48 
in every month of the year. It is somewhat more prevalent in 
spring and autumn, and there are indications of a vernal pulse in 
May and an autumnal one in September, separated by prolonged 
midsummer and midwinter minima. Vernal pulses appear in 1897 
on June 28 at 14,400,000, and in 1898 on May 17 at 7,200,000. Autum- 
nal pulses occur in 1896 on September 16 at 2,700,000; in 1897 on 
September 29 at 10,800,000; and in 1898 on September 13 at 
5,400,000. The optimum temperatures lie between 60° and 75°, the 
autumnal pulse appearing in higher temperatures than the vernal as 
a rule. This diatom is reported as often epiphytic upon alge, and 
it may be wholly adventitious in the plankton. There is nothing, 
however, in the curve of its distribution to corroborate this view. 
Cyclotella Ruetzingiana Thw.*—Average number 243,659,615, 
but slightly more abundant in the preceding year. This is one of the 
smallest as well as one of the most abundant of all the diatoms of the 
river plankton. It readily escapes through the meshes of the silk 
net, and plankton collections made by this means give no adequate 
conception of its prevalence or importance in the ecology of the 
plankton. It appears in every month in the year and in practically 
all of our collections, and is thus a perennial planktont. ‘There is a 
considerable variation in size among the individuals in the plankton, 
but the greater number lie near the smaller rather than the larger 
limits. It may be that several species have been combined in the 
enumeration. pe 
The fluctuations in the seasonal distribution of this diatom are 
considerable, and pulses occur at all seasons of the year. The vernal 
pulse is, however, preéminent, and is not approached in magnitude 
by those of any other season of the year. In 1897 this pulse culmi- 
nates at 5,724,000,000 on April 27, and in 1898 on April 26 at 2,880,- 
000,000. Throughout the summer and autumn in both years there 
is a series of minor pulses at intervals of two to eight weeks. In 1897 
an autumnal pulse of 223,200,000 appears on September 29, and 
though not of greater magnitude than two previous summer pulses, 
it does surpass anything prior to the pulse of the following spring. 
In 1898 there are seven pulses during the summer and fall, culmi- 
nating as follows: on May 10 at 2,668,000,000;0n June 28 at 291,- 
000,000; on July 19 at 561,600,000; on August 9 at 401,400,000; on 
August 23 at 122,400,000; on September 6 at 115,200,000; on Septem- 
