Different cowrie species thicken their shells with different microstructural layers. All juvenile cowrie grow in the normal prograding spiral fashion as seen in most other snails. This is a three-layer structure comarginal cross-lamellar in between a bottom layer of radial cross-lamellar and a simple surface of prismatic material. At a certain point, the progradation stops (determinate growth), the outer lips turns inward and the shell begins to thicken both internally and externally in sheets of differing arrangement. This is when they get all those great shell features, such as the teeth along the aperture and inhalent and exhalant siphonal canals. In order to expand the list of phylogenetically informative character systems, I've broken a lot of shells to see how each species gets thicker. One outstanding feature of this character system is that it can be tracked through time in the fossil record.