Idiopoma Pilsbry, 1901

Examined species: Idiopoma umbilicata (Lea, 1856) (Figs 4J, 5J, 7J); Idiopoma dissimilis (Müller, 1774) (Figs 4K, 5K, 7K).

Remarks: Idiopoma can be distinguished from other genera with a concentric inverted-comma-shaped operculum by the following combination of characters: a whole inner opercular region covered by a thick, rough, brownish-yellow sandpaper-like layer; a thin marginal region obviously differing from the enamel region in colour; and an enamel region that is smooth but has a fine grid-like pattern.

The operculum of I. dissimilis is about as high as it is wide, while in I. umbilicata the operculum is distinctly higher than it is wide. This difference can be attributed to differences in aperture shape between the two species.