The surface where the two valves of a clam join together is called the commissure. On many clams it is a simple, straight line. Rastellum has a "zig-zag" commissure, which enabled the feeding process. This "zig-zag" line is readily observable in the third picture below.
|Interior of a left valve of Ratellum carinatum. The longest|
dimension is 1.6 inches (40 mm).