Parallel lines with tongues

Abstract

The upper shell in Figure 9.1 is decorated with many fine parallel lines. This pattern suggests the same synchronous oscillations as described earlier (see Figure 3.4). However, at particular positions, the parallel lines are deformed into U- or V-shaped gaps. The pattern on the lower shell is based on the same principle; only the size and regularity of the gaps are different. On the upper shell the gaps are restricted to particular positions. On the lower shell two broad bands are nearly free of parallel lines while smaller gaps appear at more scattered positions. The shells belong to the species Clithon oualaniensis (in older literature also termed Neritina or Theodoxus oualaniensis). These small brackwater snails are frequent on shores around India and Sri Lanka and display an incredible richness of patterns. Grüneberg (1976) made a careful study of the polymorphism of these shells. He termed the deviation from parallel straight lines “tongues”. His article contains many examples of different types of patterns, transitions from one type to another, and pattern regulation after perturbation.