This is the most easily recognised family. All 9 species have laterally expanded blades (hammer-like) on their heads, with circular eyes and internal nicitating eyelids on the outermost edges of their blades. All hammerhead sharks have relatively small pectoral fins due to these blades. The origin of the first dorsal fin is above or slightly behind the base of the pectoral fins. Precaudal pits are present. Hammerhead sharks show a wide distribution in warm-temperate and tropical waters, mainly over the continental shelves down to about 270m. No species is truly oceanic. Their uniquely shaped head gives them many advantages regarding sensory organs and maneuverability. Hammerhead sharks give birth to live young (placental viviparity).