“Consider shapes that satisfy the following set of conditions. To provide both support and protection for the organism, the shape must be a hollow one, but an opening must exist somewhere. Growth can occur only by addition to the inner surface or the free edge. And the shape should change only minimally as it grows. A cubic shell with an open face won’t work: addition to walls will give more shell relative to its contained volume, and addition to cylinder doesn’t meet the conditions–addition to the edge will move it from short and fat to long and (relatively) thin. What will work are cones, whether circular or elliptical. Add to the edge and thicken the walls and one gets a bigger cone, isometric with the original.
With only slight variations of the condition of isometry, all sorts of wild derivatives of cones are possible–and these latter are the shapes in which shelled mollusks occur.” (Vogel 2003:88-89)