Canals in tegmentum exposed at its edge (1). Articulamentum reveals pink of tegmentum (2). Two apophyses (3), conjoined by shallow jugal sinus (4). Articulamentum extends laterally as short insertion plates cut by slits into 2 or 3 teeth (5). Articulamentum has rays (6) with pores where the aesthetes connect the valves to the mantle. Orkney, March 1976.