Extinct, armored, jawless vertebrates with a single pair of external gill openings. Heterostraci first appeared in the Ordovician, according to some authorities, and became common in the Silurian and Devonian. Their bony dermal armor consists mainly of aspidine (bone with apparent growth layers, lacking cavities for bone cells) and is composed of three structural layers: a basal layer of lamellar bone, a middle layer usually with a honeycomb arrangement of cavities and partitions, and a superficial layer of dentine tubercles or ridges.