42 University of California Publications in Zoology [Vol.22 



rious gradations and differences in detail make it seem probable, as 

 Kofoid (1908) found in Illinois, that at least two or three species 

 may at times be represented in the count. 



Present throughout the year at all stations. Missing irregularly 

 in almost every month at Station I, less often at the other two. Very 

 few in January and December. Distinctly more numerous at all 

 stations than in Illinois. Sewage water evidently less favorable for 

 its development. All stations show a March pulse of some moment 

 and another in September. There is some evidence of recurrent 

 pulses corresponding to lunar cycles, strongest at Station II. Maxima 

 in September at Stations I and II, October at Station III. The 

 most consistent record at Station I is in March and November, indi- 

 cating a preference in sewage water for temperatures from 13° C. 

 to 19° C. Since the only misses at Station II are in May, June 

 and December the same inference might be drawn as to temperature 

 if it were not for the fact that the representation was well sustained 

 through August to November. Somewhat similar conditions were 

 shown by Station III. At all stations the fluctuations were well 

 marked at all seasons. This was even more true of the daily series 

 at Station I (table 4) than of the regular series. The fluctuations of 

 the first half year may be mainly due to rise and fall of flood waters. 

 It is more difficult to account for those of the succeeding four months 

 under relatively stable conditions. 



Pediastrum duplex Meyen. 



Station I Station II Station III Daily Hourly 



Average 301,913 686,539 837,875 703,005 1,613,919 



Diagnosis inexact. All the coenobia with more or less bicornate 

 marginal cells showing distinct intercellular spaces were counted 

 under this head. The assemblage as a whole is fairly constant to a 

 provisional type and is quite possibly a single species. It was also 

 different from the preceding form in its larger numbers, greater con- 

 stancy of occurrence and greater regularity of development. Then 

 too, this species has a more uniform chronology at all stations. There 

 is a vernal pulse with maximum in March and an autumnal pulse 

 with maximum in October. The marked decline after March is evi- 

 dently chargeable in some way to flood conditions. The steady rise 

 from July through August and September to the October maximum 

 is just as evidently due to warmer temperature and greater stability. 

 Though P. duplex is present at all stations through the whole year 

 its optimum temperature is clearly near 20° C. The comparatively 



