Figure 21  Pattern for carving a convex shell similar to the one on the chest illustrated in fig. 12. (Drawing, Mack Headley; artwork, Wynne Patterson.) The principal arcs designate (a) the perimeter of the shell, (b) the termination of the concave lobes, (c) the crest of the shell and demarcation point in the curvature of the lobes, and (d) the perimeter of the interior design. The compass ticks along the arcs indicate the position of the lobes at the outer perimeter, crest, and perimeter of the interior design.