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  Ammonite Modern chambered nautilus
Ammonites fall into the class Cephalopoda, phylum Mollusca. They closely resemble the chambered nautilus (see picture below), but their closest living relative is probably not the modern nautilus, but rather the subclass Coleoidea, which includes octopus, squid,
and cuttlefish.
High-quality ammonites are found in North Africa and date from the Jurassic period (165 million years ago). The can vary in size from less than an inch to several feet in diameter. They only show their true potential when they are split longitudinally (See below).
Ammonites' fossil shells usually take the form of planispirals. Their spiral shape begot their name, as their fossilized shells somewhat resemble tightly-coiled rams' horns. Plinius the Elder (died 79 A.D. near Pompeii) called fossils of these animals ammonis cornua ("horns of Ammon") because the Egyptian god Ammon (Amun) was typically depicted wearing ram's horns. Often the name of an ammonite genus ends in ceras, which is Greek for "horn" (for instance, Pleuroceras).
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