The shell, anatomy, and habits of the mesogastropod Cerithium atratum (Born) are described from animals collected intertidally at the Research Station of the Oceanographic Institute, near Ubatuba, Brazil. The species has been reported from Florida and the West Indies, southward
to Argentina, but is not described in the recent literature. Its relationship with allied species, such as C. algicola C. B. Adams, C. eburneum Brugière, and C. striatissimum Sowerby, is discussed but not settled.
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