Shell obconic, with low spire, its whorls having a rather sharp shoulder which is sometimes nearly smooth, at others sculptured with revolving stride, and often nodulous; on the bodywhorl are 3 revolving rows of strong tubercles; outer lip thickened, slightly ascending the spire, swollen within and without, toothed, sometimes cut away below; columellar callus strong, finely pustulous. Color whitish, variegated with brownish or dark gray, outer lip dotted; aperture white; callus white or purplish.
Length, 23; diameter, 15; length of aperture, 23; greatest width of aperture, 2.5 mm.

Diagnosis. Shell of average size for genus, roughly cone-shaped, with flattened spire; shoulder sharply-angled, slightly rounded, bordered with 8-10 knobs; body whorl ornamented with 4 widely-spaced rows of large rounded knobs, with one below shoulder, one around mid-body, one around anterior end, and one small row around base of siphonal canal; parietal shield proportionally large, well-developed, adherent, extending over at least half of shell base; parietal shield covered with very numerous tiny rounded pustules; labrum thickened, smooth, with inner edge being lined by 12-14 large teeth; shell color white, covered with numerous dark brown speckles and small flammules, with largest and darkest flammules being on spire whorls; aperture and parietal shield pure white; protoconch white, projecting and mammilate, composed of 3 whorls.

This small harpid ranges from Bermuda to southeastern Florida (Palm Beach County) and the Florida Keys, throughout the Bahamas and the entire Caribbean Sea Basin, south to the Lesser Antilles and Barbados. The species is absent from the Gulf of Mexico. The presence of Morum oniscus in southern Florida represents the only occurrence of this species in the Carolinian Molluscan Province (see Petuch, 2013).
Morum oniscus is the most frequently-encountered moruminine harpid in the Caribbean region, being a well-known resident of most shallow water coral reef areas from Palm Beach County, Florida to Barbados. The species has been found to be especially common in the northern Caribbean and the Bahamas, and is abundant on outer islands such as the Abacos and Eleuthera. Farther south in the Caribbean area, Morum oniscus is less common, possibly due to competition with its sympatric congeners M. purpureum and M. strombiforme.