was necessary to pick out all the individuals in a sample 
and weigh them in order to obtain an average weight. The results 
of four such determinations are shown in table 3. The average 
live weight is 1.65 milligrams per individual, of which 82.4 per 
cent, is water, leaving a dry weight of 0.3119 milligram per indi¬ 
vidual. Applying these results to the numerical average per 
square meter at the five stations, we find that the live weight of 
these animals is 57.75 kilograms per hectare, or 51.4 pounds per 
acre; on the basis of dry weight the amount is 10.9 kilograms per 
hectare, or 9.7 pounds per acre. It must be remembered that these 
figures show only what may be called the standing crop which was 
present during the period of this investigation and that they do 
not indicate the amount of this material produced annually. 
With respect to the nutritive qualities of this material, it may 
be said that the percentage of ash is low in these Oligochaeta, aver¬ 
aging only 4.25 per cent, of the dry weight, while the percentage 
of nitrogen is fairly high, being 7.76 per cent., which is equivalent 
to 48.5 per cent, of crude protein in the dry material. 
Pisidium idahoense Roper 
This mollusk was counted in 216 samples from the five deep¬ 
water stations. Table 4 gives the range in the number per square 
meter at the different stations. The difference between the maxi¬ 
mum and minimum numbers is greater than in the case of the 
Oligochaeta, a fifty-fourfold difference being noted in the samples 
from the station North of Second Point. In general, however, the 
variation is much smaller than these figures seem to indicate, the 
maximum being only about twenty-five to thirty-five times as large 
as the minimum. The station off Maple Bluff showed the smallest 
mean, namely, -360 individuals per square meter, while station II 
gave the highest average, or 690 individuals per square meter. The 
mean for the 216 samples is 557 individuals per square meter. 
Various sizes of Pisidium were noted in the samples; the range 
in size was from shells that were only a little over one millimeter 
long up to those that were almost nine millimeters in length. In 
134 samples the individuals were grouped into three classes accord- 
