Which of the following differentiates a ray fish from a shark?

Rays and sharks both belong to the class Chondrichthyes i.e., they are cartilaginous fishes. All cartilaginous fishes including rays and sharks show some common characteristic features as mentioned below.

• The tail fin in cartilaginous fishes is heterocercal. This is because the vertebral column bends upward into the larger lobe giving it an unequal appearance.
• The mouth is ventrally placed bordered with sharp teeth.
• Their body is covered with placoid scales.

The key differences between a ray and a shark are: 

• A ray has a flattened, disk-like body whereas, a shark has a long and rounded body with tapering ends.
• A ray has gill openings on the ventral (lower) surface of the body while a shark has 5-7 pairs of gills on the lateral side of the head placed in gill pouches.