Placoid scales are found in

• a. Cartilaginous fishes/sharks/chondrichthyes
• b. Bony fishes/osteichthyes
• c. Lung fishes
• d. All the above

Placoid scales are a specific type of scale that are primarily found in cartilaginous fishes, which include sharks and rays. These scales are characterized by their tooth-like structure and provide protection as well as reduce drag in the water.

Types of Fish Scales

To better understand where placoid scales fit in, let's look at the different types of fish scales:

• Placoid Scales: Found in cartilaginous fishes (e.g., sharks and rays).
• Cycloid and Ctenoid Scales: Common in bony fishes (osteichthyes).
• Ganoid Scales: Present in some primitive bony fishes.

Answer to the Question

Therefore, the correct answer is a. Cartilaginous fishes/sharks/chondrichthyes. Placoid scales are not found in bony fishes or lung fishes.