Polychidium is a genus of lichen-forming fungi in the family Massalongiaceae. These lichens form tiny, shrub-like tufts with branching filaments that create woolly cushions on moss-covered rocks and tree twigs. The genus was traditionally thought to include four similar-looking species, but molecular studies revealed that three of these actually belong to a different genus called Leptogidium. Today, Polychidium in the strict sense contains only P. muscicola and three closely related species, found from tropical to subarctic regions worldwide.

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