Comparison of valve iv, T. marmorea (left), B. ruber (right), at same scale. Leg. D. M. McKay & S. Taylor, and P. Lightfoot.
Apart from growth lines, both appear to have smooth valves, but both have caps/granules that become visible at about 3X (T. marmorea) and X30 (B. ruber).
1: T. marmorea caps/granules visible if viewed on 50cm monitor.
2: B. ruber caps/granules almost or entirely invisible if viewed on 50cm monitor.
3: T. marmorea apophyses are, relative to valve size, short (anterior-posterior), wide and gently curved.
4: B. ruber apophyses are prominent and rounded, often almost semicircular.
5: T. marmorea jugal sinus (gap between apophyses) is about quarter width of one apophysis.
6: B. ruber jugal sinus (gap between apophyses) is about same width as one apophysis.

NOTE: The shapes of apophyses vary depending on position. For example, apophyses on valves vii and viii on B. ruber are much less prominent and rounded than above. For consistency, it is suggested that comparisons of chiton spp. are made with undamaged valves iv or v; the central largest ones.

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