Thallus yellow, ochre-yellow or grey-yellow, areolate, forming patches to several cm diam. Thallus margin diffuse. Areoles angular, 170–700 (–1100) µm diam, to 150 µm thick. True cortex absent but thin alveolate cortex (sensu Vondrák et al. 2009) sometimes present, to 20 µm thick. Medulla inconspicuous. Prothallus often visible at the thallus margin, white or pale grey.
Anamorph: conidiomata pycnidial, indistinct or forming orange spots, to ca 170 µm diam. Conidia 2.7–3.2 × 1.0–1.7 µm, narrowly ellipsoidal or shortly bacilliform.
Teleomorph: Ascomata apothecial, zeorine, 0.4–0.7 (–1.0) mm diam, disc pale to dark orange. Proper exciple covered by yellow-orange pigments, (30–) 40–120 (–150) µm thick, upper part formed of isodiametric cells 5–7 µm diam.; lower part formed of prosoplectenchyma. Thalline exciple covered by yellow pigments, often crenulate, (60–) 80–125 (–150) µm thick. Hymenium 70–90 µm thick. Asci (40–) 50–70 × (12–) 14–20 (–24) µm, clavate. Interascal tissue of paraphyses, branched, thickened in the upper part to (2.7–) 3.5–5 (–5.7) µm diam. Ascospores (11.0–) 14–16.5 (–17.5) × (5.0–) 6–8 (–9.0) µm, mean length / breadth ratio 2.2:1. Ascospore septa thin (2.0–3.5 µm thick), mean ratio of septum width / ascospore length 0.17:1.
Superficially similar to Flavoplaca maritima, which differs considerably in ascospore characters, mainly in a wider septum. X. crenulatella s. lat. lacks a diffuse thallus margin and its thallus is more reduced.
In GB&I, known from Wales (Carmarthenshire). Otherwise reported widely from the Black Sea region, and from one locality in eastern Spain (Catalonia).
In GB&I, reported from a slightly shaded limestone block on a stream bank.