256 
where it ‘appears almost constantly in the plankton.”’ His investi- 
gations, however, appear to cover only the months of May—August. 
Maximum numbers appeared in July, and considerable differences 
were noted in two successive years. 
I find no previous record of the occurrence of Moina micrura in 
American waters. 
Pleuroxus denticulatus Birge.—Average number, 5. Occurs in 
small numbers and irregularly during the autumn and spring months 
during declining or rising temperatures. The earliest autumnal 
record is November 2, and the latest, December 15; the earliest 
vernal is March 8, and the latest is May 31. Egg-bearing females’ 
appear in the earlier occurrences in each season. It is evidently 
adventitious. 
Pleuroxus hamatus Birge was found once—March 29, 1898. 
Scapholeberts mucronata O. F. Mull. was recorded in small num- 
bers in May and August—December through the seasonal range of 
temperatures. It is apparently adventitious in channel plankton, 
though not attending flood invasions. 
Sida crystallina O. F. Mill. is rare in the summer plankton. 
Simocephalus serrulatus Koch.—Average number, 261. This 
species appears irregularly in the plankton, generally in small 
numbers and in isolated occurrences. An exception to this is found 
in May—June, 1898 (Table I.), when it is found continuously May 
10—June 14 in numbers which furnish 61 per cent. of the total for 
all years. There is a slight preponderance of occurrences in May 
and September, 12 of the 26 recorded appearing in these months. 
Their irregular appearance in the plankton in general suggests that 
they are adventitious from the littoral area, especially at times 
of their maximum development there. The period of their occur- 
rence in the channel plankton in 1898 was one of rising water, 10 to 
14 feet above low-water mark—a stage permitting free communica- 
tion between the channel and large areas of slightly submerged 
bottom-lands. . 
Simocephalus vetulus O. F. Mull. appeared irregularly and in 
small numbers in the plankton in April-June (4 occurrences) and 
September—December (5 occurrences). It is evidently adventitious 
in the plankton, coming from the littoral area, though not confined 
to flood waters. 
