1920] Allen: Plankton of the San Joaquin River 97 



February 12 and March 5. Numbers always small at other stations, 

 otherwise distributed about the same. Hence this form may be said 

 to be definitely Hunted to temperatures below 20° C. Sewage appears 

 to be favorable. 



This species is not listed by Kofoid for Illinois. Mr. H. K. Harring 

 designates it particularly as a brackish water form. It is also so 

 listed in Susswasserfauna Deutschlands. However, its presence here 

 is not so prominent as is that of the brackish water diatom, Bacillaria 

 paradoxa. Some other factor than salinity must determine the occur- 

 rence of such forms. 



Brachionus urceus L. 



Station I Station II Station III Daily Hourly 



Average 139,352 8,558 2,871 352,237 36,606 



Identification usually satisfactory. Some confusion with other 

 forms at times. Recorded in every month of the year at Station I. 

 Missing in March and November at Station II, in April, November 

 and December at Station III. There was a well developed pulse in 

 February at Station I but there were only four catches from March 

 I to May 14 when a long period of regular appearance in considerable 

 numbers began, which finally ended on October 29. Numbers were 

 then small and absences frequent to the end of the year. Maximum 

 (899,232) reached twice in June. Records scattering at Station II 

 except in June, July and August. Maximum (105,792) on August 2. 

 Conditions similar at Station III with smaller numbers. Maximum 

 September 20, 38,400. This form shows a clear preference for sew- 

 age and for temperatures above 20° C, but stagnation is apparently 

 detrimental. Recurrent pulses are fairly distinct at Station I as 

 follows : 



Feb. 8 Julv 26 Sept. 27 



June 3 Aug. 9 Oct. 15 



June 21 Aug. 23 Oct. 29 



July 12 Sept. 9 Nov. 15 



Of the twelve dates just mentioned, eleven can be connected defi- 

 nitely with pulses of chlorophyll bearers, four preceding by from 

 four to eleven days, four following by three or four days and three 

 exactly coinciding. 



Brachionus with endoparasites. Average at Station I, 5,363. 



Different species of Brachionus were found infested with parasites 

 but no specific count was made. These parasites were rather small 

 and their relationships were not determined. In almost all cases they 



