244 



Cohn seems not to have called attention to these clearly denned 

 recurrent pulses. 



In European streams D. cuciillata also forms an important part 

 of the plankton. Lauterborn ('93) states that, with its varieties 

 kahlbergiensis and cederstromii, it appears abundantly in the plank- 

 ton of the Rhine in summer, but is not found in it in 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. Fric and 

 Vavra ('01) reportD. 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. Birge 

 ('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 Iowa. 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 maxi- 

 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. 



