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We investigate the relation
between the thickness and diameter of naturally occurring shells, such as
the carapaces of turtles and the skulls of mammals. We hypothesize that
shells used for different protective functions (for example, protection against
headbutting or falling on the ground) will exhibit different power-law trends
for shell thickness and diameter. To test this hypothesis, we examine over
600 shells from museum collections with diameters between 1 and 100 cm.
Our measurements indicate that eggs, turtle shells, and mammalian skulls
exhibit clear and distinct allometric trends. We use a theoretical scaling
analysis based on elastic thin shell theory to show that the trends observed are
consistent with the corresponding protective functions hypothesized. We thus
provide theoretical evidence that shells can be classified by their protective
function.