261 af 
The following forms are of numerical importance in the order 
named: C. bicuspidatus, young Diaptomus, Cyclops edax, Diaptomus 
siciloides, D. pallidus, Canthocamptus spp., and Cyclops albidus. 
Cyclops prasinus, C. modestus, C. phaleratus, and C. serrulatus are 
also found, but in such small numbers as to be of no quantita- 
tive consequence. 
DISCUSSION OF SPECIES OF COPEPODA. 
Argulus sp.—A small and apparently young argulid was found 
in the plankton on August 10, 1897. Members of this genus are 
abundant upon Amza calva and both species of Lepisosteus, all very 
common fish in channel waters. 
Canthocamptus spp., including C. allinotsensis Forbes.—Average 
number, 78. Canthocamptus was found in the plankton in every 
month of the year but June. The percentage of collections contain- 
ing Canthocamptus is greatest (44 to 63 per cent.) in March—May and 
November, and the numbers per m.* are highest in March—May, 
when females, females with eggs, and nauplii all occur in their 
maximum numbers. All records of totals in excess of 400 fall in 
this vernal period with the single exception of one collection in 
August, 1897. The largest number, 3,058 per m.°, was found 
April 29, 1896. 
Canthocamptus occurs throughout the whole seasonal range in 
temperatures, with smallest numbers and least regularity during 
maximum summer heat in June-August. It is thus a planktont 
of the colder rather than the warmer part of the year. 
The relations which hydrographic conditions bear to the occur- 
rence of Canthocamptus in the plankton may be inferred from the 
fact that of the 48 records in 1894-1899, 24 were made in rising 
flood waters, 14 in falling flood stages within a few days after the 
culmination of the rise, and but 10 in stable conditions or in declining 
levels when flood waters of recent origin did not fill the channel. 
From these facts it seems probable that Canthocamptus is in the 
main adventitious in the plankton from its normal habitat in the 
slime at the bottom and margins of the river and its backwaters. 
Over 88 per cent. of the total Canthocamptus recorded in the 
plankton consists of nauplii. It may be that—as is the case with the 
young Ostracoda—they enter the area of the plankton more readily 
than the adults. Adults were found in the plankton only in 
