252 
European author previously quoted. Our results in Illinois waters 
are in striking confirmation of his conclusions. He finds the first 
scattering individuals in the plankton late in May, but numbers do 
not rise until late in July or early in August, increasing rapidly 
through August or even into September, then declining rapidly, and 
disappearing entirely before November 1. The active period is thus 
at a time when a considerable part of the lake is at or above 68°. In 
our waters these temperature limits are 78° or above, but the sea- 
sonal distribution is almost identical with that in Lake Mendota. He 
finds it more abundant in the upper strata, 0-2 meters, than in the 
deeper ones—just the opposite of Cohn’s (’03) results. Marsh 
(97) has also determined its seasonal and vertical distribution in 
Green Lake, Wis., with considerable care. Occurrences from the 
last of October to the last of June are very few, and maximum 
numbers appear from the middle of August to the middle of Septem- 
ber, when surface waters have a temperature of 65°-80°. It occurs 
in all depths (0-40 m.), but 70 to 80 per cent. of the individuals 
were taken within 10to 15 m.of the surface, the upper 5 meters being 
more densely populated by night than by day and in September-— 
October than in August. 
Diaphanosoma is a typical planktont, with strong antenne, and 
an active swimmer. Examination of the literature indicates its 
wide distribution in the plankton of lakes and streams, and its very 
marked seasonal limitation to seasons of higher temperature. It is 
thus, as Birge (’97) has stated, markedly stenothermous. The 
divergent conclusions concerning its limnetic habit and its vertical 
distribution will doubtless be found to rest in some cases upon 
insufficient data, and in others, upon its reactions to varying condi- 
tions of light and temperature. 
Eurycercus lamellatus O. F. Mill.—This species occurred spar- 
ingly and irregularly in the winter plankton at minimum tempera- 
tures from November 30 to March 28. It is evidently adventitious. 
Ilyocryptus spintfer Herrick.—Average number, 4. This species 
occurred sparingly and irregularly in the plankton during the 
warmer months. The earliest record was on July 23, and the latest 
October 11 at 65°. This species is evidently adventitious in the 
plankton. I have doubtfully referred our examples to Herrick’s 
species I. sprntfer, for the reasons given by Herrick and Turner ('95), 
rather than to J. longtremts, to which Birge (’91) would refer our 
