Summary:
Features include 1) a small, yellow to orange-yellow or brownish yellow, umbonate cap that becomes mealy to wrinkled, 2) yellow gills, 3) a stem the same color as the cap, 4) a white spore deposit, and 5) microscopic characters including siderophilous granules in the basidia and a hymenidermal cap cuticle. Calocybe naucoria (Murrill) Singer, Sydowia 15(1-6): 47 (1962) [1961] was the current name in the online Species Fungorum, accessed October 25, 2018, a change from March 24, 2014 when the current name was Rugosomyces naucoria (Murrill) Boffelli. MycoBank, accessed on both dates, listed Calocybe fallax (Sacc.) Redhead and Singer based on Tricholoma fallax Sacc. separately from Calocybe naucoria (Murrill) Singer based on Melanoleuca naucoria Murrill, but Redhead and Singer had synonymized Melanoleuca naucoria Murrill with their name. All of these names are based on the same holotype, so there is only one species involved. The dispute about the correct name is nomenclatural and complicated (see Redhead(62)).

Cap:
0.6-2.7cm across, convex to umbonate to depressed when old, margin incurved at first, flat when old; "hygrophanous, orange yellow to grayish yellow to brownish yellow"; bald, (Brunner), 0.5-2(4)cm across, convex, umbonate; yellow to orange-yellow; becoming mealy (use hand-lens) to wrinkled, (Hansen), 0.5-2cm across, yellow to orange-yellow, orange-yellow-brown, center sometimes brownish, (Moser)

Flesh:
thick under disc; light yellow; in stem light yellow to brownish yellow, (Brunner)

Gills:
sharply adnexed to sinuate, crowded, 28-54, subgills 1-3, gills narrow to broad; light yellow to brownish yellow, (Brunner), wax-yellow, (Moser)

Stem:
1.7-3.7cm x 0.1-0.5cm, equal, solid at first to hollow when old; bald or sometimes coated with white to light yellow fibrils, apex yellow and pruinose; "base covered with white felt-like mycelium", (Brunner), 2.5-3 x 0.2-0.4cm, equal, solid; yellow to orange-yellow, base fulvous to red brown; fibrillose, base tomentose to strigose, (Hansen), same color as cap, but lighter, base sometimes red-brownish, (Moser)

Odor:
none (Brunner)

Taste:
mild (Brunner)

Microscopic spores:
spores (2.75)3.0-4.0(4.5) x (1.75)2.0-2.5 microns, elliptic, smooth, inamyloid, colorless, thin-walled; basidia 4-spored, 15.0-21.0 x 4.0-5.5 microns, cylindric to slightly clavate, thin-walled, with siderophilous granulation, with basal clamp connection; pleurocystidia and cheilocystidia absent; cap cuticle a hymeniderm of clavate cells, 8-22 x 5-10 microns, with light yellow contents in 4% KOH; clamp connections present, (Brunner), spores 3.5-4.5 x 2.5-3 microns; cap cuticle cellular, (Hansen), spores 4-5 x 2.5-3 microns, (Moser)

Spore deposit:
white (Brunner)

Notes:
Brunner(1) examined collections from ID, MT, and AK. It is reported from three eastern WA counties (Lorelei Norvell, pers. comm.) There is a Scott Redhead collection from BC at University of British Columbia. Murrill gives the distribution as ME, NY, and VT. It is also found in Europe.

EDIBILITY

Synonyms and Alternate Names:
Tricholoma fallax Sacc.