68 
viridis is the most abundant, and it is associated with other species 
of the genus, with species of Amiblyophis, Phacus, Lepocinclis, Chlo- 
ropeltis, Colactum, and Trachelomonas, especially the latter. 
The Peridinude are quantitatively of considerable importance 
in the plankton of our Great Lakes (Kofoid, ’95), but in the Illinois 
River they are of little significance, at least the larger forms such as 
Ceratium. Smaller species such as Peridinium tabulatum and 
Glenodinium cinctum are more abundant. As a group they do not 
show any marked seasonal preferences. 
The Volvocide, on the other hand, are of more than the usual 
consequence in the plankton of the Illinois. The group is repre- . 
sented by the curious Chloraster gyrans, by the sporadic and meteor- 
ic Carteria multifilis, and by the colonial genera Eudorina, Pando- 
rina, Pleodorina, Platydorina, and Volvox. As a group they are 
almost exclusively summer planktonts. 
The Mastigophora as a whole are, next to the Bacillariacee, the 
most abundant of the synthetic organisms of the plankton. Their 
quantitative importance has not hitherto been sufficiently demon- 
strated in the plankton of fresh water, owing it may be to their 
escape through the silk net in the ordinary methods of collection. 
It seems quite probable also that they may be present in our warm 
and fertile waters in much greater abundance than they are in the 
colder and clearer waters of most lakes. This is especially true of 
the Euglenide and Volvocide, perhaps less so of the Chrysomonadide 
and Peridiniude. 
DISCUSSION OF SPECIES OF MASTIGOPHORA. 
Amblyophts viridis Ehrbg.*—Average number, 63,014 in 1897. 
It occurred throughout the summer in 1897, from May to October, 
with a maximum of 1,440,000 on August 31. Apparently a sum- 
mer planktont but never very abundant. 
Anthophysa vegetans (O. F. Mull.) Butschlii—This was identi- 
fied in the plankton of June, 1898. It is very abundant at times on 
various substrata in stagnating water, and from such places becomes 
adventitious in detached fragments of colonies in the plankton. 
Asterosiga radiata Zach.—This interesting colonial and limnetic 
choanoflagellate, described originally from the plankton of German 
lakes, has been found but a single time in our plankton—in the latter 
