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of the male eggs in March, and winter eggs follow in April and May. 

 The second sexual period extends from the end of July to the end 

 of October, with a maximum in September-October. This bears 

 some resemblance to the distribution in the Illinois, with the 

 exception that the recurrent cycles which make the species poly- 

 cyclic were not noted, and that male or winter eggs were not present 

 in the colder months. It may be that the application of the 

 quantitative statistical method with brief intervals of collection in 

 the Rhine would reveal a still closer correspondence in the seasonal 

 routine of Polyarthra in the two streams. Wesenburg-Lund ('98) 

 finds that temperature has nothing to do with the appearance of 

 the sexual cycle of this species in Danish waters. Males were 

 found in December, as also (eggs only) in the Illinois. He also 

 found differences in different bodies of water as to the times of the 

 sexual cycles. Apstein ('96) has found this species perennial and 

 one of the most abundant rotifers in plankton of the lakes near 

 Plon, Germany, with maximum period from April to August, and 

 in November in one lake, and in July-August in another. The 

 sexual cycle was noted in May- June only. Seligo ('00) finds the 

 species perennial in lakes near Danzig, with large numbers in April 

 and July. His collections were too widely separated to trace fully 

 the seasonal fluctuations. Burckhardt ('OOa) finds Polyarthra in 

 small numbers in winter months in the plankton of Swiss lakes, and 

 in larger numbers in the summer, but does not trace their seasonal 

 fluctuations. 



Pterodina patina Ehrbg. Average number of females, 37. With 

 two exceptions all the records of this species lie between the last of 

 May and the first of October. There are but four records below 

 70. This indicates optimum conditions for the species during the 

 period of maximum heat, and further evidence of this lies in the 

 occurrence of the larger numbers during this period. Appearances 

 in January-March suggest a perennial habit ; and small and irregular 

 numbers, that the species is largely adventitions. Hempel ('99) 

 also records P. valvata Hudson from Quiver Lake. 



Rattulus tigris O. F. Mull. Average number of females, 207. I 

 have not found this species in any year later than October, though, 

 as shown in Table I. , it appears in January at minimum temperatures, 

 and continues in small numbers and somewhat irregularly until 

 autumn. These conditions and the absence of pulses suggest that 



