Mycena fuscoaurantiaca is a species of mushroom in the family Mycenaceae. First reported as a new species in 2007, the diminutive mushroom is only found in Kanagawa, Japan, where it grows on dead fallen twigs in lowland forests dominated by hornbeam carpinus and Chinese evergreen oak trees. The mushroom has a brownish-orange conical cap that has grooves extending to the center, and reaches up to in diameter. Its slender stem is colored similarly to the cap, and long—up to tall. Microscopic characteristics include the weakly amyloid spores (turning blue to black when stained with Melzer's reagent), the smooth, swollen cheilocystidia and pleurocystidia (cystidia on the gill edges and faces, respectively) with long rounded tips, the diverticulate hyphae of the cap cuticle, and the absence of clamp connections. Taxonomy, naming, and classification The mushroom was first collected by Japanese mycologist Haruki Takahashi in 1999 and, along with seven other Mycena species, identified as a new species in a 2007 publication. The specific epithet is derived from the Latin words fusco- (meaning "dark") and aurantiaca ("orange-yellow"), and refers to the color of the fruit bodies. Its Japanese name is Taisha-ashinagatake タイシャアシナガタケ(代赭足長茸). Takahashi suggests that the species is best classified in the section Fragilipedes, as defined by Dutch Mycena specialist Rudolph Arnold Maas Geesteranus. Within the section, the North American species M. subfusca appears to be closely related to M. fuscoaurantiaca. M. subfusca may be distinguished by its spindle- to broadly club-shaped cheilocystidia without a narrow neck, club-shaped to irregularly shaped caulocystidia, and lack of pleurocystidia. Description The cap, which reaches in diameter, is initially conical to convex to bell-shaped, but becomes flattened in age. It is radially grooved almost to the center, and somewhat hygrophanous (changing color as it loses or absorbs moisture). The cap surface is dry, minutely pruinose initially (that is, appearing as if covered with a fine white powder), but soon becomes smooth. The cap is brown to brownish-orange when young, with a somewhat darker center, and fades to paler toward the margin with age. The flesh is white, and up to 0.5 mm thick. It does not have any distinctive taste or odor. The stem is long by thick, cylindrical, centrally attached to the cap, slender, hollow, and dry. Its color is orange to brownish-orange, and it is initially pruinose, but later becomes smooth. The base of the stem is covered with coarse, stiff white hairs. The gills