Onoba semicostata

(Montagu, 1803)

Description (shell):
Shell solid, opaque, somewhat cylindrical with blunt apex; five or six slightly tumid whorls, with sculpture of spiral striae, and truncated costae confined to apical parts of whorls; costae absent in var. ecostata . Aperture small, oval or egg-shaped; peristome thickened, with anal sinus. Inner lip reflected over umbilical groove; umbilicus may be present in young shells (O. semicostata-drawing).

Up to 4 x 1.8 mm, most commonly 2 x 1 mm.

Shell commonly off-white, often with two red-brown bands on last whorl. Periostracal pigmentation may give rise to darker, even black, forms.

Body of typical rissoid pattern; mantle edge smooth, with short pallial tentacle on left, and in many animals on right also. Metapodial tentacle narrow, triangular, may extend to edge of foot.

Gregarious, under stones, among corallines and in other silty areas, on rocky shores from near LWM and sublittorally to 100 m; deeper in north of range.

Distributed from Mediterranean to Norway; common, especially in summer, on all suitable shores (Distr. O. semicostata).