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 was more than ten times as abundant in the collections 

 of 1896 and 1897 as in 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 March 8, 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 unicornis 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, 



