Statistics of D. pulicaria. 
473 
nearly 75 per cent, of D. hyalina were found on the average 
above the 6 m. level, 93 per cent of D. pulicaria were found 
below the same line. The 6-9 m. level was common to the 
two species, and they appear there in not very dissimilar 
numbers, the average from this level being 46 for D. hyalina , 
and 37 for D. pulicaria. It is possible that D. pulicaria de¬ 
velops in the spring, while the water of the lake is 
still cold, and remains in that cooler water as the temperature 
of the lake rises. However, the temperature can hardly be the 
sole factor in determining its distribution, or we should expect 
the species to be more abundant in the still deeper and cooler 
parts of the lake. 
The center of distribution for D. pulicaria varied from 8.46 
m. in Period IV to 10.7 m. in Period I. In Periods II and III 
it was 9,83 m. and 10.15 m. respectively. Thus the center for 
those periods when any considerable number was present lay at 
about 10 m. below the surface, assuming the distribution of the 
animals to be uniform in each of the 3 m. levels. 
STATISTICS OF D. PULICARIA. 
Table P —Vertical distribution. Day and night. Average for month. 
Depth. 
Average. 
Day. 
Night. 
Meters. 
Obs. 
No. 
Per cent. 
Obs. 
No. 
Per cent. 
Obs. 
No. 
Per cent. 
0-3... 
59 
2 
1.9 
34 
2 
1.9 
25 
1 
1.0 
3-6. 
57 
6 
5.8 
32 
5 
4.8 
25 
6 
5.9 
6—9. 
58 
37 
35.9 
33 
39 
37.5 
25 
34 
33.7 
9—12. 
57 
42 
40.8 
32 
46 
44.2 
25 
38 
37.6 
12—15. 
56 
16 
15.6 
33 
12 
11.5 
23 
22 
21.8 
15—18. 
55 
0 
0 
30 
0 
0 
25 
0 
0 
Total. 
103 
104 
101 
The average percentile distribution of D. pulicaria is shown 
in PI. IX. 
