major division of Miocene rocks and time in the Pacific Coast region of North America (the Miocene Epoch began about 26,000,000 years ago and lasted about 19,000,000 years). The Luisian Stage, which precedes the Mohnian Stage and follows the Relizian Stage, was named for exposures studied in San Luis Obispo County, Calif. Three subdivisions of the Luisian Stage are recognized, based on characteristic fossil foraminiferans—single-celled organisms having calcareous shells. The Luisian Stage is approximately equivalent in age to the Helvetian Stage recognized in Europe and elsewhere.