Thelidium rehmii  

Thallus a whitish grey to green-grey, thin tartareous crust 15–110 μm thick, with fine cracks. Photobiont coccoid green algae 3-10 µm diam., ± globose, dispersed, not aggregated in small groups. Soredia absent.
Anamorph: not known.
Teleomorph: ascomata perithecia, 160-210 µm diam. (slightly larger in European material), semi-immersed to almost sessile, dark brown in the upper part, hyaline below. Involucrellum absent or indistinguishable from the exciple, forming a slight apical thickening of the perithecium but not or hardly extending laterally. Interascal tissue absent, periphyses to 30 µm in length. Asci 67–93 × 17–40 μm, clavate, fissitunicate with a usually conspicuous ocular chamber and a thin inner layer that extends at dehiscence into a rostrum, 8-spored. Ascospores (20–) 21.5–25 (–26.5) × (10–) 10.5–12 (–12.5) µm, ellipsoidal, one-septate, hyaline, thin-walled, without a perispora or gelatinous sheath or appendages.