The White Cap Limpet, very common on the British Columbia coast, is usually found on the lower zones as compared to the intertidal zones of other limpets. White cap limpets shells are white in colour amd rather thick in nature. The animal extends from 19-44cm (0.75-1.75") in size.

The limpet, however, can only be seen as white after it is dead. When alive, it usually is covered by a pinkish coralline algae, Lithothamniom sp. which is also its main source of food. This forms an excellent comouflage. It's predators include sea stars.

References:

http://web.mit.edu/corrina/tpool/limpets.html#whitecap

http://www.gastropods.com/shell_pages/m/Shell_Acmaea_mitra.html