Placoid scales are found in

The correct option is cartilaginous fishes.

Step-by-Step Explanation:

1. Understanding Placoid Scales:
Placoid scales (also called dermal denticles) are structurally similar to vertebrate teeth. They consist of a basal plate made of bone-like tissue embedded in the dermis, and a spine that projects outward through the epidermis. The spine is composed of dentine and covered by a hard, enamel-like substance called vitrodentine.

2. Occurrence in Fish Groups:
Placoid scales are a characteristic evolutionary feature of Chondrichthyes, commonly known as cartilaginous fishes, which include sharks, rays, skates, and chimaeras. These scales help reduce drag and turbulence in water, allowing them to swim more efficiently and quietly.

3. Comparison with Other Groups:
• Bony fishes (Osteichthyes), including lung fishes, typically possess other types of scales such as cycloid, ctenoid, or ganoid scales, which have different structures and origins.
• Since placoid scales are exclusively found in Chondrichthyes, they are a defining diagnostic feature of cartilaginous fishes.