General Conclusions. 
481 
trustworthy method could hardly have led to such different re¬ 
sults, and to results so constantly different. 
When in August Diaphanosoma began to appear in numbers 
sufficient to count, its distribution agreed with that of Diap- 
tomus. This agreement persisted through September, when the 
number of Diaphanosoma caught in each series increased to 
nearly 1,000, to disappear early in October. 
While, therefore, we are not disposed to urge that the re¬ 
sults reached by us are to be considered as mathematically ac¬ 
curate, we believe that they are essentially correct. Even taking 
into account ail the various sources of possible error, the gen¬ 
eral results are so strongly marked that they can hardly be af¬ 
fected by such errors. When the number in the upper level, 
0-3 m., is ordinarily 50 per cent, to 100 per cent, more than 
that from any other level, with an average number of over 3000 
Crustacea in the upper level, it is impossible to be mistaken in 
the general fact. When this large excess of Crustacea in the 
upper level is found at all times of day and night, it is impos¬ 
sible to believe in a descent of the Crustacea by day, and a re¬ 
ascent at night. 
Zacharias, in the Third Report of the Biological station at 
Plon (p. 126), announces that the plankton of that lake shows 
no diurnal movement. His observations were made on Septem¬ 
ber 16, 17, and the report came to hand just as this paper was 
going to press. 
GENERAL CONCLUSIONS. 
1. The plankton Crustacea of lake Mendota during July con¬ 
sist chiefly of Diaptomus oregonensis (64 per cent.), three species 
of Cyclops (30.5 per cent.), Daphnia hyalina (3.3 per cent.), 
and D. pulicaria (1.5 per cent.). 
2. All of the Crustacea out the last named agree in vertical 
distribution, having 50 per cent, or more in the upper 3-4 m. 
of the lake. D. pulicaria is found chiefly between 6 m. and 12 m. 
and almost wholly between 6 m. and 15 m. is distribu¬ 
tion is maintained throughout the month. 
3. During July, only the upper 12 m. of lake Mendota are 
tenanted by Crustacea, and over 90 per cent, are in the upper 9 J 
