1920] AUrn: Plankton of the San Joaquin River 91 



(Kofoid 1908). Inasinuch, however, as ten out of seventeen pulses 

 came near to those of chlorophj^ll bearers it may be fair to assume 

 that the two groups are in some way interdependent or tliat the gen- 

 eral conditions favoring one likewise favor the other. The relation- 

 ships at the other two stations were still less definite and it was not 

 considered worthwhile to transcribe the tables of pulses of Plohna 

 there. 



Our records are at variance with the Illinois records in showing 

 more pulses of Ploima to precede pulses of chlorophyll bearers tlian 

 to follow. The discrepancy is probably due to the errors incident to 

 escape of chlorophyll organisms through the net. But it might be 

 due to a difference in the numerically dominant forms in the two re- 

 gions or to some similar factor. The problem of difference cannot be 

 solved, apparently, from the 1913 records, but after all there is suf- 

 ficient likeness to warrant the conclusion that the two groups are 

 closely inter-related, here as there. The daily record was too short 

 to help in a decision on this matter. 



Discussion of Species 

 Anuraeopsis fissa Gosse. 



.station I Station II .Station III Daily Hourly 



Average 10,2.51 8,411 18,507 2,477 40,066 



Average eggs attached 1,786 .30 3,078 98.5 



Identification frequently satisfactory, more often uncertain. Prob- 

 ably some loss through net. Not recorded anywhere during first half 

 year. Appeared at Stations I and II about mid-July, at Station III 

 in August. Maximum at Station I in November, at Station II in 

 August and at Station III in September. Occurrence frequent at 

 Stations I and III in August, September, October and November, dis- 

 appearing in December. Occurrence at Station II rare except in 

 August. Apparently a form favored by warm and quiet water. Larger 

 numbers at Station III suggest a preference for moderate quantities 

 of organic matter in surrounding water. 



All stations re.semble Illinois in showing a distinct limitation of 

 this form to a four months period after midsummer. It was recorded 

 in June there and di.sappeared in early November. A single Decem- 

 ber record here at Station II indicates the possibility of occurrence 

 in very small numbers at other times. The pulses came mainly at 

 temperatures above 20° C, the single exception on November 22 pos- 

 sibly being due to confusion in counting. 



