1930] Allen: Plankton of the San Joaquin River 95 



The larger numbers at Station I indicate favorable influence of 

 sewage and the distinct limitation to temperatures above 20° C. and 

 mark B. caudatus as a sunuuer form. The maxima in late summer 

 may also indicate stagnation as a favorable factor. Kofoid's refer- 

 ence (1908) to the contention of various observers that spinous pro- 

 cesses, etc., appear as adaptive responses to lessened buoyancy of 

 warmer waters is especially interesting in this connection. The con- 

 dition of our records more strongly supports that view than do the 

 Illinois records. In fact the evidence could hardly be stronger with- 

 out deliberate manipulation. 



Brachionus budapestinensis D. 



Station I Station II Station III Daily Hourly 



Average 970 7,564 2,322 413 9,615 



Identification doubtful. Recorded six times at Station I, thir- 

 teen times at Station II and seven times at Station III. Occurrence 

 at all stations rather scattered and mainly from July to October. 

 Maximum at Station II in August. Apparently hindered by sewage 

 but favored by warmth and stagnation. Not a very important form 

 here, though the average at Station I is somewhat higher than it was 

 in Illinois. 



Brachionus calyciflorus Pallas. . (B. pala Ehrbg.) 



station I Station II Station III Daily Hourly 



Average 109,718 10,828 32,378 516 175,708 



Identification satisfactory. No attempt to distinguish varieties in 

 final records. The above count consists entirely of females exclusive 

 of eggs. The most striking features in the i-ecord of the occurrence of 

 this form are its great abundance in the first four or five months of 

 the year at all stations, its abrupt disappearance at the close of this 

 period and its reappearance in considerable numbers in and after 

 August. The three stations vary considerably in these last two 

 points, the break in the record in May being much more abrupt at 

 Station I than at either of the others. Station I also shows only a few 

 very light catches in the fall while Station III reaches the maximum 

 for the year at that time. Station II has the maximum in February 

 but shoM^s records of considerable numbers through September and 

 October. Station I has the maximum in March. The reasonable in- 

 ference seems to be that much sewage is favorable to this species in 

 flood water but that it is detrimental in stagnation. Also that tem- 

 peratures above 20° C. are rather unfavorable. 



