Any clam of the genus Donax, inhabiting sandy beaches worldwide. Coquinas are very active; they migrate up and down beaches with the tide and can reburrow between waves. They have short siphons and feed on suspended plant material and detritus. A typical species, D. variabilis, measures about 0.4–1 in. (10–25 mm) in length. Its shell is wedge-shaped and varies from white, yellow, and pink to blue and mauve.
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Limestone formed almost entirely of sorted and cemented fossil debris, most commonly coarse shells and shell fragments. Microcoquinas are similar sedimentary rocks composed of finer material. Common among microcoquinas are those formed from the remains of crinoids (marine invertebrates, such as sea lilies, that have limy disks and a limy internal skeleton). A distinction is made between a coquina, which is a rock formed from debris, and coquinoid limestone, which is composed of coarse shelly materials with a fine-grained matrix.
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