2 
of collections containing brevispinosus in these months being 27, 
80, 62, 67, 48, 75, and 59 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.*, while between April 20 and 
October 15 the pulses often culminate at 3,000-5,000 per m.°, 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 were 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.* 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.*, in the same period in the 
disturbed hydrographic conditions of 1898 no pulse rose above 200 
per m.*, 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. 
