largest genus of foliose (leafy) lichens, which includes among its members the species commonly known as crottle and skull lichen. Crottle, the largest foliose lichen, resembles crumpled leather and sometimes grows 90 to 120 centimetres in diameter. It is characterized by a black underside. The central portion may die out, leaving a toad stool-like “fairy ring.” It is used as a reddish-brown cloth dye and was once considered a cure for epilepsy and the plague.

The so-called skull lichen (P. saxatilis) is a common variety that grows in flat gray-brown rosettes (5 to 10 centimetres across). According to folk superstition, it was believed to be an effective treatment for epilepsy if found growing on an old skull, especially that of an executed criminal.