Si 
on September 21. Optimum temperature about 70°, and disappear- 
ing from our records below 60°. 
Scenedesmus bijugatus (Turp.) Kutz.*—Average number, 155,769. 
Sparingly from May till the close of September, with slight traces of 
vernal and autumnal pulses. 
Scenedesmus denticulatus Lagerh.*—Average number, 86,538. 
A few occurrences in late summer and early autumn. 
Scenedesmus genuinus Kirchner.*—Average number, 778,846. 
From May till the first of October, but continued through this 
month in 1897. Vernal pulse not observed, though the autumnal 
pulse attains 28,800,000 on September 21 and October 26, 1897. 
Midsummer pulses appear in 1897 on July 14 (16,200,000), August 
17 (14,400,000), and in 1898 on August 9 (19,800,000). Optimum 
temperatures lie above 60°, though an occurrence in December in- 
dicates the adaptability of this organism to lower temperatures. 
Scenedesmus obliquus (Turp.) Kttz.*—Average number, 1,505,769 
(silk, 673). This form appears in our records from the last of April 
until the middle of November. Traces of vernal and autumnal 
pulses appear in both 1897 and 1898, with intervening midsummer 
fluctuations of even greater magnitude. In 1897 the vernal pulse 
on May 25 reaches 3,600,000; a midsummer one on August 10, 
5,400,000; and the autumnal one appears twice, once on September 
21 at 28,800,000, and again on October 19 at 25,200,000. In 1898 
the vernal pulse appears May 10 at 1,800,000; midsummer ones, on 
July 19 at 10,800,000, and August 9 at 36,000,000; and the autumnal 
on September 9 at 8,100,000. As in some other organisms, these 
pulses are separated by intervals of three to six weeks. The 
optimum temperatures lie above 60°, though development begins 
before that temperature is reached, and the impetus of the autumnal 
pulse, or acclimatization to lower temperatures, carries the species 
beyond this limit into temperatures of 45°. There is a marked 
absence of pulses below 60°. This seems to bea summer planktont 
with no marked preference for the lower temperatures of spring and 
autumn. 
Scenedesmus quadricauda (Turp.) Bréb.*—Average number, 
9,276,923 (silk, 8,611). In this species, as in the case of others of 
the genus and of the Chlorophycee generally, the numbers present in _ 
1897 were much greater than in 1898 (32,492,647,* silk, 5,818). 
Prolonged low water and concentration of sewage afforded stable 
