1920] Allen: Plankton of the San Joaquin River 41 



definitely than it did the typical form. Temperature is not the de- 

 ciding factor, however, as is shown by the fact that this variety ap- 

 peared last in the warmer water of Stockton Channel whence it also 

 practically disappeared first. Its lower temperature limit seems to 

 be about 14° C. Stockton Channel was distinctly less favorable to 

 the large form than to the typical form as is shown by the difference 

 in numbers being greater there than at the other two stations (tables 

 1 to 3). Gradations in size between the two forms sometimes made sep- 

 aration difficult. This species is evidently of considerable import- 

 ance here. 



Coelastrum microporum Naeg. 



Station I Station II Station III Daily Hourly 



Average 190,313 31,595 60,819 143,454 107,761 



Identification satisfactory. Possibly includes at least two other 

 species. Coenobia counted. Was not found at Stations II and III 

 until July, and only five times before that at Station I. Maximum 

 in September at all stations. Last appearance on December 14 at 

 Station I, a month later than at the other places. Obviously favored 

 by the slightly higher temperature of Stockton Channel. The dif- 

 ference in averages also suggests a distinct preference for sewage 

 water. The maximum in September, when there was little disturb- 

 ance of the waters, may indicate great susceptibility to action of 

 strong currents and to rising and falling flood waters with their rapid 

 changes in temperature. The September pulse is the only distinct 

 one. In view of the enormous losses through the net, Coelastrum must 

 be reckoned as an important planktont, though it is mainly limited 

 to temperatures above 15° C. 



Crucigenia lauterbornii Schmidle. 



Station I Station II Station III Daily Hourly 

 Average : 



Identification fairly satisfactory. Recorded once from Station 



II, four times from Station III. Average of colonies for the year at 



Station III, 14,771. Clearly too small for accurate study by net 



methods. Occurrence in September and October. 



Pcdiastrum boryanum Menegh. 



Station I Station II Station III Daily Hourly 



Average 9,913 41,265 72,153 12,612 37,474 



Identification inexact. Coenobia with bicornate marginal cells 



but without intercellular spaces were counted under this head. Va- 



