Comparison of central valves viewed from posterior.
Tonicella marmorea valve iv (upper); B. ruber valve iv (centre); & Lepidochitona cinerea valve v (lower),
B. ruber & L. cinerea leg. P. Lightfoot, N. Yorkshire, Sept. 2014. T. marmorea leg., coll. I.F.S. Orkney, April 1974
1: Tonicella marmorea, low arch-profile, elevation 20% (height/width) with pronounced keel. N.B. varies 20% to 44% so overlaps with other two spp. Upper slope often flat becoming slightly convex further down 2.
3: T. rubra, low smoothly-convex arch-profile, elevation 23% (h/w) with moderate keel. Sometimes flatter.
4: Lepidochitona cinerea, moderately-high arch-profile, elevation 32% (h/w) with keel.
N.B. elevations vary within each species, and sometimes on an individual with age. The elevation ranges of the two Tonicella spp. overlap, so cannot be used to separate them.
Kaas & Van Belle in Schwabe define elevations (h/w) of valve iv as low if less than 25%, moderate 25% to 45% and high if over 45%.

KEY ID FEATURES and similar species at: 01 Boreochiton ruber. Length 12mm. North Yorkshire. September 2014. (leg. Paula Lightfoot).