 Members of the Parmeliaceae exhibit a diverse chemistry, with several types of lichenan ("Xanthoparmelia"-type, "Cetraria"-type, intermediate-type), isolichenan and/or other polysaccharides being known from the cell walls of many species.The wide diversity in the types of chemical compounds includes depsides, depsidones, aliphatic acids, triterpenes, anthraquinones, secalonic acids, pulvinic acid derivatives, and xanthones.The compounds usnic acid and atranorin, which are found exclusively in the Parmeliaceae, are of great importance in the systematics of the family, and the presence or absence of these chemicals have been used in several instances to help define genera.