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Peltigera (order Peltigerales) A genus of lichens, in which the thallus is foliose and often large. The lower surface is loosely downy, and marked with more or less prominent, vein-like ridges. The phycobiont may be a green alga or a cyanobacterium, according to species. Apothecia are formed on the upper surface at the edges of the thallus. The spores are multiseptate, and colourless or pale brown. They are found on the ground or on mosses, trees, rocks, etc. See also DOG LICHEN.