Shell. Solid, opaque, not glossy. The shell is conical with a short spire and a large last whorl extended basally into a short siphonal canal. There are about 6 whorls, each slightly swollen, which meet at shallow sutures lying at the periphery of the upper whorl. Neglecting the curvature of the whorls the spire is straight-sided in profile, or very slightly coeloconoid. The apical angle varies, 73-84°, commonly 78-79°. The ornament consists of growth lines and of spiral ridges and grooves. The growth lines are fine, markedly prosocline, and occasionally thickened. In some shells they are raised to form a series of fine varices. The spiral ridges are low. undulate to strap-shaped in section and broader than the intervening grooves. Those on upper whorls are narrower and more raised. In many shells the ridges at the periphery of the last whorl and corresponding parts of the spire are better developed and may give a slightly angulated appearance to the whorls. There are 11 - 14 ridges on the last whorl, 3-4 on the penult, 3 on the others except the topmost, which has 2. A broad spiral keel at the base overlies the siphonal canal. The ornament is most obvious in young shells and is often so eroded as to give a nearly smooth surface in older ones.
The protoconch has 1.25 - 1.5 whorls, is smooth and measures 500 - 700 µm across.
Aperture. An elongated oval, surrounded by a peristome, angulated adapically and abapically, its long axis set at c. 16° (range 13-19°) to that of the spire. The aperture lies prosoclinally adapically but is not usually plane and tends to lie orthoclinally at the base. The outer lip arises nearly tangentially from the periphery of the last whorl at an angle of c. 140° to that of the spire. Its first section is nearly straight, but it then curves to the base where there is a short, straight siphonal canal lying nearly in the shell axis. The canal is half closed by overlap of the columellar lip which is broad, smooth, slightly concave and expanded so as to obliterate the umbilical canal and umbilicus. The inner lip forms a wide glaze over the last whorl, usually obscuring ornament and colour. In young shells the outer lip is thin and slightly crenulated by the ends of the spiral ridges; in older ones it is thick, not usually crenulated, and marked internally by up to 12 teeth, short spiral ridges extending into the throat for 3 - 4 mm and located under the spiral grooves on the outer surface. Their inner ends are connected by a ridge constricting the throat. The tooth on the edge of the siphonal canal is often bigger than the others. Occasionally an inner row of teeth may lie deeper.
Colour. Most specimens arc grey or white, but they may also be yellow, pale mauve or various shades of brown. In addition the shell may show spiral bands, usually of pale brown, but sometimes of chestnut or chocolate brown and, rarely, black. The pattern of the bands is extremely varied and their width may range from just filling a spiral groove to nearly half the last whorl. In the latter case the shell seems to have 1 - 2 light bands on a dark ground. Narrow pigment bands frequently arc found in pairs. The outer lip is often pigmented (uniformly, or only between the teeth) as far as the ridge at the inner ends of the teeth; the deeper parts of the throat may also be pigmented, either evenly or so as to repeat the external pattern.
Size. 32 x 22 mm, but occasionally up to 50 mm high. Last whorl = 83 — 88% of total shell height; aperture = 65 — 75% of total shell height.
Animal. The head is flattened. without a snout, the mouth (= opening of a proboscis pouch) a small longitudinal opening on the underside of the fold from which the two tentacles arise. These are narrow and tapering, thicker basally, with the eye where the two parts join, about one-third of their length from the base. The mantle skirt has a smooth edge and is extended on the left into a short siphon, little visible in a crawling animal. In males a curved penis arises from the floor of the mantle cavity a little behind the base of the right tentacle. It is round in section and recurved; the vas deferens is internal and opens at the tip, which forms a short flagellum.
The foot is axe-shaped anteriorly, a little embayed in the mid-line and double-edged because of the opening of the anterior pedal gland. The posterior end is bluntly rounded. In every animal there is a pore in the mid-line of the sole a short distance behind the anterior edge — the opening of a pit within which the accessory boring organ (ABO) lies when not in use. In females, but not males, a second opening lies a little behind this, the opening of the ventral pedal gland used in depositing egg capsules. The operculum is oval with basal nucleus.
Colour. White or pale yellow with white speckles. Operculum dark horn colour — chestnut brown.
Geographical distribution. From the Straits of Gibraltar north to the Arctic and extending across the Atlantic to Iceland, Greenland and south to Connecticut. It reaches into the Skagerrak and along the Swedish shores of the Kattegat but is uncommon in Denmark. It occurs on all suitable shores in the British Isles. It lives between the 0° and 19°C isotherms of oceanic waters according to Moore (1936).
Habitat. N. lapillus is an abundant inhabitant of the middle and lower reaches of all rocky shores (MLWS to MHWN, 10 - 75% emersion), and extends, more rarely, to depths of 30 - 40 m. It is often aggregated, especially in winter, in rock crevices and under stones but isolated animals are common on rock surfaces and in pools. It is particularly numerous where barnacles and mussels occur and is little affected by exposure if crevices for shelter arc available. It avoids very weedy shores and seems to stand only limited reduction of salinity.
Food. This is a carnivore relying mainly on barnacles and mussels for food but also taking any available mollusc which it is capable of attacking. Cannibalism also occurs. The faeces arc pellets 200 x 100 µm.