The combination of an inner layer of nacre and an outer layer of simple prisms is thought to be phylogenetically the oldest molluscan shell type. Although nacre is very strong it is not as widely used as crossed-lamellar shell. Factors other than tensile strength or modulus of elasticity, such as resistance to abrasion or chemical attack, may have been important in the evolution of other shell types. Nacreous shell is shown to be less resistant than prismatic or crossed-lamellar shell to simulated components of whelk attack. Homogeneous shell is more resistant than other shell types to abrasion. A relationship between life-style and shell microstructure is suggested.