Chlorodiella longimana is the only chlorodielline species presently known from the western Atlantic Ocean. Although C. longimana superficially resembles other species of the genus in general appearance of the carapace, morphological analyses revealed a suite of characters that separate it from all other known species of Chlorodiella – in particular, ambulatory legs having dactyli with a single tip and a basal antennal segment with a lateral flange that extends halfway into the orbital hiatus, excluding the antennal flagellum. In addition, a phylogenetic analysis of the Chlorodiellinae inferred from three mitochondrial markers (12S, 16S, COXI) and a nuclear marker (histone H3), confirms that C. longimana is genetically distinct from its congeners. A new genus, Ratha, is proposed to accommodate C. longimana. In addition, a phylogenetic analysis of six chlorodielline genera indicates that the subfamily is polyphyletic as presently defined.