Shell of large size for the genus height: 6-15 mm, mean: 9.81, width: 2.5-5.3 mm (mean: 3.99 ) Thin, fusiform, slender, H/W: 2.21-2.81, mean: 2.51 .
Protoconch paucispiral ,only protoconch I of 1.3 convex whorls, height: 367 µm, width: 386 µm. Sculpture irregularly cancellate. Variable in shape and size: more or less elated with more or less protruding nucleus. Protoconch-teleoconch boundary indistinct but oblique.
Teleoconch of 5-7 [7] convex whorls, suture evident, sculpture robust. No microgranules on the surface. Axial sculpture of 15-20 [19] orthocline ribs, interspaces as wide as the ribs. Spiral sculpture of 6-8 [7] cordlets above the aperture, interspaces wider (x2) than the ribs. Cancellation rectangular, with strong and elongated tubercles at the intersections. Sculpture not always visible in transparency throughout the internal shell wall. Subsutural ramp narrow. Two cordlets on the subsutural ramp, the adapical smaller with smaller tubercles. Columella simple, almost straight. Outer lip with 9-11 (10) inner denticles, strong, the most anterior delimiting the siphonal canal and closer to the subsequent than the others, the most posterior delimiting the anal sinus. Denticles 1-2 and 11-12 closer than the others. Siphonal canal short, open. Siphonal fasciole with 8-9 nodulose cords. Coloration uniformly tawny-reddish, from light to dark, with whitish blotches as wide as two axials usually vanishing towards the suture. Comma-shaped white spots on the subsutural ramp. Soft parts foot sharped bilobed anteriorly. Black eyes at proximal 1/3 of tentacles. Foot and cephalic tentacles withish/yellowish semi-transparent with many bright white spots, head semi-transparent greysh/blackish. Siphon blackish with withish spots and a whitish ring at top.

R. philberti differs from R. alternans by its less slender (H/W < 2.6), more solid shell, and the vanishing blotches (vs. sharp in R. alternans). From R. papillosa R. philberti differs by its more solid shell and more robust sculpture, and by its occasionally darker background, very rare in R. papulosa. R. farolita differs in its less slender outline and more robust shell. R. bartolinorum n. sp. could perhaps be mixed with very atypical small sized specimens of R. philberti with uniform coloration, but it will be easily diagnosed by its very less robust and more slender (H/W > 2.4 vs. < 2.5) shell.

Known from the entire Mediterranean Sea.