Shelf. Solid, opaque, and with a dull surface, originally covered with a thin periostracum, but this tends to be lost from all but the most protected parts. The spire is straight-sided, neglecting the curvature of the whorls, the apical angle 50-55°, the apex pointed. There are 7-8 tumid whorls which meet at distinct, incised sutures just below the periphery of the upper whorl; the last whorl is much the largest part of the shell. The ornament comprises costae, spiral ridges and grooves, and growth lines. The costae are undulate in section and follow a crescentic course; they are prosocline at the suture, where they are also most prominent, becoming orthocline below; they cross the upper whorls of the spire, sometimes also the penult (though often failing to do so), and usually fade below the periphery and towards the aperture on the last whorl. Those that do cross this whorl become flexuous on the base. The number on the last whorl is variable, but there are commonly 12-15 on each whorl of the spire. The whole shell also bears fine growth lines which are still more prosocline than the costae and so cross them. The spiral ridges are low and strap-shaped, and vary in height and breadth. Commonly every fourth or fifth is enlarged on the upper half of the last whorl, but elsewhere it is more often every second or third producing the effect of a denser packing; these enlarged ridges consist of two lying together. On the base of the last whorl an enlarged spiral keel, the siphonal fascicle, runs from the end of the siphonal canal to the umbilical region. In young shells in which the periostracum persists small hairs project where spiral ridges and growth lines cross; these are fairly rapidly lost.
The protoconch comprises a little over 1 whorl, smooth, measuring 1 mm high and 2 mm across. It is often bulbous.
Aperture. An oval opening, pointed adapically, its greatest breadth towards the base, surrounded by a peristome lying in the prosocline plane. The outer lip arises just below the periphery of the last whorl, somewhat obliquely. Its initial course is rather straight and presents a broad and shallow sinus; thereafter the lip curves more lightly to the base of the aperture where its direction lies more or less at right angles to the axis of the shell. In older animals its edge is a little thickened and out-turned, thin in young ones. The siphonal canal is open and often short, not extending as far as the most basal part of the outer lip, which it is twisted to face, and from which it is separated by a small notch. The columella is short and curved, the lip everted and applied to the siphonal fascicle. The inner lip forms a film over a considerable area of the last whorl, making it glossy.
Colour. Yellowish, with lighter and darker areas, the latter forming spiral bands in the adapical and basal parts of the last whorl. The throat, columella, and parietal region are usually white, but there may be areas of darker colour in the first.
Size, Up to 110 x 68 mm. Last whorl = c. 70% of total shell height; aperture = c. 50% of shell height.
Animal. The head has no snout, but forms a transverse shelf from which the tentacles arise; these are flattened and each has an eye rather less than half way from the base. The part below the eye is thicker than that distal to it. The mouth (= opening of a proboscis sac) lies under the shelf. The mantle edge is rather thick and smooth, forming, on the left a siphon which extends a considerable way from the canal as the whelk creeps. Males have a very large penis behind the right tentacle; it bears a small projection at its tip.
The fool is large, shield-shaped, with a broad, double-edged anterior end which is slightly convex, and a little angulated laterally; it tapers to a rounded point posteriorly and bears an oval operculum with a nearly central nucleus and a groove along its posterior radius. In females the mouth of a pit is visible on the sole of the foot a little anterior to its mid-point: this is the opening of the ventral pedal gland.
Colour. Cream, with innumerable black streaks and blotches. These are thickest on the dorsal surface of the head, the tentacles, the siphon, and the ventral part of the sides of the fool.
Geographical distribution. A cold-water, northern species found throughout much of the North Atlantic from the Bay of Biscay north to the north of Norway on the eastern side; in Iceland; and between Labrador and New Jersey on the western side, it occurs in the whole English Channel (not usually in the Scillies). the North Sea, the Kattegat- and the Baltic as far east as the Mecklenburger Bucht. It is often abundant. If it occurs in the Mediterranean at all nowadays it is limited to deep water.
Habitat. Though liable to occur on every kind of substratum this species is usually found on soft bottoms, occasionally at L.W.S.T. but normally sublittorally to about 1200 m deep. It tolerates brackish situations to a salinity of about 15%0.
Food. Commonly regarded as a carrion feeder (which it undoubtedly is) it has been shown by Nielsen (1975) and by Taylor (1978) that B. undatum feeds primarily on polychaetes, especially Lattice, supplemented by a variety of bivalves, mainly cockles and crustaceans.

Along the North Atlantic shores, from New Jersey, U.S.A. (Abbott, 1974), north to. Greenland (Thorson 1951) (70° N), Spitzbergen (Odhner, 1915), N Norway (Sars, 1878) and S to N Spain (Bay of Biscay, Ortea, 1977), Portugal (Hidalgo, 1917) and possibly the W Mediterranean'-(Altimira, 1975, Ghisotti, 1974, Jeffreys, 1867, et al.). Normally it occurs in shallow water, 0-100 m,, but it has been recorded from bathyal depths (Friele & Grieg, 1901, 1500 m; Sykes 1911, 1450 m, etc.). We have, however, not seen specimens taken alive from depths greater than 500 m (Korsfjorden, W. Norway).