264 
data on its seasonal distribution are given by any of the authors 
cited. 
C. albidus appears thus to be adapted to both the littoral and 
limnetic areas, but seems never to attain great numbers in the 
latter. 
yclops bicuspidatus Claus.—Average number, 373; in 1897, 
206; in 1896, 145; in 1895, 312; and in 1894, only 2. A full dis- 
cussion of the variation and synonymy of this species has been 
published by E. B. Forbes (’97). 
This species shows sharply marked seasonal limitations. Every 
one of the 68 records, with the exception of one of a single female 
found September 30, falls within November—May, and all of the May 
records were made in the delayed low temperatures of the spring of 
1898. The general distribution of this species during this period’ 
is indicated by the high percentage of collections in which it was 
found, viz., 63, 71, 67, 73, 93, 53, and 40, respectively, for November— 
May. The numbers per m.’ are, however, high only in November 
and April-May, reaching 8,000 in 1895 and 1898 in this vernal pulse, 
and 3,560 in November, 1897, in the autumnal pulse. In Decem- 
* ber-March numbers do not rise above 500 per m.* save once in 
December and on March 24-30, 1896. C. bicuspidatus is thus a 
winter and early spring planktont in channel waters of the Illinois. 
The temperature adaptations are exhibited by the fact that only 
13 of the 68 occurrences are in temperatures above 50°, only 5 above 
60°, and but 1 above 70°—that of May 24, 1898, at 73°. -On the 
other hand, the greater developments in numbers take place during 
these higher temperatures of 50°-70°, the only rises above 1,000 per 
m.* at temperatures below 50° being those of March 30 and April 10, 
1896, at 48° and 46.4°, and of November 15,‘1897, at 47°. Muini- 
mum numbers thus prevail below 45°, and the temperature opti- 
mum in channel waters of the Illinois appears to lie near 60°. 
The seasonal routine in channel waters begins with the appear- 
ance of small numbers about November 1, with an occasional pulse 
of some amplitude in that month followed by a continuance of small 
numbers through the minimum temperatures of December—Feb- 
ruary, and a rise with the temperatures in March to a maximum 
vernal pulse toward the end of April or the first of May, and a 
complete disappearance of adult individuals after temperatures pass 
70° during May—October. 
