Found a pretty nice big shell beachcombing....need ID help.

[paul monfils]

The shell is formed by a specialized organ called the mantle, which extracts minerals, primarily calcium, from sea water, and turns them into mineral salts which it secretes in crystalline form along the edge of the aperture.  This allows the snail to continuously enlarge its shell as it grows.  The growth of the shell is in the form of a widening spiral around a central axis, with the oldest part of the shell at the apex or pointed tip, and the most recently formed shell around the rim of the aperture.  The mantle also secretes pigments that give the shell its colors and pattern.  As you observed, in many species the pigment deposition gradually decreases as the animal gets older (hmmmm, sounds familiar doesn't it?), so that the older parts of the shell are often more colorful than the newer parts.