Gnathostomes
Comprising all vertebrates with upper and lower jaws: Chondrychthyes, Osteichthyes, Amphibia, Reptilia, Aves, and Mammalia. These animals usually have paired olfactory organs and nostrils and paired limbs.
Gnathostomes belong either to the Fishes or to the Tetrapods (animals with four limbs, albeit that some of the limbs may be reduced or lost).
The sharks, skates, and rays belong to the class Chondrichthyes, marine fish whose scales, with so called placoid scales, are each made of a plate of dentine covered by enamel—like teeth. Sharks and their relations lack bones; their skeletons are made instead of a softer, more flexible material, cartilage. All the other fish belong to the class Teleostei, the bony fish. Their scales, made of bony material, are called cycloid or ctenoid according to whether their outer edge is smooth or spiny. Altogether, there are some 25,000 species of fish—mainly of the bony kind.