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DescriptionThe form of this calcareous alga is very variable. It occurs in two main forms, a thin, hard crust on hard substrata as well as an unattached, fragile, branched nodules. When young, the crustose form is smooth with some scattered young mounds but develops branches with age. The loose-lying nodules may form dense beds of algal gravel. Encrusting individuals may reach up to 20 cm across and free-living plants may reach 4 - 5 cm across. In the free-living form the branches are up to 4 mm in diameter and 15 mm in length. The plants, when alive, are reddish to deep pink in colour with a violet tinge and white when dead.This genus was previously called Lithothamnium but now Lithothamnion is the preferred name. Previous classifications included two varieties (sometimes formerly given species status): Lithothamnium granii (Foslie); and Lithothamnium colliculosum. It is quite difficult to differentiate between Lithothamnion glaciale and Lithothamnion corallioides. The hard surface and the absence of numerous surface mounds on Lithothamnion glaciale may help separate them although for greater accuracy the cortical cell structure should be used.