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. 
1894. 
1895. 
1896. 
January 1-15.... 
1.3 
10.1 
January 16-31.— 
a 
19.5 
February 1-14. 
a 
4.8 
February 15-28... 
a 
3.8 
March 1-15. 
a 
No obs. 
March 16-31. 
a 
1.4 
April 1-15. 
a 
1.9 
April 16-30. . 
to 
c 
1 
s 
9.8’ 
# c 
May 1-15. 
ct 
; 
12.1 
28.0 
May 16-31. 
t 
s 
16.5 
30.8 
June 1-15..... 
■s 
36.7 
87.6 
June 16-30... 
c 
S3 
21.9 
230.8 
July 1-15. 
s 
156.8 
382.0 
July 16-31. 
s 
163,4 
245.1 
August 1-15. 
s 
78.6 
406.5 
August 16-31. 
15.0 
81.7 
426.0 
September 1-15... 
No obs. 
15.6 
748.0> 
September 16-30 ..... 
278.9 
s 
263.0 
October 1-15... 
193.3 
8.6 
423.7 
October 16-31. . 
202.0 
8.1 
191.9 
November 1-15. 
97.9 
29.9 
62.7 
November 16-30.. 
No obs. 
19.7 
69.3 
December t-15. 
9.5 
15.9 
38.2 
December 16-31... 
1.6 
20.9 
28.1 
