62 



Undetermined species of Penium, Arthrodesmus, and Docidium 

 have been found in the plankton but always singly. They are 

 doubtless adventitious. 



PHANEROGAMIA. 



The Lemnacecz are represented in our waters by several species 

 of Lemna, by Spirodela, and by two species of Wolff ia brasiliensis 

 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. 



Wolff ia 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. 



Wolff ia 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 

 organisms of the phytoplankton, but do not serve as food for any of 

 the zooplankton. 



PROTOZOA. 



Average number, 111,731,000. The number of species exceeds 

 147 ( + 38), distributed as follows: Mastigophora, 60 ( + 10); Rhizop- 

 oda,3l ( + 28); Heliozoa,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 either in connection with the subdivisions of the 

 class or with the individual species. In the totals, traces appear of 



