[Mollusc] shells

Shells grow through the secretion of rigid material from the soft lip or mantle of the animal inside, and over periods of months to years they form coiled structures that normally follow rather accurate equiangular spirals, typically right-handed. The number of turns or whorls varies widely, from less than one in a typical bivalve, to more than thirty in a highly pointed univalve such as a screw shell. Usually the coiled structure is obvious from looking at the apex on the outside of the shell, but in cowries, for example, it is made less obvious by the fact that later whorls completely cover earlier ones, and at the opening of the shell some dissolving and resculpting of material occurs. In addition to smooth coiled overall structures, some shells exhibit spines. These are associated with tentacles of tissue which secrete shell material at their tips as they grow. Inside shells such as nautiluses, there are a sequence of sealed chambers, with septa between them laid down perhaps once a month. These septa in present-day species are smooth, but in fossil ammonites they can be highly corrugated. Typically the corrugations are accurately symmetrical, and I suspect that they in effect represent slices through a lettuce-leaf-like structure formed from a surface with tree-like internal growth.