Summary:
Features include 1) resupinate growth on dead wood, slash, or plant debris, 2) a fruitbody that is pellicular, white to cream, smooth, the margin not distinct, rhizomorphs sometimes present, 3) spores that are elliptic to subcylindric and rounded at base, 4) absent cystidia, and 5) a monomitic hyphal system, the hyphae with clamp connections. Hjortstam & Larsson (Windahlia 21, 1994) consider Fibulomyces a synonym of Leptosporomyces according to Ainsworth & Bisby''s Dictionary of the Fungi, Ninth Edition: this would makes the correct name Leptosporomyces mutabilis (Bres.) Krieglst. Z. Mykol. 57(1): 53. 1991. Ginns(5) uses Fibulomyces in 1993, but Ginns(23) uses Leptosporomyces in 1998. On the other hand, the online Species Fungorum, accessed August 29, 2012, uses Fibulomyces mutabilis.

Microscopic:
SPORES (3)4-5 x (1.5)2-2.5 microns, elliptic to subcylindric, rounded at base, "in side view both sides may be somewhat convex or the adaxial side or both sides are straight", (if the sides mainly straight, they may be parallel or somewhat converging in the apical direction), smooth, inamyloid, thin-walled; BASIDIA 4-spored, 12-15(20) x 4-5 microns, clavate to subcylindric, with basal clamp connection; CYSTIDIA none; HYPHAE monomitic, "most of the hyphae thinwalled but the basal ones somewhat thickened, varying in width, in some forms of about equal width, in one form the basal hyphae are characteristically wide, 5-7(-10) microns, with clamps, anastomoses frequent, crystalline encrustation especially on the subhymenial hyphae", (Eriksson), SPORES 4-5 x 2-2.5 microns, elliptic, smooth, inamyloid, colorless; BASIDIA 2-4-spored, 11-18 x 3.5-4 microns, clavate, with basal clamp connection; CYSTIDIA not seen; HYPHAE monomitic 3-4 microns wide, BASAL HYPHAE rather thick-walled, the rest thin-walled, some hyphae encrusted with crystals, all septa with clamp connections, (Breitenbach)

Notes:
Leptosporomyces mutabilis is found in BC, OR, ID, MB, ON, AZ, CO, IA, IL, IN, MI, MN, MO, PA, WV, (Ginns), Austria, Czechoslovakia, Finland, France, Germany, Italy, Norway, Poland, Sweden, United Kingdom, Turkey, USSR, (Julich, but Eriksson suggests that the material cited is heterogeneous: "According to him to hyphae [sic] should be of about equal width... but in some specimens the basal hyphae are characteristically wider").

Synonyms and Alternate Names:
Helvella arctica Nannf.