Juday—Quantitative Studies of Fauna in Lake Mendota. 463 
servations were continued in 1917 also. The composition of the 
bottom in these instances varied from practically pure sand to 
material which was characterized as marl. In general, a complete 
series was begun at a depth of about 7 meters and was carried out 
to a depth of from 18 to 20 meters. 
The bottom of Lake Mendota has thus been separated into three 
zones for the study of its bottom fauna. The first zone extends 
from the shore line to a depth of 7 meters, and it is designated as 
the littoral zone; the second lies between the depths of 7 meters 
and 20 meters, constituting the intermediate zone, and the third 
includes the area within the 20-meter contour line, thus represent¬ 
ing the deep-water zone. The first or littoral zone has an area of 
about 12 square kilometers (4.6 square miles) ; the second or in¬ 
termediate zone includes an area of about 21 square kilometers 
(8.1 square miles) and the third about 6.6 square kilometers (2.5 
square miles). Expressed roughly in percentages of the total area 
of the lake they are respectively 30 per cent., 53 per cent., and 17 
per cent. As already indicated, Muttkowski dealt with the popu¬ 
lation of the littoral zone in his paper which was published in 
1918. The present paper is based upon the studies made upon the 
population of the intermediate and of the deep-water zones. 
During this investigation 36 samples of mud were obtained from 
the outer portion of the littoral zone, that is, at depths between 
4 meters and 7 meters; 303 samples were taken in the second or in¬ 
termediate zone, but 63 of them were obtained at the regular sta¬ 
tion in 18.5 meters of water; 276 were taken in the third or deep¬ 
water zone. With very few exceptions, the samples were secured 
with an Ekman dredge which covered an area of 473 square centi¬ 
meters. Where the bottom consisted of fairly soft material, this 
dredge penetrated the mud to a depth of 15 to 20 centimeters, but 
where a large percentage of sand was present it did not go so deep. 
On fairly pure sand, for example, it reached a depth of only two 
to three centimeters, or rarely five centimeters. A few samples 
were taken with a smaller dredge, one with an opening 15 centi¬ 
meters square, covering an area of 225 square centimeters; but 
usually two hauls were made for a sample when this dredge was 
used, so that the area covered for a catch was almost as large as 
that for a sample taken with the large dredge. In the softer mud 
the small dredge penetrated to a depth of about 15 centimeters. 
The samples of mud were transferred from the dredge to spe¬ 
cially constructed containers or buckets made of galvanized iron. 
