470 Birge — Vertical Distribution of Pelagic Crustacea. 



the percentage ranging from 2.00 in Period II to .3 per cent, 

 in Period IV. 



The actual numbers obtained by day and night were almost 

 the same. In Period I the day averaged only 75, against 132 

 at night. In Period II the day and night were almost iden- 

 tical, being 139 and 129 respectively. In Period III the day 

 catch averaged 128, that at night, 120. In Period IV, day 15 

 and night 21. 



Vertical distribution. — In spite of the small number of the 

 -catch the vertical distribution of D. pulicaria was very distinctly 

 marked. In general the species was almost entirely lacking in 

 the upper 6 m. of the lake, and equally wanting wanting below 

 the depth of 15 m. Taking the average of the month, more 

 ■than 93 per cent, of the species was found between 6 m. and 

 15 m. and over 75 per cent, between 6 m. and 12 m. In the 

 0-3 m. level the average number found for the month was 2, the 

 maximum number 18 and the minimum 0, which occurred in a 

 majority of the observations. No great confidence can be 

 placed in the accuracy of this number. It will be remembered 

 that the total number of Crustacea in this level was very great, 

 averaging more than 3000, and it very rarely happened that 

 more than one individual of this species appeared in the quan- 

 tity counted. This of course was multiplied by the factor 6, 

 and thus would give a minimum of 6 specimens in the upper 

 level if any were present. It must often have happened that 

 catches containing a few members of the species are marked 

 with 0, because no individual happened to get into that portion 

 of the catch which was counted. Making all allowance for this 

 inaccuracy it is, however, plain that the number of specimens 

 in the upper 3 m. is extremely small. The largest average was 

 found in Period II, when, it will be remembered, the weather 

 was very calm, and the nights were lighted by a full moon. In 

 that period an average of 3 was obtained, or a little over .08 

 per cent, of the total catch from that level 



In the 3-6 m. level the number was somewhat greater, averag- 

 ing 6 for the whole series of observations, or about 5.8 per 

 cent, of the total number of the species, with a maxim am of 

 30 and a minimum of 0. The same observation regarding in- 



