Ovate-conical shell, thin, transparent vitreous, smooth (except for the presence of prosocline growth lines): spire of four and a half whorls, moderately convex and growing regularly, body whorl slightly swollen but rather high (2/3 h); deep, welldefined sutures, rounded apex, aperture almost round with curved external lip, sometimes expanded at the bottom, umbilicus formed by a narrow deep slit. mm 1-1.5 x mm 0.6-0.9

Diagnostic characters
A minute, glossy, milk-white, conical shell, taller than broad, with a blunt apex; there are 4-5 swollen whorls showing obscure sculpture and deep sutures; aperture oval. A conspicuous dark, oval mark lies near the anus and is visible through the semitransparent shell. Operculum with T-shaped brown mark along the columellar edge.
Other characters
Though it appears smooth to the naked eye the shell has prosocline growth lines and these may be sufficiently marked in places as to suggest slight costae; they often appear as white streaks. The last whorl is large. There is a small umbilicus. Up to 1-1.5 mm high, 0.7-1 mm broad; last whorl occupies about 70% of shell height, aperture 45%. The tip of the snout is deeply bifid, each half appearing like a flattened, triangular tentacle. In addition the animals possess the normal cephalic tentacles, which are digitiform, each with an eye a little behind its base. The mantle edge is smooth. The animals are hermaphrodite so each has a penis behind the right tentacle. The foot has a groove along the posterior half of the sole. Cream with some darker patches. R. diaphana is an epiphytic grazer living among red weeds, mainly in rock pools between L.W.S.T. and M.T.L., where it may be abundant in summer; it also occurs sublittoraily. The animals are widespread along the coasts of the British Isles, predominantly in the south and west. Elsewhere they occur from the Mediterranean to Norway. The animals breed in spring and summer. One or two eggs are enclosed in an ellipsoidal capsule with a flat base which is deposited on green or red weeds. Young snails hatch from these in about two weeks.

Species present from the North Atlantic to the English Channel (particularly frequent along the southern and western coasts of England), to the Mediterranean. In this sea it is not very common.
Previously never recorded for the Gulf of Naples, it was found by members of the Naples Malacological Group both in littoral waters (Capri, Tragara, Bay or Carbonchio at 38 m. depth and in the Gulf of Naples offshore station at 20-160 m. depth!). The largest specimen (Bay of Carbonchio) measures mm. 1.2 x 0.7.
In literature it is considered a littoral species (often in pools), living on algae and coral, in holes in rocks or calcareous algae.