244 
Cohn seems not to have called attention to these clearly defined 
recurrent pulses. 
In European streams D. cucullata also forms an important part 
of the plankton. Lauterborn (’93) states that, with its varieties 
kahlbergiensis and cederstrémt, it appears abundantly in the plank- 
ton of the Rhine in summer, but is not found in itin winter. Zimmer 
(’99) states that D. kahlbergiensis was found constantly in the plank- 
ton of the Oder in July-September, and Schorler (’00) also finds it 
in the Elbe at Dresden in May—August, with larger numbers in 
June and August. Steuer (’01) reports it, in small numbers only, 
in August in the backwaters of the Danube at Vienna. Fri¢ and 
Vavra (’01) report D. kahlbergiensis as rare in the Elbe. Sowinski 
(’88) finds it in several varieties in plankton of the Dnieper and its 
tributaries, Rossinski (’92) finds it in the summer plankton of the 
Moskwa, and Zernow (’01) in the June-July plankton of the 
Schoschma and Wjatka. Meissner (’02 and ’03) finds it in several 
varieties in the May—August plankton of the Volga. 
D. cucullata in some of its various forms or varieties appears to 
be widely distributed in American waters. It was reported by 
Forbes (’82), as D. retrocurva, from the plankton of Lake Michigan, 
and also (’90) from Lake Superior and adjacent waters. Burge 
(91 and ’94) also finds it abundantly in Wisconsin waters and 
in Lake St. Clair. Herrick (’84) and Ross (’97) report it from Min- 
nesota and Jowa. Careful studies of its seasonal and vertical 
distribution in Wisconsin waters have been made by Marsh (’97) 
in Green Lake, and by Birge (95 and 97) in Lake Mendota. In 
Green Lake D. kahlbergiensis is reduced to a minimum or even 
extinction in December—April, rises in a late vernal maximum in 
June-July, falls again to a lower level in August-September, and 
then rises to a second and sometimes higher autumnal pulse in 
October. In its main outlines this conforms to the seasonal course 
of the cucullata form in our channel plankton. Our vernal max1- 
mum appears somewhat earlier, as a result probably of an earlier 
warming up of the water. According to Birge (’97) this species is 
more definitely periodic in its occurrence in Lake Mendota, being 
confined entirely to July-December. Here also the largest numbers 
are found in October, and the individuals gather in lower levels as 
temperatures decline. 
