A crustose species with a pale grey to greenish grey, thin or thick thallus that is frequently splashed rust-red and surrounded by a black prothallus. The apothecia are distinctively large and sessile, and the black or brown-black discs may be grey-pruinose. The disc is convex.
Medulla: K+- yellow
In Scotland, Porpidia macrocarpa is widespread on siliceous rocks and large boulders in upland areas. BLS distrbution map
scottishlichens.co.uk 