Arthopyrenia A. Massal., Ric. auton. lich. crost. (Verona): 165 (1852), MycoBank: MB 325
Non-lichenized on bark in terrestrial, temperate forest habitats. Thallus reduced and immersed in the substrate, whitish or indistinct, often forming dark brown hyphae over the substrate. Sexual state: Ascomata scattered, prominent, brown-black, hemisphaerical, carbonaceous, ostiolate, ostiole round. Involucrellum not distinctly separated from excipulum, dark brown to carbonized, K+ sordid-green in section. Excipulum dense, proso- to paraplectenchymatous in thin sections, brown. Hamathecium comprising 0.5–0.7μm wide, densely packed paraphysoids connected to both the base and the top of the centrum, hyaline, straight to irregularly bent, branched and anastomosing. Asci forming in locules between the paraphysids, 8-spored, bitunicate, fissitunicate, broadly clavate, short-pedicellate, with rather broad, non-amyloid ocular chamber. Ascospores irregularly arranged to bi-seriate, oblong with the proximal end slightly tapering, hyaline, (1–)3- septate, with thin eusepta and rectangular lumina, smoothwalled, not or very slightly constricted at the septa, the upper cells often slightly larger than the lower cells. Asexual state: Pycnidia erumpent to prominent, visible as brown-black dots. Conidia acrogenous, either macro- or microconidia; macroconidia non-septate, bacillar, hyaline; microconidia non-septate, acicular, hyaline. Chemistry: Unidentified perithecial wall pigment reacting K+ green. Notes : Arthopyrenia, once a large collective genus including over hundred accepted species, and with over 600 epithets associated with that name (Harris 1975), is now reduced to less than a handful of species which are essentially northern temperate in their distribution and not lichenized. The unique features of these species are the densely arranged paraphysoids developing from both the top and base of the centrum, and the K+ sordid-green perithecial wall (Foucard 1992; Harris 1995; Smith et al. 2009. While Harris (1975) transferred a large number of species to genera unrelated to Arthopyrenia, he suggested that Arthopyrenia could still be further reduced and might contain groups of unrelated taxa. Harris (1995) continued his refinement of Arthopyrenia by transferring a number of species from the A. rhyponta-A. saxicola group to Naetrocymbe and again noted the genus may contain further aberrant taxa. Not all agreed with Harris’ transfer of these species to Naetrocymbe and have continued to recognize species from this group as Arthopyrenia (Aptroot 1998, 2002a; Coppins 2002).

Type species : Arthopyrenia cerasi (Schrad.) A. Massal., Ric. auton. lich. crost. (Verona): 167 (1852), MycoBank: MB
Verrucaria cerasi Schrad., Nov. gen. pl. (Lipsiae) 22: 86 (1797)

Fig. Arthopyrenia cerasi (Material examined: Rabenhorst, Lichenes Europaei 145) a-c Surface view of thallus and erithecial ascomata, showing dark brown hyphae. d-f Section through perithecium showing centrum, paraphysoids, and asci. g Branched and anastomosing paraphysoids. h, i Asci with ascospores embedded in hamathecium. j, k Individual asci with ascospores. l Ascus enlarged showing ascospores. Scale bars: a–c =1 mm, d–l =10μm