Euzonus Grube, 1866: 43, 64. Type species: Euzonus arcticus, by original designation. Preoccupied by Euzonus Menge, 1854: 14 (Diplopoda, Chordeumatida)
Thoracophelia Ehlers, 1897: 7, 101. Type species: Thoracophelia furcifera Ehlers, 1897, by original designation; considered a subgenus of Euzonus Grube, 1866 by Hartman (1956: 293).
Pectinophelia Hartman, 1938: 108. Type species: Pectinophelia dillonensis Hartman, 1938, by original designation. Junior subjective synonym of Euzonus Grube, 1866 according to Hartman (1959: 434).
Hartman (1938: 103) described the genus Pectinophelia and listed distinguishing characters between the genera Pectinophelia and Thoracophelia. Hartman (1944) considered both genera closely related. The polychaete genus Thoracophelia Ehlers, 1897 was treated as a subgenus of Euzonus Grube, 1866 by Hartman (1956: 293) who cited distinguishing features between members of the subgenera Thoracophelia and ‘ Euzonus sensu stricu. ’ Later, Hartman (1959: 434) synonymized Pectinophelia under ‘ Euzonus.’ In subsequent publications, the subgeneric structure of Euzonus Grube, 1866 was mostly maintained (e.g. Hartman 1966: 51, 1967: 137 and Rozbaczylo & Zamorano 1970) but not always (e.g. Blake 2000; Misaka & Sato 2003; and Hartmann-Schröder 1971: 383 who placed the genera Euzonus and Ophelia Savigny, 1818 in the then new subfamily Opheliinae without any reference to the subgenus Thoracophelia). In a subsequent publication, Hartmann-Schröder & Parker (1995: 2) refer to the subgenus again.
Currently, 15 species are placed in Euzonus Grube, 1866 (Fauchald & Bellan 2009):
Subgenus Euzonus (Euzonus)
Euzonus (Euzonus) dillonensis (Hartman, 1938) Subgenus Euzonus (Thoracophelia)
Euzonus (Thoracophelia) furcifera (Ehlers, 1897) Euzonus (Thoracophelia) heterocirrus Rozbaczylo & Zamorano, 1970 Euzonus (Thoracophelia) mucronata (Treadwell, 1914) Euzonus (Thoracophelia) profundus Hartman, 1967 Euzonus (Thoracophelia) williamsi (Hartman, 1938) Euzonus arcticus Grube, 1866
Euzonus ezoensis (Okuda, 1936)
Euzonus flabelligerus (Ziegelmeier, 1955)
Euzonus japonicus Misaka & Sato, 2003
Euzonus mammillatus Santos, Nonato & Petersen, 2004 Euzonus otagoensis Probert, 1976
Euzonus papillatus Santos, Nonato & Petersen, 2004 Euzonus zeidleri Hartmann-Schröder & Parker, 1995