Description: Thallus foliose, orange to orange-yellow, adnate to loosely adnate, lobed, forming regular to irregular, 1-2(-3) cm wide rosettes, individual thalli often coalescing to cover much larger surfaces. Lobes dorsiventral, flattened, 3-5 mm wide, smooth, first with helmet-shaped soralia at the apex of short, inner, appressed lobes, these soon becoming crescent-shaped, so that the soralia appear almost crater-like, the soredia yellow, contrasting with the orange thallus colour. Medulla white; lower surface white to yellow, somewhat wrinkled, with many short, white hapters. Apothecia very rare, 0.5-0.7 mm across, laminal, stipitate, lecanorine, the margin initially smooth but often becoming sorediate. Asci 8-spored, clavate, Teloschistes-type. Ascospores 1-septate, polarilocular, hyaline, ellipsoid, 12-16 x 5-6 µm, the septum: 3-5 µm thick. Pycnidia common, immersed to protruding, darker orange than thallus. Conidia bacilliform. Spot tests: thallus and soredia K+ red, C-, KC-, P-. Chemistry: parietin (major), fallacinal (major), emodin, teloschistin (major) and parietinic acid.
Note: a recently-described epiphytic species differing from X. fallax in the much larger lobes with numerous rhizines, in the narrower spores with shorter septum, and in the ecology (epiphytic versus saxicolous). From a preliminary analysis of the epiphytic samples of X. fallax in TSB, all of them appear to belong to X. huculica. Thus, I have placed here all records of X. fallax from Italy collected on bark. The species seems to be most frequent in areas with a subcontinental climate (dry alpine valleys and eastern side of the Peninsula).
Growth form: Foliose, narrow lobed
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly asexual, by soredia, or soredia-like structures (e.g. blastidia)
Subcontinental: restricted to areas with a dry-subcontinental climate (e.g. dry Alpine valleys, parts of Mediterranean Italy)