1920] Allen: Plankton of the San Joaquin River 37 



thrughout the year at all stations. Much more abundant in July, 

 August and September. Loss through net certainly very great. An 

 important member of the plankton, but net catches do not afford a 

 good basis for discussion of its distribution. 



Nostoc spp. 



Station I Station II Station III Daily Hourly 



Average 156,435 301,283 459,306 547,678 1,794,781 



All the plants included under this head were filaments of the 

 contorted type. The count gives the number of filaments in some 

 cases though usually fragments of colonies constituted the units. 

 Whole colonies were rarely, if ever, found. Possibly three or four 

 species are included in this enumeration. Nostoc appeared occasion- 

 ally throughout the year and became quite prominent in July, Au- 

 gust and September. In view of the fragmentary condition of the 

 colonies it is probable that the loss through the net was considerable. 



Oscillatoria formosa Bory. 



Station I Station II Station III Daily Hourly 



Average 2,037 45,699 2,205 



Identification uncertain. This form occurred more often at Sta- 

 tion II where it was recorded frequently throughout the year, reach- 

 ing its maximum in June. It does not seem to be very important 

 since its size makes it improbable that loss through the net was very 

 great. 



Oscillatoria spp. „ „ „ . TTT „ ., TT , 



*^ Station I Station II Station III Daily Hourly 



Average 210,437 32,323 49,941 299,126 105,792 



Probably three or four species are included under this heading. 

 Found occasionally throughout the year but distinctly a summer 

 form. Maximum in August, Oscillatoria of all kinds were nearly 

 always found in single filaments or fragments of filaments. Masses 

 of filaments were rarely seen. This might be considered as support- 

 ing Kofoid's suggestion (1908) that physiological conditions may at 

 times make Oscillatoria a temporary planktont. On the other hand 

 it does not furnish very definite proof against the view that this form 

 is an adventitious planktont, cast adrift by gas bubbles or violent 

 currents. 



Oscillatoria tenuis Agardh. 



Station I Station II Station III Daily Hourly 



Average 46,340 2,404 3,040 154,363 12,207 



Identification uncertain. Not a very prominent form. General 

 occurrence much the same as for the above mentioned species. 



