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Cetraria (order Lecanorales) A genus of lichens in which the thallus is foliose to more or less fruticose with erect, flattened branches. Rhizines may be present. Apothecia are lecanoriae and occur at or near the edges of the thallus in foliose species. The spores are colourless and aseptate. They are found on the ground, on trees, on rocks, etc., according to species. See also ICELAND MOSS.