1920] Allen: Plankton of the San Joaquin River 45 



especially favorable to this form. There are recurrent pulses appar- 

 ently but the uncertainty as to the percentage of the population on 

 record makes all such points unreliable. 



Scenedesmus quadricauda Breb. 



Station I Station II Station III Daily Hourly 



Average _ 772,520 224,775 363,308 786,615 366,203 



Diagnosis sometimes confused by intergradations. Only a small 

 percentage in doubt. Losses through net certainly very heavy, prob- 

 ably over 99 per cent according to Kofoid's results (1908). Hence in- 

 ferences are to be made with caution as in case of the preceding 

 species However, the greater continuity of the record for this species 

 makes intelligent discussion possible. 



A vernal pulse in March and an autumnal pulse culminating in 

 October were quite well marked at Stations II and III. The vernal 

 pulse is not clear at Station I and the autumnal maximum was reached 

 in November. In no case was there very heavy representation until 

 about June. At Station I a distinct decline through July and August 

 suggests marked limitation by higher temperatures. Sewage water 

 with temperature between 15° C. and 20° C. is evidently nearly ideal 

 for Scenedesmus. 



There is rather distinct indication of monthly pulses at all three 

 stations (tables 1-3). These do not, however, correspond very closely 

 with Pediastrum pulses, so it is hardly worth while to attempt to 

 establish any connection with lunar cycles from present data. 



This species was rarely absent from any station, never from Sta- 

 tion I. Its appearance there in large numbers through the winter 

 months clearly marks it as perennial in this locality. While the records 

 of this and the preceding species are fragmentary so far as the whole 

 population of the genus is concerned they show very clearly that the 

 genus is of first rate importance in our waters through a large part of 

 the year and especially where there is much organic matter. 



Sckroederia setigera Lemm. 



Station I Station II Station III Daily Hourly 



Average 35,724 4,099 5,343 43,587 16,276 



Identification satisfactory. Probably only small percentage cap- 

 tured by the net. Only recorded six times at Station III and three 

 times at Station II. Irregular occurrence from last of April to end 

 of year at Station I. Well represented through June and part of 

 November. Not enough data to indicate more than a preference for 

 dilute sewage. 



