Juday— Quantitative Studies of Fauna in Lake Mendota. 469 
Some of the specimens were put in boiling water for a few seconds 
so that the bodies of the animals could be more readily removed 
from the shells, while others were removed as completely as possible 
without being treated with hot water. Some specimens were 
weighed, also, without removing the animals from the shells. The 
averages for the different classes are shown in table 5. 
With respect to the dry weight it will be noted that the medium 
size weighs, on an average, three and a half times as much as the 
small, while the large weighs about three and a half times as much 
as the medium. The percentage of ash is unusually low in the 
animals of large and medium sizes, but much higher in the small 
specimens. The percentage of ash in the large and medium classes 
is lower than that obtained by Bradley (2) for the larger mollusks 
which he collected in the vicinity of Madison. He found that the 
ash content of the various tissues and organs ranged from a mini¬ 
mum of 6 per cent, in the muscle to a maximum of 48 per cent, in 
the mantle. The distinctly higher percentage of ash in the small¬ 
sized Pisidium may be due to a greater activity of the mantle in 
the secretion of the shell. 
In all three sizes of this mollusk the body portion of the animal 
constituted from 17 per cent, to 21 per cent, of the dry weight of 
the entire organism, the average being highest in the small size; in 
other words, from 79 to 83 per cent, of the dry weight of the whole 
animal consisted of the shell. 
By taking the average number of these mollusks per square me¬ 
ter, namely, 557, and the relative percentages of the different sizes, 
the weight per unit area may be estimated. This amounts to 3.7 
kilograms of dry matter per hectare, or 3.3 pounds per acre, not 
including the weight of the shell. No live weights were determined, 
so that the quantity of living material can not be given. These 
figures show that Pisidium plays a relatively unimportant role so 
far as being a source of food is concerned. No chemical analyses 
of this mollusk were made. 
Chironomus tentans Fabricius 
The larvae of this dipteron were enumerated in 263 samples and 
they yielded an average of 593 individuals per square meter of 
bottom. The variation in the averages for the five different sta¬ 
tions are shown in the first part of table 6. The lowest average 
was obtained at the station North of Second Point, while the maxi- 
