69 



part of August, 1896'. It is one of many illustrations of the cosmo- 

 politan distribution of plankton organisms. 



Bicosceca lacustris J. Clark*. Average number, 112,896. Only one 

 third as abundant in 1896, and four times as many in 1897. This 

 minute flagellate 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 Richteri- 

 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. I regard 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 Bico- 

 sceca. Thus in the late summer and fall of 1897 Melosira fluctuated 

 without any appearance of Bicosceca. In the autumn of 1898 the 

 pulse of Bicosceca on October 1 1 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 in- 



