240 



Michigamme in small numbers, and ( '93) in that of the Alpine waters 

 of Wyoming and Montana, where it is, 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 Anab&na. Its maximum only a single well-defined 

 one occurring in each year was found in July-October. Birge 

 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 Anab&na 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 littoral 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 limnetic 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 jardinei 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 



