Plankton Studies III. On Platydorina, A New Genus of the Family Volvocidae, from the Plankton of the Illinois River

Abstract

The family Volvocidce is well represented in the plankton of fresh-water ponds and streams. Indeed, with the possible exception of Steplianosphera, all of the colonial forms included in the subfamily Volvocince—Spondylomoruni , 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 subfamily 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 September, diminishing in numbers in October, and disappearing in November.