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 species 

 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. 3 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. 3 The highest number recorded was 3,600 on October 

 5, 1897. 



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 



