258 



which follow, are drawn in the main from Sharpe ('97), to whom I 

 am also indebted for assistance in identifications which I have 

 made. A few supplementary notes are based on my plankton 

 .records. 



DISCUSSION OF SPECIES OF OSTRACODA. 



Candona sigmoides Sharpe. is rare in shore collections below the 

 plankton station. 



Candona reftexa Sharpe was taken but once in the river on 

 November 11. 



Candona simpsoni Sharpe appears commonly in April-May, and 

 again, in smaller number, in October-November in shore collections 

 on the west side of the river at the plankton station. It is occa- 

 sionally adventitious in the plankton at these seasons. 



Cypria exsculpta Fischer appears rarely in the channel plankton 

 and in shore collections in April-October. 



Cypria ophthalmica Jurine is found frequently in the plankton 

 throughout the year, but more abundantly in May-September, and 

 especially in late summer and early autumn. 



Cypria pustulosa Sharpe was taken rarely in channel plankton 

 in July and September. 



Cypridopsis vidua O. F. Mull, was perennial in the plankton, 

 though present in greater numbers in May-October. It is the 

 commonest of the Ostracoda in the plankton, and it seems probable 

 that many, though not all, of the young and immature forms belong 

 to this species. 



Limnicythere illinoisensis Sharpe was taken in the plankton in 

 March, August, and November in 1898, in two instances in flood 

 waters. 



COPEPODA. 



This is the most abundantly represented order of the Entomos- 

 traca in channel plankton. Though the species number but 12 to 

 the 25 Cladocera, the individuals among the Copepoda outnumber 

 the Cladocera over fivefold in the grand totals, the ratio varying in 

 individual years from twofold in 1894 to almost sevenfold in 1898. 



The average number in 1898 was 40,608 per m. 3 ; in 1897, in 

 more stable conditions, 80,632; in 1896, a year of recurrent floods, 

 43,764 approximately the number in 1898; in 1895, a year of low 

 w r ater in spring, 116,264 the highest average of any year; and in 



