62 
Undetermined species of Pentium, Arthrodesmus, and Docidium 
have been found in the plankton but always singly. They are 
doubtless adventitious. : 
PHANEROGAMIA. 
The Lemnacee are represented in our waters by several species 
of Lemna, by Sptrodela, and by two species of Wolffia—brasiliensts 
and columbiana. The first two genera are predominantly floating 
surface-plants, while the last occurs at all levels, is taken with the 
plankton, and has been treated in our measurements and enu- 
merations as a limnetic organism. 
Wolffia brasiliensis Weddell.—Average number, 2; in 1897, 13. 
It appears irregularly in river planktons from the last of March till 
January, and is somewhat more abundant in late summer and 
autumn. The seining operations of fishermen in the river and 
tributary backwaters have much to do with its appearance in the 
plankton of the river. 
Wolffia columbiana Karsten.—Average number, 7; in 1897, 41. 
With the preceding species. Neither of these species are sufficiently 
abundant greatly to affect the ecology or quantity of the plankton 
of the river, though they are of more importance in the backwaters. 
Owing to their size and duration they compete with the smaller 
orgamsms of the phytoplankton, but do not serve as food for any of 
the zodplankton. 
P{RIO TIO;ZIOLAL 
Average number, 111,731,000. The number of species exceeds 
147 (+38), distributed as follows: Mastigophora, 60(+10); Rhizop- 
oda, 31 (+28); Heltozoa,5; Sporozoa (3); Ciliata, 45; and Suctoria, 
5,—the numbers in parentheses indicating the additional forms 
whose specific rank was not recognized in the enumerations. 
The Protozoa occur in great numbers (Table I.) in every collection 
of the year. Owing to the fact that the totals are a conglomerate of 
two methods of collecting, of a large number of species of many di- 
vergent seasonal tendencies, and of both eulimnetic and adventitious 
forms, their seasonal fluctuations have no particular significance which 
is not better treated eitherin connection with the subdivisions of the 
class or with the individual species. In the totals, traces appear of 
