31 



on September 21. Optimum temperature about 70, and disappear- 

 ing from our records below 60. 



Scenedesmus bijugatus (Turp.) Kiitz.* 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 genuimts 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.) Kiitz.* 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 be a summer planktont 

 with no marked preference for the lower temperatures of spring and 

 autumn. 



Scenedesmus quadricauda (Turp.) Breb.* Average number, 

 9,276,923 (silk, 8,611). In this species, as in the case of others of 

 the genus and of the Chlorophycecs 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 



