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 defined 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; 11 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. leuckartt 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 leuckartt 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 
