The characters of the family are the following: the form is oval, flattened on one side, on which is placed the aperture; this is as long as the shell, narrow, and open at each end: the spire in the adult state is entirely concealed: the outer lip is in general bent inward and thickened. The surface of the shell is often highly polished, with a glassy, or porcelain-like enamel, with no trace of an epidermis, or investing coat of skin.
The animal is large; the mantle is developed into widely-expanded lateral lobes, which are, during activity, turned upward on each side, so as closely to embrace the shell. These lobes are generally gaily coloured, and are often adorned with various fringes and other appendages. The head is furnished with a retractile proboscis, and with a muzzle. The gill-plume is single. The sexes are separate. There is no operculum.
"The difference of aspect," observe Messrs. Forbes and Hanley, "between these mollusks when crawling, with all their beautifully-coloured soft parts exposed, often completely concealing their enamelled shells, and their appearance when, after being seized, they suddenly and instantaneously withdraw their bodies and mantle-lobes, and expose the shell only, is very curious and surprising."
As specimens of some or other of the numerous species that compose this beautiful genus may be found on almost every mantlepiece, sideboard, and
- Brit. Mollusca, iii. 493.