Dr. C. A. Kofoid on Platydorina. 543 



On August 2, 1888, Professor H. Garni an, while con- 

 ducting a biological survey of the aquatic life in the vicinity 

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

 Garraan, 1890), found a specimen of this interesting species 

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

 notes now on file 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 

 recurrence 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 

 characterized by an abundance of flagellates, of rotifers — 

 especially Braehionidas — and of immature (Jopepoda. A 

 water-bloom, composed largely of Euglena, Trackelomonas, 

 Carteria, and other green flagellates, often appears at the 

 suiface of waters where Platydurina is abundant. Qonium 

 is frequently associated with it in large numbers, as are also 

 Pandorina, Eudorina, and Pleodorina, though these three 

 genera may also be plentiful in the early summer, when 

 Platydorina may be absent or rare. Pleodorina californica 

 was extremely abundant in Salt Fork in August, but had 

 almost entirely disappeared by the time that Platydorina had 

 reached its maximum. A few specimens of Volvox, which, 

 in this locality, is common in the spring months, were also 

 noted, while perhaps the most interesting associate in Salt 

 Fork was Ceratium kumaonense, discovered by Carter (1871) 

 in Hindostan. Other chlorophyll-bearing associates fre- 

 quently seen are Pediastrum, IScenedesmus , Actinastrum, and 

 Closteriwn. Among the diatoms, Melosira, Fragillaria, and 

 Surirella were to be seen ; and among the Peridinidaj Peri- 

 dinium tabulatum was almost always represented. 



The zooplankton associated with Platydorina is not less 

 varied than the phytoplankton. The Protozoa were usually 

 represented by Arcella, Difflugia, and occasionally by pelagic 

 Amoeba ; by Synura, Mallomonas, Dinobryon, and Uroglena; 

 and by Codouella and Coleps. Among the Rotifera the order 

 Ploima was well represented; Triarthra and the Brachionidse 

 — notablv Brachionus militaris, B. angular is, B. punctatus, 

 and B. Bakeri and its varieties — were most common during 

 the summer months, while the Synchajtidge increase in numbers 

 in the early autumn. Polyarthra was frequently abundant, 



