1920] Allen: Plankton of the San Joaquin River 81 



Didinium nasutum. 



Average at Station I, 1,636. 



Identification positive. Recorded sixteen times at Station I at 

 irregular intervals January to April and in November. Only once 

 at Station II and twice at Station III. Distinctly a cold water form. 

 Probably heavy loss through net. 



Euplotes patella Ehrbg. 



Station I Station II Station III Daily Hourly 



Average 38,100 7,350 8,068 



Identification of species fairly certain. Recorded in first three and 

 four months and the last two montlis at all stations. Far more numer- 

 ous at Station I. Distinctly favored by sewage and low temperatures. 

 Spring maximum on March 19 at Station I at 14.5° C. Fall maxi- 

 mum on November 12, at 17.5° C. Disappearance in April at 20° C, 

 reappearance in October at 19° C. 



Euplotes sp. 



Average at Station I, 3,567. 



Recorded only twelve times at Station I (February 8-April 5), 

 twice at Station II and three times at Station III. Showed same 

 tendencies of distribution as preceding species. Probably variety of 

 same. 



Halteria grandineUa O. F. Mull. 



Station I Station II Station III Daily Hourly 



Average 3,390 126 206 12,207 



Identification probable. Loss through net heavy. Recorded sixteen 

 times in first three months and five times in the last three months at 

 Station I, only five times at Station III. Apparently adventitious 

 and not of much consequence here. 



Hastatella radians Erlanger. 



Station I Station II Station III Daily Hourly 



Average 2,245 32 96 



Identification positive. Probably very small percentage retained 

 by net. Recorded fourteen times at Station I, once at Station II and 

 twice at Station III, in February, March, April and December. Dis- 

 tinctly a cold water form and almost as distinctly favored by sewage. 

 Notwithstanding its small size this form is so distinct in appearance 

 and so little deformed by preservation that the count is unusually 

 trustworthy. 



