There are two groups of Desmids: the "Placoderm" and the "Saccoderm" Desmids.

Placoderm desmids have cells composed of two halves (semicells), often almost divided from each other by deep median incisions. The cells are symmetrical about 3 planes, and often bear elaborate ornamentation. Genera are Cosmarium, Cosmocladium, Euastrum, Micrasterias, Staurastrum, Staurodesmus, Xanthidium.

Saccoderm desmids are simple shapes, cylindrical or spindle-shaped, rarely with any median constriction. This group forms the family Mesotaeniaceae and includes the genera Cylindrocystis, Mesotaenium, Netrium, Roya, Spirotaenia, Tortitaenia, and, in the suborder Closteriinae, Closterium, Gonatozygon and Penium, and in the suborder Desmidiinae, Actinotaenium.

	COSMARIUM	Semicells simple shapes, undissected, but margins may be crenellated. Apical view elliptical.
		Cosmarium blyttii
		Cosmarium botrytis
		Cosmarium brebissonii
		Cosmarium contractum
		Cosmarium cyclicum v. arcticum
		Cosmarium depressum
		Cosmarium formulosum
		Cosmarium impressulum
		Cosmarium ?depressum v. planctonicum
		Cosmarium portianum
		Cosmarium ?punctatulum
		Cosmarium ralfsii
		Cosmarium speciosum
		Cosmarium subcrenatum
	COSMOCLADIUM	colonial desmid in a spherical mucilage sheath; cells similar to Cosmarium, linked by threads of mucilage.
		Cosmocladium saxonicum

Cosmarium	Cosmocladium	Euastrum	Micrasterias	Staurastrum	Staurodesmus	Xanthidium
Saccoderm:	Closteriinae		Mesotaeniaceae		Desmidiinae

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John Kinross