267 



times when occurrences were scattering and numbers few; that 

 is, during the rise or decline of the species to or from the summer 

 maximum. Of the 13 records below 70, there were 5 between 

 60 and 70, 7 between 50 and 60, and but 1 below 50. Cyclops 

 edax in channel waters of the Illinois is thus stenothermic in narrow 

 limits near the maximum temperatures of the year. 



The relation which hydrographic conditions bear to the seasonal 

 development of C. edax may be inferred from the fact that the 

 July-October population of this species in the disturbed waters of 

 1898 was only 35 per cent, of that in the more stable months of the 

 preceding year. 



The occurrences of C. edax take the form of pulses, though less 

 distinctly recurrent and less clearly denned than in species present 

 in larger numbers. Such pulses appear in July, August, and 

 September, 1895, and in August and October, 1897. In 1898 

 (Table I.) the numbers present are too small to clearly indicate 

 recurrent pulses, though suggestions of the phenomenon appear in 

 the records. In general these pulses tend to coincide with those of 

 other Entomostraca. 



Of the totals of all our records of C. edax in 1894-1899, 60 per 

 cent, are females without eggs ; 1 1 per cent., females with eggs ; and 

 29 per cent., males. Young and nauplii were not distinguished 

 from those of other species. Egg-bearing females were found in 

 April and in June-October, but in greatest numbers in July- August. 

 Males occur in June-November, with no marked predominance in 

 any period. 



This species has not been separated from C. leuckarti by other 

 investigators of the plankton, though E. B. Forbes ('97), after a 

 careful comparison of American forms with C. leuckarti of Europe, 

 concludes that edax is specifically distinct, and that leuckarti also 

 occurs in American waters, though apparently not in numbers com- 

 parable with those in European waters. C. edax appears in a 

 measure to replace it in our plankton. He reports it as widely 

 distributed in American lakes and streams and in the plankton of 

 our Great Lakes. 



Cyclops leuckarti Claus. A single dead specimen was recorded 

 in channel plankton August 26, 1898. E. B. Forbes ('97) records 

 it from the Fox and Sangamon (tributaries of the Illinois) , from the 

 Illinois and Mississippi rivers, and from Quiver, Flag, and Dogfish 



