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 ChlorophycecE. 



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 

 greater 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 ccenobium 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 



