Yellow excrescences resemble yellow papillae on mantle of Trivia monacha.

1: fragile, internal, white, ear-shape shell; never exposed on live Marsenia.

2: mantle halves fused so never retract to expose shell.

3: inhalent siphon.

4: head and tentacles have form similar to Trivia.

Body length 15.5 mm. Menai Strait, Wales. March 2011.