234 
Certodaphmia scitula appears but once in the records of European 
plankton, Scourfield (’98) finding it in the waters of Epping Forest 
in September. The closely related C. quadrangula as well as the 
other species have been frequently recorded by European investi- 
gators both in the littoral and the limnetic fauna, but they appear 
to be less generally found there than the other dominant Cladocera 
of our waters. 
It does not appear in the plankton of our Great Lakes (Forbes ’82 
and ’90, Birge ’95), or in that of Lake Mendota (Birge ’95 and 797), 
or Green Lake (Marsh 797), but Herrick (’84) reports it as the most 
abundant species in Minnesota, and Fordyce (’00) finds it in 
Nebraska in shallow waters. A revision of the genus is needed 
before the seasonal distribution of the various species can be worked 
out on a basis that will make satisfactory discussions of the literature 
possible. 
Chydorus sphericus O. F. Miill.—Average number, 422, of which 
26 are egg-bearing females, and 6 are immature, the remainder, 390, 
being females in which the ova were not prominent. 
The identification of species of Chydorus is attended by consider- 
able uncertainty. Comparison with named specimens from Europe 
supplied by Prof. G. O. Sars, leaves no doubt that C. sphericus is 
common in our waters, and it is apparently the dominant species. 
It is probable that several other species, as, for example, C. globosus 
Baird and C. celatus Schoedler, occur sparingly in our waters and 
have been included with C. sphericus in my enumerations. . The 
difficulties which attend the attempt to assign every individual to 
one of the several species of Chydorus can be appreciated only by 
one who makes the effort. The problem of their specific validity 
should be solved by a statistical analysis of the range of varia- 
tion. 
The seasonal distribution of Chydorus sphericus in channel 
waters is in its general outlines very characteristic and well defined. 
The following table, which gives the average number of Chydorus 
per m.* for each month of our collections, shows clearly that it is a 
vernal planktont, and that there is a slight tendency toward an 
autumnal pulse in September, when vernal temperatures return. 
The number for November (222) would probably be considerably 
reduced if more than one collection had been taken in that month 
in 1896. Omitting this year, the average for November falls to 
