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. 



Obs. 



Average. 



Day. 



Night. 



Meters. 



No. 



Per cent. 



1.9 



• 5.8 



35.9 



40.8 



15.6 







Obs. 



No. 



Per cent. 



Obs. 



No. 



Per cent. 



0-3 



3-6 



59 

 57 

 58 

 57 

 56 

 55 



2 



6 



37 



42 



16 







34 

 32 

 33 

 32 

 33 

 30 



2 



5 



39 



46 



12 







1.9 



4.8 



I 37.5 



44.2 



11.5 







25 

 25 

 25 

 25 

 23 

 25 



1 



6 



34 



38 



22 







1.0 

 5.9 



6—9 



9—12 



12—15 



33.7 

 37.6 

 21.8 



15—18 











Total 





103 







104 







101 









The average percentile distribution of D. pulicaria is shown 

 in PI. IX. 



