Shells small for family and subfamily, roughly cone-shaped, with flattened or only slightly elevated spires; shells ornamented with variable numbers of low, rounded knobs, always arranged in spiral rows around the body whorl; apertures proportionally narrow, bordered by wide parietal shields along columellar side and by strong teeth along edge of labrum; parietal shields variable in color, ranging from white and pink to dark rose-violet, ornamented with very numerous small pustules.

A survey of the genus Morum (sensu stricto) in the western Atlantic (taken from Petuch, 2013) has shown that there are two separate species complexes within the region, with each complex being confined to a separate molluscan faunal province. The Caribbean Province (Bermuda and the Bahamas, south to Suriname) is now known to house a complex of three species, including Morum oniscus (Linnaeus, 1767), Morum purpureum Roding, 1798, and Morum strombiforme (Reeve, 1842). The Brazilian Province (the Amazon River mouth south to Cabo Frio, Rio de Janeiro State) houses a complex of four species, including Morum bayeri Petuch, 2001, Morum berschaueri Petuch and Myers, 2015, and two previously undescribed species, here named Morum damasoi and Morum mariaodeteae. These seven western Atlantic species, along with the three western Atlantic Oniscidia species, altogether, represent the largest fauna of the Harpidae known from any single biogeographical region in the Atlantic and Eastern Pacific Oceans.