This section is from the book "Our Edible Toadstools and Mushrooms and How to Distinguish Them", by W. Hamilton Gibson. Also available from Amazon: Our Edible Toadstools And Mushrooms And How To Distinguish Them.
There are two other species of mushroom which might possibly be mistaken for the above by the casual eye, but which are easily distinguishable on careful examination. The first of these is the false Champignon (Plate 9, fig. 1). The most important distinguishing features are italicized. They will be seen to afford a striking contrast to the true edible species in these especial characters.
The pileus is pale buff, convex, central mound absent; the cap varies from one-half to one and a half inches in diameter, and is thus slightly smaller than the "true" fairy-ring; gills, yellowish brown, narrow, and crowded, twenty-five or more to the inch at circumference in good specimen, curving upward at junction with stem, thus "free" from actual attachment; stem, solid, clothed with whitish down, especially noticeable at the base; cup, none; taste, acrid. This last quality alone should distinguish the species, which, moreover, usually grows in woods, though occasionally found upon the lawn in association with the edible species.
Marasmius urens. Marasmius peronatus
Pileus: Pale buff in color; tough and fleshy; flat convex, becoming depressed and at length wrinkled; one to two inches in diameter. Gills: Unequal, cream colored, becoming brownish; much closer together than in the true Champignon, hardly reaching the stem proper. Stem: Solid; fibrous; pale, its surface more or less covered with white, flocculent down, and densely clothed with white down at base. Taste: Acrid. Habitat: Lawns and pastures, often in association with the edible Marasmius oreades.
Pileus: Reddish buff; convex slightly flattened at top, becoming convex by expansion; very wrinkled when old; diameter, at full expansion, between one and two inches.
Gills: Thin and crowded; creamy, becoming light reddish brown, continuing slightly down stem by a short, abrupt curve.
Stem: Solid; fibrous; pale, densely clothed with stiff yellow hairs at base.
Habitat: In woods, among dead leaves, etc.
Plate IX. Poisonous Champignons. Marasmius Peronatus. Marasmius Urens.
The other false species (Plate 9, fig. 2) still more closely simulates the "fairy-ring," but may be identified by the growth of spines at the base of the stalk. The gills are also annexed to the stalk by a small, sharp, recurved tooth. Like the previous spurious species, it is found in woods, and is rarely to be seen in association with the true Champignon or in its peculiar haunt.