Summary:
Features include 1) resupinate growth on wood, 2) a whitish to yellowish fruitbody with the surface floccose when young, becoming irregularly odontioid (without much bare hymenium showing) with conic spines that are apically bristled under a 20x lens from projecting hyphae, the margin indeterminately thinning out, more bounded when old, 3) spores that are elliptic, smooth, inamyloid, and colorless, 4) cystidia of 2 types: a) hypha-like, projecting, with an apical head covered with a cap of resinous matter, and b) gloeocystidia, enclosed, thick-walled, and long cylindric but usually with several constrictions or even torulose, 5) a monomitic hyphal system, the hyphae somewhat thick-walled, with clamp connections, in mature fruitbodies the aculeal hyphae with star-like crystals, and 6) often when mature and old, specimens infected with a hyphomycete that has pale brown hyphae and fusiform, pale brown conidia.

Microscopic:
SPORES 4-5 x 3-3.5 microns, elliptic, smooth, thin-walled, containing one large droplet or sometimes a few smaller droplets; BASIDIA 4-spored, 14-18 x 3-5 microns, at first clavate, then subcylindric, "usually with a median suburniform constriction", containing several oil droplets, with basal clamp connection; CYSTIDIAL ORGANS of 2 types: 1) numerous sterile hyphal ends in the hymenium, projecting, not well differentiated (hypha-like), the apical head covered with a cap of resinous matter, and 2) gloeocystidia 40-60 x 4-5 microns, (in some cases up to 125 microns long), enclosed, thin-walled, long-cylindric but usually with several constrictions, in extreme cases totally torulose; HYPHAE monomitic, 2.5-3 microns wide, distinct, somewhat thick-walled, "richly branched and intertwined into an open texture", clamp connections at all septa, "in mature fruitbodies aculeal hyphae with numerous, attached, starlike, composed crystals", young hyphae cyanophilic, "older ones slightly or not so"; often when mature and old, specimens "are infected by a Hyphomycete with pale brown hyphae and fusiform, pale brown conidia, developed from small teeth in the apical part of the conidiophores", (Eriksson), SPORES 4-5 x 2.5-3.5 microns, elliptic, smooth, inamyloid, colorless, with droplets; BASIDIA 4-spored, 18-23 x 3.5-4.5 microns, narrowly clavate, with basal clamp connection; CYSTIDIAL ORGANS of 2 types: 1) cystidia-like hyphae 25-55 x 2.5-4 microns, with capitate ends and sometimes with resinous secretion, smooth, capitate ends up to 5 microns across, and 2) moniliform cystidia, 35-55 x 3-5 microns, smooth, +/- thick-walled; HYPHAE monomitic, exserted hyphae of the spines 2.5-4 microns wide, thin-walled, encrusted with crystals, hyphae of the subhymenium 2.5-3.5 microns wide, branched, +/- thick-walled, septa with clamp connections, (Breitenbach)

Notes:
Xylodon breviseta has been found in BC, AB, NB, NF, NS, AR, AZ, CO, IA, MN, NC, NM, TN, and VA, (Ginns), BC, MT, Finland, Germany, Norway, Slovakia, Sweden, Tanzania, Turkey, and China, (Langer), Switzerland (Breitenbach), and Denmark (common in Scandinavia), (Eriksson).