96 
ton which belong to the type of A. radiosa Ehrbg., a type which 
presents no distinctions sufficiently well-defined to separate it spe- 
cifically from the first-named form. It seems probable that A. 
radiosa includes small individuals of A. proteus which are not, at 
the time of observation, creeping upon a substratum; that is, they 
are limnetic, floating free with filamentous pseudopodia character- 
istic of that condition. Verworn (’97) has shown that A. proteus 
takes the radiosa form in weakly alkaline solutions. Pond water 
rich in algee may have an alkaline reaction (Knauthe, ’98) in bright 
sunlight. Larger individtials, distinctly referable to the A. proteus 
type when taken in the plankton, possess at times the slender pseu- 
dopodia of the A. radiosa type as well as the blunter ones charac- 
teristic of the A. proteus form. I see no valid reason for separating 
the two as distinct species. Most of the Ameba recorded from the 
plankton collections belong to the A. proteus type, the smaller ones 
belonging to the radiosa type probably escaping through the 
meshes of the silk net. 
This species was found in 30 of the 180 collections examined, 
being observed in all months of the year except May, November, 
and December. The conditions attending its occurrence suggest 
that it is not, habitually at least, an active planktont at all seasons 
of its occurrence, but rather a tycholimnetic member, an invader 
from the littoral or bottom fauna, or a temporary accession during 
the warmer months. In the first place, both the number of occur- 
rences and the numbers of individuals found are. small, and the 
seasonal distribution, plotted from the data of the collections of 
the five years, is exceedingly irregular. Furthermore, 17 of the 30 
occurrences happened on rising floods, when the fauna of the bot- 
tom and shore of both the river and its tributaries is most mingled 
with the plankton. Further evidence of the agency of floods in 
introducing Ameba into the plankton is brought to light by a com- 
parison of its occurrences in 1897 and 1898. As shown by Plates 
XI. and XII., Part I., the hydrograph of 1897 is much less irregular 
than that of 1898, the latter year exhibiting repeated fluctuations 
in level due to floods. As a result we find Ameba occurring rela- 
tively (to the number of collections) almost twice as often in 1898 
as it did in 1897. It may also be significant that Ameba was not 
found in November and December, months of unusual stability in 
river levels. There is, however, a suggestion in the data of distri- 
