Summary:
Features include 1) a fan-shaped or kidney-shaped, thin, leathery fruitbody narrowed into a stemless base and growing on old logs, 2) an upper surface at first orangish, tomentose-downy, faintly zoned, when mature dull dark red, bald, and densely radiate-wrinkled, 3) a spore-bearing surface underneath that is yellow, becoming brick color when old, 4) microscopic characters obscure in that only 2 spores seen, but no cystidia, composed of an intermediate layer of longitudinal, densely arranged, thick-walled, rigid hyphae 3-3.5 microns wide, bordered on the upper side by an opaque brown layer that gives the color to the cap, and on the lower side hyphae curving into a hymenial layer.
Stereum atrorubrum is known only from the holotype specimen, found in BC, (Ginns).
Fruiting body:
fan-shaped or kidney-shaped, 1-3cm broad and long, coriaceous [leathery], "thin, narrowed behind into a sessile base, hollow at first (about the same color as S. complicatum) and tomentose-pubescent with a few narrow faint zones, but when mature of a dull dark red" with the surface bald and densely radiate-rugose, "margin lobed and crisped and in some specimens proliferous, young hymenium yellow, becoming when old brick color when moist, paler when dry"; in the mature state "the 3-5 concentric zones are more distinct and slightly elevated"; the specimens "roll up on drying and become hard and brittle", (Burt, using an earlier description, Latin name italicized in quotation)
Microscopic:
spores up to 7 x 2-2.5 microns present, but "may not belong for only 2 seen"; no cystidia, gloeocystidia, or conspicuous conducting organs seen; in structure about 800 microns thick, composed of 1) an intermediate layer of longitudinal, densely arranged, thick-walled, rigid hyphae 3-3.5 microns wide, 2) bordered on the upper side by an opaque brown layer 60 microns thick that gives the color to the cap, and 3) "curving on the lower side into a hymenial layer 300 microns thick", (Burt)
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