[3] Monactinus forms flat colonies, termed coenobia, of cells in powers of two, usually four, eight, or 16.

Coenobia are disc-shaped in outline, and marginal cells are drawn out into one lobe or point.

This distinguishes it from Pediastrum and closely related genera, which have marginal cells which are drawn out into two lobes or points.

[4] Although Monactinus was first described by August Carl Joseph Corda in 1839, for most of its history it was considered to be a synonym of Pediastrum.

In 2005, molecular phylogenetic analyses showed that Pediastrum sensu lato was paraphyletic with respect to Hydrodictyon.