Plectostoma episomon (Vermeulen, 1994)  

Vermeulen (1994)'s description

"Spire conical with flat sides. Apex oblique or not. Whorls 5 5/8-6. convex; last whorl rounded. Constriction with a transverse palatalis. Tuba free from the spire, distinctly bulging and then abruptly narrowed towards the constriction, rounded below. Radial ribs (widely) spaced (2-4 ribs/0.5 mm on the penultimate whorl, 2-3 ribs/0.5 mm half-way on the tuba), those on the spire with a slight, shallowly concave projection half-way, usually abraded to a sinuous scar, but those on the last whorl (slightly) sinuous, without a projection; those on the tuba not sinuous below, without a projection; the portion of the tuba closest to the constriction without radial ribs, spiral striction absent. Umbilicus open, 0.40-0.50 mm across. Aperture tilted 15-45 degree with regard to the coiling axis, its upper margin widely below the level of the apex, circular to elliptic. Peristome double; outer peristome moderately spreading beyond the inner, but gradually narrowed towards, and absent or narrow along the right side of the aperture, often slightly widened along the upper side; inner peristome distinctly protruding from the outer, moderately spreading. Spire: height 2.4-2.6 mm; width 1.7-1.9 mm; index 1.3-1.4. Total width 3.1-3.4 mm. Height aperture 0.8-0.9 mm; width 0.9-1.1 mm."