97 
bution (see Table I.) that Am@ba may become an active member of 
the plankton during the warmer seasons, like other Rhizopoda, as a 
result, perhaps, of the formation of gas or oil vacuoles in its proto- 
plasm. Of the 30 occurrences, 21 fall between April 19 and Octo- 
ber 17, with water temperatures of 58° and 56°, respectively. Of 
these 21 occurrences in warm waters but 8 accompany flood inva- 
. sions, while all of the 9 occurrences during the colder months are in 
connection with such disturbances. Finally, the maximum num- 
ber per cubic meter (6,400) was found July 21 in clear waters, free 
from the debris of flood invasion. In conclusion, it seems probable 
that Ameba in warmer seasons of the year (above 56°) may adopt 
a limnetic habit. There is, however, the possibility that local and 
minor disturbances of the water due to current, waves, etc., are 
the occasion of its presence in the plankton in the absence of flood 
conditions. Jennings (00a) reports both A. proteus and A. radiosa 
in the open water of Lake Erie. 
The range of temperature of river water in which Ameba was 
found was from 32° to 89°—the full extremes observed by us in 
the river at Havana. The temperature at the maximum occur- 
rence, July 21, 1897, was 82°. It is perhaps significant that 14 
of the 30 occurrences of Ameba were between June 21 and Sep- 
tember 6, the period of maximum heat, the river averaging 
almost 80°—apparently the optimum temperature for the occur- 
rence of Ameba in the plankton in this locality. The relative 
numbers of individuals found in the various collections of the five 
years are too irregular to suggest any conclusions as to a seasonal 
cycle. 
_Ameba verrucosa Ehrbg.—Average number, 19. This species 
was found but three times in the plankton, once each in May, 
August, and September, occurring but singly, and in each case in 
flood waters. Itis apparently a tycholimnetic member of the plank- 
ton. The temperature limits of its recorded occurrence in’ the 
plankton were 58° and 82° respectively. 
Arcella. 
This genus is represented in the plankton by four species and 
two varieties which, like most of the Rhizopoda, are exceedingly 
variable, grading in some instances into each other by occasional 
