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Scytosiphonales (division Phaeophyta) An order of algae in which the thallus is parenchymatous and growth is mostly diffuse. Scytosiphon lomentaria is a common British species, consisting of loosely tufted, tubular fronds. Petalonia has flat, leaf-like fronds and is found mainly on rocks on the lower shore. Colpomenia has a thallus consisting of thin-walled hollow spheres. See also OYSTER THIEF.