150 



PULSES OF ANUR^EA COCHLEARIS. 



conditions seem thus to be found in the river at temperatures some- 

 what below the maximum, between 60 and 70. 



The phenomena of recurrent pulses are distinctly traceable in the 

 seasonal distribution of this species, not only in 1898 (Table I) but 

 also in preceding years. The large May and June pulses of 1898 

 appear on the declines of the spring and the June rise, respectively; 

 the pulse of September 27 is in a falling river; and that of October 

 25, on a slowly rising flood (Pt. I., PI. XII.). In 1897 (Pt. L, PL 

 XI.) the first two pulses attend the spring flood and June rise in like 

 manner, but the two subsequent pulses are in stable low water. In 

 1896 five of the seven pulses lie on the declines of the recurrent 

 floods of that year and two in rising waters (cf. PI. X. of Pt. I. and 

 the table just given). In 1894 and 1895 the pulses appear either 

 in falling water or in the earliest stages of the rise. The number of 

 pulses on declining waters is somewhat greater than the relative 

 number of days of this condition w 7 ould lead us to expect, and it 

 seems probable that optimum conditions for the appearance of 

 larger numbers of Anuraza cochlearis are to be found in such hydro- 

 graphic conditions. The run-off of impounded backwaters is one 

 of the favorable phases during flood decline. On the other hand. 



