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..
This genus is characterized, according to Fries, by the gills being channeled along the edge, but singularly the only species attributed to the genus in Europe and in our country has not channeled gills, but only somewhat crisped along the edges. It is usually, therefore, a difficult matter for a beginner to determine the plant simply from this description. The gills are furthermore narrow, irregular, and the plants are somewhat soft and flabby when wet, but brittle and persistent when dry, so that when moistened they revive and appear as if fresh.
This species is the principal if not only one in Europe and America. It is widely distributed, and sometimes not very uncommon. It occurs on trunks, branches, etc., often on the birch. The plants are from 0.5-1 cm. broad, usually sessile. The upper surface is whitish or reddish yellow toward the attachment, sometimes tan color, and when young it is sometimes covered with whitish hairs. The gills are very narrow, vein-like, irregular, interrupted or continuous, and often more or less branched. The gills are very much crisped, hence the name, blunt at the edge and white or bluish gray. The caps are usually much crowded and overlapped in an imbricated fashion as shown in Fig. 134; a photograph of a fine specimen after being moistened.