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., Pl. 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 17, 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. ceruleus. 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 S. zgneus (7), but from the descriptions 
of Roux (’01) I am inclined to consider it as S. miger 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 S. igneus var. fuliginosus Forbes, which, it would seem 
from Roux’s description of these species, should be transferred to S. 
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. barrettt Barrett and S. roeselat Ehrbg. from the river, but 
I have not identified them in the plankton collections. 
