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. upicata 

 and kahlbergiensis, D. hyalina, Ceriodaphnia scitula, Chydorus 

 sphcericus, 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 a~ffinis, Ceriodaphnia 

 reticulata and C. rotunda, Scapholeberis 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. 



