Ammonites and nautili have a single shell, usually spirally coiled, with partitions, or septa, at intervals. Ammonites thrived in the Mesozoic era, becoming extinct with the dinosaurs. Nautili are found from the Cambrian to the present day. Nautili have a very simple suture line (where the septa intercepts the shell surface) but ammonites have a very complex suture line. The suture line increased in complexity from the Devonian to the Mesozoic.
The picture shows Amaltheus margaritatus a common ammonite from the Jurassic south Dorset coast, England. One should note particularly its characteristically beaded edge.