49 



ber 27 at 57,600,000; on October 25 at 25,200,000; and in December 

 a pulse well sustained throughout the month culminates on the 15th 

 at 414,000,000. 



The temperature optimum appears to be about 60, though its 

 return in the autumn does not induce a development comparable 

 with that of the closing days of April. The midsummer pulses and 

 that of December show that other causes than temperature are 

 operative in regulating the occurrence of this organism. 



The appearance of the vernal pulse of Cyclotella at the time of 

 the volumetric maximum (Pt. I., PL IX.-XII.) in April-May sug- 

 gests its function as one of the primal sources of food for the animal 

 components of that plankton. The plates are based on collections 

 of the silk net, and Cyclotella constitutes an insignificant part of the 

 volumetric total there graphically presented, since it is so small that 

 it escapes readily through the silk. 



Cymatopleura solea (Breb.) W. Sm.* Average number, 2,115 

 (silk, 1,292), but slightly more abundant in 1897. Isolated occur- 

 rences in small numbers appear during the colder months, generally 

 below 60, though several individuals appear in summer records. 

 This is apparently an adventitious planktont, whose presence is 

 often due to flood waters. 



Diatoma elongation var. tenue Van Heurck.* Average number, 

 2,471,923. This is a perennial limnetic diatom occurring in every 

 month of the year and in the majority of our collections. It is but 

 sparingly present during midsummer. There are well-defined 

 vernal pulses in 1897 on May 25 of 50,400,000, and in 1898 on May 

 3 of 18,000,000. A second large pulse appears on the approach of 

 winter, in 1897, on November 15, culminating at 2,700,000, and in 

 1898, on November 22, at 9,000,000. In the silk collections of 1895 

 and 1896 pulses also appear in the last days of April and in Novem- 

 ber or December. The records thus indicate a decided preference 

 of the species for temperatures below 70 and the possibility of 

 rapid development in midwinter as in 1895, during a fortnight of 

 minimum temperatures (32 + ), culminating at 53,424 (silk) Decem- 

 ber 18. The vernal pulses coincide approximately with the volu- 

 metric maximum, and the December pulse of 1895 attends an 

 unusual winter development of the plankton (Pt. I., PI. IX. and 

 Table III.). 



