her as an annulus around the stem, or
fragments cling around the margin of the cap. Since the =annulus= is so
frail it shrivels as the plant ages and becomes quite inconspicuous or
disappears entirely (see Figs. 1--7).

[Illustration: FIGURE 17.--Agaricus rodmani. Entirely white, showing
double veil or ring. (Natural size.) Copyright.]

Variations in the surface characters of the cap and stem have led some
to recognize several varieties. This is known as the common mushroom and
is more widely known and collected for food than any other. It is also
cultivated in mushroom houses, cellars, caves, abandoned mines, etc.

=Agaricus (Psalliota) rodmani= Pk. =Edible.=--Rodman's mushroom,
_Agaricus rodmani_, grows in grassy places along streets of cities,
either between the curbing and the walk, or between the curbing and the
pavement. It is entirely white or whitish and sometimes tinged with
yellowish at the center of the pileus. The plants are 4--8 cm. high, the
cap 5--8 cm. broad and the stem 1--2 cm. in thickness.

[Illustration: FIGURE 18.--Agaricus arvensis, fairy ring.]

The =pileus= is rounded, and then convex, very firm, compact and thick,
with white flesh. The =gills= are crowded, first white, then pink, and
in age blackish brown. The =stem= is very short, solid, nearly
cylindrical, not bulbous. The =annulus= is quite characteristic, being
very thick, with a short limb, and double, so that it often appears as
two distinct rings on the middle or lower part of the stem as shown in
Fig. 17. This form of the annulus is probably due to the fact that the
thick part of the margin of the pileus during the young stage rests
between the lower and upper part of the annulus, i. e., the thick veil
is attached both to the inner and outer surface of the margin of the
cap, and when it is freed by the expansion of the pileus it remains as a
double ring. It is eagerly sought and much relished by several persons
at Ithaca familiar with its edible qualities.

The plant closely resembles A. campestris var., edulis, Vittad. (See
Plate 54, Bresadola, I Funghi Mangerecci e Velenosi, 1899) and is
probably the same.

[Illustration: FIGURE 19.--Agaricus silvicola. White to cream color, or
yellow stains. (Natural size.) Copyright.]

=Agaricus (Psalliota) arvensis= Schaeff. =Edible.=--The field mushroom,
or horse mushroom, _Agaricus arvensis_, grows in fields or pastures,
sometimes under trees and in borders of woods. One form is often whi