ammonite n.

"fossil cephalopod mollusk," 1758, from French (Breyn, 1732), "better established" [Century Dictionary] by French zoologist Jean Guillaume Bruguière in 1789, from Medieval Latin (cornu) Ammonis "horn of Ammon," the Egyptian god of life and reproduction (see Ammon), who was depicted with ram's horns, which the fossils resemble. The resemblance also was noted in ancient times. They also were thought to be petrified snakes, hence snakestone.