The pileus is often umbonate or gibbous, and the center is often darker than the margin.
This species differs from C. atrata in having an umbonate pileus.
umbonate, bossed; furnished with a low, rounded projection like a boss (umbo).
The pileus is first nearly cylindrical, then conic, becoming bell-shaped and finally nearly expanded, when it is umbonate.
The umbonate pileus and the nearly free, broad, gray gills will distinguish it.
When the center of the pileus is raised in the form of a boss or knob it is umbonate (Fig. 242).
It may be rounded and depressed (concave), elevated (convex), level (plane), or with a little mound in the centre (umbonate).