Shell morphology. Measurements up to 14x7 mm, with 5.5 whorls. General shape slender, with elevated spire and important development of last whorl. Whorls rounded, somewhat flattened at periphery, with impressed suture. Surface smooth and glossy, with indistinct growth lines. Shell material translucent in juvenile shells, tending to become opaque and porcellanous in adults (especially along the suture). Aperture oval, oblong, with a callus where the outer Hp joins the preceding whorl. Inner lip closely appressed without any umbilicus or chink. Colour pattern. The colour pattern includes normally one single tone of yellowish brown on a whitish to tawny background. Exceptionally, this is replaced by one single tone of red. Most commonly, the area below the suture is whitish with clusters of brown lines, the remainder of the whorl bearing regular, fine oblique lines interrupted by two or more spiral bands where these lines fuse into crescent-shaped dots. There may, especially on the younger whorls, be small opaque-white flecks, also spirally arranged. The oblique lines are often replaced on part or all of the shell by small dots arranged in a similar pattern, thus recalling the design in T. tenuis Exceptional individuals may be uniformly red with some white flames under the suture (this is T. rubra Risso), or entirely yellowish, or white with only some spiral rows of dots.
Radular morphology: The outline of central tooth is slightly tapering towards its two rounded sides. The first 3 pairs of laterals are grossly equivalent in size and have a highly characteristic leaf like, finely denticulated outline, unique among the species reported in the area. The fourth pair of laterals is smaller, with an acute termination and 3-4 irregular, pointed cusps on the outer side. The inner side is denticulated as in the first three. The fifth pair of laterals is small, bearing normally 3 pointed cusps. The marginals are imbricated in the first 4-5 rows; the first pair is characteristically smaller than the next one. The pectinate structure is achieved by the 12th to 15th row.

The difference between juvenile specimens of large Tricolia shells (speciosa, pullus and tenuis) and of small Tricolia shells is that these last ones have always a trace at least of the crack-like umbilical groove, while the other ones are not umbilicated and external lip always covers umbilical zone on columella.
This species reaches larger sizes than the other ones and it is slimmer, with apex sharp. Even aperture is more stretched, about half of total height, suture deep. Aperture ovoidal in form, lip sharp and bell-shaped at the base, columella concave. Colour pattern is really variable: on a ground grey yellowish in colour, from suture there start spiral lines undulated either entire or interrupted, either sketched or dotted, often either reddish or brown in colour, visible on internal margin of lip too. Below suture there are often spots white and red in colour alternate.
There are quite a lot of varieties in colour issued by several authors, among whom Monterosato (1880), Dautzenberg (1882-1883) and B.D.D. (1884), all of them without any taxonomic value, just to prove the extreme chromatic variability of this species. Average measures of adult specimens are around 12 mm in height.

This species is recorded throughout most of the Mediterranean and in the Black Sea. We have never found it in the westernmost Alboran Sea or in the Straits of Gibraltar area, and records from this area (Bellon-Humbert,.1973) and from Portugal need confirmation. This species is usually found on Posidonia grounds, rather low in the infralittoral. It may often be found abundant on the leaves of the Posidonia.

Habitat: it often lives on Posidonia oceanica in the infralittoral zone.
Distribution: it can be found all over the Mediterranean. Common enough.