What subclass of cartilaginous fish includes sharks and rays?

The subclass of cartilaginous fish that includes sharks and rays is Elasmobranchii.

Elasmobranchii belongs to the class Chondrichthyes, whose members have skeletons made primarily of cartilage rather than bone. Modern sharks, rays, skates, and sawfish are elasmobranchs, while chimaeras belong to the separate subclass Holocephali. The name comes from Greek words associated with “plate” and “gill,” referring to their exposed gill openings.

Elasmobranchs usually have five to seven pairs of gill slits, placoid scales, several rows of replaceable teeth, and no swim bladder. Many maintain buoyancy with large, oil-rich livers. Male members typically have claspers, modified pelvic fins used to transfer sperm.

A common mix-up is treating Elasmobranchii as a synonym for sharks alone. Rays and skates are included too, although taxonomists differ over whether certain fossil cartilaginous fishes should be placed within the group. The alternatives are not equivalent: Dipnoi are lungfishes, Teleostei are bony fishes, and Holocephali contains chimaeras.