Gills adnate, pileus generally smooth, Armillaria.
"Among the Mushrooms" by Ellen M. Dallas and Caroline A. Burgin
Manner of growth is frequently in tufts, and, as with most of the Armillarias, generally parasitic on old stumps.
"The Mushroom, Edible and Otherwise" by M. E. Hard
Agaricus (Armillaria) melleus Vahl.
"Student's Hand-book of Mushrooms of America, Edible and Poisonous" by Thomas Taylor
Circles in nature — armillaria mushrooms clustered about a round stump.
In eastern Oregon, the 2,400-year-old Armillaria ostoyae mushroom spans 2,200 acres, making it the largest organism in the world.
"Shoestring" rhizomorphs or mycelial cords of Armillaria found along with dead woody roots.