1920] Allen: Plankton of the San Joaquin River 77 



Difflugia corona Wallich. 



Identification uncertain. Recorded three times at Station I, six 

 times at Station II and twice at Station III, always in small numbers. 



Difflugia piriformis Py. 



Station! Station II Station III Daily Hourly 



Average 76,G92 51,957 43,846 71,508 28,483 



Identification uncertain. Surely includes several other species 

 certainly includes some Codonella and Tiutinnidium which could not 

 be readily distinguished while counting. This was especially true up 

 to August before the presence of Tintinnidium and Codonella was 

 noticed. In view of this error the record for Difflugia cannot be re- 

 garded as trustworthy. The variations in form are bewildering at 

 best. As the record stands, the maximum seems to fall in June at all 

 stations and Station I shows the largest numbers. 



Hyalodiseus sp. r „ . ttt „ ., _ , 



J l Station I Station II Station III Daily Hourly 



Average 26,286 15,566 22,818 144,447 



Identification very doubtful. This form was not recorded until 

 late in the season because it was small and hard to diagnose. It was 

 finally decided to record it simply for the purpose of calling atten- 

 tion to considerable numbers of an organism frequently occurring 

 which could not be definitely designated. The condition of the rec- 

 ords does not warrant conclusions. 



Microgromia socialis H. and L. 



Station I Station II Station III Daily Hourly 



Average 23,280 56,487 72,004 15,976 61,034 



Identification uncertain. Probably heavy loss through net. Oc- 

 currence at Station I in every month except January and December. 

 Found mainly at the other stations in July, August, September and 

 October. Evidently favored by warm weather and deterred by 

 sewage. 



The following forms were recorded only once or twice, or were 

 thought to be present in fresh material : 



Amoeba verrucosa Ehrbg. Once, II and III. 



Difflugia acuminata Ehrbg. Once, II and III. 



Difflugia globulosa Duj. Once, I and twice at III. 



Euglypha alveolata Duj. 



Hyalosphenia cuneata St. Twice, I. 



Hyalosphenia papilio Leidy. Twice, I, once, II and III. 



Nebela collaris Leidy. Once, III. 



Pseudodifflugia gracilis Schlumbg. Once, III. 



Quadrula symmetrica F. E. Sch. Once, III and daily. 



Trinema enchelys Ehrbg. Once, III. 



