51 
which the species dwindles away and disappears in August to return 
early in November. The winter pulse culminates December 14 at 
8,159,250, at a break in the ice blockade. In 1898 the winter mini- 
mum continues into April, and the vernal pulse appears May 10 at 
253,960,000, rising with rocket-like suddenness from 390,000 of the 
previous week, and declining the week following to 4,110,400. The 
decline to the summer minimum is prolonged into July, and the 
species does not reappear until October. The winter pulse begins 
earlier than usual, on November 1, and is well sustained through 
the month, culminating on the 29th at 2,254,000. The winter mini- 
mum which follows, does not reach the low levels of that of summer. 
This species has thus a characteristic distribution, the analysis 
of which is by no means simple. The contrast between the summer 
and winter minimum may be due to the low nitrates of the summer 
and the larger amount in the winter (Pt. I., Pl. XLIII.—XLV.), 
which favor a proportionate development of this diatom, though 
not every species shows this response. The two minima separate 
the seasonal occurrences of this species into two periods of growth; 
a vernal, from March to June, and a hiemal, from October to Janu- 
ary, the limits and relative development of each being somewhat 
variable from year to year. The temperatures of the-two periods 
differ. Botharetimesofrapid change,—ofriseand fall respectively,— 
and the culminations of the periods of growth lie at widely sepa- 
rated temperatures. The vernal pulses in 1896 and 1898—-in which 
years collections were frequent enough to locate them with some 
degree of accuracy—appear at 72° (April 24) and 61° (May 10) 
respectively, and in every year the vernal pulse appears during a 
period of rapid change. The hiemal pulse, on the other hand, cul- 
minates in each year after the winter minimum approaching 32° has 
been reached, and in two years during the ice blockade. Tempera- 
ture within these limits seems not to be a determining factor in the 
pulses of this organism. ‘The nitrates (Pt. I., Pl. XLIII.—XLV.) 
have been uniformly high (above 2 parts per million) whenever the 
qiises occurred. In 1898 they decline abruptly (Pt. I., Pl. XLV) 
and remain at a low level throughout December, and in this month, 
when usually Fragilaria attains its hiemal maximum, we find it 
dropping to the unusual minimum of 20,000. The pulse which 
began in November is cut off apparently by this unusual decline in 
nitrates. Abundance in nitrates is not, however, in itself sufficient 
(5) 
