This section is from the book "Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc.", by George Francis Atkinson. Also available from Amazon: Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc..
The pileus is convex to expanded, and the margin at last revolute (upturned). The surface is marked by strong wrinkles (rugae), which radiate irregularly from the center toward the margin. The pileus is broadly umbonate, fleshy at the center and thinner toward the margin, the flesh tinged with yellow, the surface slightly viscid, but not markedly so even when moist, smooth, not hairy or scaly, the thin margin extending little beyond ends of the gills. The color is tawny (near fulvus). The gills are adnate, slightly sinuate, 5-7 mm. broad, in age easily breaking away from the stem and then rounded at this end, spotted with the black spores, lighter on the edge. The spores are black in mass (with a suggestion of a purple tinge), oval to broadly elliptical, inequilateral, pointed at each end, echinulate, or minutely tuberculate, 8-11x6-8µ. The basidia are short, cylindrical; cystidia cylindrical, somewhat enlarged at the free end, hyaline, delicate, thin-walled, in groups of two to six or more (perhaps this is partly responsible for the black spotted condition of the gills). The stem is cylindrical, even, somewhat bulbous, of the same color as the pileus, but lighter above the annulus, irregular, smooth, fleshy, hollow, continuous with the substance of the pileus. The annulus is formed of a few threads, remnants of the veil, which are stained black by the spores. Figure 29 is from plants (No. 3202 C. U. herbarium) collected near Ithaca, July 18, 1899.
Plate 8, Figure 29
Hypholoma rugocephalum (7/8 natural size). Cap tawny; gills purple-black, spotted. Copyright.