The hard agrocybe has a beige to brown dry, convex cap, sometimes with a low knob, 1-5/8 to 4 inches wide. The older caps develop prominent cracks, reminding me of the Marvel Comics superhero, The Thing, and making this mushroom easy to identify.
Hard Agrocybe, side view
Note the cap's characteristic blocky texture.
The broad white gills, which attach to the stem, become brown as the mushroom ages. The spore print is dark brown.
The white to beige solid stalk is 1-5/8 to 4 inches long, 1/4 to 5/8 inches thick. A faint ring encircles the stalk (or remnants of this partial veil may hang from the cap margin) when the mushroom is young.
Hard Agrocybe, from below
The gills are forked: they don't all reach from the edge of the cap all the way to the stalk.
This decomposer grows on wood chips in great quantity in the spring, throughout the northern US, in the spring.
Although non-poisonous, it's so terrible-tasting, anyone who tries it realizes that it's inedible.