This species has the smaller type of apex, at least in the greater part of its area.
Quite surprisingly, however, the range of variation of the dimensions of the apex is much wider among R. guerinii from NE. Bretagne, France. The length of the shells varies, 3.3-5.4 mm. Punctate spiral striae are always present in shells of R. guerinii. They may, however, be restricted to the lowermost part of the shell, most clearly visible at the back. All shells of R. guerinii are ribbed. Usually, there are 8-11,5 , rarely 12 terminal ribs per whorl. Among a sample from S. Sicily, however, which contains 16 specimens, there are 7-9,5 terminal ribs per whorl. The labial rib is white and strong. There usually are 2-4 ribbed whorls. The area between the last well-developed longitudinal rib and the labial rib rarely exceeds 0.3 whorls.

The colour of R. guerinii is most remarkable, because there seem to be five different sympatric colour patterns with few or no intermediates. As regards these colour patterns, Dautzenberg & Durouchoux (1914: 27) wrote: "On rencontre avec la coloration typique: blanche avec les intervalles des cotes longitudinales bruns, les trois varietes suivantes:
• Var. ex colore albina Dautz. et Dur. (Faunule Saint Malo, 1900, p. 9). Entierement blanche.
• Var. ex colore bipartita Dautz. et Dur. (Faunule Saint Malo, 1900, p. 9). Quatre ou cinq premiers tours d'un violet fonce, les derniers entierement blancs.
• Var. ex colore conspersa Dautz. et Dur. (Faunule Saint Malo, 1900, p. 9). Fond brun, parseme de taches tres petites, disposees en damier."
This quotation refers to the malacofauna in the Bay of St. Malo, NE. Bretagne, France. On the basis of my own observations I would like to add the following:
— The colour varieties albina and bipartita seem to be restricted to NE. Bretagne (and possibly S. England), where they account for only a small percentage of the population. The normal colour pattern and the variety conspersa occur over the entire range of R. guerinii. Everywhere, the variety conspersa accounts for only a small percentage of the population, roughly guessed about 6%.
— The chess-board colour pattern may quite often also be seen more or less clearly
in shells with the normal colour pattern. Therefore, the main difference
between the normal colour pattern and the variety conspersa is the colour of the ribs:
whitish in the normal colour pattern, brown in conspersa, though often with a white spot
on the middle of the rib.
— Two distinct colour patterns may be distinguished among the normal colour pat¬tern. In the great majority of the shells, the uppermost whorls, if undamaged, are of a remarkable grey-violet colour, and of a dark violet colour where damaged. This suggests the presence of dark violet material, coated with a thin whitish layer. In occasional samples, however, shells occur in which the uppermost whorls clearly are not coloured in a particular way.
— In the variety conspersa the colour of the uppermost whorls is always similar to that of the lower ones, i.e. brown.
— In the variety conspersa the chess-board colour pattern may change into narrow, close-set longitudinal brown zigzag lines.
— The variety albina is the only one which might be mistaken for R. similis. They, however, do not occur together, and moreover, R. guerinii usually has less ribs per whorl than R. similis.
— In my opinion, the uppermost whorls of the variety bipartita are dark brown, rather than dark violet. This colour clearly differs from the grey-violet in the normal colour pattern.

Diagnostic characters
Shell ovoid-conic in profile with relatively tall spire, coeloconoid towards the apex. The apical five whorls smooth and nearly flat-sided, the youngest three tumid and with costae; all with fine spirals. Costae disappear on last whorl near aperture but there is a thick labial varix. Brownish, costae white.
Other characters
The shell has eight whorls and there are ten costae on each of the penult and antepenult whorls. They cross these but fade below the periphery of the last; they are usually a little opisthocline. The spiral ridges and grooves are clearest between costae. The aperture is oval, narrowing adapically. There may be some brown streaks crossing the smooth whorls, and the apical region is often slate-coloured; a similar brown-lilac colour appears in a band in the throat. Up to 6mm high, 3mm broad; last whorl occupies half the shell height, the aperture a third.
The animal is like R. parva) but is yellower. The tentacles have a central yellow line and the snout and the sides of the foot are brown. There is a long metapodial tentacle with a broad base. R. guerini is a southern species found from the Canary Islands north to the south coast of Ireland and to the English Channel. It lives at L.W.S.T. and sublittorally on weeds and under stones. It is locally not uncommon.
Breeding (at Plymouth) takes place from February to April, when clear, lens-shaped capsules are laid on weeds. They are about 1 mm in diameter and contain 80-100 colourless eggs which develop to veliger larvae. The velum has two prominent brown blotches on each of its two lobes.

R. guerinii is washed ashore more or less numerously along the Atlantic coasts of Europe and in the western part of the Mediterranean. I saw one specimen from Agadir, S. Morocco. As yet, I have seen no specimens from the eastern Mediterranean, or from the Adriatic Sea. In the British Isles it has only been reported from the south-western coasts of England and Ireland (Fretter & Graham, 1978: 204).