Arthopyrenia platypyrenia  

Thallus absent, colonies not lichenized.
Anamorph: not observed in this study. Conidia rod-shaped, 3-5 × 1-1.5 µm in size (Coppins & Orange 2009).
Teleomorph: ascomata perithecia, formed below an involucrellum, rather variable in shape but usually ± circular, oblate to shallowly conical in section, only very shallowly domed, with a broad  lateral non-papillate ostiole developing through a split in the side of the covering layer, lined with periphyses ca 10 µm long and 2 µm diam. Involucrellum (350-) 400-600 µm diam., 20-30 µm thick, composed of heavily melanized and occluded tissue, fused with the upper peridium except at the edges of the ascoma, sometimes  extending beyond the ascoma by 20-30 µm, the hyphae ± parallel and radially arranged at the periphery and ± intertwined in the central portion. Peridium 8-12 µm thick, composed of tissue intermediate between epidermoid and hyphal in structure, the cells 2-3 µm diam., dark brown in both upper and lower parts. Interascal tissue composed of pseudoparaphyses, 2-2.5 µm diam. and rarely branched at the base, 1.5-2 µm diam., irregular and copiously anastomosed above the ascus apices. Asci (85-) 105-125 x 18-25 µm, clavate, the apex rounded and the base narrow and tapering, thick-walled and fissitunicate, the apical wall strongly thickened with a broad ocular chamber, (4-) 8-spored. Ascospores arranged biseriately, (19.5-) 23-27.5 x 8-11 µm, ellipsoidal to fusiform-ellipsoidal, sometimes slightly ovoid, 3- to 4- (to 7-) septate, constricted at all the septa, rather thick-walled, smooth, hyaline, with a gelatinous sheath 2-3 µm thick; pale to mid-brown and appearing distoseptate when overmature.