Adult with thick shell-walls and line of well-developed blunt bosses within outer lip of aperture. Sheltered-shore form with small aperture, 45% of shell height. Features described are suited to resist attack of crabs (common on sheltered shores). High mortality rate among young before these features develop (90% yr 1, 50% yr2). Adapical angle becomes rounded as shell thickens. White shell with green algal growth.