128 



this species in stagnant water has often been observed. Roux ('01) 

 finds it especially abundant in September, October, and February in 

 stagnant waters about Geneva. 



Stentor niger Ehrbg. Average number, 3,124. In our waters 

 this species also is a winter planktont (Table I.). There have been 

 but four records of occurrence between May 1 and September 1 . In 

 1895-96 the species appeared November 14 at 44 and reached a 

 maximum of 68,635 December 18, after three weeks of minimum 

 temperatures and approaching stagnation under the ice. Numbers 

 declined in the December- January flood (Pt. I., PI. X.), but rose 

 again in March, as the flood declined, to 39,087 on the 24th at 40. 

 It disappeared from the plankton April 30 at 70 and did not re- 

 appear until November 1 7 , from which time it continued until March 

 22. In 1897-98 it returned September 21 at 71, attained a maxi- 

 mum of 42,000 November 23 at 43, declined during December, and 

 rose to 47,000 on January 21 at 34 under the ice, and in the con- 

 ditions approaching stagnation described in connection with the dis- 

 cussion of S. c&ruleus. A decline in numbers continued until April 

 12 at 52. Favorable conditions for growth are thus found in our 

 waters between 32 and 50, and the optimum seems to lie near 40 

 or below. 



This species reaches its greatest development in our waters during 

 the time of greatest sewage pollution and bacterial development. It 

 is known as a bog-water species, and was found by Roux ('01) in 

 stagnant waters about Geneva during the colder months. Hempel 

 ('99) reports this species as 5. igneus (?), but from the descriptions 

 of Roux ('01) I am inclined to consider it as 5. niger Ehrbg. It may 

 be that both species are included in our data, but they are predomi- 

 nantly of the niger type. They include also individuals of the black- 

 ish variety 5. igneus var. juliginosus Forbes, which, it would seem 

 from Roux's description of these species, should be transferred to 5. 

 niger. The fuliginosus form was very abundant in the margins of 

 Pine and Round lakes, Michigan (Kofoid, '95), during the summer 

 in surface temperatures of 61-70, where sewage contamination was 

 but slight. 



Stentor polymorphus (O. F. Mull.) Ehrbg. was found sparingly in 

 July and August during maximum temperatures. Hempel ('99) 

 reports S. barretti Barrett and 5. roeselii Ehrbg. from the river, but 

 I have not identified them in the plankton collections. 



