Vertical Distribution of the Crustacea — Summer . 383- 
resulting from the temperature conditions. In both years the pop¬ 
ulation of the lower half of the lake in the latter part of June 
is equal to or greater than that in the same region in the early 
or even the latter part of April. The population of the 9-12 m. 
level remained substantially stationary until the middle of June, 
1895, and the same was true of this level and the level below 
until the middle of July, 1896. In June, 1895, the population 
of the bottom level was high, owing to the accumulation there 
of large numbers of diseased and dying Daphnia hyalina; but 
as soon as these had died, the numbers rapidly fell off, and the 
population in the 15-18 m. level was very small in the first half 
of July. 
In 1894, observations begun with the 1st of July, and at that 
time the population below 9 m. was extremely small, far smaller 
than in either of the succeeding years. At that time the 
temperature conditions below the surface were not observed, 
but it is fair to infer that the thermocline was established at a 
comparatively early date in that year. A second fact which 
influenced the distribution in 1894 is the unusual preponder¬ 
ance of Diaptomus among the Crustacea in that year. A very 
high percentage of this species is found at all times in the up¬ 
per water, while Cyclops , whose per cent, in the lower water is 
greater than that of any other species, was represented by very 
small numbers. 
During July the population of the waters below 9 m. declines 
very rapidly, as will be seen from the table which gives the 
population of the lower water during the months of June, July and 
August. 
Table XXIX. — Population per cubic meter. 
1895. 
1896. 
9-12 m. 
12-15 m. 
15-18 m. 
9-12 m. 
12-15 m. 
15-18 m. 
June 1-15. 
45,000 
36,000 
46,600 
24,000 
12,600 
30,600 
June 16-30... 
13,300 
9,200 
17,500 
24,200 
18,800 
15,000 
July 1-15.... 
14,200 
4,200 
2,200 
25,ICO 
20,900 
2,600 
J uly 16-31.. 
10,900 
2,100 
1,400 
9,700 
700 
300 
August 1-15. 
27,500 
4,100 
600 
11,200 
1,200 
20 
August 16-31. 
32,700 
3,500 
550 
49,600 
6,900 
500 
