One side of shell stuck to hard surface. Shell thick with long hollow
spikes. Attached to hard surfaces. Common on all our shores.
Plain shell, shell opening close to the surface, not regularly zig-zag.
Common on rocks on the Northern shores.
Shell opening away from the surface, regularly zig-zag. Common rocks
on the Southern shores.
6cm. Shell is thick with clear ridges. Under stones near reefs.
Lower valve is cup-shaped and stuck to a hard surface. Upper valve
flat covered with flat short spines. Among living reefs. Sometimes
seen on some of our shores.