420 Illinois State Jjdhonttury of Natural History. 



we have found Platydorina in greatest numbers in Phelps 

 Lake, which in 1896, '97, and '98 afforded the conditions 

 above described. It was likewise abundant in Thompson's 

 Lake in the late summer and early fall of 1897 and '98, 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 drouth to a series of stagnant pools 

 with no vegetation. In the early part of August it was full 

 of algte and other aquatic vegetation, and Platydorina was 

 then present in considerable numbers, although not so abund- 

 ant as it was in the following month. 



On August 2, 1888, Professor H. Garman, while conduct- 

 ing a biological survey of the aquatic life, in the vicinity of 

 Quincy, 111., in the bottoms of the Mississippi Pdver (see 

 Garman '90), found a specimen of this interesting species in 

 the waters of Libby Lake. He records and sketches it in 

 notes now on tile at this Laboratory, but published nothing 

 concerning it. 



The occurrence of this new genus in the waters of the 

 Wabash, Illinois, and Mississippi river systems and its 

 recurreince in our collections for several successive years 

 indicate its wide distribution and firm establishment in the 

 Mississippi Valley in waters of some permanency. It has 

 not yet been noted in temporary pools. 



The associates of Platydorina in the plankton have varied 

 with the season, the locality, and the year. It may be said, in 

 a general way, that the plankton in which it occurs is charac- 

 terized by an abundance of flagellates, of rotifers, — especially 

 BrachionidfC, — and of immature Copcpoda. A water-bloom 

 composed largely of Euglcna, Trachelomonas, Carteria, and 

 other green flagellates, often appears at the surface of waters 

 where Platydorina is abundant. Goniuni is frequently 

 associated with it in large numbers, as are also Pandorina, 

 Endorina, and Pleodmina, i\\o\\g\\ these three genera may also 

 be plentiful in the early summer, when Platydorina may 



