40 University of California Publications in Zoology [Vol. 22 



Thirty forms were recorded in the count and it was thought that 

 several others were recognized in the living material. Certainly there 

 were more species present. So far as net catches indicate, Pediastrum 

 ■was the leader both numerically and volumetrically, at Stations II 

 and III. At Station I it was not far different from Scenedesmus in 

 numbers (coenobia counted) and of course, exceeded it in volume of 

 catch. Scenedesmus was clearly second in importance, Actinastrum 

 third and Coelastrum fourth. Crucigenia, Raphidium, Richteriella 

 and Schroederia were frequently found. In view of the common oc- 

 currence of Botryococcus in other places its scarcity here needs ex- 

 planation. Failure to identify seems to be the most probable reason, 

 though it may actually have been absent usually. 



Present methods do not show the dilute sewage water of Stock- 

 ton Channel to be much more productive of Chlorophyceae than the 

 river. Hence such evidence as we get from this study only weakly 

 supports Kofoid's suggestion (1908) that sewage laden waters favor 

 the increase in numbers of the group. At any rate there is clear 

 indication that the Chlorophyceae contribute largely to the plankton 

 at all stations. 



Discussion of Species 

 Actinastrum hantzschii Lagerh. 



Station I Station II Station III Daily Hourly 



Average 51,050 216,851 257,954 36,939 392,439 



Identification satisfactory. The combined averages of two varie- 

 ties of this species are given here. They are recorded separately in 

 tables 1 to 5. The only difference noted was in size, the one recorded 

 as ' ' large ' ' being from two to four times as large as the typical form 

 measured by length of the cell. As might be expected, it is largely 

 responsible for the enormous average here as compared with the 

 silk net average (338) in Illinois. Without it, however, the average 

 is much greater, thus indicating a distinctly greater prevalence of 

 the species here. It will be noticed too that Stockton Channel with 

 its dilute sewage shows only about one-fourth of the average num- 

 bers produced by the other stations. The typical form was found 

 occasionally at all stations throughout the year and it also reached 

 its greatest abundance at the same three periods at all stations, i.e., 

 March, June and September, the last showing the maximum. The 

 large variety came in late (April and May), produced a weak pulse 

 in July, and strong pulses in August and October; it dropped out 

 again in November. Apparently temperature affected it much more 



