The Collaborative International Dictionary of English v.0.48:

Cornu Ammonis \Cor"nu Am*mo"nis\ ([a^]m*m[=o]"n[i^]s); pl.
   Cornua Ammonis. [L., horn of Ammon. See Ammonite.]
   (Paleon.)
   A fossil shell, curved like a ram's horn; an obsolete name
   for an ammonite.
   [1913 Webster]