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 (796) 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 (’00a) as a form of D. hyalina—from the 
region of Kénigsberg, 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. Fri¢ and Vavra 
(01) state that D. microcephala—regarded by Burckhardt (’00a) 
as a form of D. hyalina—is abundant in the plankton at a depth of 
O-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 
