209 
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 
Plén, 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 (’00a) 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 
