Comments
i) The placoid scales are arranged in regular oblique rows, covering the entire surface of the body and from the exoskeleton of the shark.
ii) Placoid scales are small pointed and tri-radiate denticles found embedded in the dermal layers of the skin.
iii) A typical placoid scale consists of a diamond-shaped or rhomboidal basal plate having an opening of the pulp cavity & flat trident spine.
iv) The basal plate is formed of a trabecular calcified tissue, the cement.
v) The spine is composed of a hard calcareous substance, the dentine which is coated externally with a hard & dense enamel.
vi) The pulp cavity contains the vascular connective tissue, pulp containing numerous odontoblasts, blood vessels, nerves and lymph chambers.
vii) Placoid scales are found in sharks, rays & skates.
