Semimytilus patagonicus (d'Orbigny, 1842) (Bivalvia: Mytilidae) revealed as the valid name with priority over Semimytilus algosus (Gould, 1850)

An examination of the type material of Mytilus patagonicus d'Orbigny, 1842 housed at the Natural History Museum, London, has established a synonymy with Semimytilus algosus (Gould, 1850). Following the principle of priority, d'Orbigny's name is resurrected, and a new combination, Semimytilus patagonicus, is proposed. To remove any ambiguity regarding the identity of this taxon, a lectotype and paralectotypes are designated among the syntypes of M. patagonicus, which originate from a population now extinct in Argentina. Additionally, the type localities of all nominal species are analyzed, and a revision of several synonyms is provided. Type materials of Mytilus algosus Gould, 1850, Modiola pseudocapensis Lamy, 1931, and Modiolus nonuramus Pilsbry & Olsson, 1935 are illustrated, some for the first time.