1920] Allen: Plankton of the San Joaquin River 67 



Station II there were prominent pulses in January, February. 

 March, May, July and October, while at Station III the prominent 

 pulses came in February, March, April, July, August, September and 

 October. It is thus apparent that the combined history is considera- 

 bly different at the different stations and that the inclusion of the 

 Mastigophora destroys the coherence of the Protozoa as a group. The 

 totals are given both with and without the Mastigophora in tables 

 1 to 5 and the reader may suit himself with the list of his preference. 

 See also Plates 12-14. 



Mastigophora 



Station I Station II Station III Daily Hourly 



Number of forms recorded 33 30 31 20 20 



Ay. number per eu. meter.. 3,708,569 2,629,112 2,631,702 1,853,429 3,931,892 



Almost all of the Mastigophora were too small to be retained by 

 the silk net to any great extent. This is probably the main reason for 

 the fact that the averages recorded are exceeded from twenty-five to 

 forty times by Kofoid's averages (1908) in Illinois. It is certainly 

 true that other factors might be expected to operate, as mentioned 

 before in this paper, but this is so obviously sufficient in itself to 

 account for the difference that it seems useless to inquire further. 



Mastigophora were present at all stations throughout the year, 

 although the numbers were quite small for the first few catches. This 

 may be partly due to the fact that the net used before January 15 

 was of slightly larger mesh than the regular number 25 which was 

 ready by that date. By far the larger proportion of the flagellates 

 came in the last six months, even December showing much more than 

 June. There were, however, two or three strong pulses in January 

 and February at all stations. Hence the general indication seems to 

 be that quiet water or even stagnation is about as important as tem- 

 perature in controlling production. The greater numbers at Station 

 I also indicate that sewage is favorable to this group. The maximum 

 at Station I was reached in September, at Station II in October and 

 at Station III in November. These dates are largely due to Chromu- 

 lina sp., a form very unreliable for suggesting conclusions, both on 

 account of its small size and the uncertainty of identification. 



The recurrent pulses of Mastigophora were not so distinct as they 

 were in the case of the total chlorophyll bearing organisms. The 

 semi-weekly collections at Station I seem to obscure them rather than 

 make them more distinct. At any rate the intervals vary from eleven 

 to forty days in a rather indefinite way. Indications at the other 



