Detailed studies of the distribution of fossils in the Onondaga limestone (Devonian) in central New York permit division of the formation into four members each of which is subdivisible into two or more zones and several lateral facies. The Edgecliff (lowest) member is a coral biostrome over much of the State but locally passes into a bioherm facies. A basal zone of reworked sand (Springvale of various authors) is included in the Edgecliff wherever present. The Nedrow member is a shaly limestone characterized by a variety of platyceratid gastropods. The upper part of the member is a more massive limestone with a sparse fauna. The Nedrow passes westward into cherty limestone with few fossils and eastward into coarse limestone. The Moorehouse member is a massive, fine-grained limestone characterized by abundant brachiopods and certain large coiled cephalopods. This member passes west and east to coarser limestones with brachiopod-coral faunas. The base of the Seneca (upper) member is defined as the “Tioga bentonite”, a prominent marker bed and an important paleontologic break. The Seneca is characterized by an abundance of Chonetes lineatus and few other forms. The Seneca member passes to the east into the Union Springs black shale of the overlying Marcellus formation. This relationship is indicated by the eastward thinning of the limestone and thickening of the black shale unit, and by the eastward disappearance from the top down of the Seneca zones which are recognized in the type area.