Protoparmeliopsis muralis

Thallus to 10 cm diam., forming placodioid circular patches or rosettes, usually closely appressed but becoming uneven when old; marginal lobes 0.5–0.8 mm broad, flat to concave, corticate below, thallus centre sometimes areolate, in extreme habitats reduced to dispersed granules or areoles, pale green-yellow to yellow-brown, tending to be darker centrally, ± shiny to slightly pruinose, the undersurface and edges of the marginal lobes pale grey. Apothecia 0.5–1.5 (–2) mm diam., sessile, densely aggregated in the centre of the thallus; thalline margin well-developed, entire to crenulate or flexuose, often angular by compression, pruinose, generally persistent, sometimes almost excluded in old apothecia; disc yellow-brown to red-brown, flat to slightly convex, not pruinose; epithecium pale yellow to brown, with granular crystals on the surface that do not dissolve in K; hymenium (50–) 65–75 (–80) µm tall; paraphyses 2–3 µm diam., mainly unbranched or sparsely branched above, apices not swollen or capitate. Asci 30–40 × 8–12 µm. Ascospores 9–15 (–16) × (4–) 5–7 µm. Conidia 20–25 × 0.75–1 µm, thread-like to curved. Thallus C–, K–, KC+ pale yellow, Pd± yellow, UV+ dull orange (usnic acid, zeorin, ± murolic acid, ± psoromic acid).

Unlikely to be confused with any British species apart from the northern, exclusively coastal Myriolecis straminea which has a C+ orange thallus with convex lobes and the very rare Protoparmeliopsis achariana which has more loosely attached and ± convex lobes. Specimens with a thallus reduced to round, isolated, apothecium-like areoles are easily confused with Lecanora polytropa. Urban populations may represent a different ecotype from the montane, ornithocoprophilous ones. Popularly called the chewing-gum lichen.