Juday—Quantitative Studies of Fauna in Lake Mendota . 481 
C. Intermediate Zone 
In the intermediate zone the bottom shows much variation in its 
composition, varying from gravel and sand, or even rocks, at one 
extreme to the fine mud of the deeper water at the other. In cer¬ 
tain parts of the shallower regions of this zone there are fairly 
large areas of sandy bottom, and this compact material does not 
furnish a very good habitat for many forms, such as the Corethra 
larvae, for example. While such areas are not entirely devoid of 
inhabitants, they are by no means as densely populated as the re¬ 
gions which have a softer bottom. In the more protected parts of 
the lake, mud bottom is found at a depth of 5 to 6 meters, while 
along the more exposed shores sand may extend to a depth of 12 
meters or more. On the steeper slopes practically no loose mate¬ 
rial accumulates, so that the bottom is compact even where it con¬ 
sists of marl. These steep slopes also possess a sparse population. 
In the study of this zone an attempt was made to get samples from 
the various types of bottom in fair proportions so that the results 
would constitute a good general average for the entire intermediate 
zone. 
Table 16 shows the general averages for the different forms that 
were found between the depths of 8 and 20 meters. The results 
shown for the 5--7-meter area are taken from Muttkowski’s report 
(4) ; for further comparisons, also, the general averages for station 
I, station II, and the general average for all samples from the five 
deep-water stations are included in this table. All of the results 
given in table 16 for Corethra and Protenthes represent general 
averages for the entire series of samples and not the spring crops 
of these larvae. This method of computation gives a lower average 
than that noted for the vernal generation because the number of 
larvae is so much smaller during the summer than during the other 
three seasons of the year. The averages have been computed on 
this basis for this table in order to obtain results which are directly 
comparable with each other. 
No samples were obtained from the intermediate region in the 
month of April, so that the vernal crop of larvae of Corethra and 
Protenthes has not been determined directly. The vernal weights 
have been estimated for these larvae, however, by means of the 
general averages for all samples from this region; this general av¬ 
erage has been multiplied by the ratio of the April crop to the 
31—S. A. L. 
