1920] Allen: Plankton of the San Joaquin River 109 



Copepoda 



Station I Station II Station III Daily Hourly 



Forms recorded 4 3 3 4 3 



Individuals per eu. meter.. 536,312 14,882 14,549 1,488,421 189,4(59 



Copepoda were recorded in every month of the year at Station I, 

 and they were entirely missing only at Stations II and III in Decem- 

 ber and November, respectively. Numbers were small at all stations 

 through the first three months and in the last month. At Stations II 

 and III they only reached 100,000 in three catches in September at 

 the former, and one catch at the latter. There was no increase in num- 

 bers at either place until May and the decline was very rapid after 

 September. On the contrary, Station I showed very marked and 

 steady increase in numbers after March and the decline after Sep- 

 tember was gradual, though starting abruptly with the close of the 

 month. The maximum at Station I came in August according to the 

 record but it was so nearly eqalled in September that a recount might 

 show it really to be in that month. However, this does not affect the 

 general conclusion that the warmest months are most favorable, the 

 culmination coming with approaching stagnation. The record also 

 clearly shows that the dilute sewage of Station I was distinctly favor- 

 able to this group. 



Specific identifications were not attempted and there was certainly 

 some confusion of forms during the count. Some of these errors could 

 be corrected by a recount, but in the writer's judgment there would 

 not be enough advantage to warrant the great effort involved. 



Dr. C. D. Marsh, of the United States Bureau of Plant Industry, 

 very kindly identified a few forms from a very limited amount of 

 material sent to him. He noted the presence of Cyclops americanus 

 Marsh, Cyclops prasinus Fischer, and Cyclops albidus Jurine. No 

 other Copepoda were found in the samples sent to him but the writer 

 is certain that some other forms occurred at times in limited numbers. 

 Since Cyclops completely dominated the other genera in numbers, dis- 

 cussion of seasonal distribution will be deferred till discussion of 

 that group is reached. 



Discussion op Genera 



Canthocamptus sp. 



Identification fairly certain. Recorded only twice at Station I, 

 thrice at Station II and five times at Station III in small numbers and 

 at wide intervals, but mainly in spring and fall. This genus has been 



