Semi-slug
Semi-slugs, also spelled semislugs, are land gastropods whose shells are too small for them to retract into, but the shell is not quite vestigial. The shell of some semi-slugs may not be easily visible on casual inspection, because the shell may be covered over with the mantle.
This is a type of gastropod that is intermediate between a slug (without an external shell) and a land snail (with a large enough shell to retract completely into).
There exist a number of gastropod families that have semi-slugs species. There exist about 1000 species of semi-slugs in comparison to about only 500 species of slugs.
Examples
Semi-slugs from the Neotropics:
family Amphibulimidae: Amphibulima and Gaeotis
family Xanthonychidae: Cryptostrakon, Semiconchula and Xanthonyx
family Pleurodontidae: Coloniconcha prima
Semi-slugs do exist but are exceptional in the Camaenidae.
References