271 



of collections containing brevispinosus in these months being 27, 

 80,62,67,48,75, and 5 9 per cent , respectively , while in other months 

 it does not rise above 20 per cent. The number of individuals is 

 also greater during the warmer season. No record between October 

 15 and April 20 exceeds 200 per m. 3 , while between April 20 and 

 October 15 the pulses often culminate at 3,000-5,000 per m. 3 , and 

 over 98 per cent, of the total individuals were recorded. 



This variety appears throughout the whole seasonal range of 

 temperatures from summer's maximum to winter's minimum, but 

 predominantly during the warmer season. Only 15 of the 71 

 occurrences and 2 per cent, of the individuals w r ere recorded at 

 temperatures below 60. As soon as the vernal rise in temperatures 

 passes 50-60, the minimum numbers and scattered occurrences of 

 the winter months give way to a vernal pulse of considerable mag- 

 nitude in April-May, attaining 4,452 on April 25, 1895, and 4,960 

 on May 25, 1897, but only 2,600 on June 7, 1898. This is followed 

 by a period of depression in July, when the summits of the pulses 

 did not often surpass 1,000 per m. 3 In the late summer and autumn 

 of 1895 and 1897, and to a less extent in 1896 .and 1898, a second 

 period of maximum pulses appears, attaining 9,711 September 12, 

 1895, and 4,800 October 5, 1898. When temperatures decline in 

 September-October below 50, this variety falls at once to minimum 

 numbers. 



The records of brevispinosus in channel plankton exhibit some- 

 what clearly the phenomenon of recurrent pulses whenever collec- 

 tions at brief intervals make it possible to delimit the pulses. Thus, 

 in 1895 there are pulses culminating in July, August, September, and 

 October; in 1896, in April, May, June, July, August, and September; 

 in 1898, in July, August, and October; but in 1898 (Table I.) the 

 numbers are too small to exhibit fully the phenomenon of recurrent 

 pulses. 



The relation to hydrographic conditions may be inferred from 

 the fact that while in the stable conditions of July-October, 1897, 

 pulses culminated at 800-4,800 per m. 3 , in the same period in the 

 disturbed hydrographic conditions of 1898 no pulse rose above 200 

 per m. 3 , and the total of all records in those months is only 8 per 

 cent, of that in 1897. Evidently brevispinosus does not thrive in 

 flood waters. 



