Summary:
Crustoderma testatum is distinguished from others in the genus by having both dendrohyphidia and two types of cystidia. Other features include 1) resupinate growth on wood, 2) fruitbodies that are irregularly elongated or fusiform, cream buff, waxy, and tightly attached, the surface smooth with scattered warts and slightly velvety, the margin thinning out abruptly, 3) spores that are broadly elliptic, smooth, and inamyloid, and 4) hyphae with clamp connections.

Microscopic:
SPORES 6-7 x 4-5 microns, broadly elliptic, "somewhat flattened and appearing nearly straight on one side with lateral apiculus", smooth, inamyloid, thin-walled; BASIDIA few in specimen examined, 4-spored, 30-40 x 4.5-5.5 microns, ventricose in lower part, cylindric in upper part, "extending above the level of the hymenium about one-half of their length at maturity", sterigmata 5-6 microns long, slightly arcuate, subulate; GLOEOCYSTIDIA 30-55 x 8-12 microns, variable, broadly clavate or clavate-ventricose in lower part and subcylindric-flexuous in upper part, "walls thin or the clavate ones sometimes with walls slightly thickened and indistinctly lamellate, contents guttulate becoming resinous"; subiculum made up of fine interwoven hyphae with gelatinized walls, "irregularly enlarged at intervals and these enlargements developing to cylindrical or pyriform structures with thick walls that appear irregularly scattered, developing in the hymenium to the more typical gloeocystidia", clamp connections present throughout, (Jackson, describing BC specimen), SPORES 7-9.5 x 4-5 microns, elliptic, flattened on adaxial side, smooth (some smaller, presumably immature spores minutely rough-walled), inamyloid, acyanophilic, colorless, slightly thick-walled; BASIDIA 4-spored, 30-50 x 5-6.5 microns, clavate, colorless, thin-walled, with basal clamp connection, sterigmata 4-5 microns long; DENDROHYPHIDIA 40-50 x 1-5 microns, arising from hymenium, lobed or branched at tip, colorless, smooth, thin-walled, with basal clamp connection; CYSTIDIA of two types: 1) 40-75 x 8-30 microns, abundant throughout subhymenium and hymenium, not protruding, clavate to cylindric, colorless, slightly thick-walled to thick-walled, smooth, with clamp connection at base, "containing nonstaining, refractive and phloxine staining, nonrefractive materials", 2) 60-100 x 7-10 microns, numerous in hymenium, protruding up to 65 microns, ventricose, colorless, smooth, thin-walled, with clamp connection at base, "containing nonstaining, refractive and phloxine staining, non-refractive materials"; subhymenium "thickening, up to 200 microns thick, composed of agglutinated, vertically arranged thick-walled cystidia and hyphae"; subiculum thin or absent, hyphae 3.5-7.5 microns wide, arranged parallel to substrate, tightly agglutinated, "evenly or irregularly thick-walled, nodose septate, much branched", (Nakasone(4), describing Oregon specimen, said to be well developed in contrast to BC specimen also examined, which lacked both dendrohyphidia and distinct, thickening subhymenium, and was considered poorly developed)

Notes:
Crustoderma testatum has been found in BC and OR (Ginns).