1920] Allen: Plankton of the San Joaquin River 105 



could not be accurately distinguished in preserved material. 8. tr< in- 

 ula,, S. stylata and S. pectinata were probably most often present. Al- 

 though the assemblage was recorded at Station I in every month of 

 the year, the catches were few and small during the first three months 

 and in July, August, September and October. The maximum occurred 

 on June 25 in a strongly developed pulse. The occurrence at Station 

 II was somewhat similar except that there were no large numbers till 

 July when there was a fairly strong pulse. The maximum came, how- 

 ever, in the larger November pulse. At Station III the numbers were 

 more evenly distributed from March to December, August being the 

 only later month with very small numbers. The November pulse was 

 largest on the whole but the maximum came on May 31 in a catch 

 following a lapse and preceding two lapses. Such irregular records, 

 in addition to the difficult identification make it necessary to be cau- 

 tious as to conclusions. Still it seems clear that sewage is beneficial 

 and that stagnation with high temperature is harmful. 



Trichocerca capucina W. & C. (Rattulus capucinus.) 



Station I Station II Station III Daily Hourly 



Average 248 4,777 1,004 413 9,123 



Identification uncertain. Genus probably correct in all cases. 

 Recorded only four times at Station I in small numbers and at wide 

 intervals ; seven times at Station II, once in fairly large numbers ; 

 eight times at Station III, all in small numbers. Unimportant numeri- 

 cally in our plankton. Probably adventitious. 



Trichocerca iernis Gosse. (Hal lulus gracilis.) 



Station I Station II Station III Daily Hourly 



Average 13,097 21,577 46,414 88,913 285,187 



Identification uncertain. May include other species. Not recorded 

 at Station I till May 27 and not appearing regularly till July :3. 

 Fairly constant through July and August and for a time in Septem- 

 ber, and October. Maximum in July. Occurrence at Station II 

 almost limited to July, August and September with maximum in Sep- 

 tember. Conditions similar at Station III with a slightly longer 

 period of regular occurrence. Maximum in September. Evidently 

 favored by higher temperatures and quiet waters. Larger numbers 

 at Station III than in river suggest that more organic matter in the 

 water is helpful, though the organism does not do so well in the dilute 

 sewage of Station I. 



