154 
surpassed by several other species. Zimmer (’99) finds that this 
species is the most common winter rotifer in the plankton of the 
Oder, with a maximum in the spring and a predominance Of var. 
tecta from July to September. Schorler (’00) finds it to be the most — 
common rotifer in the Elbe—from April to November; and Skor- 
ikow (97) finds it in the Udy, in Russia, throughout the summer 
in great numbers, but surpassed by Syncheta, Polyarthra, and 
Brachionus angularis. The variety. tecta greatly exceeds var. 
stipitata in these waters. Seligo (00) finds it throughout the year 
in Prussian lakes near Danzig, with a maximum in May. There are 
indications, in his data, of recurrent pulses during the summer, but 
his interval of collection is too great to follow their history. Burck- 
hardt. (00a) finds it throughout the year in Swiss waters, with its 
single maximum in August. Jennings (’94, ’96, and ’00) reports it 
in the summer plankton of Lake Michigan and Lake Erie and of 
inland waters of Michigan. 
Anurea hypelasma Gosse.—Average number of females, 2,390; 
of eggs, 1,917. This species has a very definite limitation to a 
period extending from early in June to the first days of November. 
There are but two records outside of these limits—a single female and 
egg on Jan. 11, 1898, and another upon April 19 of the same year. 
The probabilities of occurrence in very small numbers at all tempera- 
tures is thus indicated. The following table gives the data of pulses 
and temperatures. 
All of the pulses save one occur at temperatures above 70°, and 
with this exception the species declines rapidly and disappears 
shortly after temperatures pass below 60°. It is plainly, in our 
waters, a summer planktont, with its optimum temperature close 
to the summer maximum. This species takes no share in the vernal 
pulse, and there is no satisfactory evidence of any fluctuation 
corresponding to it at any other season. There are three or four 
pulses in each summer, and the species is apparently polycyclic, for 
winter eggs were found in 1898 either at the maximum of the pulse 
or the week or fortnight following. Thus 24,000 winter eggs were 
recorded on Sept. 27, 1898, the date of the maximum of the Septem- 
ber pulse. The parthénogenetic eggs preponderate during the rise 
of the pulses in a very marked manner in this species. For example, 
in this September pulse 55,400 eggs were recorded during its rise 
to 500 during its decline. In like manner, in the case. of the 
