crustose (adjective; variant spelling: crustaceous; the spelling crustose is preferred since crustaceous is commonly used to describe hard-shelled animals, i. e., crustaceans) – general growth form where the entire thallus forms a crust, i. e., the lower surface without a cortex and directly attached to the substrate, removed only with difficulty and most generally not without destroying or damaging the thallus. The crustose growth from includes granular, rimose, areolate, placodioid, lobate, squamulose, and peltate thalli. Poorly differentiated or loosely aggregated thalli such as filamentose or leprose are also treated here, even though they are not, strictly speaking, forming distinct crusts. Other categories treated here represent transition forms. Thus squamulose and lobed thalli may be considered subfoliose. Minutely fruticose or dactyliform thalli are subfruticose. Crustose thalli are rarely continuous but usually organized in smaller units like areoles or squamules. Related terms: scale, secondary areole, rosette.