Summary:
Features include 1) a greasy, hygrophanous, dark reddish brown to chestnut brown to dull brown or ocher brown cap, 2) adnexed to notched gills, 3) a smooth stem colored like the cap when old but often buff when young, usually with a wider base, 4) growth on the ground, 5) a creamy to yellowish, buff or pinkish buff spore print, and 6) elliptic smooth spores that are often dextrinoid. The gray brown form or variety asema, regarded by some as separate species, is similar but its cap is paler brownish beige to grayish cream, or almost white when dry, the umbo ochraceous, and the stem pale rather than reddish vinaceous. Of the descriptions below, only the one derived from Breitenbach(3) is specific to var. butyracea. Rhodocollybia butyracea is common in the Pacific Northwest.

Cap:
3-8(12)cm across, convex to conic becoming flat or uplifted, often with a broad umbo, margin incurved at first; dark reddish brown to chestnut brown to dull brown, fading as it dries or ages to tan, reddish tan, or ocher buff; "smooth, greasy or slippery when moist but not truly viscid", (Arora), 2-4.5(5)cm across, convex with incurved margin, becoming flat-convex to flat or subumbonate with downcurved margin, occasionally uplifted and eroded at margin when old; hygrophanous, reddish brown to violet-brown when young, fading to light brown or cinnamon brown; bald, lubricous when young, fresh and moist, (Halling), 2.7-7cm across, with prominent broad umbo when mature; hygrophanous, buff to warm tan to light fulvous, generally darker on disc and lighter toward margin, drying only slightly lighter, changing from reddish hues to more gray hues, sometimes becoming more yellowish over disc; bald, lubricous and shining at first, later dull-matte on drying, (Lennox), 2.5-5(7)cm across, convex when young, soon flat, almost always bluntly umbonate; hygrophanous, dark red-brown to ocher-brown fading to cream to beige as it dries, with darker center; smooth, buttery-shiny, margin sharp and finely translucent-striate, (Breitenbach), forma butryacea is red-brown to violet-brown when young and fresh, fading to lighter browns, (Trudell(4))

Flesh:
thin, soft; whitish or watery, (Arora), 0.5-1(1.4)cm thick; white to pale watery gray, in stem white, (Halling), soft, thick, up to 0.4cm thick over disc; watery moist and then colored as cap surface, (Lennox), thin; whitish, light to dark brown under cap surface and above gills, (Breitenbach)

Gills:
free or adnexed, close or crowded; white; "edges usually uneven, eroded, or finely scalloped" when mature, (Arora), "adnexed to nearly free, close or crowded", moderately broad (up to 0.7cm), thin; white, developing a slight pinkish cast when old; "edges wavy or straight to uneven, rarely pubescent, becoming eroded or laciniate" when old, (Halling), sinuate uncinate, easily seceding, crowded, 48-60 reaching stem, broad, 0.35-0.5cm x 1.1-2.7cm, ventricose, blunt at stem and rather pointed toward margin, 5 tiers of subgills; white with faint pinkish or cream cast, becoming yellowish in drying; "edge even or finely serrate at first, later strongly eroded", (Lennox), "deeply notched and finely adnexed", 38-42 reaching stem, broad, 3-6 subgills between neighboring gills; "whitish when young, later sometimes with a red-brownish tint"; edges crenulate [finely scalloped], (Breitenbach)

Stem:
2-10cm x 0.4-1cm, equal or more often wider in lower part and/or pinched at base, rather tough and cartilaginous, hollow at least when old; cap-colored when old but often buff when young; smooth, "often longitudinally striate or twisted", "base usually with white mycelial down", (Arora), 3-6(7)cm x 0.4-1cm at top, club-shaped, sometimes pinched at base, fibrous to brittle, "pithy when young, soon hollow and spongy"; buff to pinkish buff overall at first, or cap-colored toward the base and pallid above when young, becoming cinnamon brown when old, whitish at base below ground level; moist when wet, otherwise dry, striate to sulculate [finely grooved], sometimes whitish fibrillose at base, (Halling), 5-9cm x 0.5-1.2cm, widening downward, subbulbous at base, flexuous [wavy]; subhygrophanous, pallid or tan, sometimes slightly darker toward base; bald, striate-grooved, somewhat shining when moist, white matted-tomentose at base, (Lennox), 4-8cm x 0.4-1.5(2.0)cm, cylindric, stuffed to hollow, usually club-shaped toward base; red-brown; "dull, dry, finely longitudinally fibrillose, sometimes longitudinally grooved, base often white-tomentose", (Breitenbach)

Veil:
[none]

Odor:
mild or absent (Halling), mild, fungoid, or not distinctive, (Lennox), pleasantly spicy-resinous (Breitenbach), rancid (Lincoff(1))

Taste:
mild or absent (Halling), mild, fungoid, or not distinctive, (Lennox), mild, pleasant, not distinctive, (Breitenbach), sweetish (Lincoff(1)), mild or slightly bitter (Miller)

Microscopic spores:
spores 6-8 x 3-3.5 microns, elliptic, smooth, many of them dextrinoid, (Arora), spores (6.2)7-9(10.5) x 3.5-4.2(4.8) microns, oboval to subelliptic in face view, lacrymoid [tear-shaped] to elliptic or almond-shaped in side view, smooth, with a dextrinoid and cyanophilic endosporium and a colorless apiculus; basidia 4-spored, 23.8-37.8 x 6.4-7.8 microns, clavate, not siderophilic; pleurocystidia absent, cheilocystidia inconspicuous, 19-35 microns long, "recurved to repent along the edge, clavate cylindric to irregularly lobed or diverticulate"; cap cuticle "a compact layer of radially oriented, repent hyphae, often encrusted with a brownish pigment, sometimes with a superficial gelatinous matrix", hyphae 2.8-5.6 microns wide; clamp connections present in all tissues, (Halling), spores 6.5-9(11) x 3.0-4.5 microns (Lennox), spores 5.9-7.3 x 2.9-4.3 microns, elliptic-dacryoid [elliptic - tear-shaped], smooth, some dextrinoid, colorless, with droplets; basidia 4-spored, 25-28 x 5-6.5 microns, narrowly clavate, with basal clamp connection; pleurocystidia and cheilocystidia not seen; cap cuticle of parallel to irregular hyphae 2-3.5 microns wide, brownish-pigmented, slightly gelatinized, septa with clamp connections, (Breitenbach)

Spore deposit:
"creamy to yellowish, buff, or pinkish-buff" (Arora), pale pinkish buff (Halling), cream to light orange (Lennox), "whitish-light cream" (Breitenbach)

Notes:
There are collections from BC at the Pacific Forestry Centre and the University of British Columbia. There are collections from WA at the University of Washington. It was recorded from WA by Edmonds(1) and Trudell(1), and from OR by Kauffman(5). It has been reported from CA by Arora and from ID by Andrew Parker, pers. comm. It is also widely distributed elsewhere in North America, and also (according to Breitenbach(3)) Europe, North Africa, and Asia.

EDIBILITY

yes but careful with identification (Arora), yes, mediocre, (Lincoff(1))

Synonyms and Alternate Names:
Collybia butyracea (Bull.: Fr.) P. Kumm.