72 



ing it as a variety of that species. It occurs most frequently in 

 August and September, and is apparently a warm-water planktont. 



Ceratium cornutwn 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 paramcecium 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 

 31. 



Colacium calvum Stein. The attached stage only of this flagel- 

 late was observed , and was recorded only in 1 8 9 6 and 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 sphczricus. 

 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. 



