A hammerhead shark with broad, narrow-blade side extensions on the head forming a hammer (width 23-27% of TL); front margin of head nearly straight in adults, with shallow central and side indentations; front teeth blade-like, with 1 point, lower teeth straight, upper teeth oblique, deeply notched on rear side; rear teeth like front teeth; first dorsal fin very tall and curved, with pointed tip; second dorsal fin large, height > length of 3rd gill slit; second dorsal and anal fins with strongly notched rear edges, their bases about equal; pelvics large, with concave rear borders; transverse pit above tail base crescent shaped, a pit below tail base; tail fin strongly asymmetrical, notched under tip of top lobe, large lower lobe.
Grey brown on back and sides, whitish below; no prominent markings on fins.
Said to reach 610 cm and 450kg, but uncommon above 350 cm; size at birth 50-70 cm.
A coastal pelagic and semi- oceanic species.
Depth range 1-300 m.
Circumtropical distribution, in the eastern Pacific from southern Baja California and the Gulf of California to northern Peru, Malpelo and the Galapagos.