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	Cosmocladium	Euastrum	Micrasterias	Staurastrum	Staurodesmus	Xanthidium
Saccoderm:	Closteriinae		Mesotaeniaceae		Desmidiinae

KEY to Genera:

		Go to:
1a	Cells solitary, symmetrical		2
1b	Cells in colonies or linked together in filaments		15
2a	Cells more or less cylindrical		3
2b	Cells a more complex shape		9
3a	Cells with a median constriction		4
3b	Cells without a median constriction		6
4a	Median constriction slight	Penium Actinotaenium Mesotaenium
4b	Median constriction conspicuous		5
5a	Apex without a median notch	Pleurotaenium
5b	Apex with a median notch	Tetmemorus (no illustration available)
6a	Chloroplasts 2 simple ribbons	Gonatozygon
6b	Chloroplasts 2, stellate or ridged		7
7a	Walls smooth, without pores		8
7b	Walls with pores / granulation / striation		22
8a	Choloroplasts stellate	Cylindrocystis
8b	Chloroplast ridged with notched edges	Netrium
9a	Cells flattened from front to rear, with distinct median isthmus	(side view) (frontal view)	10
9b	Cells radially symmetrical, with distinct median isthmus	(end-on view)  (Frontal view)	14
10a	Lateral margins not incised		11
10b	Lateral margins slightly incised; apex with a deep notch or slightly depressed	Euastrum
10c	Lateral margins deeply incised	Micrasterias
11a	With marginal spines		12
11b	Without marginal spines		13
12a	Cell face has a central protuberance	Xanthidium
12b	No central protuberance	Staurodesmus
13a	Angles of semicells extended into processes	Staurastrum
13b	Angles of semicells not extended into processes	Cosmarium
14a	With spines at angles	Staurodesmus
14b	Angles of semicells extended into processes	Staurastrum
15a	Colonies of Cosmarium-like cells connected by mucilage strands	Cosmocladium
15b	Cells linked together as filaments		16
16a	Cells linked by apical processes		17
16b	Cells not linked by apical processes		18
17a	linked by a single apical process	Sphaerozosma
17b	linked by paired apical processes	Onychonema
18a	Cells flattened, not radially symmetrical in apical view		19
18b	Cells radially symmetrical in apical view: filament appearing to twist as cells are linked slightly offset from each other		20
19a	Cells divided by a deep median sinus, somewhat Cosmarium-like: linked together at the poles		21
19b	Cells rather barrel-shaped, without a median sinus. Filament in a mucilage sheath sometimes with a fibrillar structure clearly visible: single stellate chloroplast.	Hyalotheca
20a	Cell with a median notch	Desmidium
20b	Cells markedly barrel-shaped, with a shallow median sinus	Bambusina
21a	Cells linked by small apical granules	Teilingia
21b	Cells linked at flattened apices, no granules	Spondylosium planum
22a	Cells short, cylindrical	Penium
22b	Cells tapering, straight to strongly curved	Closterium

This key owes much to the diagrammetic key in Desmids of the English Lake District, by EM Lind and AJ Brook, Freshwater Biological Association, Ambleside

John Kinross