The Vertical Distribution of the Crustacea. 377 
frequently during the winter. But as the observations were few 
in number in comparison with those made at the regular point 
of observation, they have not been used in the preparation of 
the tables. 
During the last half of the year 1894, 75 serial observations 
were made, 127 during 1895, and 131 during 1896. These were 
most numerous during the summer months. In general it may 
be said that on every day on which observations were made as 
stated in Table A of the appendix, a series was taken, and on 
some occasions more than one. The general distribution, of 
the observations, however, can be ascertained from the table. 
At least five were made in each two week period from the mid¬ 
dle of April to the middle of November. During the winter of 
1895, some observations were made by six meter intervals in 
the lower water of the lake, and the result of these observations 
was equally divided between the two levels covered by them. 
In Table B, accompanying this part of the report, the popu¬ 
lation of each level is given in thousands per cubic meter, the 
total population of the level being divided by three on the as¬ 
sumption that the Crustacea are equally distributed throughout 
the level. Under some circumstances this assumption is incor¬ 
rect. In the 0-3 m. level, the upper meter contains more than 
one-third of the Crustacea, especially when there are large 
numbers of young. It may contain twice as many as any meter 
below. On the other hand, on bright calm days, when few 
young Crustacea are present, the upper meter may contain less 
than one-third of the total catch from the upper level. 
In the level which includes the region of the thermocline the 
population of the single meters varies greatly, as will be shown 
later in this paper; the Crustacea being found in considerable 
numbers above this stratum and practically absent below it. A 
third error arises at times when large numbers of Crustacea are 
settling to the bottom and dying. This occurs with Cyclops 
during the winter and spring, and with Daphnia hyalina in the 
early part of June. At such times the lower meter of the lower 
level would contain more than one-third of the Crustacea present 
in that level. These variations from an approximately uniform dis¬ 
tribution are however so varying themselves that it has not been 
