The Collaborative International Dictionary of English v.0.48:
Hammerhead \Ham"mer*head`\ (-h[e^]d`), n.
1. (Zool.) A shark of the genus Sphyrna or Zyg[ae]na,
having the eyes set on projections from the sides of the
head, which gives it a hammer shape. The Sphyrna
zyg[ae]na is found in the North Atlantic. Called also
hammer fish, and balance fish.
2. (Zool.) A fresh-water fish; the stone-roller.
3. (Zool.) An African fruit bat (Hypsignathus monstrosus);
-- so called from its large blunt nozzle.