240 
Michigamme in small numbers, and (’93) in that of the Alpine waters 
of Wyoming and Montana, where it 1s, however, more abundant in 
smaller pools. Birge (’94) finds it generally distributed in collec- 
tions, including plankton, in Lake St. Clair and (’97) a member of 
the plankton of Lake Mendota, where its abundance is dependent 
on the supply of Anabena. Its maximum—only a single well-defined 
one occurring in each year—was found in July—October. Burge 
regards it as an accidental member of the limnetic fauna, maintained 
there as long as suitable food is present. Its mode of occurrence 
does not, however, differ from that of typical plankton organisms, 
which would doubtless likewise disappear from the plankton if their 
food should be lacking. 
It is noteworthy in this connection that it was only sparingly 
present in the channel of the Illinois in the midsummer—autumn 
plankton, when—as, for example, in 1897—-Anabena and its allies 
were abundant. It seems not improbable that temperature even 
more than food is an important factor in controlling its seasonal 
and local distribution. It is unquestionably a member of the 
plankton in our waters, though also abundant here, as elsewhere, in 
the lttoral fauna. In our locality in channel plankton it shows 
distinctly seasonal limitations which suggest the operation of tem- 
perature rather than food. Its occurrence in large numbers in 
Wisconsin lakes in midsummer and its absence in the Illinois at 
that time may also be correlated in part with the contrasted tem- 
perature conditions in the two localities. Its occurrence in our 
littoral fauna may also in part be due to the lower temperatures 
consequent upon spring-fed areas and the shade of aquatic vegeta- 
tion. Chydorus is one of those organisms capable of both the littoral 
and limnettc habit under suitable conditions of food and temperature. 
In our waters, at least,—and, as it seems from the data of distribu- 
tion, elsewhere,—temperature, rather than food directly, appears 
to be the factor controlling the occurrence of Chydorus in the 
plankton. 
Daphnia cucullata G. O. Sars.—Average number, 181. In 1897, 
very much greater,—5,483 per m.3 
For the reasons given by Burckhardt (’00) I use Sars’s name 
cucullata rather than jardinez of Richard to designate those forms 
of the subgenus Hyalodaphnia in our plankton. In channel waters 
this species varies considerably, but not to the extent that it does 
