1: posterior and usually largest of three pitch-brown/black dorsal marks on shell .

2: middle of three pitch-brown/black dorsal marks weakest on this specimen, but usually anterior mark is smallest (Forbes & Hanley, 1853); considerable variation in size and colour saturation.

3: pale, median, dorsal line marks meeting position of sides of expanded mantle; little pigmented shell deposited in slight gap between mantle sides.

4: whitish ribs have very slight pink tint where exterior mantle has deposited minimal amount of pigmented material.

Adult. Shell height 10.8 mm (longest dimension). Menai Strait. August 2010.