Tonicella marmorea on left.
Boreochiton ruber on right, at same scale; intermediate valve iv from 14.5 mm long animal. North Yorkshire. September 2014. Leg. Paula Lightfoot.
Apart from growth lines, both appear to the naked eye 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 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.