AMMONITE, a group of extinct Mollusca, belonging to the class Cephalopoda (q.v.), which also includes the octopus and cuttlefish. They possessed an external shell, coiled in a flat spiral and divided into chambers very similar to that of the modern nautilus (q.v.). Many reached a great size, the largest, Pachydiscus seppenradensis, from the Cretaceous of Wqstphalia, having a shell 6ft. Bin. in diameter. The Ammonites began in the Carboniferous period and became extinct at the end of the Cretaceous.