CAP 2.5–7 (10) cm across, broadly conical to acutely convex at first, becoming more bell-shaped, to obtusely bell-shaped. Margin incurved to slight- ly uplifted, immediate edge inrolled to downcurved. Surface viscid to glutinous when wet, smooth, hygrophanous; dull when dry. Dark olive-green overall, or with a darker disc and brighter yellowish green margin when young, becoming duller greenish olive to greenish brown, then darkening to dark olive to olive- brown in age. GILLS narrowly attached to nearly free, close to crowded, broad. Pale yellow, dingy cream or greenish buff when young, developing ochraceous tones, to olivaceous brown in age. STIPE 3–7 cm long above ground, 20+ overall, 0.4–1.3 cm thick at apex, equal, enlarged or tapered slightly towards ground level, rooting pseudorhiza tapering (abruptly at times) belowground to a fragile cord. Surface dry to moist, with scattered scurfy fibrillose patches, to nearly smooth in age. Greenish to pinkish buff at apex, transitioning to a dark green to yellow- brown or red-brown base; darkening overall in age. Pseudorhiza dark orange to red-brown. FLESH thin, firm, dull greenish yellow to whitish. Stipe solid, with a thin cartilaginous rind, stuffed with pale greenish white to pinkish-buff pith when young, sometimes becoming hollow in age or when insect infested. ODOR indistinct, or occasionally sweet or pungent. TASTE indistinct. SPORE DEPOSIT pinkish cinnamon-brown. MICROSCOPY: Spores (6.5) 7–9 (9.5) x (4) 4.5–5.5 μm, averaging 8 × 5 μm, ovate with long narrow beak in face view, inequilaterally lemon-shaped with central beak and eccentric apiculus in side view, coarsely to warty roughened, except near beak. Medium amber in KOH, inamyloid and non-dextrinoid in Melzer’s reagent. Basidia 25–32 x 6–9 μm, clavate, 4-spored. Cheilocystidia 25–80 μm long, lageniform to tibiiform, (bottle-shaped, or with extended cylindri- cal necks), lower portion thin-walled, 3–7 μm wide, with narrow, thick-walled necks, 0.5–2 μm wide, with, or without a capitate head. Pleurocystidia absent. Cap cuticle a two-layered ixocutis; upper layer 100–250 μm thick, radially aligned, cylindric, highly gelatinized, hyaline hyphae, 1–3 μm wide; in thick gelatinous matrix. Lower layer diffusely pigmented, gelatinized hyphae 8–15 μm wide; en-crusted with pale greenish amber pigments, which turn orange in KOH. Clamp connections absent.
ECOLOGY: Ectomycorrhizal with conifers and possibly Tanoak (Notholithocarpus densiflorus); apparently restricted to mature and old-growth forests. Solitary, scat- tered or clustered in small patches in soil; typically in areas with thick duff. Fruiting in fall and early winter.
SIMILAR SPECIES: Phaeocollybia olivacea is typically slightly larger, and typically has a paler yellowish green stipe apex. The cheilocystidia are thin walled, variably clavate, and often have threadlike extensions in age (versus relatively thick-walled cheilocystidia, which are bottle-shaped or cylindrical with or without abruptly rounded heads on P. pseudofestiva). Microscopic examination is usually necessary to distinguish these two species; especially with older fruit bodies. Phaeocollybia fallax also has a green cap, but has distinctly lilac gills when young, and a hollow stipe. In age, the gills lose their distinctive color; in such cases, the thin-walled, clavate to capitate cheilocystidia readily distinguish it.