What's the difference between ammonite and baculite?

Ammonite

Definition:

(n.) 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.

Example Sentences:

(1) Start your exploring at Bearreraig Bay, where, if you are lucky, you may find belemnites, ammonites and bivalves.
(2) An ambitious project to showcase the prehistory of the south coast of England, famous for its marine fossils from ammonites to giant sea reptiles, has attracted support from David Attenborough and Eden Project founder Tim Smit.
(3) Everest is published by Ammonite Press on 22 May, priced £40.
(4) Cretaceous-Tertiary, 65 million years ago An giant asteroid impact on Mexico, just after large volcanic eruptions in what is now India, saw the end of the dinosaurs and ammonites.