Six genera having an interior septum ad an apical or subapical foramen are defined on characters of shell sculpture, shell profile, radula, epipodium, and ctenidial structure. Four genera obliterate the protoconch by expansion of the foramen at maturity Altrix Palmer, 1942, Fissurisepta Seguenza, 1862, and the new genera Clathrosepta and Cornisepta. Two new genera retain the protoconch at maturity: Manganesepta and Profundisepta. All described species previously assigned to Fissurisepta are tentatively assigned among these genera.
New species described are Manganesepta hessleri on manganese nodules from the north equatorial Pacific near Clipperton Island, 4500 m; Clathrosepta depressa from Volcano 5, Eastern Pacific Rise at 13oN, 1160 m;Clathrosepta becki from hot vents at Manus Basin, east of Papua New Guinea, 2494 m; Cornisepta levinae from Volcano 6, Eastern Pacific Rise at 13oN, 1775 m; and Cornisepta verenae from Axial Seamount, Juan de Fuca Ridge, 1530 m.
A hypothesis for the evolution of these genera is offered, based on a cladistic analysis of morphological characters. Outgroup genera are the scissurellid genus Anatoma Woodward, 1859, and the fissurellid genus Emarginula Lamarck, 1801, which is first recorded from the Middle Triassic. Additional genera included in the analysis are Cranopsis A. Adams, 1860, and Puncturella Lowe, 1827, in which the apical whorl is retained, and Diodora Gray, 1821 in which the septum is reduced to a truncate callus.
Analysis of 22 characters for the 10 genera produced a single most parsimonious tree. The traditional sequence of Emarginula, Cranopsis, Puncturella, and Diodora is confirmed. The genera Clathrosepta, Fissurisepta, and Cornisepta showed the highest number of derived character states.