Summary:
Notable are the 1) very broad, frequently eroded gills and 2) the white rhizomorphs. Other features include 3) a non-viscid, often radially streaked cap that is dark brown to grayish brown or pallid with a darker center, 4) notched to adnexed or adnate, well-spaced, broad gills that are white to grayish, 5) a stem that is white or flushed cap color, 6) growth on or near rotting logs or stumps, and 7) microscopic characters. Molecular work by Hughes(3) divided this group into several species of which the one found in western North America is considered to be Megacollybia fallax (A.H. Sm.) R.H. Petersen & J.L. Mata, a synonym of which is Tricholomopsis fallax A.H. Sm. Of note is that A.H. Smith included in a 1960 Tricholomopsis monograph both T. fallax and T. platyphylla, differentiating T. fallax by the yellow color in the stem cortex (as opposed to pallid in T. platyphylla) and microscopically by "the yellow pigment in the pilocystidia, the lack of diversity in the shape of the cheilocystidia, and the yellow pigment in the gills and cortex of the stipe".

Cap:
4-12cm across, convex to flat or centrally depressed; dark brown to grayish brown, "or sometimes pallid with a darker center and/or fibrils", often streaked; smooth, not viscid, (Arora), 5-12(20)cm across, convex with inrolled margin soon becoming broadly convex to flat or with recurved [upcurved] margin, sometimes retaining low broad umbo, sometimes with disc slightly depressed; blackish to avellaneous on disc, paler and grayish towards margin, sometimes very pale gray overall except slightly darker disc; moist to dry, streaked with fibrils, at times almost bald but usually furfuraceous to subscaly [somewhat scaly], margin opaque or when wet striatulate [finely striate] and when old often frayed, (Smith(15)), "dark gray-brown with an ochraceous tint, sometimes lighter; somewhat streaky surface from radial fibers sometimes forming minute scurfy scales toward the margin", (Phillips), "gray-brown, light olive-brown, more rarely almost dingy white" (Breitenbach)

Flesh:
thin; pallid, (Arora), thin and moderately pliant; watery gray, (Smith(15))

Gills:
"adnate or more often notched, well-spaced, very broad", often splitting or with eroded edges when old; white or grayish, (Arora), adnate but becoming adnexed to subsinuate [somewhat sinuate], subdistant, broad, 1-2cm broad, 2-3 tiers of subgills, often interveined; white to grayish, usually darker in forms that have deeply colored caps; edges even to eroded, (Smith(15)), broadly adnate to subdecurrent, also notched, 55-77 reaching stem, 3(5)-7(8) subgills between neighboring gills, (Breitenbach)

Stem:
6-12cm x 1-3cm, equal or widening in lower part, hollow when old with a tough outer rind; white or flushed cap color; "base usually with white mycelial cords attached", (Arora), 6-12cm x 1-2(3)cm, equal or sometimes widening slightly downward, stuffed becoming hollow, with a somewhat cartilaginous rind; white or tinged grayish and sometimes more or less cap-colored; bald to fibrillose, striate, usually appearing unpolished, (Smith(15))

Veil:
absent (Arora)

Odor:
not distinctive (Smith(15)), fungal, fresh earth, (Miller)

Taste:
not distinctive (Smith(15)), bitter (Phillips), mild or slightly bitter (Miller)

Microscopic spores:
spores 7-9 x 4-7 microns, elliptic, smooth, [presumably inamyloid], cheilocystidia abundant, (Arora), spores 7-9(10) x 4.5-6 microns, elliptic, smooth, inamyloid; basidia 4-spored, 34-36 x 5-7 microns, narrowly clavate; pleurocystidia not differentiated, "cheilocystidia abundant, of various shapes, some basidia-like but usually broader than immature basidia" and 26-37 x 8-11 microns, "others somewhat irregular or with one or two inconspicuous obtuse projections", others in the form of hairs 24-32 x 2.5-3 microns; pileocystidia irregularly arranged, 30-60 x 8-12 microns, with smoky brown contents, narrowly clavate with rounded apices, (Smith(15)), same description except an addition to description of cheilocystidia "and some fusoid-ventricose and 36-48 x 9-13 microns" and statement that clamp connections are present (Smith(42)), cap cuticle of parallel hyphae 4-9 microns wide, "with brownish vacuolar pigment, hyphal ends clavate to cylindric", some erect, especially toward the center of the cap (some authors call these pileocystidia); clamp connections mentioned for basidia (Breitenbach)

Spore deposit:
white (Arora, Smith(15))

Notes:
A.H. Smith examined collections of T. fallax from Idaho and T. platyphylla from NS, ON, AL, CA, ID, KY, MD, ME, MI, MT, NM, OR, PA, TN, UT, VA, and WA, (Smith(42)). Megacollybia platyphylla group is widely distributed in eastern North America. There are collections from BC at the University of British Columbia (as Tricholomopsis platyphylla). Megacollybia platyphylla group has been reported from BC (DNA not fruiting body), WA, OR, ID, and AZ, (M. fallax), TX (M. texensis), NS, ON, QC, AR, MA, MO, TN, Costa Rica, and Mexico, (M. rodmani), AR, NY, and TN, (M. subfurfuracea), Colombia, Guyana, (M. fusca), Austria, Czech Republic, Finland, Germany, Russia, Spain, and Sweden, (M. platyphylla), Russia (M. marginata), and China, Japan, and Russia, (M. clitocyboidea), (Hughes(3)).

EDIBILITY

not recommended, some adversely affected, (Arora), severe gastrointestinal symptoms have been reported (Goos(1))