Are Titanoderma and Lithophyllum (Corallinaceae, Rhodophyta) distinct genera?

An analysis of southern Australian and selected type collections of species that can be ascribed to Lithophyllum and Titanoderma (Corallinaceae, Rhodophyta) has led to the discovery of considerable intraspecific variation in the occurrence of palisade cells. Within given plants, both palisade and non-palisade cells may be present in primigenous filaments; gradual transitions from palisade to non-palisade may occur along a filament, palisade and non-palisade cells may be randomly intermixed, or branches with and without palisade cells may be present. In some species, certain plants may have primigenous filaments composed entirely of palisade cells, while other plants have only non-palisade cells. As a result, these two genera can no longer be separated on the basis of the presence (Titanoderma) or absence (Lithophyllum) of palisade cells. No other criteria have been found to delimit them, and thus Titanoderma is considered to be a heterotypic synonym of Lithophyllum.