542 Dr. C. A. Kofoid on Platydorina. 



exception of Stephanosphwra, all of the colonial forms in- 

 cluded in the subfamily Volvocinas — Spondylomorum, Gonium, 

 StephanosjjJicera, Pandurina, Pleodorina, and Volvox — are 

 pelagic in habit and are found only in the freshwater 

 environment. For the past four years, during the summer 

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

 family has occurred 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 September, 

 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, Matanzas, 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 Vermilion 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 favour its multiplication. The bottom of the 

 lakes in question is usually composed of soft mud, rich in 

 decaying organic matter, and often covered by a mat of 

 Oscillaria, but otherwise quite free from vegetation. At 

 Havana we have found Platydorina in greatest numbers in 

 Phelps Lake, which in 1896, 1897, and 1898 afforded the 

 conditions above described. It was likewise abundant in 

 Thompson's Lake in the late summer and early fall of 1897 

 and 1898, when the lake was at a low level and contained 

 little vegetation. In the shallow open waters of Matanzas 

 Lake it was much more abundant than in Quiver Lake, where 

 there was usually a large amount of vegetation. At the 

 time of the maximum abundance of Platydorina in Salt Fork 

 in September the stream was reduced by drought to a series 

 of stagnant pools with no vegetation. In the early part of 

 August it was lull of algEe and other aquatic vegetation, and 

 Platydorina was then present in considerable numbers, 

 although not so abundant as it was in the following month. 



