Pusia tricolor: Sometimes between this species and the congenerous ones, littoralis and savignyi, the separation is not so easy as far as the shell is concerned: what it can be noted on the animal, on the radula and on the protoconch is determinant for a right specific distinction.
Shell elongated in form, with flattened axial ribs (sometimes absent) which are not extended below the half of the last whorl. Aperture narrow with three well defined plicae. Lip simple, internally finely denticulated.
The ground colour pattern is greenish or yellowish with a white line well defined at the central point and brown little spots laterally corresponding with the interspaces. The animal is black in colour, with small pits yellow in colour. The average measures are abt. 10 mm.

Pusia tricolor: Distribution: common all over the Mediterranean, more common in the western zone.
It does not exixt an area where P tricolor, savignyi and littoralis are all present, while tricolor can be found either with savignyi or with littoralis.
Habitat: it lives under the stones. It is often associated to Halimeda tuna or it can be found among the Posidonia rhyzomes between 3 and 10 m depth.