30 
by the experimental method. The data here cited are suggestive 
only; not conclusive. 
The relation of Pediastrum to the volumetric pulses of the plank- 
ton is not a constant one, though there 1s some correspondence in 
their fluctuations. The extreme maximum (3,264,800) of June 19, 
1895, is coincident with a plankton pulse of 30.42 cm.*, but the num- 
ber of collections is insufficient to show the relative fluctuations of 
the plankton and Pedzastrum at that season. In May and June, 
1897, and in October, 1898, the Pediastrum pulses culminate shortly 
after the volumetric pulses. In July and September, 1897, and 
in August, 1898, they coincide. 
Polyedrium trigonum Nag.*—Average number, 432,692. Ap- 
pears from June through September, disappearing when falling 
temperatures reach 60°. In 1897 it continues through October 
with the higher temperatures (averaging 65°) of that year: There 
are slight indications of a September pulse. 
Polyedrium trigonum forma minus Reinsch and var. tetragonum 
(Nag.) Rabh., P. bifurcatum Wille, and P. gracile Reinsch, were 
also recorded in a few collections during the period of occurrence 
of P. trigonum. They are all evidently summer planktonts. 
Raphidium polymorphum Fresen.*—Average number, 21,450,000. 
Occurs in every month of the year and in a majority of the collec- 
tions. In 1897 a vernal maximum of 201,600,000 occurs on April 
27 and an autumnal one of 28,800,000 on September 21. In 1898 
a vernal pulse culminates May 3 at 24,000,000, and thereafter 
throughout the summer at intervals of three to six weeks there 
occur five other pulses, the greatest of which culminates July 19 at 
75,600,000. A pulse of 90,000,000 on a declining flood in February, 
1899, indicates an adaptation on the part of this organism to the 
whole range of temperatures. A pulse of 25,200,000 December 3, 
1896, further illustrates this adaptability. Records in 1897 and 
1898, however, suggest that the optimum lies above 60°. It is thus 
a perennial planktont. 
Raphidium longissimum B. Schréder.—Appeared sparingly in 
February, August, October, and December, suggesting that it has 
also a perennial distribution. 
Richteriella botryoides _(Schmidle) Lemm.*—Average num- 
ber, 6,399,705 (in 1897). From May to November, with a vernal 
pulse of 25,200,000 on May 25, and an autumnal one of 100,800,000 
