n. One of the fossil shells of an extensive genus (Ammonites) of extinct cephalopodous mollusks (cuttle-fishes), of the family Ammonitidæ, coiled in a plane spiral, and chambered within like the shell of the existing nautilus, to which the ammonites were allied.n. A name applied to certain explosive materials, patented by Favier, containing ammonium nitrate with other substances, chiefly nitro- or dinitro-naphthalene.