192 °] Allen: Plankton of the San Joaquin River 73 



steady increase to a maximum on August 31, followed by a three 

 weeks decline, after which there were three isolated records. Station 

 III showed no distinct maximum but the nine catches were distributed 

 over about the same period from July to mid-November. It seems 

 clear enough then that too much organic matter, and possibly too 

 great stagnation, is detrimental and that the optimum temperature 

 is above 20° C. 



Pleodorina illinoisensis Kofoid. 



Identification uncertain. Probably some loss through net. Re- 

 corded once at Station I, twice at Station II and four times at Station 

 III, all in small numbers. A puzzling thing about this form is that 

 when examining fresh catches in 1913 the writer was sure he found 

 it frequently and in considerable numbers. Hence he was somewhat 

 surprised at not finding it readily while counting. About the only 

 conclusion possible is that it is very hard to distinguish this form 

 from Eudorina elegans in preserved material. Chodat (1902) and 

 others would doubtless say that this fact supports their view that P. 

 illinoisensis is only a stage of development of Eudorina. 



Pteromonas sp. 



Identification probable. Loss through net heavy. Occurrence 

 irregular and rare. Recorded ten times at Station I between June 

 21 and October 8, three times at Station II and twice at Station III. 

 Apparently favored by sewage, stagnation and higher temperatures. 



Spondulomorum quatcrnarium Ehrbg. 



Identification certain. Loss through net heavy. Occurrence very 

 rare in small numbers. Recorded twice at Station I, once at Station II. 



Synura uvella Ehrbg. 



Station I Station II Station III Daily Hourly 



Average 6,415 30 4,462 5,222 



Identification uncertain. Colonies only counted. Loss through 

 net probably very heavy. Occurrence greatest at Station I but at 

 wide intervals there. Numbers recorded usually rather small. From 

 our records it seems to do best at about 20° C. Maximum at Station 

 I in August. Also evidently favored by sewage and stagnation, since 

 it is recorded only once at Station II and five times at Station III. 

 Certainly much less important than in Illinois, even with full allow- 

 ance for escape through the net. 



