194 



Marsh — Limnetic Crustacea of Green Lake. 



did not find some representatives of this genus in every five 

 meter stratum, and yet from sixty to seventy-five per cent, 

 were commonly in the upper ten meters. 



In order to find out whether there was any difference in the 

 vertical distribution in summer and in winter I took, the 

 averages in the upper three levels of collections 7.96 to 17.96 

 inclusive, and 24.94 to 3.95 inclusive. I took these years 

 because in 1894-5 I made a large number of collections in cold 

 weather, and in 1896 I made the largest number of collections 

 in warm weather. 



The following table indicates the results: 





0-5 



5-10 



10-15 



Summer, 7.96-17.96 



49.31 

 50.02 



24.49 

 13.50 



12.26 



Winter, 24.94-3.95 



10.12 







It appears from these averages that the seasons make no 

 difference in the vertical distribution of Diaptomus, but that it 

 is uniform throughout the year. 



Apstein comes to the same conclusion. ('96, p. 180.) 

 The day and night collections of October, 1894, compared as 

 follows : 



5-10 



Day.. 



Night 



18.42 

 18.40 



Here is no evidence of diurnal migration. 



I think, then, that I am safe in saying that the vertical dis, 

 tribution of Diaptomus varies but little from one end of the 

 year to the other and is not appreciably affected by changes in 

 the amount of light. 



Birge finds the same thing to be true of D. oregonensis. 

 (Birge, '95, 450.) 



