A cap that is so rounded as to resemble half of an egg, or half of a sphere, is said to be hemispherical. Here, the emerging buttons on the right are hemispherical.
The terms ovate and ovoid are sometimes used in this sense.
An umbo is a softly curved bump in a flat cap. A cap with an umbo is said to be umbonate, as in the two larger mushrooms at the left. The two small mushrooms at the right have a central bump on a convexpileus, and are said to be campanulate, or bell-shaped. In this case, the bump is usually not considered a separate feature.