It was shown recently that Tetradium Dana, 1846 (type species: Tetradium fibratum Safford, 1856), a common Middle and Upper Ordovician fossil, is a calcareous filamentous florideophyte alga (Phylum Rhodophyta) (Steele-Petrovich, 2009a, 2009b), and not a tabulate coral or a chaetetid sponge, as traditionally classified. Reassigning Tetradium Dana from an animal to an alga subjects it to the International Code of Botanical Nomenclature (ICBN) (McNeill et al., 2006), which makes it a later homonym of Tetradium Loureiro, 1790 (see Farr et al., 1979), a Recent tree of the citrus family (Rutaceae). According to the ICBN (Articles 14, 53.1, 54.1), the name Tetradium Dana cannot be conserved and must be replaced, since Tetradium Loureiro is currently in use.