100 Uwiversity of California Publications in Zoology [Vol.22 



concerned. In view of the enormous numbers at Station I it may be 

 safe to conclude that the food factor is more potent than temperature 

 and that the smaller Illinois numbers at lower temperatures were only 

 indirectly due to that condition. With abundant food in the sewage 

 the lower temperatures seem quite favorable here. This form may 

 then be regarded as very dependent upon sewage. The higher tem- 

 peratures and stagnating waters seem to be deterrent. 



Keratclla cocldearis Gosse. (Anuraea.) 



Station I Station II Station III Daily Hourly 



Average 78,769 281,504 354,304 164,517 6,530 



Identification usually certain. No attempt to distinguish varieties. 

 Some confusion probably of spineless varieties with spineless varieties 

 of A', quadratic/, or other species. The numbers were greater at all 

 stations than recorded for Illinois, but the records resemble Illinois 

 records in the fact that the organism was found at some station in 

 every month of the year. Also in the fact that there was a period of 

 regular occurrence in the first seven months of the year, separated by 

 a period of irregular occurrence or absence from a period of regular 

 occurrence in the last three months. The location of the maximum is 

 distinctly different from the Illinois condition at all stations. The 

 maximum there was early in May, while our records show a maximum 

 at Station I in July and at Stations II and III on November 1, the 

 last two being in remarkably large numbers. The inference from 

 our records is that sewage is detrimental in large amounts, that stag- 

 nation is even more so, and that temperature in moderate limits is 

 less important directly than are other factors. The optimum tem- 

 perature seems to be slightly below 20° C. The presence of largest 

 numbers at Station III indicates the probability that a larger amount 

 of organic matter than that in the river may be favorable. 



Recurrent pulses are distinguishable at all stations, about half of 

 those at Station I corresponding closely with those of chlorophyll 

 bearers. 



This was numerically the most important planktont at Stations II 

 and III. 



