Studies of numerous southern Australian populations, two populations from tropical Western Australia, and the type material of Neogoniolithon fosliei (Heydrich) Setchell et Mason (Corallinaceae, Rhodophyta), the type species of Neogoniolithon, have revealed new data that necessitate a recharacterization of the genus. The arrangement of cells in the ventral region of the thallus, a feature previously used to distinguish Neogoniolithon from Spongites, varies from coaxial to non-coaxial and can no longer be used. Male conceptacles and carposporophytes of Neogoniolithon fosliei are described for the first time. Male conceptacles produce simple spermatangia on both the floor and roof of the conceptacle chamber, and gonimoblast filaments arise dorsally during carposporophyte development from apparently discontinuous fusion cells. In terms of spermatangial arrangement and carposporophyte morphology, Neogoniolithon appears to be unique amongst genera of the subfamily Mastophoroideae, and the generic description has been emended accordingly. Historical information on N. fosliei and relevant nomenclatural data are also included.
