348 



Birge — The Crustacea of the Plankton. 



catches. The life history of this species practically belongs to* 

 the period when the temperature of the upper water of the lake- 

 is above 20°. 



Apstein ('96, p. 166), Fric and Vavra ('94, p. 103) find the 

 relations of Diaphanosoma quite as I have done. It does not 

 seem to belong in lake Ploen. Marsh ('97, p. 215) gives the 

 species as present from June to November in Green lake. All 

 find it a little earlier in the spring than I have done. 



Chydorus sphaericus O. F. M. 



Figure 20. — Table J, Appendix. 



Table XXII.— Chydorus sphaericus. Average number per square meter 



expressed in thousands. 



January 1-15 — 

 January 16-31 .. 

 February 1-14 .. 

 February 15-28.. 



March 1-15 



March 16-31 



April 1-15 



April 16-30 



May 1-15 



May 16-31 



June 1-15 



June 16-30 



July 1-15 



July 16-31 



August 1-15 



August 16-31.... 

 September 1-15. 

 September 16-30 

 October 1-15 .... 

 October 16-31 . . . 

 November 1-15 . . 

 November 16-30 

 December 1-15 . . 

 December 16-31. 



1894. 



1895. 







1.3 







a 







a 







a 







a 







a 







a 



CO 



a 



s 

 12.1 



> 



u 

 

 w 



■8 



16.5 

 36.7 



O 

 $5 



21.9 



s 



156.8 



s 



163,4 



s 



78.6 



15.0 



81.7 



No obs. 



15.6 



278.9 



s 



193.3 



8.6 



202.0 



8.1 



97.9 



29.9 



No obs. 



19.7 



9.5 



15.9 



1 



.6 



20.9 



1896. 



10.1 

 19.5 



4.8 



3.8 



No obs. 



1.4 



1.9 



9.8 



28.0 



30.8 



87.6 



230.8 



382.0 



245.1 



406.5 



426.0 



748.0 



263.0 



423.7 



191.9 



62.7 



69.3 



38.2 



28.1 



