Lamellibranchiata, a class of Mollusca (q.v.), including those forms which have a bivalve shell, such as oysters, mussels, and cockles. The group is characterised by the absence of a distinct head, from which circumstance it is often known as the Lipocephala; by the presence in most cases of a muscular, more or less triangular, foot, with which the animals can burrow into the bottom of the sea, pond, or river in which they live; by the complete absence of eyes and of a rasping tongue such as is found in the Glossophora (q.v.). It is divided according to the nature of the adductor muscles, which close the shell, into (1) the Isomya, which possesses two such muscles of about equal size, one anterior and one posterior - e.g. Anodon; (2) the Heteromya, where the anterior muscle is much the smaller; (3) the Monomya, in which the anterior muscle is wanting - e.g. the oyster (Ostrea). Some of the animals of this group are of considerable importance commercially, not only as articles of food - e.g. the oyster - but also in various other ways. Thus some produce pearls; the shell of others is used for making buttons and other articles. They are also interesting as being among the most ancient forms of life known, their fossil remains occurring from the Cambrian period onwards.