26 
above 70°, though isolated occurrences in March and December in- 
dicate its presence throughout the whole range of temperatures. 
It appears to be a summer planktont without the marked prefer- 
ence for the close of the period of maximum heat noted in some 
other Chlorophycee. 
Oocystis solitaria Wittr.*—Average number, 121,153. In 1897 
much more abundant, averaging 2,170,588. In this year it 
occurs in numbers above 1,000,000 from the end of July till the end 
of October, reaching a maximum of 36,000,000 on September 21, 
1897. Its optimum conditions occur during the latter part of the 
period of maximum heat, at temperatures approaching 80°. It 
disappears at 60°, save for isolated appearances in December, at 
33°— a fact which suggests its persistence in small numbers 
thoughout the year. It is characteristic of the plankton of late 
summer,—that is, of low water, high temperatures, and stable con- 
ditions. 
Ophiocytium capitatum Wolle*.—Average number, 1,465,385. 
More abundant in 1897, averaging 2,858,823. Present from the 
last of April until the beginning of November. There is some indi- 
cation of a vernal pulse, which on May 25, 1897, attains 3,600,000, 
and on April 26, 1898, 10,800,000. The major pulse appears in 
late summer or early autumn, attaining 57,600,000 on September 21, 
1897, and 28,800,000 on August 9, 1898. The two pulses are 
separated by an interval in which occurrences are less frequent and’ 
numbers smaller. This planktont thus exhibits the tendency 
towards seasonal maxima near the average temperature. The 
ereater development in 1897 is followed by a prolongation of the 
occurrences into November. The optimum temperature appears 
to be about 60° or above, the vernal pulse appearing at that tem- 
perature, and the major one at 71°. No records occur below 46°. 
Pediastrum boryanum (Turp.) Menegh.—Average number, 4,510. 
This alga was found in every month of the year, though not in 
every collection examined. The numbers present fluctuate greatly 
and are usually much less than those of P. pertusum, with which it is 
associated, and with which it fluctuates, often with remarkable 
coincidence. I have included under this head those individuals 
in which the coenobium is a plate with no intercellular spaces or 
only -insignificant ones. Individuals are not lacking which serve 
to connect this species with P. pertusum, and, indeed, with others 
