Chondrichthyes are a taxonomic class of cartilaginous fishes that encompass sharks and rays (elasmobranchs) and chimaera. Though there are exceptions, in general, Chondrichthyes have exposed gills, no swim bladder, internal fertilization, and placoid denticles. These characteristics differentiate them from the more evolutionarily derived branch of fishes, bony fish (Osteichthyes).
Placoid denticles are found on sharks, rays, and chimaeras. Not really a true “scale,” like ctenoid or cycloid scales, placoid denticles are actually modified teeth. They have an inner tissue component, which contains both blood vessels and nerves, that is covered by a layer of dentin and an outer enamel. They form a tough protective skin layer for sharks, rays, and chimaeras and also have shown to reduce friction and drag so that these fish can swim more efficiently through water.