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(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).

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

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

Animal:
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.

Habitat:
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.

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

Onoba semicostata