Sharks and dog fishes differ from skates and rays because
Sharks and dogfishes are included under Pleurotremata. They have a spindle-shape body. Their pectoral and pelvic fins are clearly demarcated from the body. Their gill-slits are laterally placed (hence the name Pleurotremata). Skates and rays are included under Hypotremata. They are bottom-dwellers with a dorsoventrally placed body. Their pectoral fins are joined to the sides of the body and head. Their gill-slits are ventrally placed (hence the name Hypotremata).