Cassidulus infidus Mortensen 1948b is one of the few recent species of the family Cassidulidae Agassiz & Desor 1847 and known only from its poorly described holotype. In Brazil, this family is represented only by two extant species and, to date, both are endemic to Bahia (C. infidus) and Rio de Janeiro (Cassidulus mitis Krau 1954). After a century of its first capture, some topotypes of C. infidus were found enabling its redescription, which are provided herein. The characterization of some of its ecological aspects and a key to the known species of Cassidulus are also given.