MOLLUSCA. CEPHALOPODA 285 



Distribution of the Cephalopoda 



Nautiloidea. At the present day the Nautiloidea 

 are represented by only four species of Nautilus, which 

 are found in the Indian Ocean and the East Indian 

 Archipelago (from Sumatra to Fiji). Nautilus is an 

 active swimmer, and lives in fairly shallow water. 



This group appears much earlier in the geological 

 series than either the Ammonoidea or the Dibranchia ; 

 the early forms are either straight or slightly curved ; 

 subsequently genera with spiral shells appear. Primitive 

 forms ( Volborthella) are found in the Lower Cambrian ; 

 and two other types {Orthoceras and Cyrtoceras) appear 

 in the Upper Cambrian (Tremadoc Beds). In the Ordo- 

 vician the Nautiloidea are very much better represented 

 than in the Cambrian, and the group attains its maximum 

 development in the Silurian, where the number of species 

 is very great ; it decreases slightly in importance in the 

 Devonian and Carboniferous, and is but poorly represented 

 in the Permian. The only genus which extends beyond 

 the limit of the Palaeozoic period is Orthoceras, which is 

 found in the Trias. Nautilus occurs first in the Trias 

 and is abundant in the Jurassic and Cretaceous ; in the 

 Tertiary it is rare ; Aturia appears in the Eocene and 

 Miocene. 



Ammonoidea. The geological range of the Ammonoi- 

 dea is shorter than that of the Nautiloidea. The earliest 

 representatives of this sub-order are found in the Devonian; 

 the latest in the Chalk. The group is especially abundant 

 in the Mesozoic formations. Clymenia is limited to the 

 Devonian ; ' goniatites ' also occur in the rocks of that 

 system, but are more numerous in the Carboniferous — 



