Thallus thin, crustose, forming a white stain or immersed and inconspicuous, prothallus indistinct. Photobiont trebouxioid.
Anamorph: not known.
Teleomorph: ascomata apothecia, 100-300 µm diam., scattered, plane to concave, sessile or slightly constricted at the base, sometimes leaving shallow pits when shed, translucent when wet. Thalline exciple hardly visible. Proper exciple prominent, sometimes swollen and raised above the disc, ± concolorous. Disc orange, sometimes inconspicuously pruinose. Epithecium orange-brown, K+ purple. Hymenium and hypothecium hyaline. Interascal tissue or robust fairly thick-walled paraphyses, branched at the apex with the apical cells progressively swollen, to ca 5 µm diam. Asci 55-65 x 13-17 (-21) µm, clavate, 8-spored. Ascospores arranged biseriately, 11-14 × 6-8.5 μm, 1-septate, the septum 1-2 μm thick, less than 1/8 of length of the ascospore, hyaline, smooth, without an epispore, gelatinous sheath or appendages.
Chemistry: thallus K–, apothecia K+ purple.
Not assessed by Woods & Coppins (2012). Further research is needed.
Historical records of Xanthocarpia lactea are now considered to belong to X. marmorata, which has reddish-orange apothecia and somewhat larger ascospores (measuring 16-21 (-25) x (5-) 5.5-7 µm according to Navarro-Rosines & Hladun 1996, though Powell & Vondrák 2011 found samples with intermediate-sized spores). However, the true species may occur in Great Britain.