1920] Allen: Plankton of the San Joaquin River 103 



to December. Numbers always small. Maximum, 16,000, April 5. 

 Recorded nine times at Station III, seven times from January 23 to 

 April 19 and twice in November. Numbers always small but con- 

 siderably larger in November than at other times. It is clear from 

 these records that this form is intolerant of sewage and of summer 

 conditions, probably including temperatures, though the monthly 

 occurrence at Station II may indicate the influence of a food factor. 

 At any rate the optimum temperature seems to be below 15° C. The 

 seasonal distribution at our Station III most nearly resembles that 

 noted for Illinois. 



• Polyarthra trigla Ehrbg. (P. platyptera.) 



Station I Station II Station III Daily Hourly 



Average 410,770 35,241 58,0.37 662,712 1,267,469 



Identification certain. Recorded in every month at all stations. 

 Occurrence at Station 1 very constant after January 12, only two 

 absences, both in November. Maximum on May 7, culminating a grad- 

 ual increase from first appearance, numbers well sustained after that 

 except for a drop in June, another in November and the final decline 

 in December. Continuous record at Station II except for two misses 

 in June, one in August, one in October and three in November. Maxi- 

 mum on September 6 in a well defined pulse. Record at Station III 

 began on January 19, after which there was one miss in March, one 

 in June, one in October and three in November. Maxinuim on Sep- 

 tember 20 in a minor pulse. The character of the record at Station I 

 suggests the idea that temperatures may vary widely without appre- 

 ciable influence except as they approach the lower recorded limits. 

 Even here the influence may be througli the food supply instead of 

 direct. Stagnation appears to be slightly favorable. This is next to 

 the most important species uumericall.y, of rotifers at Station I. 



Recurrent pulses are prominent at all stations as follows : 



station I Station II Station III 



Jan. 1.^ July 30 Feb. 8 Feb. 8 



Jan. 29 Aug. 9 March 8 Apr 26 



Feb. 8 Aug. 20 March 29 May 31 



Feb. 23 Sept. 13 May 10 July 12 



Mar. 8 Oct. 18 May 24 Aug. 2 



Apr. 13 Nov. 1 Juue 28 Aug 31 



Apr. 26 Nov. 19 Sept. 6 Sept 20 



May 7 Nov. 30 Oct. IS Oct. 18 



June 3 Dee. 14 Pec. 14 Dec. 6 



June 25 Dee. 31 

 July 12 



These pulses at Station I are even more distinctly marked than 

 in Illinois, which of course might be expected from the very large 



