Comparison with the related Trivia monacha. Both from the Menai Strait, North Wales.
Lamellaria spp. are gonochoristic Caenogastropoda, closely related to European cowries, Trivia spp., rather than to hermaphrodite Nudibranchia and the majority of seaslugs. But, having a concealed internal shell into which they cannot retreat, they qualify as ‘seaslugs’, a vernacular, non-scientific term.
1: respiratory siphon; both also have a feeding proboscis hidden within the head except when feeding.
2: smooth, slender cephalic tentacles.
3: foot.
4 & 5: mantle lobes, can extend to cover whole shell, or retract to fully expose the shell on Trivia.
6: mantle lobes fused, cannot retract to expose the internal shell of Lamellaria.

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