Shell small, semitransparent, glossy, nearly circular in apical view, with low spire and large umbilicus. Whorls with spiral keels (six on the last), crossed by numerous small costae. Aperture lying in a very prosocline plane, with a small anal spout adapically. White to orange. There are about 3.5 whorls of which 1.5 belong to the protoconch and are without ornament. The last whorl occupies most of the shell, its six spiral keels placed (1) alongside the suture; (2) halfway to the periphery; (3. 4. 5) above, at, and below the periphery; (6) round the umbilicus. Up to 1.5 mm high, 2.5 mm broad; last whorl occupies about 80% of shell height, aperture 60%.
The head has a long, narrow snout and carries two long tentacles, but the animal is blind and lacks visible eyes. There are two pallia! tentacles at the mantle edge on the right and the ctenidium may be extended out of the mantle cavity on the same side. Males have a penis. Flesh pale yellow or pinkish.
T. subcarinatus lives at L.W.S.T. under large boulders embedded on sandy beaches, where the sand is yellow, not black. The animals are rarely found, perhaps because the habitat is not commonly known. The species is southern in distribution, from the Mediterranean north to the British Isles, where it has been recorded from the western Channel and along western coasts.
The only reproductive stage of this species which is known is the veliger larva, which may be recognized by the bilobed velum which is dark red over its anterior half. The same colour is shown on the mesopodium though the propodium is pale.
Notes on the anatomy of this species were given by Woodward (1899) and Graham (1982).

Shell orbicular depressed, nearly lenticular, very solid. Protoconch of about 1,75 smooth whorls, of about 430-440 µm diameter, and a nucleus of about 37 pm for the Atlantic specimens; the Mediterranean specimens have a protoconch (Fig. 13-14) with a little more than 1 whorl, of about 350 µm diameter and 65 µm diameter of the nucleus. Teleoconch of about 1,5 whorls, with not very rapidly increasing, sculpture consisting usually of few spiral cords and numerous axial ribs; the adapical cord is in subsutural position; the two next ones are in the convexity of the whorl, the following one forms the angulation of the periphery, and there are two more below, the lowest one delimiting the umbilicus. The axial sculpture is formed by irregular, sometimes curved, nearly orthocline narrow ribs. Under magnification, the whole shell surface is covered with microtubercles disposed in spiral rows. The umbilicus is deep, narrow and with a wide infundibulum. Aperture rounded and a little depressed, columella curved, nearly vertical, the upper part of the outer lip sharp and extended dorsally, undulating by the end of the spiral cords. Dimensions: Usually between 2.0 and 3.0 mm of maximum diameter.

Animal was already described by FRETTER & GRAHAM (1978): "The head has a long, narrow snout with the mouth a longitudinal, subterminal and ventral slit. The tentacles are also long. The pallial edge is simple but carries 2 finger-like tentacles on the right; the attachment is well within the edge". The color is yellowish with reddish-orange tinge and semitransparent appearance. The long cephalic tentacles dorsoventrally flattened, not ciliated; black eyes hardly visible. Two pallial tentacles arise from the inner surface of the mantle skirt near its right margin. The gill is partly bipectinate and may project from the mantle cavity. The foot has a straight anterior edge and two opercular rear lobes; there is no metapodial tentacle. In the male, a penis arises from the dorsal surface of the head behind the right cephalic tentacle. Radula taenioglossate, formula 2-1-R-1-2. Rachidian tooth wide, base with a pair of denticles, ventral enlargement with a well-developed U-like shape, lateral margins expanded; margin area with a main cusp and 4-5 smaller denticles at each side. Lateral tooth similar to the central one, but narrower basally, border area with a central cusp and 4 smaller denticles at each side. Marginal teeth elongate, the internal one with 8-10 small denticles on the upper third of its right margin. The external marginal tooth is sickle-like, strongly inclined towards the outer side and presenting 2-3 slight denticles on the upper end of its internal margin. Operculum fine, transparent, with an irregular border and subcentral nucleus.

Shell solid, roundish in form with spiral depressed. Sculpture made by 5-6 spiral carinae quite evident, crossed by strong axial little ribs giving a reticulated aspect to surface. These little ribs are present at the base too in a larger number.
Aperture roundish in form, lip simple, base not much convex with two carinae evident and umbilicus broad. Uniform white in colour. Atlantic forms, more solid and careened, correspond to variety robustior Dautzenberg. Variety interrupta Marshall, 1902, found around Sicily (Italy), is more depressed in form and it has a lower number of carinae. This species is easily recognizable from its similar one T. mienisi (found along Israeli coasts) since this last one is much more depressed in form, it has larger sizes, it has a larger number of whorls of teleconch, it has larger sizes of protoconch and it has a considerable carina at the periphery of last whorl. Average measures of adult specimens are around 2-2.5 mm in diametre.

T. subcarinatus var. interrupta described by MARSHALL (1902) and commented on in AARTSEN EX AL. (1998) is probably a variation of the Atlantic shells.
The form robustior mentioned by DAUTZENBERG (1912) and commented on and illustrated in ADAM & KNUDSEN (1969) is really the European form. This form is mentioned in contrast to other more attenuated African forms, which we consider a different species (see Tornus attenuatus). Also the British shells shown in FRETTER & GRAHAM (1978) are more attenuate in shell sculpture, but conspecific in our opinion.
As pointed out by AARTSEN ET AL. (1998) the specimens from the Mediterranean (our material is from Tarifa, Gibraltar Straight in Spain, Algeria up to Kusadasi, Turkey) have a shorter protoconch with only a little more than 1 whorl and a smaller diameter) than the shells described by FRETTER & GRAHAM (1978) and ADAM & KNUDSEN (1969) (see dimensions in the description) and those studied in the present work from the Atlantic, with a larger protoconch. Anyway, T. subcarinatus is a variable species living in a wide range of water temperature and that may change the rate of development, as seems to occur in other species, such as Nassaridae (ROLAN, 1986). Thus, we consider all of them conspecific, although these differences and the reasons for them should be studied in the future.

Distribution: European Atlantic from Norway (POPPE & GOTO, 1991), British Islands (FRETTER & GRAHAM, 1978), North Spain (HIDALGO, 1917, ROLAN, 1983), Portugal (NOBRE, 1938-40) and Mediterranean (HIDALGO, 1917, POPPE & GOTO, 1991) to the Black Sea (MACEDO EX AL., 1999). NORDSIECK & GARCIA-TALAVERA (1979) cited it from Canary Islands. NICKLES (1950) and ADAM & KNUDSEN (1969) also mentioned Morocco, Mauritania (PALLARY, 1912 also), Ivory Coast up to Gabon, and DAUTZENBERG (1912) mentioned it from Congo. We have collected it as far south as Senegal. Perhaps, that range must be reduced because some of the records, such as those from Ivory Coast and Congo may be due to misidentification of other species (like T. attenuatus spec. nov.).

Habitat: HIDALGO (1917) mentioned that he found two live specimens in Luana, North Spain, attached to rocks at low tide. GOFAS & PONDER (1991) give complete information on the habitat of this species, attached to the surface of stones buried in sand down to 30 cm, in pure sand without any silty/muddy deposits, where the water can have sufficient circulation and is clean and oxygenated. In this habitat, it lives with other species such as: Onoba semicostata, Alvania lactea, Plagyostila asturiana, Striarca lactea, and Caecum sp. In Fuengirola, Alboran Sea it lives attached under stones buried in sand together with Alvania lactea (Michaud, 1832), Bornia sebetia (O.G. Costa, 1829) and Pseudophitina macandrewi (P. Fischer, 1867). In Tarifa, Gibraltar Straigth, it lives in sandy bottom with boulders and stones partially buried, in places of strong waves and currents with the sames species previously mentioned. In Galicia, North Spain, specimens of T. subcarinatus have been collected in rocky bottom under stones with Litbopbyllum tortuosum and Hymeniacidon sanguinea. In this habitat, Cingula cingillus (Montagu, 1803), and Bittium reticulatum (Da Costa, 1778) were also collected.

Habitat: it lives among algae in the infralittoral zone.
Distribution: it is collected all over the Mediterranean. Rather common.