The Cephalo-pods are largely represented in all the primary groups of stratified rocks from the Upper Cambrian up to the present day. Of the two orders of Cephalopoda, the Tetrabranchiata is the oldest, attaining its maximum in the Palaeozoic period, decreasing in the Mesozoic and Kainozoic epochs, and being represented at the present day by the single form Nautilus pompilius, together with some varieties or nearly allied species. Of the sections of this order, the Nautilidae proper and the Orthoceratidaa are pre-eminently Palaeozoic, and the Ammonitidae are not only pre-eminently, but are almost exclusively, Secondary. Of the abundance of the two former families in the Silurian seas some idea may be obtained when it is mentioned that over a thousand species have been described by M. Barrande from the Silurian basin of Bohemia alone. The Nautilidae proper have gradually decreased in numbers from the Palaeozoic through the Secondary and Tertiary periods to the present day. The Orthoceratidae. died out much sooner, being exclusively Palaeozoic, with the exception of the genera Orthoceras itself and Cyrtoceras, which survived into the commencement of the Secondary period, finally dying out in the Trias.

Fig. 237.   Shells of Secondary Cephalopods. 1. Ancyloceras Matheronianus; 2. Scaphites (equalis; 3. Crioceras Duvalii; 4. Hamites attenuates; 5. Turrilltes catenatus.

Fig. 237. - Shells of Secondary Cephalopods. 1. Ancyloceras Matheronianus; 2. Scaphites (equalis; 3. Crioceras Duvalii; 4. Hamites attenuates; 5. Turrilltes catenatus.

The second family of the Tetrabranchiata - viz., the Ammo-nitidae - is almost exclusively Secondary, being very largely represented by numerous species of the genera Ammonites, Ceratites, Baculites, Turrilites, etc. The principal Palaeozoic genera are Goniatites and Bactrites, of which the former is found from the Upper Silurian to the Trias, whilst the latter is a Devonian form; but true Ammonites have been found in strata of Carboniferous age in India (Dr Waagen). The genus Ceratites is characteristically Triassic, but it is said to occur in the Devonian rocks. All the remaining genera are exclusively Secondary, the genera Baculites, Titrrilites, Hamites, and Pty-choceras being confined to the Cretaceous period. The only genus which passes up into the Tertiary is Ammonites, which occurs in beds believed to be of this age in America.

Of the Dibranchiate Cephalopoda the record is less perfect, as they have few structures which are capable of preservation. They attain their maximum, as fossils, shortly after their first appearance in the Secondary rocks, where they are represented by the large and important family of the Bdemnitidae. Some of the Teuthidae and Sepiadae are found both in the Secondary and in the Tertiary rocks, and two species of Argonaut have been discovered in the later Tertiaries. No example of a Dibranchiate Cephalopod is known from the Palaeozoic deposits, and the order attains its maximum at the present day.