225 
(5,040), indicating a probable maximum between these dates. In 
other seasons, for example in 1896 and 1897, the maxima of this 
species coincide generally with those of other Cladocera, so that this 
divergence seems to be anomalous. An inspection of the table of 
records for 1898 gives a remarkably uniform and coincident rise and 
decline of the pulses of the several species which constitute this 
characteristic vernal pulse. 
No effort has been made by me to determine the total cladoceran 
fauna of the Illinois River. Only those species are here given which 
have appeared in our plankton enumeration. A number of others 
are known to occur in the littoral fauna, and a few scattering indi- 
viduals found in the plankton were not identified. | 
Of the 25 forms here listed, only 10—named in the sequence of 
their relative numbers as shown in grand totals—may be regarded 
as typical planktonts, autolimnetic in channel plankton, viz.: Moina 
micrura, Bosmina longirostris, Daphnia cucullata and vars. aptcata 
and kahlbergiensis, D. hyalina, Certodaphmia scitula, Chydorus 
sphericus, Diaphanosoma brachyurum, and Leptodora hyalina. Of 
the ten, the last named and the varieties of D. cucullata appear to 
be of little quantitative importance in the channel plankton, though 
it may be that our methods of collection fail adequately to represent 
Leptodora. Of the remaining 15 species, Alona affinis, Certodaphnia 
reticulata and C. rotunda, Scapholeberts mucronata, and the two 
species of Simocephalus are the only adventitious Cladocera of 
quantitative importance, and this only to a relatively small extent. 
DISCUSSION OF SPECIES OF CLADOCERA. 
Alona affinis Leydig.—Average number, 36. This species has 
a well-defined seasonal distribution. It appears in autumn in the 
last of October, as temperatures approach 40°, and remains until 
the end of June, when the summer maximum of 80° is re-established. 
The numbers are too small (Table I.) and irregular to define its 
seasonal fluctuations, though there are suggestions in the records 
of late autumnal and of vernal pulses. Egg-bearing females were 
recorded in January—February at minimum temperatures. No close 
dependence on hydrographic fluctuations is apparent to account for 
their occurrence in the plankton. 
Alona costata Sars.—Average number, 11. Only a few scattered 
occurrences of small numbers. Earliest autumnal record, Novem- 
ber 22, at 40°; latest vernal, May 24, at 73°. 
