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247 
in this lake but exhibits great differences in its seasonal course from 
year to year. The vernal development in May—June (the only one 
in our channel plankton) is relatively large in each year, but is 
sometimes exceeded by an autumnal one in October. A midsummer 
minimum sometimes appears between these pulses, and a winter 
minimum in December—April is always present. 
From the data here reviewed it seems probable that the very 
limited seasonal distribution and irregular annual recurrence of 
D. hyalina in our channel plankton is in a measure indicated in 
streams elsewhere, and may have its cause in the instability of the 
fluviatile environment as compared with the lacustrine, where the 
species evidently finds its environmental optimum. - 
Diaphanosoma brachyurum (Liévin).—Average number, 479, of 
which 154 are females, 49 females with eggs, and 276 immature. 
This species in our waters is monocyclic, with sharply defined 
seasonal distribution. With the exception of two records of young 
individuals in March—April, 1895. (and the identification of these 
individuals is questionable), all our records of occurrence in 
1894-1899 fall between May 25 and October 19, the first vernal 
records appearing at temperatures of 55.8° to 72.3°, and the last 
autumnal at 52.5° to 65°. The one pulse in each year—except in 
1894, when none was recorded—falls in a period of 3-6 weeks in 
July-September, the first record above 2,000 per m.* appearing 
July 26, and the latest (with one exception, 2,175 on September 27, 
1895) on September 7. The pulse varies in duration in different 
years from 3 to 6 weeks, and attains a maximum on dates ranging 
from July 26 to August 31, and varying in amplitude-from 8,580 
to 19,602 per m.* An analysis of the distribution of 61 recorded 
occurrences in channel plankton shows that of these only 13, or 21 
per cent., occur outside of July-September, and that the records 
outside of the seven weeks of the pulse include less than 12 per cent. 
of the total individuals. 
A comparison of the seasonal curve of distribution with the 
annual thermograph reveals the fact that the pulse occurs toward 
the close of the period of maximum summer heat, and in every case 
at a temperature of 78° or above, and that the decline of the pulse 
often begins with declining temperatures, and is always accom- 
plished during the autumnal decline. The effect of summer heat 
pulses upon the Diaphanosoma curve is strongly suggested by the 
