384 
Birge—The Crustacea of the Plankton. 
While the absolute population of the lake during the summer 
months has varied very greatly in the three years of my obser¬ 
vation, the vertical distribution of the animals has been almost 
exactly the same, as may be seen from the following table: 
Table XXX.— Average percentile distribution of Crustacea June 15-Sept. 
15. {In 1894 , July 7-Aug. 23.) 
Average No. 
Pee cent, in each 3 m. level. 
0-3 in. 
3-6. 
6-9. 
9-13. 
i3-i5.: 
15-18. 
1894... 
.406,000 
45.5 
30.2 
16.0 
6.7 
1.3 
0.4 
1895... 
44.0 
24.6 
18.4 
8.9 
2.2 
1.9 
1896... 
..1,116,000 
45.1 
27.5 
14.9 
7.7 
3.4 
1.2 
From this it appears that from 44 to 45.5 per cent, of the 
Crustacea were present in the upper three meters of the lake 
from the middle of June to the middle of September, and from 
25 to 30 per cent, more between 3 and 6 meters, from 15 to 18 
between 6 and 9 meters, leaving from 8.5 to 13 percent, for the 
lower half of the lake. 
The percentile distribution of the Crustacea during the summer 
and its relation to the thermocline are shown in Figs. 26 and 27. In 
each diagram the depth is computed above which were found in each 
half month, respectively 25, 50, 75, 90, and 95 percent, of the 
Crustacea, on the assumption that the Crustacea in each of the 
3m. levels were equally distributed through it. The points repre¬ 
senting the depths for the corresponding percentages were 
platted on the diagram a. I then connected by lines. There is 
added in each diagram the position of the isotherm of 20° 
which lay in the thermocline in both years, although in 1896 
the lake cooled below 20° before the thermocline disappeared. 
In Fig. 26, the temperature for each date was computed from 
the average of the week preceding and that following the date. 
The temperature-line of Fig. 27 is taken from Fig. 4. 
The diagrams show that 25 per cent, of the Crustacea are al¬ 
most always found in the upper two meters of the lake. No 
doubt the position of this line would be higher if it had been 
