83 
collections. The optimum temperatures plainly lie near the maxi- 
mum, that is, about 80°, and the season of growth approximates 
that of the land flora, being limited to the months of April-Septem- 
ber. This isa variable organism, and a number of species have been 
described in the genus in recent years. Many of these occur in our 
waters, but no attempt has been made to separate them, since they 
are based on minute characters. 
Mallomonas plésslit Perty. and M. producta Zach.—These two 
forms will be treated together, as in my opinion they are merely 
divergent variants—perhaps seasonal—of a single species. In 1898 
M. plossht was found but three times—in June and July—and M. 
producta eight times—from May through September. In 1897 the 
latter only was recorded, and in September and October. In 1896 
M. plosslit appeared in July and WM. producta in April and August. 
In 1895 M. producta alone was recorded, and that in November. 
The data are hardly sufficient for generalization, but so far as they 
go they indicate that producta is more prevalent in late summer and 
autumn and fpldssli in early summer, the more attenuate form 
(producta) in the warmer season. 
Butschli (80-89) has intimated that there may be some genetic 
connection between Mallomonas and Synura_ uvella. Certain 
features of its occurrence in our plankton lend their support to this 
view. Synura in our waters is a winter planktont, with December 
and February or March pulses. Mallomonas is a summer planktont, 
making its first appearance during the time of the decline of Synura, 
and when many of the colonies of the latter are breaking up into 
their individual zodids. Again, the differences in structure and size 
- between the two genera are quite superficial, and might result from 
the growth attending the free life of a Synura zoéid and its prepara- 
tion for sporulation. It is a noticeable phenomenon that the pro- 
portion of sporulating individuals of Mallomonas in the plankton is 
exceptionally large among all plankton organisms. “Free cells” 
of Synura are plainly referable to that genus by their resemblance, 
and by the fact that they are often united in clusters of several indi- 
viduals forming fragments of disintegrating colonies. It may be 
that some reproductive phase, as conjugation, intervenes between 
the free-cell condition of Synura and the Mallomonas stage, and 
that the relatively smaller numbers of the latter are due to the in- 
frequency of this process. While the features of seasonal distribu- 
(7) 
