The mentum, or "hanging lip" that defines the myrmecophilous scarab genus Cremastocheilus was examined and compared between several species. Examination of this unique, modified mouthpart revealed that it can be used as a key character in distinguishing closely related groups within the genus. It appears as a simple disc in the currently recognized sister group Genuchinus, and is variably modified as a cup-shaped shield in all examined Cremastocheilus species. The variability of the mentum within Cremastocheilus suggests that it probably serves a particular function that corresponds with specific ant host or habitat. The mentum is introduced here as a single-character system that presents possibilities for ongoing and future phylogenetic work on Cremastocheilus and related genera. Other characters that are useful for identifying Cremastocheilus species and their relationship are introduced as a basis for future phylogenetic studies.