This is large genus of broad, loosely-attached, foliose lichens. Most grow on the ground or logs over soil or moss, however many can be found at the base of trees or, in a few cases as true epiphytes (P. collina). There are even a couple of aquatic species (P. gowardii and P. hydrohyria). Most are cyanolichens (photobiont is a cyanobacterium instead of a green alga) with variable color ranging from pale to dark brown or gray, sometimes with a distinct bluish tint, especially when damp. Others contain both green algae (primary photobiont, in the thallus) and cyanobacteria (secondary photobiont, confined to small cephalodia on the upper surface or in the case of P. venosa) lower surface). These are vivid grass-green when damp, fading to pale gray or brownish gray when dry.
All species have rhizines and veins to varying extents on the lower surface (though see P. malacea and P. elisabethae for example). The only other genus with this characteristic is the closely-related Solorina. Many species also have a thin tomentum on the upper surface. Most species are fertile, however a few are sorediate, isidiate, or phyllidiate. Apothecia are typically borne on distinctive erect, finger-like projection on the margins of the thallus (though see P. venosa and P. horizontalis).