Cosmarium tetragonum

Cosmarium tetragonum is a species of green algae in the family Desmidiaceae. It is a freshwater species with a worldwide distribution.

Description
The nominate variety (var. tetragonum) is about 35 to 48 μm long, and 20 to 26 μm wide, 14 to 18 μm thick, with an isthmus about 6 to 12 μm long. Semicells are roughly rectangular in outline, with undulate margins; each side has about four bumps and the top side has about two bumps.

Varieties
Cosmarium tetragonum is variable and several varieties have been described. They are:
 * Variety lundelii differs from the nominate variety that the lateral bumps are unequal; the two median bumps are larger.
 * Variety bipapillatum also has two median bumps that are larger, but also has a protuberance right at the base of the semicell, near the isthmus.
 * Variety ornatum has a crease that follows the margin of the cell; the crease may also have projections near the apex of the semicell.
 * Variety intermedium has stout cells that are 1.5 times as long as broad; semicells are more trapezoidal.