Morum oniscus is one of the few species of the gastropod family Harpidae living in the western Atlantic Ocean. The species may reach 32 mm (about 1.26 inches). The species is known from the Gulf of Mexico, south Florida, the Caribbean, and south to eastern Brazil. Until the late 1980s, Morum species were assigned to the helmet-shell family Cassidae, most likely because of shell shape and presence of a relatively large *parietal shield* (area of the shell adjacent to the *aperture* or opening). They share, however, many anatomical and behavioral features with the larger species in the genus Harpa and their relatives. This unusual *sinistral* (left-handed) shell was collected on the coast of Eleuthera, Bahamas.
