56 
in September in 1898; 1n November in 1896, 1897, and 1898; and in 
December in 1894. 
The optimum temperatures, omitting the pulse of August 30, 
1897, at 80°, all lie below 72°, averaging 65° for the vernal pulse 
and 62° for the autumnal. But three pulses in all, exceeding 
100,000, lie at temperatures above 70°, and but three below 50°. 
In the case of this species likewise temperatures seem to be potent 
factors in limiting its seasonal occurrence. The fluctuations in 
nitrates do not seem to bear any constant relation to its develop- 
ment. The midsummer minimum of the diatom may appear, as 
in 1896, during an abundance of nitrates (0.5 to 3.0 parts per mul- 
lion—Pt. I., Pl. XLIII.) unusual for the season. On the other hand, 
a minimum of nitrates (.1 to .35) in August and December, 1898, 
coincides with a suppression of this species in the plankton. Thus 
in the presence of food, temperature seems to be a determining 
factor in the seasonal distribution of this organism. Whipple (94) 
expresses the opinion that the growth of diatoms occurs at those 
seasons of the year when the water is in vertical circulation; that is, 
when it passes 39.2°. In our waters this generally occurs early in 
March and late in November. In this species the only pulses 
which it seems might exhibit the effect of this phenomenon are 
those of December and March, and neither of them are in any way 
constant or prominent. Neither of the major pulses, vernal nor 
autumnal, can be attributed to it. The latter pulse occurs prior 
to the autumnal overturning of the water. 
The vernal pulse usually follows the spring volumetric maxt- 
mum,and the autumnal one generally appears during a volumetric 
minimum. No immediate quantitative effect of this species upon 
the plankton 1s apparent. 
In European waters this is a common planktont, and Apstein 
(96) reports vernal maxima in March, April, and May, and an 
autumnal one of minor value in November. 
The number of cells in the filaments varies from one to sixty, 
and in filter-paper collections averages four, while in the silk 
catches it varies from seven to fifteen from year to year. The fila- 
ments average somewhat longer during the periods of maximum ° 
growth, reaching twelve to twenty-five. This species also occasion- 
ally bears the flagellates found upon M. granulata var. spinosa, but 
not in such abundance. It is quantitatively much less important 
