The thallus is flattened and leaf-like, with the upper and lower surfaces differing in color or surface features. In fruiticose lichens the branches of the thallus are round in cross section, with few differences between top and bottom. They may be bushy, hairy, or strap-shaped. The thallus of crustose lichens is appressed to the surface of the substrate, and is sometimes in the sub-strate, in which case the outer margin is delimited by a dark line or color difference. Some small foliose lichens can be confused with crustose species.
Crustose lichens are common on rocks and tree bark.
These lichens are characterized by the squamules, small lobe-like structures which lack a lower cortex. Usually chitin and glucan from the mycobiont and cellulose from the phycobiont. The taxonomy of this phylum is artificial and similar to that of the Deuteromycota since both the alga and fungus also can exist as separate entities. Peltigera, Cladonia, Parmelia, Usnea . Cora, Omphalina .