Ammonites, a group of fossil molluscs, related to the living Pearly Nautilus, being, like it, tetrabranchiate cephalopods. Ammonites differ from Nautilus in having the chambers of their shells divided by foliated partitions, and in having the siphuncle, or tube passing through the chambers, lateral instead of central. The genus is confined to Secondary rocks, being first found in the Trias, and dying out in the Chalk. The species number several hundreds, and some of them reach a diameter of over three feet. As many of the species lasted but a very short time, and are fairly abundant, they have been used by geologists to divide the Secondary rocks into "zones." The name is derived from the resemblance of the shells to the ram's horns with which Jupiter Ammon was represented.