Agrocybe praecox

Synonyms
Agrocybe semiorbicularis

Key to Gilled Mushrooms Key

This is a key to gilled mushrooms, that is, mushrooms having a definite cap with a fertile surface consisting of gills. The fruiting body usually also has a stem, although that may be lateral or absent (usually, then, the mushroom is growing from wood). You can use this key to identify mushrooms that you find.

Agaricales Order

Fruiting body containing fibers (usually in the stalk)

Brown, Olive, Orange or Tan Spored Suborder

Gills not free
Spore print tan, orange, deep ochre, yellowish olive, olive brown, rusty or cinnamon brown or deep brown
Ring usually either absent or not membranous

Lignicolous Brown Spored Family

Growing on wood

Medium To Big Lignicolous Brown Spored Subfamily

Cap usually more than 2" across, and sometimes up to 8; usually tan, yellow, or pumpkin-colored

Agrocybe Genus

Cap dull brown to tan; glabrous (occasionally slimy or viscid); often cracking open like an overripe tomato in age; usually 2-3" across
Fruits in great quantity in wood chips in the springtime
Growing either on hardwood or in grass

Agrocybe praecox (Persoon: Fries) Fayod

Here are the characters that distinguish this species from the others in its group. For its more general characters, see higher up on the page.
If there's just a few words or a microscopic feature here, a more thorough description can be found above.

Diagnosis

Cap up to 4" across; some shade of light brown
With a fragile but membranous partial veil forming a persistent annulus
On hardwoods, especially wood chips
Often with white rhizomorphs

Microscropic Characters

Spores 8-11 x 5-6.5 µm

Comments

If it were not for the tremendously important weight of tradition, and the fact that every one of my books distinguishes them, I would long ago have synonymized this species with Agrocybe acericola (okay, I sort of did, but that's only so you can read about them both together here). Their spores are the same size; they're Agrocybes, so everything else is similar, too. The only consistent differences I can detect between the two species concepts are:

Agrocybe acericola sometimes has subdecurrent gills
Agrocybe praecox is sometimes yellowish, but Agrocybe acericola is always yellowish

The distinctions in the field guides:

Arora (1986) :Agrocybe acericola is always yellow brown when young, and has a "better-formed ring".
R. Phillips (1991) : Agrocybe acericola has a "large, flaring" ring and sometimes subdecurrent gills; the cap of Agrocybe praecox sometimes cracking when dry.
Lincoff (1987) : the difference is cap color
A. E. Bessette, D. W. Fischer & A. R. Bessette (1997) says that Agrocybe praecox is the one with the subdecurrent gills (not Agrocybe acericola, like all the other authors), and that it further differs from Agrocybe acericola in having sinuate gill edges, and an upper size limit of 2 3/4" for the cap. Their book is more recent than the others, so they may have access to a more up-to-date set of species concepts than the other works
Two books attribute a different colored spore print to the two species (the others make no such distinction): R. Phillips (1991) gives a dark brown spore print for Agrocybe praecox and a rusty/cinnamon brown one for Agrocybe acericola; A. E. Bessette, D. W. Fischer & A. R. Bessette (1997) makes the same distinction, but with the colors reversed

You now know as much as I do
Good luck

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