140 
portance in the plankton, especially of the backwaters. Average 
number, 8,796. 
DISCUSSION OF SPECIES OF RHIZOTA. 
Apsilus lentiformis Metsch.—An Apsilus doubtfully referred to 
this species was taken December 25, 1895, and April 29 and July 23, 
1896, at temperatures of 41°-78°, in each case with rising river 
levels. 
Conochilus dossuarius Hud.—Average number of females, 517. 
This species wes more than ten times as abundant in the collections 
of 1896 and 1897 asin 1898. Hempel (99) reports it from January to 
September, with a maximum in March. In the plankton collections 
of 1896 I did not. record it until June 11, at 73°. It reached a maxi- 
mum of 25,800 July 18, and another of 142,800 August 15, at 86°, 
and disappeared from the plankton September 30 at 58°. In 1897 
it reappeared May 25 at 66° and reached greatest numbers September 
7 at 80°, and was not recorded after the 14th. In 1898 (Table I.) it 
first occurred March8, at 37°,and attained its greatest number, 14,400, 
on September 27 at 73°. In 1899 it returned towards the end of 
January, under the ice, and continued till the cessation of operations 
in March. It thus occurs in the Illinois throughout practically the 
whole range of seasonal and thermal conditions, but not continu- 
ously. 
Colonies are of few individuals, and isolated individuals are often 
found in the preserved plankton. Females with 1-4 eggs were-taken, 
and were most numerous during the rise of the pulse. About 4 per 
cent. of the females observed, were carrying eggs. Males were found 
on the decline of the pulse of July, 1896. 
Conochilus unicornts Rouss.—Average number of females 8,208; 
eggs, 100. Recorded from March 15, at 46°, to July 5, at 80°. A 
pulse of 8,000 on April 26 and one of 392,000 on June 7 constitute 
the only fluctuations. It was not found in 1897, and only sporad- 
ically, during the summer, in 1896. Females with 1-3 eggs attend the 
rise of both pulses in small numbers. The colonies of this species 
also are composed of but few individuals. 
Conochilus volvox Ehrbg.—Average number of females, 129. A 
few large colonies were taken March 29 and April 5 at 49°. 
Megalotrocha alboflavicans Ehrbg.—Colonies of this species are 
found in numbers on Ceratophyllum in the backwaters, and in 1894, 
