SUMMARY1.Secotium agaricoidesis a widespread species, occurring in Europe, Asia, Africa, North America, Australia, and New Zealand.2.In the young carpophore, the fundament of the hymenophore first appears, followed by the demarcation of the margin of the pileus and the appearance of a gill cavity.3.The development of the“peridium“is like that of the cap ofAgaricus campestris, showing a primitivevelum universaleand a pileus. No truevelum partialeis found, though a layer of“neutral tissue”occurs which may represent it. The universal veil disappears during the maturation of the plant.4.The hymenophore consists of radiating, branched, and anastomosing gills decurrent at the top of the columella.5.The trama and hymenium are simple in structure, without pseudoparenchyma or cystidia.6.The young basidium has two nuclei, which unite to form one. This latter divides twice to form four nuclei for the four spores.The mycelial cells are binucleate.7.Secotium agaricoidesis nearly related toAgaricus (Psalliota), being an arrested or paedogenic form. It is not closely related to the Gasteromycetes or Phalloids.The author is indebted to the Botanical Department of the State University of Iowa for a loan of high power lenses, and to the management of the Missouri Botanical Garden for the use of books. These favors are gratefully acknowledged.