noun

• (countable) Any of several small marine clams, of the species Donax variabilis, common in United States coastal waters.
  "The pretty little coquinas (genus Donax, family Donacidae), often called butterfly shells or wedge shells, are distributed throughout the world. Perhaps the most noted species is the small Donax variabilis Say, so commonly found on the beaches of Florida. Coquinas live in droves just beneath the surface of the sand. Collecting coquinas is both a fascinating and tantalizing experience."
• (uncountable, geology) A soft form of limestone made of fragments of shells, sometimes used as a building or road paving material.
  "The isthmus is nowhere more than about 8 or 9 feet above the water surface. The top soil is generally sand, with underlying coquina rock—a conglomerate of small broken shells held together by some natural cement."