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 10 to 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 antennae, 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. Mull. 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 spinifer 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 /. spinifer, for the reasons given by Herrick and Turner ('95), 

 rather than to /. longiremis, to which Birge ('91) would refer our 



