The Collaborative International Dictionary of English v.0.48:

bonnet head \bonnet head\, bonnethead \bonnethead\n. (Zool.),
   small harmless hammerhead shark (Sphyrna tiburio) of the
   southern United States and West Indies, having a spade-shaped
   head; abundant in bays and estuaries.

   Syn: shovelhead, bonnet shark, Sphyrna tiburo.
        [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

bonnethead
    n 1: small harmless hammerhead having a spade-shaped head;
         abundant in bays and estuaries [syn: shovelhead,
         bonnethead, bonnet shark, Sphyrna tiburo]