Dunkleosteus was the biggest Placoderm of all time. It could reach 20 feet in length, or more. The front third of its body was encased in thick armor. The rest of the body had unprotected, scaleless skin. Instead of teeth, it had bony projections coming out of its massive jaws. These jaws acted like scissors, the upper jaw slightly ahead of the lower jaw, so when it closed its mouth, the bony projections slid past each other. This meant that the bony "teeth" were always self-sharpening.
The definition of Dunkleosteus, is "Dunkle's Boney one" named after the founder.