1. Amplisiphonia represents a new member of the Rhodomelaceae close to Placophora in vegetative structure. It differs from that genus in size and habit, but chiefly in the fact that the tetrasporangia are borne in modified lobes of the thallus rather than in cylindrical branches. 2. The dorsiventral thallus grows as a result of the activity of a row of marginal cells which give rise to forking rows of cells which are laterally united and which function as central cells, cutting off three pericentral cells above and two below in regular sequence. 3. Fruiting portions of the thallus lack dorsiventrality, three pericentral cells being cut off above and three below each central cell. As in a number of other members of the family, the tetrasporangia arise from certain modified pericentral cells after the latter have cut off outwardly two cover cells in each case. They divide more or less cruciately. Sexual plants are unknown.