1920] Allen: Plankton of the San Joaquin River 75 



Gymnodinium sp. 



Hyalobryon sp. 



Mastigamoeba sp. Once, Station I, Station II and Hourly. 



Oikomonas sp. Once, Station I. 



Phacus longicauda Duj. Station I. 



Salpingoeca sp. 



Syncrypta sp. Once, Station II and III. 



Uroglena volvox. Once, Station III and Hourly. 



Rhizopoda 



■ Station I Station II Station III Daily Hourly 



Number of forms recorded 13 14 17 7 9 



Average number of indi- 

 viduals per cubic meter 167,328 150,817 288,365 101,802 396,950 



Rhizopoda, according to our records are from five to ten times 

 as numerous as in Illinois. This is probably erroneous, however, 

 since Difflugia contributes mainly to this showing and the Difflngia 

 count includes an unknown percentage of Tintinnidium. Conceding 

 this error to be very large, it still seems safe to say that the Rhizopoda 

 were at least as important as in Illinois where Kofoid (1908) gives 

 them a high rank because they are bottom living forms, actually 

 present in far greater numbers than the catch of floating forms could 

 possibly indicate. This adventitious character is just as strongly 

 marked here as there, since all the recorded forms are irregular in 

 distribution and erratic in occurrence. The group as a whole con- 

 tributes most during the warmer season. This might be expected as 

 a result of greater activity of the animals rendering them more liable 

 to dislodgment. Larger numbers produced would also mean larger 

 numbers dislodged. Heavy food supply would also account for it in 

 part because, as Kofoid (1908) says, it means greater oil and gas 

 production in the body of the animal thus reducing the specific 

 gravity. 



Discussion of Species 



Amoeba proteus Rosel. 



Station I Station II Station III Daily Hourly 



Average 2,782 14,487 3,645 206 61,034 



Identification uncertain. Probably includes several species. Amoeba 

 proteus was the large form most frequently observed and so the name 

 is used as being probably most frequently correct. Occurrence rather 

 irregular at all seasons of the year. All stations agree in showing the 

 largest numbers at about the height of the warm season, i.e., in Au- 

 gust at Stations I and II and September at Station III. All show 

 only very small numbers in cold weather. Larger numbers at Station 



