266 
pools near Lincoln, Neb. None of the investigators quoted give 
statistical data of the seasonal limitations of C. bicuspidatus. 
The absence of this species from the summer plankton of the 
Illinois River and its abundance in that of the Great Lakes is perhaps 
explained by the temperature conditions. Surface waters in Lake 
Michigan are reported by Ward (’96) to range from 62° to 67° 
August 11-29, while deeper waters at and below the thermocline 
reach a minimum of 42°. The warmest waters there (62°-67°) are 
thus considerably cooler than the coolest in the waters examined 
by us (which are usually above 70° and often above 80°) during the 
months in which C. bicuspidatus is not found in our plankton. That 
its absence is not due to sewage contamination in low water which 
usually prevails during the warmer months is shown by the prompt 
reappearance of the species in the autumn; as, for example, in 1897, 
when sewage was even more abundant than usual. It may be that 
temperature is also one of the factors limiting its distribution 
elsewhere. 
Cyclops edax Forbes.—Average number, 49; in 1897, 194; in 
1896; 159+ in 1895, 321: and in 1894, 187. . This is the third spemres 
of Cyclops in numerical importance in channel plankton of the 
Illinois. , 
With the exception of a single record on November 2, 1897, all 
occurrences of this species in channel plankton are confined to 
April-October, and all but 9 of the 48 occurrences are in July— 
October, and 32 of them in July-September—the period of maxi- 
mum summer heat. During these three months the percentage 
of collections containing C. edax is highest (44 to 75 per cent.), and 
they are the only months in which the C. edax population rises 
above 1,200 per m.* in channel waters excepting a single instance on 
October 5, 1897, in the high temperatures of that delayed autumn. 
In other months the records are all below 800 and generally below 
400 per m.? The highest number recorded was 3,600 on October 
aoous | 
The seasonal distribution, with maximum numbers in July— 
September, exhibits a temperature adaptation on the part of C. edax 
to maximum summer temperatures (70° to 80°) in channel waters. 
An examination of the records shows that only 13 of the 48 records 
of this species fall in temperatures below 70°, and these were all in 
the months of April, May, September, October, and November, at 
