CEPHALOPODA. 825 



the Cephalopoda (Orthoceras and Cyrtoceras) appear in the Upper 

 Cambrian rocks. All the known Palaeozoic representatives of the 

 class belong to the order of the Tetrabranchiata, whereas the order 

 of the Dibra?ichiata makes its first appearance in rocks of Triassic 

 age. The Tetrabranchiates attain their maximum in the Secondary 

 period, about six or seven thousand fossil forms being already known 

 from strata of Palaeozoic and Mesozoic age. On the other hand, 

 but few Tertiary Tetrabranchiates have been recorded, and the 

 order is represented at the present day by the single genus Nautilus. 

 The order of the JDibra?ichiata, on the contrary, attains its maximum 

 development at the present day. 



