Shell size to 9 mm; shell sub-cylindrical, heavy for size, with three spiral "rows" of squarish knobs. Parietal shield white, with pustules. Outer lip with tiny teeth. Protoconch usually present in well-preserved specimens. Color whitish or light gray with brown patches. Periostracum dark-gray, velvety. The genus was included in the family Cassidae (helmet shells) until 1987, when Hughes & Emerson demonstrated that it belongs within the Harpidae. Morum oniscus is relatively common in the Caribbean, Bahamas, and Florida Keys, where it thrives in coral habitats, but is rare in SW Florida. The shell herein illustrated was found by Kimberly Nealon on Captiva, on August 7, 2010.