This is a large, important, and, generally speaking, well-defined group of bivalves, the characters of which are thus given by Cuvier. In all the mantle is open anteriorly, but posteriorly it presents two siphons or orifices, one for respiration, the other for the egesta. These form tubes sometimes distinct, sometimes united into a single mass. There is always a transverse adductor muscle at each extremity of the shell, and a foot which generally serves for the purpose of locomotion. It may be laid down as a rule, that the species with elongated siphons live buried in the mud, or sand. This condition of organization may be recognised on the shell, by the more or less developed contour which the impression of attachment of the borders of the mantle describes, before uniting with the impression of the posterior transverse muscle;" or, in other words, by the course of the pallial impression.
These Mollusks are all marine, with one or two doubtful exceptions; they frequently attain large size, and are valued for the delicacy and wholesomeness of their flesh; their abundance often renders them easy to be procured.