SYNAPSID

[syn.ap.sid] n [NL Synapsida, fr. Gk syn- + apsid-, apsis arch, vault--more at apsis] (1956): any of a subclass (Synapsida) of extinct reptiles existing during the Pennsylvanian, Permian, and Jurassic, having a single pair of lateral temporal openings in the skull, and usu. held to be ancestral to mammals -- synapsid adj