Description: These thick white crusts which decorate siliceous rocks in the uplands require a great deal of attention and often, laboratory analyses. What draws the observer to this species is the chalky-white purity of the thallus and minutely warted surface. Discs are common, never immersed, margin raised and exciple black. All reactions are negative.

Key Identification Features:

• Stunning, chalky-white crusts (often several cms diam.), surface minutely warted and with scattered, sessile (not sunken) discs
• Disc edges raised and exciple black; reactions negative; on siliceous rock.

Original text submitted by Vince J. Giavarini

Distribution Map from NBN: Porpidia contraponenda at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Porpidia contraponenda at iNaturalist World Species Observations database.