Cliostomum is a genus of lichen-forming fungi in the family Ramalinaceae. It has about 20 species. The genus was established in 1825 by Elias Magnus Fries, who characterized it by its rounded fruiting bodies that are integrated into the lichen's crust with distinctive folded or corrugated openings. These lichens form tight crusts on various surfaces, ranging in colour from whitish to pale grey or yellowish, and reproduce primarily through conspicuous flask-shaped structures that release spores rather than through sexual fruiting bodies.

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