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..
In Crepidotus the pileus is lateral, or eccentric, and thus more or less shelving, or it is resupinate, that is, lying flat or nearly so on the wood. The species are usually of small size, thin, soft and fleshy. The spores are reddish brown (ferruginous). The genus corresponds to Pleurotus among the white-spored agarics, or to Clau-dopus among the rosy-spored ones. Peck describes eleven species in the 39th Report, N. Y. State Mus., p. 69 et seq., 1886.
This little Crepidotus has a pure white pileus which is covered with a soft, whitish down. The plants grow usually on the underside of rotten wood or bark, and then the upper side of the cap lies against the wood, and is said to be resupinate. Sometimes where they grow toward the side of the log the cap has a tendency to be shelving. In the resupinate forms the cap is attached usually near one side, and then is produced more at the opposite side, so that it is more or less lateral or eccentric. As the plant becomes mature the edge is free from the wood for some distance, only being attached over a small area. The cap is somewhat reniform, thin, and from 6-12 cm. in diameter. The gills radiate from the point where the cap is attached to the substratum, are not crowded, rounded behind, that is, at the lateral part of the cap where they converge. They are whitish, then ferruginous from the spores. The spores are sub-elliptical, sometimes inequilateral, and measure from 8-12 x 4-6 µ
Crepidotus versutus. Cap white, downy; gills whitish, then rusty (twice natural size). Copyright.
Crepidotus herbarum Pk., is a closely related species, separated on account of the smaller spores. Both species grow either on herbs or decaying wood. As suggested by Peck they are both closely related to C. chimonophilus Berk., which has "oblong elliptical" spores. The shape of the spores does not seem to differ from the specimens which 1 have taken to be C. versutus.
Crepidotus applanatus Fr., is a larger species, shelving and often imbricated. Crepidotus fulvotomentosus Pk., is a pretty species with a tomentose cap and tawny scales, usually occurring singly. It is closely related to C. calolepis Fr.
Figure 154 is from plants of Crepidotus versutus Pk., (No. 2732 C. U. herbarium) collected on rotting wood at Freeville, N. Y., eight miles from Ithaca. The plants are represented twice natural size.