Porpidia is a genus of crustose lichens in the family Lecideaceae. Porpidia species primarily inhabit siliceous rocks, pebbles, and stonework, with rare occurrences on bark, wood, and compacted soil. The thallus, or body of the lichen, varies in appearance from thick and crusty to barely visible. It may form a continuous layer or develop cracks resulting in a segmented, structure. The colour of the thallus ranges from grey and white to orange.
Porpidia was circumscribed by the German lichenologist Gustav Wilhelm Körber in 1855, with Porpidia trullisata designated as the type species.
Genus Porpidia consists of crustose lichens, meaning the thallus (the main body of the lichen) is closely attached to the surface it grows on. The thallus can vary in appearance, from thick and scaly to almost invisible, and may be solid or cracked into small pieces called . Its colour typically ranges from grey to white, though some species may show orange hues. A, a thin, often black or orange border between neighbouring lichens, may be present but is not always visible. The lichen's inner structure, or medulla, may or may not react with iodine (I+ or I–), and some species produce powdery soredia for reproduction, while others do not. One species of Porpidia even has isidia—small, finger-like structures that aid in reproduction. The lichen's green algae partner is usually from the genera Trebouxia or Asterochloris.
The apothecia (fruiting bodies) are common and can appear either scattered or in clusters. These round, dark brown to black are often relatively large, ranging from 0.5 to 3 mm in diameter, and may be coated with a frosty layer (a). The apothecia have a distinct outer margin, which may fade as the lichen ages, and are seated directly on the surface of the thallus without a (a rim formed by the thallus itself). The rim surrounding the apothecia is usually dark blue-black or brown, and made of tightly packed fungal filaments.
Under the surface, the reproductive layer (the hymenium) reacts with iodine (I+) to turn blue, a useful trait for identification. The asci, the spore-producing sacs, each hold eight spores and have a distinctive shape called the "Porpidia-type". The are typically ellipsoid and measure 15–22 μm long, with a thick outer layer. In some species, other reproductive structures, known as conidiomata, are present and often have a rough surface.
, Species Fungorum accepts 41 species of Porpidia: