67 
shifting in the position of the maxima. Thus the pulse of January 
25, 1898, appears after a 35-day interval, but in the midst of the 
rising winter flood, to whose effect the delay may be attributed. 
In both 1896 and 1898 the 28-day rhythm is interrupted at the time 
of the vernal pulse in April-May. It appears as though these re- 
current pulses—if such exist—were submerged in the greater ver- 
nal increase. The double summit of the vernal pulse in the curve 
of the Bacillariacee and Mastigophora (Pl. 11.) for 1898 suggests the 
compound character of this pulse in the case of these groups of 
organisms at least. The time interval in the case of the vernal in- 
terruption is also significant. In 1898 there are two pulses between 
March 22 and July 19, at intervals of 42 days—a total of 84 days, 
which is the equivalent in duration of three 28-day intervals. 
The total number of species of Mastigophora recorded by me 
from the plankton of the Illinois River is over sixty. This number 
will be increased to more than seventy if forms not separated in our 
cnumerations be distinguished as separate species. 
The Protomastigina (including the Bicosecide and the Cras- 
pedomonadide) are well represented in the plankton by passive 
limnetic species which are principally sessile on other planktonts. 
These are Bicoseca lacustris, Salpingeca brunnea, S. minuta, and 
Diplosiga frequentissima. Asterosiga radiata is a eulimnetic repre- 
sentative and Anthophysa vegetans an adventitious one. As a group 
they are more abundant during the warmer part of the year. 
The Chrysomonadide are also well represented, and include the 
most abundant flagellates of the plankton of the colder months. 
Synura uvella is quantitatively the largest factor furnished by this 
group. It is supplemented by Syncrypta volvox, and the various 
forms of Dinobryon, Uroglena, and Mallomonas. The last two 
genera have more of a summer range of occurrence, but are not of 
quantitative importance in the waters of the Illinois. 
The Cryptomonadideé are represented only by Chilomonas and 
Cryptomonas, and are of somewhat constant, though of minor, 
importance quantitatively. 
The Euglenide, on the other hand, are, in our waters at least, 
second to no codrdinate group in their quantitative importance. 
They are individually of relatively large size, and they occur in 
great numbers throughout the summer months, replacing the 
Chrysomonadide of the colder seasons of the year. Euglena 
(6) 
