a3 
part of some of the rhizopods, or to the appearance of limnetic forms, 
varieties, or species—according to the systematic value placed upon 
these eulimnetic individuals. I am inclined myself to regard them 
as seasonal forms of species which are predominantly of the bottom 
or littoral fauna, which have multiplied rapidly under the stimulus 
of abundant food. Owing to this fact, to the storage in their 
tissues of the products of metabolism, such as gas and oil vacuoles 
which tend to lighten their specific gravity, and to the frailer 
structure of their shells under conditions of rapid multiplication, 
they abandon their customary benthal or littoral habitat and assume 
temporarily a limnetic distribution in the plankton where they con- 
tinue to find abundant food. Their appearance here under these 
circumstances is a result of their physiological condition, and with 
its cessation they decline, as shown by their pulse-like occurrences. 
Whatever the systematic valuation placed upon these limnetic 
forms may be, there is no doubt of their occurrence. They have 
appeared in every year of our operations, but were most prevalent 
in 1897, a year of most stable conditions, and also in the quieter 
backwaters, and on the declining spring flood or June rise when hydro- 
graphic conditions are less catastrophic than those of early flood 
stages. In 1897 there was a pulse of 68,400 (silk-net only) on August 
8 and another of 1,268,400 on September 7, both in stable conditions 
and almost exclusively of limnetic types, differing in this respect 
from the pulse of 141,524 on February 22, 1898, which was pre- 
dominantly of an adventitious character, resulting from the flood 
of that period (Pt. I., Pl. XII.). The contrast in the numbers of 
Rhizopoda in the plankton during warm and cold seasons of the 
year is very striking in 1897. The average per m*, per collection 
from May 1 to October 1, that is, above 60°, is 161,045, omitting all 
filter-paper collections, while in the seven months of lower tempera- 
tures this average is only 4,771. During the warmer period the 
June rise was the only hydrographic disturbance (Pt. I., Pl XI.) 
to which any adventitious increase might be attributed. This con- 
trast is less evident in 1898, when the summer hydrograph was more 
disturbed. These larger numbers during warmer months may be 
attributed in part to the greater numbers of the Rhizopoda in their 
littoral habitat, and in part, doubtless, to the fact that at low water 
the shore and bottom fauna are brought into more intimate relation 
with the plankton, and in the river the disturbance of these regions 
