Superposition of stable and periodic patterns

A widely distributed subgroup of shell patterns result from the superposition of a stable and a periodic pattern. The upper shell in Figure 4.1 shows two sets of parallel relief-like lines. One set is oriented parallel to the growing edge and results from a thickening of the shell at periodic time intervals. The other set is oriented parallel to the direction of growth and results from a permanently enhanced deposition of shell material at regularly spaced positions. In this example, the two patterns do not interfere with each other, a situation that is more the exception than the rule, but it shows that the assumption of two superimposed systems is reasonable.