350 



Birge — The Crustacea of the Plankton. 



Earlier and later catches agree with those given. On the 8th 

 and 9th of the month there was a violent wind from the north 

 and northwest, which probably brought this species out from 

 shore water where it had been developing. 



These facts indicate that Chydorus is not properly a limnetic 

 form but that it gets into the limnetic region by accident and 

 maintains itself there so long as suitable food is present. I agree 

 with Apstein in regarding this form as characteristic for lakes 

 abounding in Chroococcaceae or, perhaps, Schizophyceae. He 

 has not observed its dependence on the seasonal appearance of 

 these plants in the lake, as is the case in lake Mendota. In the 

 limnetic region the species is acyclic so far as my observations 

 go. The largest catches of this species were 440, 000 per sq. m. 

 Sept. 21, 1894; 221,000, July 28, 1895; 661,000, July 7, 1896;. 

 674,000, Aug. 15, 1896. 



Leptodora hyalina Lillj. 



Table XXIII. — Leptodora hyalina. Average catch per square meter of 



surface. 



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. . 



1894. 



1895. 



No obs. 



63 



No obs. 



680 



324 



986 



362 



827 



445 



2,512 



1,081 



3,078 



No obs. 



1,068 



871 



775 



1,469 



457 



966 



661 



95 



292 





25 





1896. 



254 

 1,208 



585< 



642 

 1,881 

 2,850 

 2,945 

 2,375 

 1,026 



247 

 31 



The table given above shows the average number of Lepto- 

 dora during the seasons of 1894, 1895, and 1896. The species 

 first appears in May, being first observed May 29th, 1895, and 

 1896. The nauplii must appear earlier, but I have never seem 



