Synonyms: Celidium sordidum Anzi; Celidium varians (Davies) Arnold
Distribution: N - Frl (Tretiach & Hafellner 2000, Brackel 2016), Ven (Brackel 2016), TAA (Brackel 2016), Lomb (Brackel 2016), Piem (TSB 33690), VA, Emil (Tretiach & al. 2008, Brackel 2016), Lig (Brunialti & al. 1999). C - Tosc (Brackel 2016), Marc (Nimis & Tretiach 1999), Umb (Panfili 2000, Ravera & al. 2006), Laz (Brackel 2016), Mol (Nimis & Tretiach 1999, Brackel 2016), Sar (Rizzi & al. 2011, Giordani & al. 2013, Brackel 2016). S - Camp (Brackel 2016), Pugl (Brackel 2016), Bas (Nimis & Tretiach 1999, Brackel 2016), Cal (Puntillo 1996, Brackel & Puntillo 2016, Brackel 2016), Si (Ottonello & al. 1994, Brackel 2008b, 2008c, 2016).
Description: Thallus inapparent, not lichenized, the hyphae developing inside the apothecial discs of species of the Lecanora rupicola- complex. Apothecia arthonioid, developing in the hymenium of the host and blackening the disc, 0.3-1 mm across, with a flat to slightly convex disc, without a proper margin. Exciple poorly developed, not evident; epithecium brown to olive-brown, K+ pale green; hymenium colourless, 45-60 μm high, I+ blue; pseudoparaphyses 1.5-2(-2.5) μm thick, the apices with a brown cap; hypothecium colourless. Asci 8-spored, broadly clavate to subglobose, semi-fissitunicate, with a large, non-amyloid apical dome, and a distinct ocular chamber, Arthonia-type. Ascospores (1-)2-3-septate, hyaline, ovoid to narrowly ellipsoid, sometimes subclavate, 11-17 x 5-6.5 μm, rather thick-walled, without an epispore. Photobiont absent. Spot tests: K-, C-, KC-, P-. Chemistry: no lichen substances.
Note: a holarctic lichenicolous fungus with a wide altitudinal and latitudinal range, occurring throughout the country wherever the host is present; it parasitizes the hymenium of the host.