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69 
part of August, 1896. It is one of many illustrations of the cosmo- 
politan distribution of plankton organisms. 
Bicoseca lacustris J.Clark*.—Average number, 112,896. Onlyone 
third as abundant in 1896, and four times as many in 1897. This 
minute flagéllate is found in our waters sessile upon the filaments of 
Melosira, principally M. granulata var. spinosa. It occurs more 
frequently upon the dead frustules than upon live ones, and upon 
those of the shorter form than upon the longer. It has appeared 
also upon Dinobryon sertularia, Pediastrum pertusum, and Rtchteri- 
ella botryoides. It exhibits a considerable range of variation in 
proportions, in the amount of lateral compression, and in the length 
of the pedicels. These variable forms are, however, connected 
with the type as described by Clark, and are not, in my opinion, to 
be designated as distinct species. Zacharias (’94) has described 
one of these variants as B. oculata. IJregard it as a growth condi- 
tion of B. lacustris, and not as specifically distinct from it. 
Its seasonal distribution in 1898 is somewhat peculiar. It 
appears as two quite symmetrical pulses, the first extending from 
early in June till the middle of July, and culminating on June 14 at 
3,801,600. The approach of this pulse is abrupt and its decline 
somewhat gradual. The species does not reappear until September 
13. The autumnal pulse culminates October 11 at 486,000, then 
gradually declines, and disappears November 1. There is no record 
of its occurrence in 1898 outside of these two pulses. In 1897 it is 
found irregularly from May to August, and in 1896 in February and 
from May to December, with pulses in May, June, July (2), August, 
and October. . 
In 1898 its optimum temperatures appear at 82° and 65°, and its 
pulses in other years do not occur below 57°. It thus belongs to the 
plankton of the warmer months. 
Its seasonal distribution falls within that of the limits of its host 
Melosira, and in 1896 and 1898 their vernal pulses coincide, and the 
same correlation appears in all but one of the pulses of 1896. Not 
all Melosira pulses, however, are attended by an increase in bzco- 
seca. Thus in the late summer and fall of 1897 Melosira fluctuated 
without any appearance of Bicoseca. In the autumn of 1898 the 
pulse of Bicoswca on October 11 appears on the decline of the Sep- 
tember pulse of Melosira, in which the host made no corresponding 
increase. Melosira is thus apparently essential for any marked 1n- 
