Diaptomus oregonensis. 
447 
59 sets of observations was 4221 or about 64 per cent, of the 
total number of Crustacea. Since the opening of the dredge was 
400 square centimeters, this catch would be at the rate of over 
100,000 per square meter of surface. The co-efficient of the net 
has not been determined, but partial observations show that it 
lies probably near four so that the number of Diaptomi per 
square meter of surface of the lake would average over 400,000. 
The maximum would be nearly 900,000 and the minimum about 
200,000. 
The minimum number caught m a single series of hauls was 
1991, and the maximum, 8703, or a variation between four and 
five fold. The range of variation was, therefore, not so great 
as that noted by Apstein and Zacharias, who speak of a ten 
fold variation in the plankton. It agrees however with the 
numbers noted by Zacharias in lake Plbn in 1894. The close 
correspondence in number of sucessive hauls was often very 
striking. As examples may be noted: 
July 16. 
July 19. 
9 A. M 
12 M... 
3 P. M. 
.5584 
.5988 
5 <86 
6 A. M 
9 A. M 
12 M. .. 
.5478 
5072 
.5039 
The three observations of July 16 did not agree as closely in 
the distribution of the numbers as in the numbers themselves. 
The arrangement in the three upper levels was as follows: 
Depth. 
9 A.M. 
12 Noon. 
3 P.M. 
0 -3 m. 
per cent. 
57.3 
per ceDt. 
45.4 
j> r cent. 
72.8 
3—6 m . .. 
37.5 
4^.7 
20.8 
6-9 m . 
4.4 
5.8 
6.4 
In the observations of July 19 the distribution is almost the 
same in each observation. 
