1920] Allen: Plankton of the San Joaquin River 71 



definitely distinguished before. Very few at any station after mid- 

 October. Far more numerous at Station I than at either of the others. 

 Certainly favored by temperatures near or above 20° C. by quiet 

 water and by sewage. 



Mallomonas sp. 



Identification of genus certain. Recorded three times at Stations 

 I and III in small numbers. 



Pandorina morum Bory. 



Station I Station II Station III Daily Hourly 



Average 2,752 16,804 15,457 103 L02,150 



Identification satisfactory in most cases. May sometimes include 

 young colonies of Eudorina or Pleodorina. Probably some loss through 

 net. Occurrence at Station I frequent in small numbers through first 

 three months, after that at irregular intervals usually several weeks 

 apart. Fewest in warmer months. Maximum on March 26. At Sta- 

 tion II most constant occurrence was through February and March in 

 moderate numbers. Maximum of 105,792 reached in each of the four 

 months of June, July, August and September. At Station III fairly 

 constant in February and March, November and December. Records 

 grouped at irregular intervals between. Maximum in May. Although 

 the records differ strangely at the three stations it seems fair to infer 

 that sewage is deterrent, especially during stagnation but that warmer 

 waters are favorable if stagnation could be avoided. It is possible, 

 however, that the organism was not captured so constantly in warmer 

 waters because those were the quiet waters, hence there were no cur- 

 rents to mix it through the whole body of water. If this be true it is 

 easy to see that the net might at one time traverse a "swarm" of 

 the organisms but miss it entirely at another time. 



Peridinium cinctum Ehrbg. 



Station I Station II Station III Daily Hourly 



Averag e 34,262 5,209 18,732 111,324 12,207 



Identification uncertain. Probably very small percentage retained 

 by net. Occurrence rare in first half year at all stations. Recorded 

 only six times at Station II and seven times at Station III. Evidently 

 favored by sewage, since it occurred frequently in rather large num- 

 bers at Station I from late June to late November. Maximum appar- 

 ently in July with most consistent record in September. Evidently 

 favored also by temperature of 20° C. or over, and by quiet water. 



