strniiiiiiy pall was made of Diifour's No. 2^) l)olliii.u- doth, wcfe used. 

 The quantity of water strained for a cutcli was the amount produced by 

 forty strokes of the pump, whicli averaged 22.5 liters. 



The counting method AA'as used to determine the relative alnindanc(> 

 of the varions plankton forms. In most cases 20 per cent, of the material 

 ol)tained in a catch was counted, and the results thus obtained for the 

 various forms were multiplied by five in order to determine the number 

 of individuals in a whole catcli. Whenever a catch contained a compara- 

 tively small numl)er of individuals, for example, the catches near the 

 surface in day time, the whole catch was counted. Also, all the indi- 

 viduals of the larger forms, such as Epischm-a and Leptodora, wlilcli are 

 readily recognizable with the nalscd eye, were counted. 



The sets of observations may be divided into live groups: 



1. Twenty sets of day catches which were made not earlier than 9 

 a. m., nor later than 4 p. m. 



2. Six sets of night catches which, with one exception, were made 

 between 9 p. m. and midnight. On September 2, a series Avas made as 

 early as 8 p. m., but this, however, was an hour and a half after sunset. 



3. Four sets of evening catches were made. These were begun short- 

 ly before sunset and continued at half hour intervals an hour or more 

 after sunset. 



4. The morning observations were begun one and a half to two hours 

 before sunrise and were continued at thirty minute intervals until after 

 sunrise. Six sets of these were made. 



5. In August there w'ere two sets of observations in which catches 

 were made at the surface at regular intervals during the entire night. 

 Both series were begun before sunset and continued until after sunrise. 

 Catches Avere made at half hour intervals until 8 and 9 p. m., respective- 

 ly, then every hour until 3 and 4 a. m., respectively, and again at half 

 Imur intervals until after sunrise. Tlie results of these observations are 

 shown in Figs. 1 to 4. 



The first and second groups covered the entire depth of the lake 

 (tAventy-tive meters), Avhile the catches of the third and fourth groups 

 Avere confined to the upper four meters. The tifth group consisted of 

 surface catches. 



