Juday—Quantitative Studies of Fauna in Lake Mendota. 479 
the lakeside laboratory of the University of Wisconsin, and it has 
been designated as station I in the various observations that have 
been made here. The water has a depth of 18.5 meters at this 
point. The data obtained at this station are discussed separately 
for three reasons: (1) Because the station is isolated somewhat 
from the main body of the deeper water and lies well toward its 
southern edge; (2) because the water is somewhat shallower here 
than at the other stations; and (3) because the results show some 
characteristic differences. The mud at this station contains a 
somewhat larger proportion of coarse vegetable debris than at the 
other stations, which is due, doubtless, to the fact that it is situ¬ 
ated near shallow areas supporting fairly dense growths of the 
larger aquatic plants. 
Limnodrilus and Tubifex. Sixty-four samples were taken at 
station I. The Oligochaeta were counted in 52 of them, and an 
average of 2,320 individuals per square meter was obtained. This 
number is distinctly smaller than the mean of any of the stations 
within the 20-meter area (table 1). The average for the Maple 
Bluff station comes nearest this figure, while the mean for West 
Bay is twice as great. 
Pisidium idahoense. This form was enumerated in 52 samples 
from station I and a mean of 570 individuals per square meter was 
found (table 4). This average was exceeded by only two of the 
stations in the deeper water; both the maximum and the minimum 
numbers were larger than at any of the other stations. The dif¬ 
ferent sizes were counted separately in 33 samples, and in them 
were found 14 large individuals, 124 of medium size, and 742 small. 
Stated in percentages of the total number, these figures represent 
1.7 per cent., 14 per cent., and 84.3 per cent., respectively. As 
compared with the five stations in deep water, the percentage of 
small individuals at station I is substantially the same, but that for 
the medium size is larger and that for the large size is smaller. 
Chironomus tentans. Table 6 shows that station I ranked sec¬ 
ond in the average number of the large and medium sizes of this 
chironomid larva, but it was first in the small size and second in 
the general average. The different sizes in 54 samples consisted 
of 11 per cent, of large individuals, 32 per cent, medium, and 57 
per cent, small. These percentages are substantially the same as 
those for the deep water stations, which are, respectively, 13 per 
cent, large, 31 per cent, medium, and 56 per cent, small. 
