(2 
ing it as a variety of that species. It occurs most frequently in 
August and September, and is apparently a warm-water planktont. 
Ceratium cornutum Ehrbg. was found but once—in June, 1896. 
Ceratium hirundinella O. F. Mull. was not noted in our plank- 
ton in 1898, but in 1896 was found from June to October, with a 
pulse of 19,200 on June 6. It was recorded only at temperatures 
above 57°, and is apparently a warm-water planktont. It has but 
an insignificant part in the potamoplankton of the Illinois River 
and its backwaters, though quite abundant in the summer plank- 
ton of Lake Michigan (Kofoid, ’95). It seems not to have survived 
the transit through the sewage-laden waters of Chicago River or to 
thrive in the conditions prevailing in the Illinois River, though 
common generally in fresh-water plankton of the temperate zone. 
Chilomonas paramecium Ehrbg.*—Average number, 555,000. 
This flagellate, which is frequently abundant in aquaria or stag- 
nant water, appears also in the plankton of. the Illinois River. 
There is in 1898 a vernal pulse, culminating at 10,800,000 on April 
26, and there are scattered records from October. to February. 
Chloraster gyrans Ehrbg.—This rare and unique flagellate was 
found in but two collections—in July and August, 1898—and only 
in small numbers. 
Chloropeltis monilata Stokes.*—Average number, 362,941 in 
1897. This is a summer planktont, appearing at irregular inter- 
vals from the last of May until the middle of September. It was 
not found in 1898. A maximum of 10,800,000 appears on August 
Silt 
Colacium caluum Stein.—The attached stage only of this flagel- 
late was observed, and was recorded only in 1896and 1897. It appears 
from the middle of April to the first of October, and is usually 
found upon Polyarthra platyptera. It has occurred occasionally 
upon several species of Brachionus and upon Chydorus sphericus. 
The largest number recorded (162,792) appeared on April 17, 1896, 
upon Polyarthra, usually upon the body and more rarely upon the 
oar-like appendages. It is often exceedingly abundant upon the 
planktonts of backwater ponds. 
Colacium vesiculosum Ehrbg.—This species is much less abun- 
dant than the preceding species in our waters, and was found only 
in June and September, upon Cyclops albidus and Polyarthra. 
