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 Plon in 1894. The close 

 correspondence in number of sucessive hauls was often very 

 striking. As examples may be noted: 



July 16. 



9 A. M 5584 



12 M 5988 



3 P. M 5786 



July 19. 



6A.M 5478 



9A.M 5072 



1 2 M 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. 



per cent. 

 57.3 

 37. * 

 4A 



12 Noon. 



3 P. M. 



0-3m 



per cent. 

 45.4 



4^.7 

 5.8 



p-r cent. 

 72.8 



3— 6m 



6-9m 



20.8 

 6.4 









In the observations of July 19 the distribution is almost the 

 same in each observation. 



