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Kainonereis Chamberlin, 1919 was proposed to include only one species, K. alata, based on epitokes provided with elytriform structures in chaetigers 5-7. The species was thoroughly described and illustrated, but its unique features were enigmatic and the genus is currently regarded as taxon inquirendum. In order to have a better understanding of its diagnostic features and determine how this genus differs from similar ones, type material of K. alata and two other species bearing elytriform structures-Nicon polaris Hartman, 1967 and Rullierinereis elytrocirra Sun and Wu, 1979, including additional material from Gulf of California and Caribbean Sea-were examined. Because the K. alata type material is in good condition and the presence of elytriform structures in chaetigers 5-7 is confirmed, this paper regards it as a valid genus. Given that the elytriform structures, or dorsal discs, together with a set of diagnostic features, are consistent in structure and position in all epitokes examined, they are regarded as homologous structures delineating Kainonereis. Consequently, K. alata is redescribed, the new combinations K. polaris comb. n. and K. elytrocirra comb. n. are proposed and redescribed, and two new species are described: K. chamberlini sp. n. from the Caribbean Sea, and K. peltifera sp. n. from the Gulf of California. Also, the discovery of an atoke on K. polaris comb. n. allowed us to better distinguish Kainonereis from the similar genera Nicon Kinberg, 1865 and Rullierinereis Pettibone, 1971. A key to identifying all known Kainonereis species is also included. Inglés