250 
by a fall to 5,120 on the 24th, and, in the rising temperatures which 
then ensue, by a recovery to a second maximum of 11,000 on the 
31st. Diaphanosoma then declines though temperatures continue 
to rise. These fluctuations all take place in comparatively stable 
hydrographic conditions. There is a suggestion in the records of 
this year that rising temperatures in midsummer conditions tend 
to accelerate, and falling temperatures to depress, development of 
the Diaphanosoma pulse, and also that after the pulse has continued 
for some time (six weeks in this instance) rise in temperature ceases 
to be effective. The autumnal decline in Diaphanosoma may 
therefore not always of necessity be due tc temperature decline alone. 
In 1898 there are also two midsummer heat pulses, culminating 
on July 26 at 89°, and August 30 at 82.5°, separated by a depression 
which reaches 77° on August 16. The depression to 78.3° on August 
2, with the consequent appearance of a third summit at 83° on 
August 9, is due mainly to the fact that the temperature was taken 
at 9:15 a. m., while all the others were in the late afternoon. The 
seasonal curve of Diaphanosoma shows likewise two apices, the first 
at 8,580 on July 26, and the second at 2,520 on August 30, separated 
by a depression to 60 per m.* on August 16, when temperatures are 
lowest. In this year the flood of the middle of: August doubtless 
plays a large part in depressing alike the thermograph and the 
seasonal curve of Diaphanosoma, but. in the light of the evidence 
from 1897 in stable hydrographic conditions the direct influence 
of temperature is also possible in this instance. 
Diaphanosoma is thus a late summer planktont which in develop- 
ment is very responsive to changes in temperature. It appears in 
the plankton in small numbers shortly after the establishment of 
summer temperatures in May—June, but does not begin its maximum 
development until maximum summer temperatures have existed 
for six to eight weeks, and is apparently incited to this by a summer 
heat pulse. 
Males were recorded on July 18 and August 1, and ephippial 
females on August 1 and September 5. Dead individuals were 
most numerous during or subsequent to the maximum of the pulse. 
This species is reported by Apstein (’96) in the plankton of 
Dobersdorfersee, where it is also monocyclic, first appearing in 
May, and attaining its maximum in September, when the males 
first appear. In contrast with conditions in our waters the maxima 
