Ventral, dorsal and lateral views of semicircular tail valve viii. Semicircular growth lines (1) surround a small, unobtrusive central mucro (2) from which the straight posterior slope (3) descends at an angle. The articulamentum extends all round the curved posterior-lateral edge where 14 slits cut it into rectangular teeth which insert into the girdle. The articulamentum has pores in pale rays leading to each of the slits. Orkney, Scotland, March 1976.