Summary:
Features include 1) resupinate growth on wood, 2) color cream to yellow-buff, coral, cinnamon or darker, 3) membranaceous to leathery consistency, 4) slender spores that are curved, smooth, and inamyloid, 5) narrow basidia, 6) very long cystidia, 7) a dimitic hyphal system, and 8) clamp connections. The name Peniophora crassa was proposed with the transfer to that genus because there already was a Stereum karstenii.

Microscopic:
SPORES 4.5-6 x 1.2-1.5 microns, allantoid, smooth, inamyloid, thin-walled; BASIDIA 4-spored, long and narrow, 35-45 x 2-3.5 microns, apically 2-2.5 microns wide, conspicuously constricted just behind sterigmata and then widened to 3-3.5 microns wide, with basal clamp connection; CYSTIDIA of two types 1) very long (to 250 microns), 5-8 microns wide, projecting 50-75 microns, penetrating pseudocystidia, inamyloid, apically thin-walled, thickening toward base, the projecting part often with crystal grains, 2) hymenial cystidia 60-80 x 3-4 microns, projecting 20-30 microns, thin-walled or somewhat thick-walled, with scattered crystal grains; HYPHAE dimitic: context of two layers: 1) subiculum next to wood, with some hyphae thick-walled and sparsely septate, other thin-walled with conspicuous clamp connections and numerous branches, 2) trama consisting of a network of vertical, thin-walled, collapsed hyphae, penetrated by thin-walled generative hyphae and very long thick-walled cystidia, (Eriksson), |SPORES (4)4.5-6(8.5) x (1)1.25-2(2.5) microns, allantoid, often narrowed toward the apicular end, smooth, inamyloid, thin-walled; BASIDIA very slender, broadening below the apex to a width of 2.5-3.5 microns, rising above hymenial surface when mature; CYSTIDIA "not always clear, sometimes completely embedded, otherwise projecting 15-30-70(-over 100) microns, very long, with basal septa hard to find . . . usually more or less obovate, with thick stem walls, nonamyloid, dissolving in KOH . . . lumina expanding more or less gradually toward obtuse apices, these usually 4-10 microns broad, sometimes heavily encrusted, thin-walled or sometimes with rather thickened walls best differentiated in Melzer''s; cystidial stems frequently very slender, 2.5 microns broad 30 microns from apex, at other times as much as 7 microns broad a distance of 100 microns from apex, with variations between and beyond"; hymenium often zonate, consisting of vertically arranged hyphae 1.5-3 microns wide, thin-walled but usually firm walled, often prominently clamped, along with broader hyphae 5 microns wide, thick-walled, perhaps cystidiophores or cystidial stems, (Weresub(3))

Notes:
It has been found in BC, WA, OR, ID, MB, NS, ON, PQ, AK, AL, AZ, CA, CO, MA, ME, MT, NC, ND, NH, NJ, NM, NY, PA, SC, TN, and VT, (Ginns). It occurs also in Europe including Scandinavia (Eriksson).