219 



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 if 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 poly cyclic habit in some waters. 



Trochosphczra 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. Trochosph&ra was found in greatest abundance at 

 the outlet of Flag Lake (Pt. I., PI. II.) in July, reaching 9,664 per 

 m. 3 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., PI. 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, 



