Comparison with the related Trivia monacha. Both from the Menai Strait, North Wales.

1: respiratory siphon; both also have a feeding proboscis hidden within the head except when feeding.
2: smooth, slender cephalic tentacles.
3: foot
4: mantle lobes, can extend to cover whole shell, or retract to fully expose the shell (5) , on Trivia
6: mantle lobes fused, cannot retract to expose the internal shell of Marsenia.
Marsenia and Lamellaria are closely related to British cowries, Trivia species, and are not heterobranchs like the majority of seaslugs. But, having a concealed internal shell into which they cannot retreat, they qualify as ‘slugs’, which is a non-scientific, vernacular category.