Bathynerita naticoidea Clarke, 1989
The shell is neritiform, almost naticiform, depressed and unpigmented. It has about 1-1.5 whorls, and is up to ca. 12.5 mm wide. In young specimens (2-3 mm wide) the nuclear apex is rounded, slightly elevated above the succeeding whorl, finely punctata, and about 0.4 mm in diameter. In more mature specimens part of the nuclear whorl is corroded away and the apex appears larger (ca. 1 mm diameter) and does not project above the next whorl. The whorls are rapidly enlarging, slowly descending, and coiling about 2.5 times during growth to maturity. The earliest whorl in all post-0juveniles is partially obliterated, so that only about 1-1.5 whorls are present at any growth stage. The suture is narrow and impressed. The body whorl is rounded and dominant. The surface, where uncorroded, is nearly smooth and white throughout, with poorly defined collabral lines. The aperture is prosocline, broadly expanded, with a subcircular margin which is continuous in some specimens and interrupted by an incomplete callus on the body whorl in others. The aperture has a D-shaped opening. The outer lip is thin and sharp at the edge, and slightly thickened and chalky behind the edge. The aperture is subnacreous within, with both elongated adapical and basal muscle scars clearly visible. The columella is smooth, broad, flattened and shelf-like, with a callus extending onto the body whorl.
The operculum is calcareous and brittle, but with thin corneous layers covering the outer and inner surfaces. The outer surface has prominent, sinuous, collabral threads and striae, faint spiral lines, and a broad, spiral, brownish horn-coloured admarginal band and a grayish abmarginal spiral band about half as broad as the outer band. The nucleus is below and on the labral side of centre but not submarginal or marginal. The inner surface of the operculum is without a rib or apophysis but with a conspicuous, curved flange on the labral side which arises from behind the margin, fits behind the labrum and it is embedded in the foot.
The body is pale purplish brown in preserved specimens. The subtriangular demibranch, posteriorly-located egg-filled ovary, and some other neritid features are visible through the mantle but the internal anatomy was not studied. The radula is rhipidoglossate, with a formula of many + 4 + 1 + 4 + many. The central tooth is small and wider than it is high. The first lateral tooth is long and blade-like, the other laterals are multiform and complex, and the marginals are filiform, fine and very numerous.
Ecology and Distribution
Types collected from Bush Hill, Lousiana Slope (27°47.87'N, 91°30.51'W) at a depth of 541 m (Clarke, 1989).