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 sigmotides Sans is rare in shore collections below the 
plankton station. 
Candona reflexa 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. 
Cypridopsits 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.*; 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 
water in spring, 116,264—the highest average of any year; and in 



