Shell trochoid, medium to small (diameter < 16 mm in adults), wider than high (H/D = 0.75 - 0.81); spire of about 6 1/2 flat-sided whorls, separated by a shallow suture. Base
nearly flat, narrowly umbilicate, concave umbilical area. Protoconch dirty white,
smooth, of approximately 3/4 whorl and a diameter' of about 200 mu, with a strong and sinuous terminal lip. Teleoconch of 5 1/2 whorls, the first one with three smooth spiral cords of bright red color; in the subsequent whorls, the cords become nodulose and change to a uniform dark brown color; surface with fine oblique growth lines. Penultimate whorl with 6-7 cords distributed between the suture and the periphery; the last one with 10-12, with rectangular and very close nodules of different size; spaces between cords with a spiral thread which, in subsequent whorls, becomes a new cord; the subsutural cord is a little stronger than the others, being more evident in the first whorls; the outermost cord has an angulation in the last whorl. Base with 9 to 10 depressed spiral cords, different in size, with few nodules evident, sometimes imperceptible in the cords closer to the umbilicus. In the spaces between cords some spiral threads can be seen; umbilical margin crenulated, with 6 to 7 strong denticles. Aperture trapezoid, prosocline; inner lip with 3 denticles which decrease in size as they are farther from the columella; inside of the outer lip with spiral wrinkles, no denticles at its inner margin and no anal tooth; parietal area with a fine callus, few wrinkles evident, and a wide parietal tooth. Columella strongly sloped, curved centrally, with two denticles, one larger on its upper part, and 4 rounded smaller ones distributed along its umbilical margin; at the base of the columella, there is a strong simple tooth; the apertural margin is smooth. The background color of the shell is dark olivaceous-brown, with some black and other smaller white blotches corresponding to the nodules; the base has a lighter cream background, and the cords are alternating dark brown and white, each color corresponding with a pair of nodules. Inside of the aperture pearly.
Dimensions: nearly all adult specimens of C. spadiceus have very similar size, height between 12 and 13 mm and diameter between 15 and 17 mm. The shell is a little depressed (H/D = 0.76-0.80). One specimen from of Sao Tome City was somewhat larger: 16 mm x 17 mm, with a ratio H/D = 0.94, surpassing the size mentioned by PILSBRY (1889) for this species: 10 mm x 15 mm.
Operculum corneous, multispiral, with a central nucleus and 11 whorls: there are not oblique lines; the opercular insertion area is triangular.

Clanculus spadiceus is close to C. villanus from which it can be differentiated by the uniform red color of the first whorls and the uniform dark brown of the rest of the teleoconch, its more depressed spire (H/D < 0.81) and by other anatomical and radular differences. C. spadiceus is apparently an insular endemic because we have no confirmed record beyond Annobon, Sao Tome and Principe Islands.