Summary:
Features include 1) resupinate growth on decayed wood, 2) a fruitbody that is thin, cottony, whitish to yellowish, and several centimeters across, 3) spores that are round, spiny, inamyloid, and yellowish, 4) basidia that are short subcylindric, and 5) a monomitic hyphal system, the hyphae short-celled and relatively wide.

Microscopic:
SPORES 7-9.5 microns in diameter including ornamentation, round, with spines, inamyloid, yellowish, rather thick-walled, some with droplets; BASIDIA 4-spored, 15-22 x 9-10.5 microns, short-subcylindric, without basal clamp connection; CYSTIDIA not seen; HYPHAE monomitic: BASAL HYPHAE up to 12 microns wide, yellowish, thick-walled, with right angled branches, SUBHYMENIAL HYPHAE 6-8 microns, short-celled, colorless, thin-walled; "All septa with clamps" [but note contrast with Eriksson below, and hyphae are illustrated without clamp connections], (Breitenbach), SPORES 7-10 microns in diameter, round, with numerous spines 1-3 microns long, inamyloid, yellowish, cyanophilic, with distinct apiculus; BASIDIA 4-spored, 15-22 x 8-10 microns, at first rounded, then obovate to subcylindric, oil droplets present; HYPHAE monomitic loosely interwoven, septa simple-septate [without clamp connections], BASAL HYPHAE about 10 microns wide, yellow, sparsely branched at right angles, hymenial hyphae 6-8 microns wide, colorless, thin-walled, (Eriksson)

Notes:
Botryohypochnus isabellinus has been found in BC, WA, OR, MB, NS, NT, ON, PQ, AZ, CO, CT, FL, IA, LA, MA, MI, NM, NY, PA, TN, VA, and WI, (Ginns), Europe including Switzerland, and Asia, (Breitenbach), and Denmark and Norway, (Eriksson). Desjardin(6) illustrate it for CA.

Synonyms and Alternate Names:
Gloeophyllum mexicanum (Mont.) Ryvarden
Trametes protracta Fr.