DIAGNOSIS. — Shell morphology. Shell external, solid, oval, transversely sulcate; spire short. Aperture short and wide, anteriorly rounded. Inner lip simple, with no folds or teeth. Columella smooth, with no folds. Deeply umbilicate. Operculum present.
Anatomy. Radula composed of numerous denticulate and small teeth.

REMARKS. — Iredale (1925) introduced the genus Obrussa for the single species Obrussa bracteata Iredale, 1925 as a probable member of the Acteonidae. The radula of Obrussena contains numerous, denticulate lateral teeth similar to those of Acteon. Differences between these two groups include the shorter and wider shell, the smaller number of lateral teeth on each row of the radula and the more elongate tooth shape in Obrussena.
The genus Inopinodon Bouchet, 1975 also has a similar radular morphology, with the lateral teeth having several apical denticles or cups. However, Inopinodon has more than 60 lateral teeth (Bouchet 1975), whereas Obrussena has less than 20. Also, the outermost teeth of Inopinodon are more elongate. Some members Tomlinula also have a similar radular morphology, with multidenticulate teeth (Marcus 1972), but with a larger number of tooth rows. It is possible that Obrussena will eventually be synonymized with another member of the Acteonidae, but since the rest of the anatomy remains unknown, this genus is provisionally maintained as valid within the Acteonidae until more information becomes available. The diagnosis of Obrussena has been extracted from the shell characteristics of the speci¬mens here examined.

Members of this genus are known to be difficult to place even on the family level. Some were moved to AMATHINIDAE, going back regularly to ACTEONIDAE and the like. Other families were also involved. We now decided for ACTEONIDAE, genus Obrussena, and have put bracteata, moeshimaensis and sagamiensis in there, based on their conchological characteristics. Unexplained O. sagamiensis was placed by authors earlier in the genus Maxacteon with a mixture of other genera. In fact, the only Maxacteon is hancocki Rudman, 1971, a minute New Zealand species with a spiral sculpture on the upper whorls and a "Punctactaeon" sculpture on the lower part of the body whorl.