Article IX. — Plankton Studies. III. On Platydorina, a 

 new Genus of the Family Volvocidce, from the Plankton of 

 the Illinois River. By C. A. Kofoid. 



The family Volvocidce is well represented in the plankton 

 of fresh- water ponds and streams. Indeed, with the possible 

 exception of Stephano splicer a, all of the colonial forms in- 

 cluded in the subfamily Volvocince — Spoiidylomorum, Gonium, 

 Stephanosphcera, Pandorina, Pleodorina, and Volvox — are 

 pelagic in habit and are found only in the fresh-water 

 environment. For the past four years, during the summer 

 and autumn months, a colonial form belonging to this sub- 

 family has occured in plankton collections from the Illinois 

 River and its adjacent waters, to which I have given the 

 name of Platydorina caudata. It appears as early as June 15, 

 and becomes abundant in the months of August and Septem- 

 ber, diminishing in numbers in October, and disappearing 

 in November. It thus attains its greatest development 

 toward the close of the maximum period of summer heat, 

 when the temperature of the water in which it is found often 

 reaches 36° C. This species has occurred in all the waters 

 examined in the course of the operations of the Illinois 

 Biological Station; viz. in the Illinois River, in Thompson's, 

 Quiver, Flag, Mantanzas and Phelps Lakes, at Havana, 111., 

 and in the Illinois River and Meredosia Lake at Meredosia, 

 111. During the summer and fall of 1899 it was also found 

 in abundance near Urbana, 111., in Salt Fork, a small stream 

 tributary to the Embarras River — a confluent of the Wabash. 

 'It was not equally plentiful in all these localities, but showed 

 a decided preference for shallow water free from vegetation, 

 reaching its maximum development when the turbid water 

 was but a few feet, or even less than a foot, deep. In such 

 situations the shallowness of the water and the absence of 

 vegetation conduce to a maintenance of the high temperatures 

 which seem to favor its multiplication. The bottom of the lakes 

 in question is usually composed of soft mud, rich in decay- 

 ing organic matter and often covered by a mat of Oseillaria, 

 but otherwise quite free from vegetation. At Havana 



