Summary:
Features include 1) resupinate growth generally on hardwoods, 2) a gray-lilac to bluish-gray fruitbody with an undifferentiated margin, 3) spores that are elliptic, smooth, inamyloid, and colorless, 4) cystidia of 2 types: thick-walled encrusted metuloids and thin-walled gloeocystidia (no sulfocystidia), and 5) a monomitic hyphal system, the hyphae with clamp connections, a basal layer usually absent.

Microscopic:
SPORES 7.5-9 x 3-3.5 microns, elliptic, smooth, inamyloid, colorless, spore deposit pale pink, BASIDIA 4-spored, 40-50 x 5-6 microns, cylindric-clavate, with basal clamp connection; CYSTIDIA: lamprocystidia "cylindrical with round to conical tips, incrusted part 20-25 x 7-9 microns, thick-walled, young cystidia fusiform, smooth, thick-walled, brownish"; HYPHAE monomitic, 2-5 microns wide, colorless to brownish, thin-walled to thick-walled, with clamp connections, no basal layer, (Breitenbach), SPORES 7-9(10) x 2.5-3.5 microns, allantoid to subcylindric, smooth, colorless, thin-walled, spore print pale red; BASIDIA 4-spored, 30-40 x 5-6 microns, subcylindric-subclavate, somewhat sinuous, thin-walled, with basal clamp connection, "pleurobasidia observed in young fruitbodies"; CYSTIDIA of two types: 1) a few gloeocystidia sometimes present in young specimens, 20-30 x 5-10 microns, oblong, thin-walled, "terminal or lateral on the bearing hyphae, protoplasm not darkening in sulfovanillin", 2) encrusted metuloid cystidia, starting in the hymenium as thin-walled cystidioles, "then with thickened walls, in time strongly pigmented brown in the basal part", distal part 15-20 x 7-10 microns, "not or only weakly pigmented with an outer layer of encrustation, often dissolving in old, enclosed cystidia, and an inner crystalline filling", "number of cystidia varying, as a rule numerous, in young specimens often sparse"; HYPHAE monomitic, with clamp connections, embryonal ones in growing marginal zone 2-3 microns wide, thin-walled, colorless, secondary hyphae about 3 microns wide, with thickened walls, more or less pigmented brown; subhymenium "of vertical hyphae, very closely united into a pseudoparenchymatic tissue"; subiculum "as a rule lacking or present only as a very thin layer", occasionally 10-20 microns thick, "composed of horizontal, densely united hyphae", (Eriksson)

Notes:
Peniophora cinerea has been found in BC, WA, OR, ID, AB, MB, NB, NF, NS, ON, PE, PQ, AL, CA, CO, CT, FL, GA, IL, IN, KY, LA, MA, MD, ME, MI, MN, MO, MS, NC, ND, NE, NH, NJ, NM, NY, OH, PA, TN, VA, VT, and WI, (Ginns). It occurs in Europe including Switzerland, Asia, (Breitenbach). It has also been found in Denmark, Finland, Norway, and Sweden, (Eriksson).