Morphological phylogenetic analysis of the mactrid subfamilies Mactrinae and Kymatoxinae, plus the mactroidean family, Anatinellidae, indicate that the Kymatoxinae shares closer common ancestry with the Anatinellidae than with the Mactridae. Kymatoxinae is, therefore, reassigned from Mactridae to subfamily rank within Anatinellidae. The lack of a pallial sinus in Anatinella G. B. Sowerby I, 1833 suggests that its family was derived from anatomically plesiomophic members of Mactroidea with at best only weakly developed siphons.