44 
temperatures below 40°. The data are insufficient to trace the 
pulses satisfactorily. This species is distinguished with difficulty 
from A. gracillama, and may include only old, and in our planktons 
often heavily incrusted, individuals; or it may be only a low-tem- 
perature variety of the species above named, which in the grand 
total of all our collections outnumbers it ten thousand to one. 
Astertonella gracillima Heib.—Average number of individual 
cells, 28,860,160. In 1897 the species was only one third as abun- 
dant, a contrast which finds its explanation in the fact that the 
June rise’ of that year (Pt. I., Pl. XI.) .did not reach the stagesar 
overflow, and a June pulse is absent in the collections of that year. 
The seasonal distribution of this organism is one of the best-defined 
and most striking of all the components of the river plankton. It 1s 
pecuhar in the fact that it appears in numbers only during spring and 
the beginning of summer, and in the absence of any autumnal pulse 
upon the return of the temperatures in which the spring pulse ap- 
peared. This species was recorded in every month of the vear but 
October, but always in small numbers after July 1. In 1894, collec- 
tions were not commenced until after the time of the spring pulse. 
In 1895 the spring collections were few, and at intervals so great as to 
preclude the detection of the full course of the spring pulse. The 
maximum number in the collections of that vear appears April 9 at 
1,203,100 and falls to 445,995 on April 29—which is approximately 
the time of the maximum of subsequent years. This was a year of 
unusually low water during the spring, and overflow stage was at no 
time reached (Pt. I., Pl. I[X.), which may account for the apparent 
suppression of the spring pulse. The species does not reappear in the 
collections of that year until December, but it continues in small 
numbers (less than 5,000 per m.*) until the end of March, 1896, when 
there is a rapid increase which culminates April 24 at 26,281,400. It 
disappears entirely from the records at the end of a fortnight, and save 
for a single entry in June and two in September it does not again 
appearin 1896. In1897 theculmination of the spring pulse occurs April 
27 at 324,633,600—three hundred-fold larger than in the previous 
year. There is a normal March flood (Pt. I., Pl. XI.), on the declining 
stages of which this pulse appears. With the close of June the 
species disappears from the records. The June rise does not reach 
the stage of overflow, and the scanty records show but this single 
pulse throughout the year. Beyond a single entry in August and in 
