Opisthostoma gibbosum Vermeulen, 1994  

Vermeulen (1994)'s description

"Spire cylindrical. Apex distinctly oblique. Whorl 3 3/8 - 3 3/4, convex; last whorl rounded. Constriction with distinct parietalis which continues to slightly beyond the sharp inner curve of the tuba; with distinct, short angularis covering the suture, with a transverse palatalis, with columellaris. Tuba touching the spire over its entire length, slightly to distinctly bulging and them gradually to rather abruptly narrowed towards the constriction, rounded below. Radial ribs not sinuous, on the spire densely placed (11-17 ribs/0.5 mm on the penultimate whorl); those on the tuba moderately spaced (5-6 ribs/0.5 mm half-way the tuba). Spiral striation present, fine. umbilicus open, 0.10-0.15 mm across. Aperture not or hardly tilted with regard to the coiling axis, its upper margin widely below the level of apex, about circular to slliptic. Peristome double, outer peristome spreading beyond the inner, but gradually narrowed towards, and absent along right side of the aperture; inner peristome distinctly protruding from outer, somewhat spreading, on the right side of aperture spreading over the periphery or over the lower half of the widest whrol of the spire, its outer surface with closely placed, fine lamellae. Spire: height 1.0-1.2 mm; Width 0.9-1.0 mm; index 1.0-1.2. Total width 1.4-1.5 mm. height aperture 0.40 mm; width 0.40-0.45 mm."