Stromata when young typically surrounded by a conspicuous bright yellow-orange region ca 2 mm wide that sometimes fades with age, black, very variable in size and shape, often coalescing to form large continuous areas that are sometimes several cm long, eventually breaking off together with the covering and underlying bark to leave bare wood. Conidiomata preceding and accompanying ascomata on the same stroma, black, variable in extent and difiicult to distinguish externally, the surface undulating, opening by splits or wide breaks in the covering layer; embedded beneath several layers of collapsed tanned bark cells, the upper wall composed of a single layer of brown angular cells. Individual locules separated by columns to ca 50 µm diam., composed of dark brown angular cells 3-6 µm diam. Lower wall to ca 40 µm thick, also of dark angular cells. Conidiogenous cells arising directly from the lower wall, 18-24 x 2-3 µm, colourless, thin-walled, smooth, cylindrical but slightly tapering towards the apex, proliferating percurrently, with marked periclinal thickening. Conidia (3.5-)4-5(-6) x 2-2.5(3), ellipsoidal, colourless, thin-walled, aseptate, smooth, probably spermatial in function. Ascomata black, poorly-defined externally, generally larger than the conidiomata and raising the substrate surface much more markedly, often extensive, opening by irregular longitudinal or radial cracks. Upper wall inserted below several layers of collapsed periderm cells, blackened , to ca 80 µm thick, composed of angular brown cells 3-8 µm diam., lower wall not blackened. Subhymenium 20-30 µm deep, intergrading with underlying hyphal stromatic tissue. Interascal tissue of thin-walled, septate, apparently unbranched paraphyses 1·5-2·5 µm diam., slightly wider at the apex, slightly longer than the asci, embedded in a conspicuous brown epithecium. Asci produced from croziers, 180-230 x 14-18 µm, cylindrical, short-stalked, thin-walled, not fissitunicate, the apex obtuse and without circumapical thickening, not changing colour in iodine, discharging spores through a large and irregular apical split, 8-spored. Ascospores arranged uniseriately, 28-36 x 11-14 µm, ellipsoidal, colourless, thin-walled, smooth, aseptate, when freshly collected with a group of radiating mucous hair-like appendages visible at one or both ends.
Description adapted from Minter (2001, publ. 2002). Cryptomyces maximus. IMI Descriptions of Fungi and Bacteria. Set 148 No. 1473; and Granmo et al., (2012). The secrets of Cryptomyces maximus (Rhytismataceae). Ecology and distribution in the Nordic countries (Norden), and a morphological and ontogenetic update. Karstenia, 52, 59-72.