Hapalopilus nidulans

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.

Fertile surface usually a layer of vertical tubes, of which the mouths are visible as pores on the underside of the cap or shelf.
Fruiting bodies usually tougher or harder than the "normal" gilled mushrooms, being leathery, corky, or woody. But they can be quite tender while actively growing
Once grown, they do not decay easily, remaining on the substrate for months or years
They often grow on wood, although a few are terrestrial (even those are usually growing on buried wood)
Fruiting body is usually a flat shelf, or hoof-shaped, protruding directly from the substrate, although sometimes it may have a short stalk.
Some forms never grow away from the substrate at all, so that all that is visible of the fruiting body are the pores.
Sometimes the pores are so minute that the fertile surface seems solid, until you look closely

Growing on wood

Not fitting the other choices, not stipitate
In making choices below based on texture and size, place the emphasis on texture: if left alone, some of the smaller ones can eventually get quite big; and even the huge ones have to start out small at first

Individual caps small (max. 2-3" across)
Soft or tough, but not hard when fresh, although they may become hard when they dry out
All thin-fleshed fruiting bodies go here, especially if compound or imbricate

Hoof-shaped to round fruiting bodies growing singly or in troops
Sometimes covering most of a substrate, but never clustered or compound

Fruiting body soft, even watery when fresh, corky to brittle when dry
Entire fruiting body changes color to purple or red on contact with KOH

Hapalopilus nidulans     (Fries) Karsten