Shell medium-sized, attaining height of 15.6 mm, width of 13.9 mm, conical, umbilicate, finely sculptured. Protoconch about 350 µm maximum diam¬eter, 1 whorl. Teleoconch whorls about 8.5, flat; first 4 whorls with low, rounded axial riblets and 2-7 beaded spiral cords; axial sculpture reduced to fine collabral threads or absent on subsequent whorls; spiral sculpture increasing by intercalation to 7-11 beaded cords and 5-6 threads, of which abapical 3 cords forming periphery; periphery narrowly rounded. Base weakly convex, with 17 narrow, finely beaded spiral cords. Umbilicus 22%-24% maximum shell diameter, wall almost vertical, white. Aperture subquadrate, lips thin, weakly crenulate; columella weakly sigmoid, rather thin, terminating in small, rounded denticle. Shell ground color uniformly ivory or with very faint peripheral light brown maculations.
Holotype: USNM 860257, height 15.6 mm, width 13.9 mm.

Shells of Calliostoma alternum are most similar to those of C. aulicum but differ by being evenly conical rather than coeloconoid; by having a more rounded, less strongly projecting periphery; by having fewer, stronger, more coarsely beaded spiral cords, most of which are separated by a fine spiral thread; by having axial riblets that persist on the first four whorls rather than only on the first two; and by having the abapical of the two primary spiral cords remaining conspicuously strong on first five whorls.
The specimens of C. alternum were collected on bottoms composed of carbonate rock covered by calcareous algae and sponges (UMML, unpublished).

Type locality: Northeast of Caracas, Venezuela, JOHN ELLIOTT PILLSBURY Station P-737, 10°44.0'N, 66°07.0'W, in 60-73 m.
Distribution: Shells of this species are known from northeastern Colombia, Venezuela, and Suriname, in 55-146 m.