A key to the genera and species of the transversely-dividing Flabellidae (Anthozoa, Scleractinia, Flabellidae), with a guide to the literature, and the description of two new species.

The transversely-dividing flabellids consist of five genera (Truncatoflabellum, Placotrochides, Blastotrochus, Placotrochus, and Falcatoflabellum) and 45 species. A dichotomous key is provided for these five genera as well as the species of the genus Truncatoflabellum and Placotrochides, the other three genera being monotypic. A tabular key is also provided for the 38 species of Truncatoflabellum. Two new combinations are suggested (Truncatoflabellum gambierense and Truncatoflabellum sphenodeum) and two new species are described (Truncatoflabellum duncani and Truncatoflabellum mozambiquensis). All but one species are illustrated and accompanied by their known distribution and a guide to the pertinent literature for the species. New records of 19 of the 45 species are listed. The transversely-dividing flabellids range from the Middle Eocene to the Recent at depths of 2-3010 m, and constitute 60% of the 65 known extant species of transversely-dividing Scleractinia.