Summary:
Features include 1) resupinate growth on hardwood, 2) a waxy to membranous fruitbody that is well attached, often layered, pale to distinctly yellow, becoming tan when old, and smooth or with small scattered warts, the margin indistinct, pruinose to mealy, and white to pallid, 3) spores that are elliptic to nearly round, warted (but sometimes appearing smooth), amyloid, and colorless, 4) basidia often in multiple layers, 5) gloeocystidia that are numerous, embedded, flask-shaped with elongated neck, or tubular to fusoid, sulfo-positive or not, often with secondary septa, and 6) a monomitic hyphal system, the hyphae with clamp connections.

Microscopic:
SPORES 3.8-4.8(5.0) x (2.6)2.8-3.2(3.4) microns, verrucose (sometimes appearing smooth), amyloid, colorless, thin-walled, with a small, blunt apiculus; BASIDIA 4-spored, 17-24 x 4 microns (but 30-35 microns long in one specimen), cylindric; GLOEOCYSTIDIA 40-75 x 6-12 microns (but 140-230 x 13-22 in the specimen with long basidia), typically flask-shaped with an elongated neck, "others tubular to fusoid, sulfo-positive in about one-third of the collections"; HYPHAE monomitic; subiculum "typically a very thin, indistinct layer next to the substrate", however as the fruitbodies thicken "a horizontally arranged hyphal layer develops over the surface of the current hymenium and it gives rise to the new basidial layer"; hyphae 2-3 microns wide, with clamp connection at each septum, colorless, inamyloid, and thin-walled, (Ginns(24)), SPORES 4.5-5.5(6) x 3.5-4(4.5) microns, elliptic to nearly round, asperulate, amyloid, colorless, acyanophilic, thin-walled; BASIDIA 4-spored, 25-30 x 5-6.5 microns, clavate, colorless, thin-walled, with clamp connection at base; PSEUDOCYSTIDIA numerous, arising in hymenium, embedded, (40)55-85(100) x 6-12(16) microns, lageniform to obclavate, colorless, thin-walled or walls slightly thickened at basal end, clamped at base, "blue-black in sulfobenzaldehyde, apex usually moniliform or constricted to form a bulb, often developing secondary septa"; "subhymenial trama thickening, consisting primarily of pseudocystidia, indistinct, compact hyphae, and numerous basidiospores"; HYPHAE monomitic, subiculum very thin, not well differentiated, SUBICULAR HYPHAE 2-4 microns wide, colorless, thin-walled, nodose-septate [septa with clamp connections], frequently branched, (Nakasone), SPORES 3.8-4.8(5) x (2.6) 2.8-3.5 microns, nearly round to ellipsoid, distinctly warty; BASIDIA 17-35 x 4-5 microns; GLOEOCYSTIDIA 40-75 x 6-12 microns, cylindric to fusiform, sometimes swollen at the base, sulfobenzaldehyde reaction +/-, no hyphidia; no gloeoplerous hyphae, GENERATIVE HYPHAE 2-3 microns wide, colorless, thin-walled, somewhat agglutinated, (Stalpers)

Notes:
Collections were examined from BC, OR, ON, PQ, AK, AZ, CO, CT, IL, LA, ME, MI, MN, MS, NM, NY, and WI, and it has also been reported from MA and WY, (Ginns).

Synonyms and Alternate Names:
Polyporus sulphurellus Peck
Polyporus xanthus Fr.