(da Costa, 1778)
Shell oval-conical with seven or eight semi-transparent, glossy, slightly tumid whorls. Ornamentation variable: two basic forms may be identified; costate and smooth, the latter often referred to as var. interrupta. Uppermost whorls of costate shells rarely ornamented, lower whorls with strong, steep-sided ribs, equal in breadth to intervening spaces, not extending to base of last whorl, terminating at, or slightly below, a peripheral spiral ridge (not conspicuous in all shells). The distribution of costae is somewhat erratic and specimens may show alternation of smooth and costate whorls such that on some costate shells the last whorl may be smooth. Both costate and smooth shells have fine spiral striae on most whorls, most conspicuous between costae of costate shells. Aperture D-shaped, with a thickened peristome; often angulate peripherally in young shells and with varix in mature shells. No umbilicus (R. parva-drawing).
Up to 5 x 3 mm.
Colour variable, cream or buff to horn coloured or chocolate brown; apical whorls often tinged violet. On many shells background colour of lower whorls is enhanced by darker spiral streaks and blotches, in most mature specimens a prominent comma-shaped mark present on last whorl, commencing behind and passing through varix.
Body of typical rissoid pattern; pallial tentacle on right of mantle only; metapodial tentacle well-developed, cylindrical.
Abundant below MTL and sublittorally to 15 m, under stones and on weeds on rocky shores; most commonly associated with corallines and other finely branched algae in which diatoms and detritus collect. The proportion of costate: smooth shells is higher on more sheltered shores, particularly in summer populations.
Distributed from Mediterranean to northern Norway (Distr. R. parva).