Traditional classification divides the Chondrichthyes into two groups. The Elasmobranchii includes the familiar sharks, skates, and rays, as well as some strange fossil relatives. Elasmobranchs have an upper jaw that is not fused to the braincase and separate slitlike gill openings.
The second group, the Bradyodonti, includes forms with an upper jaw fused to the braincase and a flap of skin, the operculum, covering the gill slits. The Bradyodonti includes the chimaeras and ratfish, which are relatively rare, deep-water, mollusc-eating forms.