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.  

While these Great White placoid denticles may look like scales, they are actually modified teeth (Trevor Sewell/Electron Microscope Unit, University of Cape Town).


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. (fishionary.fisheries.org/placoid-denticles)