Shell. Like that of C. zizyphinum in general but semi-transparent and pearly. It has about 7 slightly swollen whorls meeting at inconspicuous sutures. The apical angle has an average and modal value of 64° (range 60-72°), whilst that between the side of the spire and the base is commonly 76-78° (range 65-83°). The spiral ridges and furrows are much more prominent than in zizyphinum or papillosum. The growth lines are prosocline, all alike and slight, most easily seen in the furrows. The spiral ridges number 4-5 on the body whorl, 4 on the next two, 3 on the next two, and 2-3 on the whorls next (o the protoconch. They form narrow step-like projections separated by rather flat furrows 2-3 times broader. In the oldest whorls all the ridges arc beaded but in younger ones there are no beads, though they usually persist on the most adapical ridge on the body whorl. The beads arise on older whorls where slight ribs cross the ridges but ribs cannot be traced after the third whorl. The peripheral ridge on the body whorl is broader than the others, forming a prominent keel. The base shows 1-2 ridges, separated by narrow grooves, directly under the keel, and 3-5 similar ridges and grooves round an umbilical spiral groove. Between these two sets the base forms a broad, flat belt marked only by growth lines. A small umbilical chink is present. The protoconch has a large reticulate pattern and measures 250nm across.
Aperture. A rather rounded quadrilateral of which the two adapical corners are more pointed than the others. The outer lip is thin and springs at right angles from the periphery of the body whorl. It is crinkled where the spiral ridges end since they are here hollow folds rather than solid ridges. Near the columella it is slightly out-turned. The columella is a rounded pillar with an indistinct basal swelling but clearly marked by its glossy, nacreous surface. The inner lip is reflected outwards over part of the umbilical groove.
Colour. Fresh shells, particularly when wet, show an opaline refulgence in the furrows, glowing with reddish colour. In most shells the spiral ridges are yellowish, in a few white. The inside is nacreous.
Size. Up to 11-12 mm high, 10-11 across. Body whorl = 55% of total height; aperture = 40% of total height.
Animal. Like zizyphinum except that the cephalic lappets are small or absent, the anterolateral corners of the foot more expanded and the number of epipodial tentacles is 3 (occasionally 4) on each side.
Colour. Cream or buff, speckled or striped with brown or purple.

C. occidentale is a boreal form found on both sides of the Atlantic. In Europe it occurs off the whole western Norwegian coast, in the northern N. Sea, off British coasts N. of the Humber on the east but not on western British or any Irish coasts. It is not found in the Skagerrak nor off Jutland. In America it has been recorded from Newfoundland S. to New Jersey. It also occurs in Iceland, but not Greenland.