246 



viduals) in May-October to a maximum in November-January, 

 which is followed by a rapid decline (with preponderance of adults) 

 to the minimum. His results agree with those of Apstein ( '96) in 

 the main rather than with ours in the Illinois. Seligo ('00) finds 

 D. hyalina in Hintersee, though it is apparently absent from the 

 adjacent Barlewitzersee. In the former lake it appears in May, 

 rising to the year's maximum early in June, continuing throughout 

 the summer in diminished numbers, and disappearing in October. 

 In his infrequent records there are suggestions of several recurrent 

 minor pulses during the summer. Cohn ('03) reports D. galeata 

 regarded by Burckhardt ( 'OOa) as a form of D. hyalina from the 

 region of Konigsberg, but refers it rather to the cucullata group. I 

 shall therefore consider his results only in connection with D. 

 cucullata. 



D. hyalina appears but rarely in the records of European potamo- 

 plankton. Steuer ('01) reports it, in small numbers only, in May 

 from the 'backwaters of the Danube at Vienna. Fric and Vavra 

 ('01) state that D. microcephala regarded by Burckhardt ('OOa) 

 as a form of D. hyalina is abundant in the plankton at a depth of 

 0-1 m. in April-November in the Elbe and its backwaters at 

 Podiebrad. It is also reported by Zykoff ('00 and '03) in the late 

 vernal (June- July) plankton of the Volga at Saratoff, and by Meissner 

 ('02 and '03) in the same stream in May- June. The examination 

 of the plankton of the Volga made by these authors is far less 

 extensive than that made of the Illinois River plankton, but as far 

 as it goes it indicates a similar distribution of D. hyalina in the two 

 streams. Volk ('03) reports it from the Elbe at Hamburg without 

 data. 



The species appears to be widely distributed in American waters, 

 being reported, in some of its various varieties or synonyms, especially 

 from lakes and ponds. Smith ( '74) finds it in the plankton of Lake 

 Superior, Forbes ('82) in that of Lake Michigan, and Birge ('94) 

 in Lake St. Clair. It was also found in the Illinois by Forbes ('78) 

 and in the backwaters of the Ohio River by Herrick ('84), who 

 reports it also from Minnesota waters. Birge ('91) finds it in lakes 

 about Madison, Wis., and Fordyce ('00) in deep pools in western 

 Nebraska. The only investigation of its seasonal distribution in 

 American waters is that of Birge ('95 and '97) in Lake Mendota, 

 where it forms about 3 per cent, of all the Crustacea. It is perennial 



