Marsh-Limnetic Crustacea of Gh'een Lake. 213 
Bosmina (see PI. XII) was present at all times of the year. 
In only one collection during something over two years,— that 
of May 4th, 1896, — did I fail to find some individuals of this 
genus. Its time of maximum occurrence is in November. The 
numbers found in successive collections vary within very wide 
limits. For instance, Oct. 20, 1894, in a collection made 
between 2:15 and 3:15 p. m., I found only seven individuals, 
while in a collection made about two hours later, I found 212; 
and yet the conditions were apparently precisely the same. 
In regard to its vertical distribution, its home is in the upper 
layers, although it is found occasionally at all depths. 
In order to determine whether there was any difference in the 
vertical distribution at different seasons, I averaged the sum¬ 
mer collections of 1896, from June to September,—7.96 to 17.96 
inclusive,— and the winter collections of 1894-5 from November 
to April, — 24.94 to 3.95 inclusive, with results as follows: 
0-5 
5-10 
10-15 
15-20 
Winter, 21.94 to 3.95. 
61.07 
10.89 
8.08 
7.60 
Summer, 7.96 to 17.96... 
78.18 
5.05 
3.02 
9.91 
While this would indicate a somewhat larger percentage in 
the 0-5 layer in summer, the difference is not very marked, 
and we may say that the vertical distribution is very little 
affected by the changes of season. 
The averages of the night collections of 1894 compare with 
those of the day collections as follows: 
0-5 
5-10 
10-15 
15-20 
Night. 
35.77 
22.70 
22.20 
5.88 
Day. 
60.93 
18.38 
9.16 
7.71 
These figures would indicate that there is a distinctly larger 
number in the 0-5 layer in the day time than in the eight, and I 
infer that is attracted, to some extent, at least, by the light. 
