Callithamnion verticale Harvey, 1855b: 561 (type locality: Garden Island, Western Australia).—J. Agardh, 1876: 26.— Sonder, 1880: 10.— J. Agardh, 1892: 20.
Antithamnion verticale (Harvey) J. Agardh, 1896: Index p. 1.—De Toni, 1903: 1397–1398.— Lucas, 1909: 51.— Wollaston, 1968: 299.— Huisman & Walker, 1990: 419.
Taxonomic synonym:
* Callithamnion horizontale Harvey, 1855b: 560 (syntype localities: Garden and Rottnest islands, Western Australia).—J. Agardh, 1876: 26.— Sonder, 1880: 10.— J. Agardh, 1892: 20.
Antithamnion horizontale (Harvey) J. Agardh, 1896: Index p. 1.—De Toni, 1903: 1398.— Lucas, 1909: 51.
Callithamnion dimorphum Harvey, 1863: liv, nom. illeg.
INDIAN OCEAN DISTRIBUTION: Australia.
Note: Harvey (1863: liv) realized that Callithamnion horizontale and C. verticale were two growth forms of one species, which he redescribed under the superfluous name C. dimorphum. We hereby select C. horizontale as the nomenclatural synonym of C. dimorphum. Wollaston (1968: 299) thought that this species might be a form of Antithamnion hanovioides, but reproductive structures necessary to prove the point are lacking.