48 University of California PubIiratio)i.'i iit Zoologi/ |Voi,. 22 



While the records of Station I might lead one to think there pos- 

 sibly were reproductive cycles and successive rest periods as suff^ested 

 by Kofoid (1908) the records for Stations II and III point rather the 

 other way. The strong and somewhat rapid rise through June and 

 July to a maximum in August with a similar decline to the winter 

 level suggests, indeed, that factors of the immediate external environ- 

 ment are responsible and that increase in numbers would continue 

 indefinitely if the proper balance of temperature, food materials and 

 natural enemies could be secured along with sufficient removal of 

 injurious accumulations. 



The number of forms of diatoms listed (98) may seem unwar- 

 ranted inasmuch as there could be no hope of accurate determination 

 of all the species present. The large list wa.s the cumulative result of 

 an effort to give expression to differences noted. The futility of it was 

 not realized until too late to change it easily. After all, it happens 

 that there are only a few forms found frequently and in great num- 

 bers and most of these can be approximately determined. 



Twenty forms occur at one or more stations with sufficient con- 

 tinuity to give the impression that they are true planktonts. These 

 are Asterionella gracillima, Bacillaria paradoxa, Cyclotella spp. Cyma- 

 topldtra sfllea, Cymbelki. affiiiis, Cymbella cymhifornm, Cynibclla 

 tumida, Fragillaria capucina, Gyrosigma kiltzingii, Gyrosigma scal- 

 proides, Melosira granulata, Melosira varians, Navicula alpestris, 

 Navicula hacillnm, Navicula gracilis, Nitzschia aciciilaris, Pleuros- 

 tauron parvnlum, Snrirella spp., Synedra radians, and Synrdra ulna. 

 Distinctly the most important of these are Asfcnanella, Bacillaria, 

 Cyclotella, Melosira and Synedra, all of which are satisfactorily de- 

 termined as to genus, though the species are sometimes uncertain. 

 Most of the other genera just mentioned are also believed to be cor- 

 rectly designated. Much of the specific determination is largely guess 

 work for the inexperienced observer under the conditions of counting. 

 Fortunately such errors do not materially affect the generalizations 

 within reach of this present study. Schonfeldt's "Bacillariales" in 

 the Susswasserflora Deutschlands series was the main dependence for 

 identification. 



