Donax variabilis is a species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of mollusc found on the east coast of the United States from Virginia to the Caribbean. Its common name is coquina.
It has a small shell that is usually colorful. It can be found alive in the sand where the waves come up in between waves. Some people collect their shells

Seashell

A seashell or sea shell, also known simply as a shell, is a hard, protective outer layer created by an animal that lives in the sea. The shell is part of the body of the animal. Empty seashells are often found washed up on beaches by beachcombers...

, or use them for crafts or to decorate with. They are edible.