Acarospora impressula  

Thallus to 20 mm diam., often in small patches, areolate. Areoles rather small, 0.5-1.5(-2) mm diam., contiguous, interlocking, sharply angular, becoming elongate, smaller near the thallus edge, dark red- to black-brown, flat to slightly convex, not all areoles fertile. Photobiont layer irregular, continuous, not tooth-like below.
Anamorph: not known.
Teleomorph: ascomata apothecia, 0.2-0.3 (-0.5) mm diam., 1(-2) per areole, mostly punctiform, immersed and concave; disc round or variously angular, even elongate, concolorous with thallus, with a thin, shiny, darker swollen margin; hymenium 60–80 μm high. Interascal tissue of paraphyses 1.5-2 μm diam. at base, widening to 2-3 μm at apex. Asci ca 100-spored, clavate, apical dome not blueing in iodine. Ascospores 3-4 × 2-2.5 μm, shortly ellipsoidal, hyaline, aseptate, smooth-walled, without a gelatinous sheath or appendages.
Chemistry: lichen products not detected by TLC (a collection from Cornwall has a K+ yellow thallus, a similar reaction to that described for Myriospora (as Silobia) myochroa by Westberg et al. (2011).