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. 
1894. 
1895. 
1896. 
June 1-15... 
No obs. 
63 
s. 
June 16-30. 
No obs. 
680 
254 
July 1-15. 
324 
986 
1,208 
July 16-31. 
362 
827 
585 
August 1-15... 
445 
2,512 
642 
August 16-31 — ..... 
1,081 
3,078 
1,881 
September 1-15.. 
No obs. 
1,068 
2,850* 
September 16-30. 
871 
775 
2,945 
October 1-15. 
1,469 
457 
2,375 - 
October 16-31.. 
966 
661 
1,026’ 
November 1-15. 
95 
292 
247 
Nr»Vftmhf»r 16-30. 
25 
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 
