What is "cornu ammonis"

Ammonite \Am"mon*ite\, n. [L. cornu Ammonis born of Ammon; L. Ammon, Gr. ? an appellation of Jupiter, as represented with the horns of a ram. It was originally the name of an. Egyptian god, Amun.] (Paleon.) A fossil cephalopod shell related to the nautilus. There are many genera and species, and all are extinct, the typical forms having existed only in the Mesozoic age, when they were exceedingly numerous. They differ from the nautili in having the margins of the septa very much lobed or plaited, and the siphuncle dorsal. Also called serpent stone, snake stone, and cornu Ammonis.

n. 1 (context paleontology obsolete English) A fossil shell, curved like a ram's horn; an ammonite. 2 (context anatomy obsolete English) The hippocampus of the brain.