Listed on Index Fungorum as A. micromegethus Peck. Both Mushrooms Demystified, 2nd Ed. and the Agaricus section of Agaricales of California by Kerrigan list it as A. micromegathus. Both Index Fungorum and Kerrigan list Peck as the author.
Strange…both my copy of MD 2nd Ed. & Agaricales of CA list it a Micromegathus, as it should be (MGW)