Question

what is the difference btween cyenoid scales and placoid scales

Answer 1

Ctenoid scales is type of scales present in Osteichthyes (bony fish), they have tiny teeth called ctenni on the posterior margin which gives them rough texture.
Placoid scales are tough scales that cover the skin of cartilaginous fishes. They are modified teeth that are covered with a hard enamel and packed tightly together growing with their tips facing backwards.