The Collaborative International Dictionary of English v.0.48:

Limulus \Lim"u*lus\ (l[i^]m"[-u]*l[u^]s), n.; pl. Limuli
   (-l[imac]). [L., dim. of limus sidelong, askance.] (Zool.)
   The only existing genus of Merostomata. It includes only a
   few species from the East Indies, and one (Limulus
   polyphemus) from the Atlantic coast of North America. Called
   also Molucca crab, king crab, horseshoe crab, and
   horsefoot.
   [1913 Webster]

WordNet (r) 3.0 (2006):

horseshoe crab
    n 1: large marine arthropod of the Atlantic coast of North
         America having a domed carapace that is shaped like a
         horseshoe and a stiff pointed tail; a living fossil related
         to the wood louse [syn: horseshoe crab, king crab,
         Limulus polyphemus, Xiphosurus polyphemus]