Aerial stems jointed, mostly whorled, of two kinds : sterile and fertile (bearing spikes). Spikes ovoid or oblong-cylindrical; sheaths below spike 1 or more. Sporangiophores in close whorls, each with (4-) 5-7 (-l0) sporangia hanging in a circle from its lower surface.

     Fifteen species, almost cosmopolitan, mainly in wet habitats; toxic (containing alkaloids and thiaminase).