Pileus: Armillaria: glabrous to scaly, convex to plano-convex
Armillariella: fibrillose to scaly, typically with pointed fibrillose scales
: typically present in both, "strongly developed" and often double in Armillaria (Singer
Stipe: central, fleshy
Ecology: Armillariella, at least, is a serious wood destroying pathogen in forests.
Basidiospores: Armillaria - smooth, white, amyloid, ellipsoid, binuclear
Armillariella smooth to longitudinally ridged, white, inamyloid, mononuclear
Hyphae: inamyloid, usually without clamp connections.
Cystidia: Armillariella cheilocystidia present, pleurocystidia usually absent.
Spore Print: white to cream
: Armillaria adnexed to decurrent Singer
Armillariella adnate to deeply decurrent, sometimes notched.
Lamellar Trama: Armillariella originally bilateral (divergent), becoming "regular" in adult specimens.
Armillaria- originally bilateral, becoming regular
: Armillariella fibrillose, often somewhat viscid (Singer
Armillariella has basal mycelium and often-black rhizomorphs, and grows on wood, often in large clusters.
Armillaria grows on the ground, and lacks the black rhizomorphs, although Largent and Baroni, 1988 say "is typically found on wood or at the base of dead or living trees."
According to Stuntz, 1978, some present day agariciologists abandon the genus Armillaria completely. Largent, 1986 states "under modern concepts it is almost impossible to separate Armillaria from Tricholoma, unless one restricts Armillaria to the mushrooms with smooth amyloid spores and a divergent gill trama". Singer, 1986, who did much of the work on the genus states that it (the "old" genus Armillaria) is one of the most artificial groupings in the agarics. He essentially assigned its species to nine different genera: Cystoderma, Leucocortinarius, Catathelasma, Armillaria, Oudemansiella, Armillariella, Pleurotus, Calocybe, and Tricholoma. Bessette et al., 1997 consider, for our region, that there are 5 species (all edible if thoroughly cooked).
They apparently do not recognize the genus Armillariella; they include the species mellea in Armillaria, as does Barron, 1999. On the other hand, Roody, 2003 and Huffman et al., 1989 use Armillariella mellea, for the common honey mushroom.
Arora, 1986 is very helpful here (as usual). He resolves the conflict nicely:
"Grouped here are three small genera of fleshy, white spored mushrooms with a cottony veil that usually forms a distinct ring on the stem. There are no warts on the cap nor is there a volva as in Amanita, the gills are not free as in Lepiota and Limacella Tricholoma intergrades somewhat with Armillaria, but as defined here does not usually have a veil"
Armillaria terrestrial forest mushrooms
Armillariella grows on wood, often in large clusters
Catathelasma terrestrial with a double-layered veil, decurrent gills, amyloid spores and a hard, often massive fruiting body. It is restricted to coniferous forests and is rather rare.
It appears, then, that the common, wood-rotting mushroom we most often encounter in the Berkshires should be referred to as Armillariella mellea.