Diaptomus oregonensis. 451 



while there is nearly ten per cent, difference in favor of the up- 

 per level in the day and the 6-9 m. level at night. In view of 

 the small number of observations this indication of a nocturnal 

 descent should not be pressed, especially as the greater part of it 

 is due to one of the four sets of observations. In Period II, when 

 the lake was exceptionally calm, the day observations, fifteen in 

 number, gave 68.4 per cent, in the 0-3 m. level, but only 4.84 

 per cent, in the 6-9 m. level. The night observations gave 51.75 

 per cent, and 17.6 per cent, respectively. Not only is this true, 

 but every one of the night observations in the 6-9 m. level 

 yielded numbers greater than any of those taken by day. One 

 can hardly resist the conclusion that there is a tendency to the 

 surface by day. The nights in this period were moonlight and 

 cloudless. These facts are graphically shown in PI. IX, fig. 2. 



Even when the whole month is taken into account, there 

 are in the 6-9 m. level only 8 of the 32 day observations 

 which are above the average for the month, while only 4 of the 

 26 night observations are below the average of the month. In 

 the 0-3 m. level the distribution on each side of the average o^ 

 the month is more nearly equal, since there are in the day obser- 

 vations 14 below the average and 19 above it, and in the night 

 observations 19 below and 7 above the average for the month. 

 All of the exceptionally large catches in the upper level were 

 made by day. There were 12 cases, a little over 20 per cent, of 

 the total number, where more than 65 per cent, of Diaptomus 

 occurred in the upper three meters. All of these were found 

 by day. There is thus a clear indication of a rise of Diaptomus 

 by day, but it is plain that the tendency is not strong and is 

 easily overcome by wind, etc. 



The observations of Period I, 5 day and 4 night, all in the 

 early part of the night, show a slight excess in the upper level 

 at night, — an excess of 2.2 per cent, of the whole number of 

 Diaptomus. Period II shows a day excess of 7.85 per cent in 

 the 0-3 m level, and of 10.8 per cent, by night in the 6-9 m. 

 level. In Period IV the night shows an excess over the day of 

 3.54 per cent in the 0-3 m level. These numbers while not 

 decisive of any marked movement, are conclusive against any 

 considerable sinking of the Crustacea by day. 

 30 



