The brain case of primitive placoderms was layered in bone but in later species it was formed entirely of cartilage which helped to reduce weight. The jaws were simple rods of bone which had pointed tooth-like structures for gripping prey or, in species whose diets involved crushing hard-shelled prey, the jaws were areas of thickened tubercles. The jaw joint was quite simple with the lower jaws articulating against a knob-like quadrate bone which is usually fused to the cheek bones. The eyes were surrounded by a simple ring of bone made up of sclerotic plates and connected to the brain case by an eyestalk. The eyes were situated either at the sides of the head or together with the nostrils in the center of the skull.

John A. Long, 1995