Summary:
Features include 1) resupinate growth on decayed wood, 2) a thin, whitish to lurid yellow fruitbody, soft and fibrous in consistency, porose-reticulate or somewhat furfuraceous under 50x lens, bristly from projecting cystidia, the margin thinning out, and the periphery often pruinose, 3) spores that are allantoid, smooth, inamyloid, and colorless, 4) cystidia that are numerous, long-cylindric, constricted in places and sometimes with secondary septa without clamp connections, thick-walled except in the distal part, the projecting part with resinous encrustation that dissolves in mounting media, and 5) a monomitic hyphal system, the hyphae with clamp connections. The online Species Fungorum, accessed October 7, 2020, listed the current name as Kneiffia subalutacea, but MycoBank, accessed the same day, listed the current name as Hyphodontia alutacea.

Microscopic:
SPORES 6-8 x 1.5-2 microns, allantoid, smooth, thin-walled; spores and basidia may be smaller in old fruitbodies than in young ones; BASIDIA at first subclavate, then subcylindric with suburniform constriction, thin-walled except for the basal part that is somewhat thickened, 12-20 x 3-4 microns, normally 4-spored, with basal clamp connection; CYSTIDIA numerous, usually 80-150 microns but may reach 200 microns or more, width 5-7 microns, long-cylindric or somewhat widening in the apical direction, more or less constricted in places and sometimes with secondary septa without clamp connections, thick-walled except in the distal part, "projecting part normally with a resinous encrustation, which, however, normally dissolves in the mounting media"; HYPHAE monomitic; subicular hyphae 2-3 microns wide, distinct, richly branched and irregularly interwoven, cyanophilic, with thin or slightly thickened walls, with clamp connections; "subhymenial hyphae thinner and more densely united with predominantly vertical hyphal direction", (Eriksson), SPORES 6.5-8 x 1.5-2 microns, narrowly cylindric, allantoid, smooth, inamyloid, colorless; BASIDIA 4-spored, 14-18 x 3-4 microns, narrowly clavate, with basal clamp connection; LEPTOCYSTIDIA up to 130 x 6-7 microns, (according to the literature up to 200 microns long), projecting a long way, long-cylindric, thick-walled, thin-walled toward apex, some with secondary septa; HYPHAE monomitic, 2-3 microns wide, strongly branched, septa with clamp connections, (Breitenbach)

Notes:
Hyphodontia subalutacea has been found in BC, WA, ID, NS, ON, PQ, AL, AZ, CO, GA, IL, LA, MD, MI, MN, NC, NM, NJ, NY, UT, and WI, (Ginns). The distribution includes BC, Argentina, Finland, France, Italy, Norway, Spain, Sweden, Turkey, United Kingdom, Kenya, Taiwan, (Langer), and Denmark, (Eriksson), and Switzerland (Breitenbach).

Synonyms and Alternate Names:
Hymenoscypha caucus W. Phillips
Phialea caucus Gillet