The Scotch bonnet (Semicassis granulata) is a medium-sized to large species of sea snail, a marine gastropod mollusk in the subfamily Cassinae, the helmet shells and bonnet shells. The common name "Scotch bonnet" alludes to the general outline and color pattern of the shell, which vaguely resemble a tam o' shanter, a traditional Scottish bonnet or cap. The shell is egg-shaped and fairly large, 2 to 4 inches (5 to 10 cm) in maximum dimension, with a regular pattern of yellow, orange or brown squarish spots. The surface sculpture of the shell is highly variable: the surface can be smooth and polished, have grooves, be granulated, or even be nodulose on the shoulder of the whorls.

This species lives intertidally and subtidally on sandy substrates, and is found primarily in the tropical and subtropical Western Atlantic Ocean, from North Carolina to Uruguay. It is the most common species in this subfamily in North America. A similar-appearing sea snail in the Mediterranean Sea and Northern Atlantic Ocean, Semicassis undulata, is currently considered to be a separate species. The exact taxonomy of Semicassis granulata has been unclear in the past: as well as the current combination, 38 other combinations and synonyms exist.

In the spring, the adult females of this species lay eggs in tower-shaped structures. The eggs hatch as veliger larvae, which can float in the plankton for up to 14 weeks before settling onto the seabed as tiny snails. Crabs are a predator of this sea snail. After the death of the snails, if the shells are still intact they are often used by hermit crabs.

In 1965, in the US, the Scotch bonnet shell was named as a state symbol of North Carolina, the first designation of a US state shell.

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