Velutina plicatilis (O. F. Müller, 1776)
Maximum length when live c.18mm (excluding protruding foot and tentacles).
Thick unfused mantle lobes (1) can overlap a short way onto exterior of shell, but not sufficiently to cover it.
Thin translucent shell consists mainly of periostracum with velvet like texture through which yellow/orange/brown viscera visible within ; often mistaken for the mantle of a dorid seaslug. Epizooic organisms such as barnacles (2) may adhere to the shell.

Moray coast, north-east Scotland. May 2018. © C. Rickard.