Gnomonical structure
(top) structure, which grows with maintainance of it's shape

The gnomon (second row), indicated, is the added part of the structure. The triangle grows by adding new material on the existing structure (filled).
(third row) Expanding triangle, with a layer-shaped gnomon.
(bottom) In this cross section of a shell you distinguish the half-circular cross section of the animal that once secreted and occupied it. There is a very small step towards the plant's stem diagram at the right.