Thelotrema lueckingii  

Thallus thickish, yellowish white to greyish green, uneven to verrucose. Cortex K+ quickly brownish red. Medulla white to yellowish, with numerous large crystals, K+ brownish red (but slower reacting than the cortex).
Apothecia similar to those of Thelotrema lepadinum: numerous, immersed in thalline warts. Thalline warts to 2 mm in diam., broadly adnate or basally slightly constricted, rounded, apically incurved, ostiolar opening to 0.5 mm diam.; medulla yellowish, containing large crystals. True exciple externally yellow, K+ red, the margin split to lacerate; disc blackish, usually greyish pruinose; hypothecium colourless; hymenium: colourless, clear, 140–170 μm tall; periphysoids to 30 μm long; epithecium minutely granular. Ascospores 2-4 (-6) per ascus, muriform, widely fusiform, with a thick outer wall, 55–110 × 15–28 μm, remaining colourless, with a gelatinous perispore.

Description taken from Aptroot et al. (2023).