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 algae may have an alkaline reaction (Knauthe, '98) in bright 

 sunlight. Larger individuals, 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 Amoeba 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 w T as 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 Amceba into the plankton is brought to light by a com- 

 parison of its occurrences in 1897 and 1898. As shown by Plates 

 XL 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 Amoeba 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 Amceba was not 

 found in November and December, months of unusual stability in 

 river levels. There is, however, a suggestion in the data of distri- 



