68 



viridis is the most abundant, and it is associated with other species 

 of the genus, with species of Amblyophis, Phacus, Lepocindis, Chlo- 

 ropeltis, Colacium, and Trachelomonas , especially the latter. 



The PeridimidcB 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 cinctwn are more abundant. As a group they do not 

 show any marked seasonal preferences. 



The Volvocida, 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 multifttis, 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 BacillariacecB, 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 EuglenidcB and Volvocidcz, perhaps less so of the ChrysomonadidcB 

 and Peridiniidce. 



DISCUSSION OF SPECIES OF MASTIGOPHORA. 



Amblyophis 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.) Butschli. 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. 



Aster osiga 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 



