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. [X.—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 (Bréb.) W. Sm.*—Average number, 2,115 
(silk, 1,292), but shghtly 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. 
Datoma elongatum 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., Pl. IX. and 
fable TI: 
