139 



m. 3 on April 24, 1896, 2,287,160 on May 25, 1897, and the maximum 

 record of all years, 5,247,800, on May 3, 1898. Pulses in excess of 

 1,000,000 per m. 3 occur 14 times in our records: in July, August, 

 November, and December in 1895; in April, 1896; in April, May, 

 September, and October in 1897; and in May, June, August, Sep- 

 tember, and October in 1898. There is, apparently, in years or 

 seasons best represented in our records, a tendency for a vernal 

 pulse, often the maximum one of the year, to occur in April-May, 

 and for an autumnal pulse of large amplitude to appear between the 

 last of August and the middle of October. The pulses contiguous 

 to these major pulses of the year are often of considerable magnitude ; 

 as, for example, in 1897, when the maximum of September 7 (5,121,- 

 000) is followed by another large pulse on October 12 (2,906,400), 

 and in 1898, when the vernal pulse of May 3 (5,247,800) is followed 

 by a June pulse, on the 21st, of large amplitude (2,601,200). The 

 recurrent character of the pulses appears throughout maximum and 

 minimum periods, and may be traced in Plates III. and IV. In the 

 period of 15 months from July, 1895, to October, 1896, there are 10 

 such pulses, and 6 months in which pulses do not appear. In the 21 

 months from July, 1897, to March, 1899, there are 18 pulses, and 

 3 months in which they do not occur. They often coincide with or 

 approximate those of the Entomostraca (PL III. and IV.) and of the 

 chlorophyll-bearing organisms (PL I. and II.). 



With the exceptions of the November-December pulses of 1895 

 at 33 (1,595,359 on November 27 and 1,636,640 on December 11) 

 and the pulse of October 25 (1,048,620) at 48, no pulse of con- 

 siderable amplitude is found at temperatures much below 60 in 

 channel waters. 



In the discussion which follows, 104 forms are listed, 6 belonging 

 to the Rhizota, 6 to the Bdelloida, 91 to the Ploima, and 1 to the 

 Scirtopoda. 



RHIZOTA. 



The Rhizofa by virtue of their fixed habit are represented in the 

 plankton either by adventitious species, torn from their location on 

 water plants or other aquatic substrata by disturbances in the water, 

 or by colonial species with a free-swimming habit, such as Conochilus. 

 As represented by the latter type they are of some quantitative im- 



