Image of Placoid scales of shark microscopic view

 

Each placoid scale of Scoliodon  has a flat rectangular base plate embedded in the dermis. From the basel plate  arise sharp, trident spines which project posteriorly on the surface. Structurally they show inner area of pulp, middle layer of dentine.It is a hard calcareous material .Outer layeris enamel covering vitrodentine and hence modification of the same develops teeth in elasmobranchs.