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A single bipectinate ctenidium on left side.

Spire of shell much reduced; two bipectinate ctenidia, the right being the smaller; no operculum.

The ctenidium is monopectinate and attached to the mantle along its whole length, except in Adeorbis and Valvata; in the latter alone it is bipectinate.

The sub-class is now divided into two orders: the Aspidobranchia in which the branchia or ctenidium is bipectinate and attached only at its base, and the Pectinibranchia in which the ctenidium is monopectinate and attached to the mantle throughout its length.

Ctenidium bipectinate, free; hermaphrodite; fluviatile.