A large hammerhead shark with broad hammer (width 24-30% of TL), narrow-bladed side extensions on the head; front margin of head broadly convex, and scalloped due to a prominent middle indentation, a more conspicuous notch near the end of each side of the hammer (which sets off the end lobe bearing the eye), and a slight indentation in front margin between the central and side notches on each side; front teeth blade-like, with 1 point, upper teeth oblique, deeply notched on rear side, lower teeth straight, rear teeth like front teeth; first dorsal fin moderately large and erect, rounded tip, rear margin concave; 2nd dorsal fin small, height < length of 3rd gill slit; free rear tip of second dorsal fin nearly reaching caudal fin; base of anal fin noticeably larger than that of second dorsal fin; pelvics with ~ straight rear edge; 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.
Brownish grey, shading to white ventrally; undersides of pectoral fins tipped with black.
Size: attains 430 cm; size at birth 42-55 cm.
Habitat: coastal and oceanic, young in shallow muddy areas.
Depth: 0-275 m.
Worldwide in tropical and warm temperate seas, California to northern Peru, plus all the oceanic islands.