Ammonite

AM’MONITE, n.[Cornu ammonis, from Jupiter Ammon, whose statues were represented with ram’s horns.]

Serpent-stone, or cornu ammonis, a fossil shell, curved into a spiral, like a ram’s horn; of various sizes, from the smallest grains to three feet in diameter. This fossil is found in stratums of limestone and clay, and in argillaceous iron ore. It is smooth or ridged; the ridges strait, crooked or undulated.