Ammonites are an extinct group of molluscs of the subclass Ammonoidea in the class Cephalopoda. If you imagine an octopus with a coiled shell you'll have the general idea.

They are called Ammonites because their shells resemble ram's horns which the Egyptian god Ammon was often depicted as wearing.

As with trilobites, we know them primarily by their shells which often fossilized and are less familiar with their soft parts which were rarely preserved.