76 University of California Publications in Zoology [Vol.22 



II may be due not so much to favorable effect of clean water as to the 

 slight flow in the river tending to greater dislodgment. 



Amoeba radiosa Duj. 



Station I Station II Station III Daily Hourly 



Average 18,202 11,528 17,852 206 12,209 



Identification satisfactory. Losses through net probably great. 

 Although Kofoid (1908) counts A. radiosa with A. proteus, it is given 

 separate listing here because it was the one form that seemed to be 

 always clearly recognizable under conditions of counting. Either 

 siugly or combined the two forms were decidedly more numerous than 

 the combination was in Illinois, and this is true of all stations. A. rad- 

 iosa was not recorded at any station until the last of April. It devel- 

 oped a small pulse at Station I in May, disappeared entirely in July 

 and August but reappeared in considerable strength in September. 

 The maximum came in a strong abrupt pulse in late October after 

 which the form disappeared again showing only two small records in 

 the rest of the year. At Station II there were two occurrences in April 

 and May, one (the maximum) in September and a small series of four 

 records in October and November. At Station III there were scat- 

 tered records from July to a September maximum, a small group in 

 October and November and one catch in December. In spite of con- 

 siderable differences at the three stations the record shows a rather 

 definite preference for water a little below the maximum temperature. 

 The larger numbers at Station I indicate a preference for sewage. 

 An almost equal number at Station III warns against too much con- 

 fidence in such a conclusion, however. 



Arcella vulgaris Ehrbg. 



Station I Station II Station III Daily Hourly 

 Average 238 523 1,790 103 



Identification of species uncertain. Probably includes three or 

 four species. Certainly there were marked differences in shells found. 

 Not very important in our plankton so far as our records show. Oc- 

 curred only seven times at widely separated intervals at Station I, 

 four times in April and June at Station II, and six times at wide 

 intervals at Station III. Numbers always rather small. Is known 

 to be very abundant in ditches near Stockton. Clearly adventitious. 



Cyphoderia ampulla Leidy. 



Identification probable. Recorded five times at Station II, and 

 four times at Station III in rather small numbers and at wide inter- 

 vals. Clearly adventitious. 



