HAMMERHEAD SHARK. Any of several species of ovo viviparous sharks of the genus Sphyrna, characterised by the unique configuration of the head, the anterior portion of which resembles a hammer. They are formidable and ferocious and oc cur in all tropical and sub-tropical seas, particularly in the Indian ocean. The common hammerhead shark (S. zygoena), which at tains a length of 16 f t., occurs on the milder coasts of Europe and ranges northward along American shores to Massachusetts and California. See SHARK ; SELACHIANS.