Summary:
Sistotrema athelioides is a member of the Sistotrema brinkmannii group, the group described as follows, "Fruitbodies effuse, smooth or grandinioid; hyphae with clamps, walls hyaline; gloeocystidia absent; basidia urniform, normally with 6-8 sterigmata; spores less than 9 microns long, ellipsoid, subcylindrical, suballantoid, or subglobose, uninucleate (except for S. binucleosporum)", (Hallenberg, Latin name underlined). Sistotrema athelioides is "distinguished from closely related taxa by the smooth, athelioid fruitbody with fibrillose margin (no cordons)".

Microscopic:
SPORES 6-8 x 3-4 microns, subcylindric, smooth, thin-walled; BASIDIA (4)6-8-spored, 16-32 x 5-6 microns, urniform, with basal clamp connection, mostly with a swollen base, some distinctly stemmed; HYPHAE monomitic, with clamp connections that frequently are ampullate, 3.3-6.5 microns wide, "usually with a yellowish, resinous content, stained brownish in sulfo-vanilline"; SUBHYMENIAL HYPHAE "richly branched with crystalline deposits"; SUBICULAR HYPHAE "somewhat thick-walled, frequently united in strands", (Hallenberg)

Notes:
The type was found in BC, (Hallenberg).

Synonyms and Alternate Names:
Leptoporus ptychogaster (F. Ludw.) Pilat
Oligoporus ptychogaster (F. Ludw.) Falck & O. Falck