Marsh — Limnetic Crustacea of Green Lake. 189 



tect the presence of any unusual form. This work was done 

 with the aid of a dissecting lens such as is furnished with a 

 Reichert dissecting microscope. This lens answered every pur- 

 pose so far as determining the species of the Crustacea, except 

 that I could not distinguish with certainty D. minutus from D. 

 sicilis. As the object of the counting was mainly to determine 

 distribution, the fact that I did not distinguish between these 

 species was of little importance, as their habits are the same. 

 In every case, however, a test of the collection was carefully ex- 

 amined under the compound microscope, and in this way a fairly 

 accurate idea was obtained of the seasonal distribution of these 

 species, and notes were made also in regard to the occurrence 

 of other smaller organisms. No attempt, however, was made 

 to keep any record of diatoms. 



The accuracy of this method of counting was carefully tested, 

 and the amount of error was found very small, — so small that I 

 do not think the general results would be appreciably affected. 

 As stated before, it is very doubtful if the method could be 

 applied so successfully to plankton rich lakes. 



These results were afterwards reduced to percentages in order 

 to show the relative abundance in vertical distribution. 



In the following table I have tabulated the conditions under 

 which the various collections were made. The table is, in the 

 main, self-explanatory. To indicate the condition of the surface 

 I have used four terms, "smooth, ripples, waves, and rough. " 

 In the tables given for the various species the "total " column 

 indicates the actual number obtained in my dredge. These 

 numbers might easily be reduced to give the actual number 

 per square meter by multiplying by the coefficient of the dredge, 

 but my object was simply to get comparative results, and, as 

 indicated later in this paper, I myself have only limited confi- 

 dence in the value of plankton determinations. In the columns 

 following "total" are given the percentages found for every five 

 meters of depth. 



