Description. Thallus of small contiguous to distinctly crowded areoles, 0.3 – 0.5 (–1.0) mm wide, 0.2– 0.4 mm thick, flat to convex, round to irregular in shape (Fig. 4A). Upper surface dark to light brown. Epicortex absent or 5 μm thick. Cortex 30 – 40 μm thick, hyphae breaking into small cells mostly 2 μm wide, upper layer dark 10 –20 μm thick, lower layer hyaline and indistinct in water. Algal layer 80 –100 μm thick, even to uneven, algal cells scattered below apothecium, 6 –14 μm diam., irregularly interrupted by narrow hyphal bundles 10 –15 μm wide but not always observed in sections. Medulla continuous with attaching hyphae, mixed with substrate crystals, 100 –250 μm thick. Apothecia 0.2– 0.4 mm wide, usually one per areole, distinctly blackish when dried, disc flat in center of elevated parathecial ring, epruinose, brown (Fig. 4B). Parathecium 25 – 40 μm wide, forming a usually black ring around the disc. Hymenium (60 –)80 –120(–140) μm high, epihymenium 10 –15 μm tall, reddish brown, lighter brown than parathecial crown, paraphyses mostly 1.0 –1.5 μm wide, apices not expanded, hymenial gel IKI+ red or blue turning to red. Asci 80 –100 × 10 –20 μm. Ascospores 100+ per ascus, 3 – 4 × 1–1.5 μm. Subhymenium 10 –30 μm tall, IKI+ blue. Hypothecium 20 –35 μm, IKI-. Pycnidia not observed. Chemistry lacking secondary metabolites in TLC (Knudsen et al. 2021a).
Ecology and Distribution. At low elevations (probably below ca. 600 m) on siliceous rock, including sandstone and pebbles, and on anthropogenic substrates (monuments, gravestones, and tops of walls). Widespread but local in United Kingdom (see map in Knudsen et al. 2021a), rare in Czech Republic and Romania known only from single collections. Westberg’s report of M. rufescens from Germany refers to specimens of A. fusca B. de Lesd. determined by Magnusson (Magnusson 1929, Westberg et al. 2011).
Discussion. The species can be confused with Acarospora fusca (Knudsen et al. 2017, Knudsen et al. 2021a & b). Magnusson notes that A. fusca and M. rufescens are anatomically similar which we can confirm (Magnusson 1929). The most distinctive difference is that M. rufescens has an elevated black parathecial ring around the apothecia while A. fusca, if it has a parathecial ring, it is brown, i.e., the color of the thallus (see pictures of A. fusca in Knudsen et al. 2021b with also a full description and discussion). While M. rufescens has a contiguous, even crowded, thallus, A. fusca has a dispersed thallus consisting of small areoles. Acarospora fusca can also have build-up of pigment on the surface of the discs which is lacking on M. rufescens.