Heterogen Annandale, 1921

Examined species: Heterogen longispira (E. A. Smith, 1886) (Figs 2S, 3S, 6S); Heterogen japonica (Martens, 1861) (Figs 2T, 3T, 6T).

Remarks: Heterogen longispira has always been considered as the only species of the genus Heterogen, which is endemic to Lake Biwa, Japan. Recently, ‘ Cipangopaludina ’ japonica has also been assigned to the genus on the basis of molecular systematic data (Hirano et al., 2019a); the opercular characters of this species also match those of H. longispira, including a relatively large, smooth and shiny nuclear region and a thin marginal region. The combination of these opercular characters differs from those found in other viviparid genera with a concentric inverted-comma-shaped operculum.

The operculum of H. japonica is clearly higher than it is wide, while in H. longispira the opercular height and width are roughly the same. In addition, H. longispira has a larger inner opercular region as compared with H. japonica .