Marsh — Limnetic Crustacea of Green Lake. 



213 



Bosmina (see PI. XII) was present at all times of the year. 

 In only one collection during something over two years, — that 

 of May 4th, 1896, — did I fail to find some individuals of this 

 genus. Its time of maximum occurrence is in November. The 

 numbers found in successive collections vary within very wide 

 limits. For instance, Oct. 20, 1894, in a collection made 

 between 2:15 and 3:15 p. m., I found only seven individuals, 

 while in a collection made about two hours later, I found 212; 

 and yet the conditions were apparently precisely the same. 



In regard to its vertical distribution, its home is in the upper 

 layers, although it is found occasionally at all depths. 



In order to determine whether there was any difference in the 

 vertical distribution at different seasons, I averaged the sum- 

 mer collections of 1896, from June to September, — 7.96 to 17.96 

 inclusive, — and the winter collections of 1894-5 from November 

 to April, — 24.94 to 3.95 inclusive, with results as follows: 



Winter, 21.94 to 3.95. . 

 Summer, 7.96 to 17.96, 



0-5 



61.07 

 78.18 



5-10 



10.89 

 5.05 



10-15 



8.08 

 3.02 



15-20 



7.60 

 9.91 



While this would indicate a somewhat larger percentage in 

 the 0-5 layer in summer, the difference is not very marked, 

 and we may say that the vertical distribution is very little 

 affected by the changes of season. 



The averages of the night collections of 1894 compare with 

 those of the day collections as follows : 



Night 

 Day.. 



0-5 



5-10 



10-15 



35.77 

 60.93 



22.70 

 18.38 



22.20 

 9.16 



15-20 



5.88 

 7.71 



These figures would indicate that there is a distinctly larger 

 number in the 0-5 layer in the day time than in the night, and I 

 infer that is attracted, to some extent, at least, by the light. 



