The genus Synelmis Chamberlin, 1919 has included only three recognized species: S. dineti Katzmann, Laubier & Ramos, 1974, S. albini (Langerhans, 1881) and S. sinica Sun & Chen, 1990. The first one was described from the Mediterranean Sea and later recorded from Japan, the second has been described from the Canary Islands and is regarded as cosmopolitan, and the third is apparently restricted to China. Examination of specimens collected worldwide has resulted in the restriction of S. albini, re-assignement of S. simplex Chamberlin, 1919 as a junior synonym of S. rigida (Fauvel, 1919) n. comb., the transfer of S. dineti to the genus Pseudexogone Augener, 1922, re-instatement of two species, and the description of nine new species. Thus, Synelmis now includes 15 species: S. albini from the eastern Atlantic Ocean, S. amoureuxi n. sp. from the Lesser Antilles and northern Brazil, S. britayevi n. sp. from the Mozambique Channel, S. emiliae n. sp. from the Gulf of California to Costa Rica, S. gibbsi n. sp. from the Red Sea to Papua New Guinea and Vietnam, S. glasbyi n. sp. from the Mozambique Channel, S. gorgonensis (Monro, 1933) n. comb. from western Colombia, Galapagos, and Chile, S. gracilis (Hessle, 1924) from Japan, S. harrisae n. sp. from California, S. kirkegaardi n. sp. from the eastern Atlantic Ocean, S. knoxi Glasby, 2003, from New Zealand, S. levinae n. sp. from submarine mountains in the Pacific Ocean, S. rigida n. comb. from the Red Sea to the Tuamotou Archipelago, S. sinica from China, and S. sotoi n. sp. from the Caribbean Sea and northern Brazil. One record of Synelmis cf. albini from the northern Gulf of Mexico might be an undescribed species but more and better specimens are needed to clarify its position. A key is provided for all described species.
Annelida, Polychaeta, Pilargidae, Synelmis, Pseudexogone, Atlantic Ocean, Indian Ocean, Pacific Ocean, revision, new species.