245 
Daphnia hyalina Leydig.—Average number, 417. In channel 
waters this species has appeared in but two years, in 1895 in 
April—July, attaining on June 19 a maximum of 166,208 per m.*, of 
which 150,626 were immature. The collections were too infrequent 
in these months to trace the course of this vernal pulse. D. hyalina 
did not reappear until the spring of 1898, on May 24, in a single 
vernal pulse culminating at 11,600 per m.* on June 7, and disappear- 
ing a fortnight later. Its occurrences with one exception were all 
at temperatures above 70°. There is no apparent cause for its 
absence in later months or in other years. Males and ephippial 
eggs were not found. 
Daphmia hyalina is an exceedingly variable species, and a large 
number of forms have been described which belong to the hyalina 
group. Burckhardt (’00), for example, recognizes 26 such forms 
as varieties of this cosmopolitan planktont. This variability and 
the difficulties attending the resulting synonymy cause any discus- 
sion of the species in other waters to be attended by much uncer- 
tainty. I shall therefore not attempt to distinguish in my dis- 
cussion between the various varieties included by Burckhardt in 
the hyalina group. 
In lakes of northern Germany, Apstein (96) finds that D. hyalina 
is essentially a winter planktont with a seasonal range of September— 
July, and with maximum numbers in November—January. The 
maximum thus appears there at the time of complete extinction in 
our waters. Stenroos (’97) records it (as D. galeata) in the summer 
plankton of Karelia, Huitfeldt-Kaas (’98) finds it in Norwegian 
lakes in July and September in considerable numbers, and Scour- 
field’s careful studies (’98) of its seasonal occurrence in waters of 
Epping Forest reveal an interrupted distribution in April-Novem- 
ber. Scott (99) finds it in numbers in Scottish lochs in the plankton 
examined at long intervals in March—January. Fuhrmann (’00) 
reports it as perennial in Neuenbergersee, with a maximum in June 
followed by a midsummer minimum. Burckhardt (’00a) finds 
great diversity in different Swiss lakes and in different years in the 
relative numbers present. His intervals of collection were too great 
to detect any pulse-like movement in the production, and it may be 
that the diversity is due in part to the incompleteness of his records. 
He concludes that D. hyalina is at a minimum in March—May, 
increases in numbers slowly (with a preponderance of young indi- 
