472 Birge — Vertical Distribution of Pelagic Crustacea. 



any other catch from this level. Almost all of the specimens 

 obtained in this level were caught in the first two periods, 

 where they averaged 34 and 25 respectively, while in Period III, 

 an average of only two was obtained, and none at all in Period 

 IV. The decline in numbers of Diaptomus and Cyclops in this 

 level makes the absolute small number of D. pulicaria constitute 

 an important percentage of the catch from this level. In 

 Period I, 17.6 per cent, of the catch belonged to this species, 

 arid in Period II, 39 per cent. For the second period Cyclops 

 with 34 specimens on the average was the only species which 

 exceeded D. pulicaria in absolute number. In the third and 

 fourth periods the number is so small as not to constitute any 

 considerable fraction of the total catch. 



In the 15-18 m. level only 18 specimens were obtained 

 throughout the entire month. Since all the catch from this 

 level was counted, unquestionably all the specimens of D. puli- 

 caria have been enumerated. It is highly improbable that the 

 species visits the deeper part of the lake in considerable num- 

 bers. No specimens in the control observations were obtained 

 below 15 m. to a depth of 22 m. 



Diurnal distribution. — The number of D. pulicaria is so small 

 that little need be said regarding its diurnal distribution. In 

 general the species agrees with all the others which have been 

 studied, in showing no constant variation between day and 

 night. If we average the catches for the total period there is a 

 slight tendency of the species toward the bottom by night, since 

 in the day catches only 11.5 per cent, of the total catch was found 

 in the 12-15 m. level, while nearly 22 per cent, of the entire 

 catch was found in the same level at night. The absolute 

 numbers, however, are so small and the quantity of the catch 

 so depeudent upon chance, that no conclusion should be drawn 

 from these results. In general, we may say that almost the 

 entire number of this species occupied the space between 6 and 

 15 metres both by day and night. 



No reason can be assigned at present for this singular verti- 

 cal distribution. It will be observed that the distribution of 

 this species is in striking contrast to that of D hyalina from 

 which it did not differ greatly in absolute number. While 



