ABSTRACTThe genusThamnidiumis restricted to a single taxonT. elegansthat produces deliquescent-walled sporangia at the apex of a primary sporophore or its branches along with persistent-walled sporangiola on lateral dichotomous branchlets. In some instances only sporangia or sporangiola may be formed on a given sporophore. The dichotomous branchlets may arise singly, in pairs, or in verticels of three or more. The species is illustrated with photographs and line drawings.Thamnidium verticellatumis regarded as a synonym ofT. elegans.An extensive list of synonyms is supplied.