1Q PLANKTON OF WINNEBAGO AND GREEN LAKES. 



continuous run of % niin. ; 4th, y 2 niin. in the opposite direc- 

 tion ; 5th, one minute straight ahead. The centrifuge was run 

 at from 1200 to 1500 revolutions per minute, any greater 

 rapidity being almost certain to break the tubes. It was found 

 that the work was most successful when the percentage of alco- 

 hol in the collected material was rather high. This method 

 has some inaccuracy, as well as the method by settling, for the 

 different kinds of material are not equally well thrown down. 

 Some of the algae, in spite of long continued running of the 

 centrifuge, will always lie in a more or less flocculent mass on 

 top, and in some cases, even after being thrown down, will 

 afterwards rise into the supernatant liquid. The supernatant 

 fluid, even when apparently perfectly clear, always contains a 

 little of this plant material. The amount is so small, however, 

 that it has no evident effect on the measurements of the plank- 

 ton. 



The examination of the plankton was by the method ex- 

 plained in my former paper. (Marsh '97, p. 188.) At first 

 all the counting was done by means of the dissecting lens, but 

 later, for the smaller forms, a compound microscope was used, 

 with a stage especially arranged to receive the counting plate. 

 Only the Crustacea were counted exactly, with the exception of 

 Gloiotricliia, of which a careful count was made. But notes 

 were made of the occurrence of the other forms and a rude esti- 

 mate of the numbers made and recorded under the terms "few," 

 "many," "very many," and "nos." 



