The skull of "Dimetrodon" is tall and compressed laterally, or side-to-side. The eye sockets are positioned high and far back in the skull. Behind each eye socket is a single hole called an infratemporal fenestra. An additional hole in the skull, the supratemporal fenestra, can be seen when viewed from above. The back of the skull (the occiput) is oriented at a slight upward angle, a feature that it shares with all other early synapsids. The upper margin of the skull slopes downward in a convex arc to the tip of the snout. The tip of the upper jaw, formed by the premaxilla bone, is raised above the part of the jaw formed by the maxilla bone to form a maxillary "step." Within this step is a diastema, or gap in the tooth row. Its skull was more heavily built than a dinosaur's.