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Gelidiales (division Rhodophyta) An order of red seaweeds, in which the thallus is filamentous, cylindrical or compressed, and typically is branched. Several species are of economic importance as sources of agar. There are two British genera, Gelidium and Pterocladia. Three British species of Gelidium are currently recognized, but these are notoriously variable and difficult to identify. They are very common in intertidal regions, often forming a fringe around rock pools.