61 
seems not to lie near either extreme, and above rather than below 
the average for the year. 
Clostertum gracile Bréb.*—Average number, 49,616 (silk, 305). 
This species was found in small numbers from March to December, 
and shows pulses on May 17 (1,600) and September 27 (6,400) at 
temperatures of 64° and 73°. The tenuity of the form of the 
frustule of this species suggests a limnetic habit. 
Closterium lunula Ehrbg.—Average number, 556. This also is 
a perennial species, and is somewhat more abundant and constant 
than C. acerosum. It likewise has a vernal pulse, which in 1895 
appears on April 29 (2,915); in 1896, on May 1 (5,364); in 1897, on 
May 25 (3,200); and in 1898, on May 24 (6,000). In both this 
species and C. acerosum there are slight indications of recurrent 
minor pulses which are often coincident in the two species. Nine 
such movements appear in 1898. The autumnal pulses are less 
regular in their appearance and size than the vernal, and appear 
from September to November. The optimum temperatures seem 
to lie between 45° and 70°. This species is only semi-limnetic, and 
never attains the fluctuations which characterize most lmnetic 
organisms. Doubtless other so-called species of Clostertum have 
been included among the variable organisms referred here to C. 
lunula and C. acerosum. 
Cosmarium constrictum Delp. was found occasionally from March 
to September, and is probably adventitious. 
Cosmocladium saxonicum De By.—A single isolated pulse of this 
minute limnetic desmid appeared in the filter collections of Septem- 
ber, 1897. It was first noted on August 31 and disappeared after 
September 29, and was never found at other times in the plankton. 
The pulse culminated September 9 at 13,500,000*. 
Gonatozygon brebissonit De By.—The filaments of this desmid 
were noted in the plankton only in March, 1899, attaining a maxi- 
mum of 136,800 on the 14th. 
Staurastrum gracile Ralfs——Average number, 31. About two 
hundred times as abundant in the plankton of 1897. It occurs from 
March to January. No vernal pulse was detected, but an autumnal 
one of 14,000 appears September 29. It appears in much larger 
numbers in the filter-paper collections, and is probably a limnetic 
planktont in our waters. 
