Definitions

(Taxon-specific: Subclass Branchiura) On third pair of thoracic appendages (thoracopods) of male, socket-like depression in protopod. Receives peg of fourth thoracopod in a peg-and-socket arrangement and serves to hold both thoracopods together during copulation. [Stachowitsch, 1992]
(Taxon-specific: Class Ostracoda) Hinge structure: pit-like (typically anterior or posterior) depression in hinge margin. Receives tooth of opposite valve. Major depression at end of hinge is termed cardinal socket; those with subdivided floors are termed compound sockets. [Stachowitsch, 1992]