61 



seems not to lie near either extreme, and above rather than below 

 the average for the year. 



Closterium gracile Breb.* 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 hmula 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 limnetic 

 organisms. Doubtless other so-called species of Closterium have 

 been included among the variable organisms referred here to C. 

 lumila 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 brebissonii 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. 



