Mekongia Crosse & P. Fischer, 1876

Examined species: Mekongia jullieni (Deshayes, 1876) (Figs 2L, 3L, 6L).

Remarks: Mekongia is not a monophyletic group according to two recent molecular systematic studies (Hirano et al., 2019a; Stelbrink et al., 2020). Here, we only examined the type species of Mekongia, M. jullieni . Mekongia can be distinguished from the other genera with a concentric inverted-comma-shaped operculum by the following combination of characters: a nucleus very close to the inner margin, an inner enamel region that is very narrow in width, a large inner opercular region and a relatively small nuclear region with fine shiny grains.