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Megacollybia

Field characters

Pileus: dull gray, quite large, often collapsing under it own weight, convex to almost planar, edges often splitting.
Annulus: absent
Stipe: rather thick, white, becoming hollow, with white rhizomorphs
Stature: Tricholomatoid
Ecology: saprophytic, growing on wood
98D25E_2_1_M_platyphylla_Pers_Kotl_Pouzar.jpg

Micro characters

Basidiospores: 7-10 x 5-7 micrometers, smooth, inamyloid
Hyphae: inamyloid with clamp connections
Cystidia: cheilocystidia very large, conspicuous (see photo); pleurocystidia absent
Spore Print: white
Lamellae: large, white (when young), mostly sinuate - adnate
Lamellar Trama: regular or subregular
Pileipellis: rectocutis (trichoderm) (www.mycokey.com)

Notes

There are some real nomenclatural problems here. Megacollybia is a synonym for Tricholomopsis, according to Singer, 1986, and Largent and Baroni, 1988 don't even list it as a "modern genus", yet Roody, 2003 and Bessette et al., 1997 use the name for the species I used for my analysis (Megacollybia. platyphylla = Tricholomopsis platyphylla). It is the only species in this genus in North America (www.ilmyco.gen.chicago.il.us/Taxa/Megacplaty354.html). The genus was created in 1972 by two Czech mycologists, Kotlaba and Pouzar.
I admit I have not looked at their paper and certainly need to do so.

So at this point, given that only one species exists around here, and it is quite recognizable, it doesn't really matter which of the two names you use.