The genus is re-described and its status discussed. A key to all known species is provided, and a full synonymy and description for each, with discussions on the complex synonymies of A. terminans and A. opulentum. Ten species are recognized and 23 basionyms reduced to synonymy. One new species, A. tildeniae from Tahiti, is described and the following new combinations are made: A. tonkinense (C. Chr.) Holttum, A. subattenuatum (Rosenst.) Holttum, A. paraphysophorum (v. A. v. R.) Holttum, A. attenuatum (O. Kuntze) Holttum, A. distinctum (Copel.) Holttum, A. ceramicum (v. A. v. R.) Holttum.