Very beautiful and unmistakable shell due to its sculpture. Solid, conic in outline, sharp. First whorls granulous, while the other ones are convex, with big nodules in the upper part. Below the whorls there is one wide and deep spiral groove internally adorned with small lamellae oblique, imbricated. Still below, one oversutural cord follows. The last whorl is more inflated. The whole surface is crossed by fine spiral striae present even at the base which is convex. Umbilicus narrow and deep, partially covered by the columellar margin. Aperture roundish, columella arched, external lip sharp with the central fold in correspondence with the furrow. Umbilical cavity white in colour, inside of the mouth pearly. Operculum corneous, thin and multispiral. Colour pattern really changeable: there are present axial flammulae in different colour and widths, irregular, on a ground yellowish. There can be often noted considerable subsutural stains. Base dotted with several colours on the concentric little cords. The adult specimens average measures are around 15 mm in height.

The juvenile forms are similar to the juvenile shells of G. guttadauri but they are slimmer in outline, they have one more prominent peripherical carina and a different sculpture. The juvenile forms of G. magus are different in sculpture and in outline. B.D.D. (1884) described some varieties in colour pattern (rubra, varia, albosordida), as a proof of the chromatic variability of this species.

Habitat: it lives in poseidon meadows of the meso and infralittoral zones. It was found even in bigger depths but rarely. Distribution: it is known all over the Mediterranean but never common.