The pigment patterns on tropical shells are of great beauty and diversity. Their mix of regularity and irregularity is fascinating. A specific pattern seems to follow certain rules, but these rules allow for variations. No two shells are identical. The immobile patterns appear static and are actually made up of calcified material. However, as shown in this book, the underlying mechanism that creates this beauty is extremely dynamic. It has much in common with other dynamic systems that produce patterns, such as a wind-sand system that forms large dunes, or rain and erosion that create complex branching river systems. In other shells, the underlying mechanism bears similarities to waves, as often observed in the spread of an epidemic. A mollusk can only enlarge its shell at the shell's edge. In most cases, new elements of pigment are added only at this edge.