Intermediate valve iv from 14.5 mm long animal. North Yorkshire. September 2014. (leg. Paula Lightfoot).
Upper image, dorsal view.
Outermost proteinaceous layer (properiostracum and accumulations) containing much of colour has been removed.
1: tegmentum: outer layer of conchiolin impregnated by calcium carbonate (aragonite) and riddled with canals, consequently softer than most shell-material.
2>3>3: jugal area distinguished by longitudinal (relative to whole body) colouration.
2>4: diagonal ridge, separating lateral area (above ridge in image) from pleural area, is undeveloped and virtually undetectable.
5: almost-rectangular, concentric, distinct growth lines; origin (2) on valve-posterior.
6: barely visible (at 30X magnification) caps on sensory aesthetes.
7: apophysis (a.k.a. sutural lamina) on anterior of valve is extension of underlying articulamentum; markedly prominent and rounded, almost semicircular (when undamaged, but brittle and easily broken).

Lower image, ventral view.
8: articulamentum: thick, white and (on jugal tract) pink, inner shell layer with fewer canals than tegmentum, so harder.
9>9: slit ray (a.k.a. notch ray); row of openings to aesthete canals that penetrate both articulamentum and tegmentum.
10: slit (a.k.a. notch) separating teeth of insertion plate.
11: ventral openings of aesthete canals in articulamentum near posterior edge.
12: ventral openings of aesthete canals in jugal tract of articulamentum.
13: openings of aesthete canals on posterior margin of tegmentum.
14: apophysis (a.k.a. sutural lamina) on anterior of valve is extension of articulamentum; markedly prominent and rounded, almost semicircular (when undamaged, but brittle and easily broken).
15: piece of strong muscle that connected apophysis to body.

Very thin, discontinuous, innermost layer (myostracum) not labelled as can only be distinguished by microscopic examination of cross section of shell.

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