44 University of California Publications in Zoology t VoL - 22 



Since the genus shows very distinct response to flood conditions, 

 perhaps it is worth while to emphasize that point, especially in view 

 of the fact that there are no reliable water gauge or tide records 

 available. The rapid decline of numbers in March follows very closely 

 on the arrival of the heavy stream flow from the mountains. The 

 rapid rise in numbers in June is just as closely connected with the 

 disappearance of flood waters. 



Raphidium polymorphum Fres. 



Station I Station II Station III Daily Hourly 



Average 131,037 48,363 47,002 129,935 29,744 



Diagnosis inexact. Probably includes two or more species. Occur- 

 rence rare in winter months at all stations. Thrives best in Stockton 

 Channel (Station I). While the numbers are large at times the rec- 

 ords are so fragmentary, particularly in view of the enormous num- 

 bers escaping through the net, that no generalization can be made. 

 There is, however, some support for Kofoid's observation (1908) that 

 the organism has an optimum temperature above 15° C. Furthermore 

 the larger numbers in Stockton Channel indicate the benefits of sewage. 



Richtericlla botryoides Lemm. 



Station I Station II Station III Daily Hourly 

 Average 5,723 20,806 27,499 



Identification satisfactory. Occurrence at very irregular inter- 

 vals at all seasons of the year and at all stations. Rather large num- 

 bers at times. Records too scant to warrant conclusions. Percentage 

 captured evidently very small. 



Scenedesmus obliquus Kiitz. 



Station I Station II Station III Daily Hourly 



Average 319,244 43,593 63,234 305,638 166,826 



Diagnosis sometimes confused on account of apparent intergradat- 

 ions. May perhaps include two or three species. Rare in January 

 and February. Few in March, April, May and December at all sta- 

 tions. Maximum in June at Station I, although the numbers are best 

 sustained there through September and October. Other stations also 

 show greater constancy in that period. Vernal pulse very slight so far 

 as silk net can show. Average silk net catch in Illinois was given by 

 Kofoid (1908) as 673. The largest Stockton Channel catch is about 

 500 times as great. If the silk net only captures a fraction of 1 per 

 cent of the Scenedesmus present the numbers there were certainly 

 enormous. In spite of the losses through the net it is at least safe 

 to say that sewage and moderate temperature, 15° C. to 20° C. are 



