127 
Levander (’94) finds it in numbers under the ice in Finnish waters. 
On account of its motility and small size it readily escapes through 
the silk net. 
Pleuronema chrysalis (Ehrbg.) Stein.—Average number, 9. Re- 
corded only in January, 1898, at minimum temperatures. 
Prorodon farctus Clap. and Lach.—Only a few scattered occur- 
rences—from the last of September to the first of March at tempera- 
tures from 73° to minimum. An unidentified species of Prorodon was 
also found irregularly from November to April. 
Pyxacola affinis S. Kent.—Average number, 58. ‘This species is 
usually attached to aquatic plants, especially to Lemna. It has been 
found in the summer plankton from June to August during maximum 
temperatures, especially in 1896, when recurrent floods brought much 
Lemna from the backwaters into the river. It was found October 18 
at 52°, attached to Melosira varians. 
Rhabdostyla spp.—Average number, 110. Peritrichan ciliates re- 
ferred to this genus have been noted on Cyclops, Canthocamptus, 
Oligocheta, and even in considerable numbers upon the body, append- 
ages, and eggs of Polyarthra platyptera. They have appeared thus 
passively in the plankton during winter months from December to 
March, especially in 1899. 
Stentor ceruleus Ehrbg.—Average number, 882. This species 
presents a characteristic seasonal distribution in our plankton. Its 
numbers are never very large, and its full cycle can not always be 
traced in the records. It is a planktont of the colder season in our 
waters. But three records—one July 28, 1896, at 82°, one August 3 
of the same year at 80°, and a third, August 15, 1894, at 84°—lie 
outside of the period between ‘September 1 and May 1. In 1898 
(Table I.) the autumn cycle begins September 6 at 79°, but in 
both 1895 and 1897 the species does not appear until late in 
November or in December at 34° or below. In years prior to 1898 
the numbers were small and irregular, but on January 21, 1898, 
the maximum number of 28,800 was reached at 34°, under the ice, 
during the slowly rising flood of that month (Pt. I., Pl. XII.). It 
accompanied an increase in Stentor niger, and there are indications 
elsewhere that the two species may fluctuate together. The high 
(Pt. I., Pl. XLV.) chlorine (38.), nitrites (.175), and free ammonia 
(4.6) at the season of greatest development in the plankton are in- 
dicative of conditions approaching stagnation. The appearance of 
