Superficial mycelium abundant, forming a dense reticulum; dark brown, with a sculptured surface, composed of elongate cells 3·5–9·0 µm diam., septate, markedly constricted at the septa, branched (often more or less rectangularly) and anastomosed.
Anamorph: conidiomata pycnidia. Conidia 2·0–4·5 × 0·5 µm, bacilliform, hyaline.
Teleomorph: ascomata perithecia, (30–) 50–80 µm diam., with an ostiole 5–15 µm diam., black, subglobose to somewhat conical above, smooth-walled, sessile, scattered. Peridium dark brown throughout, sculptured, composed of a few layers of angular cells (3–) 5–8 (–12) µm diam. Hymenial gel I−, K/I−. Interascal tissue absent. Asci (26–) 31–41 (–55) × (10–) 14–20 (–21) µm, ellipsoidal to pyriform, sessile or occasionally with a short stalk, fissitunicate, strongly thickened above but without a distinct ocular chamber, not staining in iodine, 8-spored. Ascospores arranged biseriately, (8·5–) 11·0–13·5 (–16·0) × (3·5–) 4·5–5·5 (–6·5) µm, l/b ratio (1·8–) 2·3–2·9 (–3·9), clavate to narrowly obovoid, usually with a wider and sometimes shorter upper cell, occasionally located upside down in the ascus, almost always 1-septate, mostly constricted at the septum, hyaline, smooth-walled or rarely (when overmature ?) pale to medium brown and with a rough wall, sometimes with a gelatinous sheath 0·5–2·0 µm thick.
Not assessed, only known from a single GB&I collection.
On Thamnolia vermicularis, the single infected thallus appearing old but not obviously damaged by presence of the parasite.
Only known in GB&I from a single collection in VC92 S Aberdeenshire, with Thamnogalla crombiei and Cercidospora thamnoliae also present.