Lioplax Troschel, 1857

Examined species: Lioplax subcarinata (Say, 1817) (Figs 2A, 3A, 6A).

Remarks: Lioplax is the only genus in the family with a paucispiral operculum at early developmental stages, which is different from the concentric operculum of all other genera of Viviparidae . In addition, the semi-elliptical shape and lower central location of the nucleus are also diagnostic opercular characters for this genus.