43 
of the plankton, and perhaps with the result of increased photo- 
synthesisin periods of lunar illumination, which tends to establish the 
limits of the pulses. 
The number of forms of diatoms noted in our records in the 
plankton of the Illinois River is thirty-one. This number could be 
greatly increased by the inclusion of the many adventitious species 
which flood-waters bring into the plankton and by the addition of 
rarer limnetic species. Of these thirty-one at least twelve are 
eulimnetic, while the others are in the main adventitious. There 
are no species among them peculiar to the potamoplankton, and 
the dominant forms here are also abundant in the fresh-water plank- 
ton of our own Great Lakes and of European streams and lakes, 
barring a few mooted points of specific identity. 
The limnetic species are fourteen in number, viz.: Astertonella 
formosa, A. gracillima, Cyclotella kuetzingiana, Diatoma elongatum 
var.tenue, Fragilaria crotonensis, F. virescens, Melosira granulata 
var. spinosa, M. varians, Meridion circulare, Rhizosolema ertensts, 
Stephanodiscus niagare, Synedra acus, S. acus var. delicatissima, 
and Tabellaria fenestrata. Of these limnetic forms the more impor- 
tant ones are Asterionella gracillima, Cyclotella, Fragilaria virescens, 
Melosira granulata var. spinosa, and Synedra acus and its varieties. 
The absence or small number of certain limnetic species is notice- 
able. These are several species of Tabellaria and Attheya. On ac- 
count of the abundance of silt and the transparency of Attheya it 
may have been overlooked. It has hitherto been reported from 
waters much nearer the sea, and this coupled with its affinities to 
marine diatoms may explain its absence in our waters. 
The remainder of the forms are adventitious, or largely so, and 
With the exception of the species of Navicula they have little effect 
upon the ecology or quantity of the potamoplankton. 
DISCUSSION OF SPECIES OF BACILLARIACE#. 
Asterionella formosa Hassall.—Average number of individual 
cells, 960. Average size of colony, 4.8 cells. Recorded only in 
November, December, and from February through April, and never 
in large numbers. The greatest pulse attained at any time cul- 
minated on March 30, 1896, at 54,540. Aside from an isolated 
occurrence on June 27, 1896, no individuals were recorded at tem- 
peratures above 48°, and three fourths of the occurrences are at 
