Periwinkles and Topshells

Single valved (as opposed to BIVALVED) shells on the shore could be two MAIN types, periwinkles or topshells. Limpets also have single shells but are unlikely to be confused.

The best way to tell apart the periwinkles and topshells whilst you are 'getting your eye in' is as follows. The shell in both cases is a spiral built on a central rod. In the winkles this rod is solid, but in the topshells it is hollow. This means that, when turned over, the winkles have no small hole next to the shell opening, but topshells do. This hole is called the UMBILICUS.

It can be clearly seen in the comparative photograph below.

Two common shore 'snails' winkle on the left, topshell on the right