29 
ennial in lakes near Danzig, rivaling Polyarthra in abundance, and 
exhibiting maxima in the warmer months from April to October. 
It is also a member of the potamoplankton of European streams. 
Skorikow (’97) finds it in summer months in the Udy, and Zimmer 
(99) reports it as present in small numbers and irregularly in the 
Oder from April to November. Schorler (’00) finds it in the Elbe 
-in May—October with maxima in May and September, and Lauter- 
born (’98a) includes it in ‘his list of perennial rotifers in the plankton 
of the Rhine. It has two sexual periods, the first in March—May 
and the second in July—October, and he suggests the probability of 
a polycyclic habit in some waters. 
Trochosphera solstitialis Thorpe was found June 27, July 2, and 
August 15,in 1896; in 1897,on May 25 and July 14-30. Free winter 
eggs were taken August 15, 1896. All occurrences were above 66°. 
These records were all from plankton taken in mid-channel of the 
main stream. Tyrochosphera was found in greatest abundance at 
the outlet of Flag Lake (Pt. I., Pl. II.) in July, reaching 9,664 per 
m.? at 72°. It was also found in August in the weedy backwaters 
of Dogfish Lake. Both of these backwaters connect with the river 
(Pt. I., Pl. II.) below the point at which our collections were made. 
It was either introduced from some similar backwater higher up the 
stream than our plankton station, or developed in the river itself. 
SCIRTOPODA. 
This order is represented in the plankton by a single species, 
whose discussion will suffice for the order. 
Pedalion mirum Huds. Average number, 4,524. This is a 
summer planktont of somewhat definite temperature limits. The 
following table combined with the data in Table I. will suffice to 
characterize its seasonal fluctuations. 
Its limitation to temperatures above 60°, indeed almost 70°, is 
apparent. There are in all but two records below 60°, and but four 
below 70°. It is a typical midsummer planktont, with several 
recurrent pulses during the period of maximum temperatures. 
The location of these pulses with reference to those of the 
chlorophyll-bearing organisms is significant. AS shown in Table I., 
they follow immediately, or coincide with, those of the synthetic 
organisms. For example, the apices of the pulses of Mastigophora, 
