The edge of the cap is called the margin, and the center of the top surface of the cap is called the disk (or disc). This specimen of Hygrophorus russula has a white margin, and the cuticle darkens considerably at the disk.
In some species, the margin is inrolled, and this can be a helpful feature in identifying the mushroom. The term incurved is sometimes used in this sense.
If the margin is looks like it has irregular, large, blunt, blobby extensions, it is said to be lobed. The margin of a mushroom often becomes lobed in age, as in this picture.
If the margin is broken up into many smaller, sharper protrusions that look like the cap has been torn slightly in order to make them, the margin is called lacerate. As with the lobed condition, a margin often becomes lacerate as the mushroom ages; but here it starts out quite young.
