54 
XLIII.-L.). Other diatoms, as in the hiemal pulse of Fragilaria, 
develop in numbers at temperatures approaching 32°, but not M. 
granulata var. spinosa. Whipple (94) concludes from the records of 
examinations of potable waters in Massachusetts that temperature 
has possibly a slight influence on the growth of diatoms, but that it is 
of so little importance that it does not affect their seasonal distribu- 
tion; and, on the other hand, that a sufficient supply of nitrates is one 
of the most important conditions for their growth. The seasonal dis- 
tribution of Melostra was not separately discussed in his paper 
though included in his general statements. In our waters the data 
at hand seem to show conclusively that abundance of nitrates is of no 
avail in the case of Melosira when the temperature falls below 60°. 
There are times, therefore, in the case of this, our most important 
diatom, when temperature is more potent than food as a factor con- 
trolling its growth. 
Melosira does not appear in its maximum pulses at the time of the 
major volumetric pulse of the total plankton of April-May, nor do 
its fluctuations seem to bring about directly any considerable 
changes in the volume of the plankton. For example, the extreme 
pulse of 111,456,000 on July 21, 1897, occurs at the time of a sudden 
drop in the amount of plankton (Pt. I., Pl. Xi). ~ The amount ior 
plankton on July 14, 21, and 30 is 8.16, 0.92, and 1.05 cm.* per m:*%, 
and the corresponding numbers of Melosira are 66,528,000, 111,456,- 
000, and 13,176,000. 
The diatoms here discussed are predominantly of the type 
designated as var. spinosa, marked by the spinous prolongations 
from the valves at the ends of the filaments. The cells of the forms 
in our plankton are proportionately much longer, as a rule, than 
those figured by Schréder ('97), usually attaining one and a half to 
two times the length without proportional increase in diameter. 
Not infrequently in the height of the growing season much elongated 
and curved cells and filaments are to be found. In one instance an 
unusual number of filaments approaching M. varians in form though 
still of the spinous type were found. It is not improbable that 
several so-called species of Melostra have been included with this 
variable species in the enumeration. 
Melosira is the bearer of numerous passive planktonts, the most 
abundant of which is Bicosweca lacustris Clk. Associated with this, 
and often on the same filament, is the elegant little craspemonad 
