436 



MANUAL OF ZOOLOGY. 



is tKe 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 



Fig. 237. Shells of Secondary Cephalopods. i. Ancyloceras Matheronianus ; 2. 

 Scaphites (zqualis; 3. Crioceras Duvalii; 4. Hamites attenuatus ; 5. Turrilites 

 catenatus, 



pompilius, together with some varieties or nearly allied 

 species. Of the sections of this order, the Nautilida proper 

 and the Orthoceratida are pre-eminently Palaeozoic, and the 

 Ammonitidce are not only pre-eminently, but are almost ex- 

 clusively, 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 de- 

 scribed by M. Barrande from the Silurian basin of Bohemia 

 alone. The Nautilidtz proper have gradually decreased in 

 numbers from the Palaeozoic through the Secondary and Ter- 

 tiary periods to the present day. The Orthoceratidce. 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. 



The second family of the Tetrabranchiata viz., the Ammo- 

 nitidcz is almost exclusively Secondary, being very largely 

 represented by numerous species of the genera Ammonites, 

 Ceratites, Baculites, Turrilites, &c. 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 



