MOLLUSC A. CEPHALOPODA. 181 



fossil state ; examples of it are Octopus and Argonauta, 

 the latter has been found in the Pliocene Beds. 



Distribution of the Cephalopoda. 



The Nautiloidea at the present day are represented by 

 four species of Nautilus only ; these occur in comparatively 

 shallow water in the Indian Ocean, the Persian Gulf, the 

 China Sea, and off New Guinea and Fiji. The group 

 appears much earlier in the geological series than either 

 the Ammonoidea or the Dibranchiata, and soon attained 

 its maximum development. Two genera, Orthoceras and 

 Cyrtoceras, are found in the Cambrian (Upper Tremadoc 

 Beds), and these, especially the first, continue to be im- 

 portant in the succeeding Paleozoic formations as far as 

 the Carboniferous. In the Ordovician the group is very 

 much better represented than in the Cambrian, the chief 

 genera being Orthoceras, Cyrtoceras, Endoceras, and Nau- 

 tilus ; but it is most abundant in the Silurian, where the 

 number of species is very great; afterwards in the De- 

 vonian and Carboniferous, although still very numerous, 

 there is a slight decline. In the Permian it is poorly 

 represented; and only two genera extend beyond the 

 Palaeozoic, of these, one (Orthoceras) becomes extinct in 

 the Trias, the other (Nautilus) lives on to the present day, 

 and becomes especially abundant in the Cretaceous, in 

 the Tertiary it is rare, and is associated with one other 

 form only, namely, Aturia. 



The distribution of the Ammonoidea differs from that 

 of the Nautiloidea in that the former are mainly Mesozoic. 

 In the Palaeozoic the only genera are Goniatites, Clymenia, 

 and (in the Carboniferous of India) Ammonites. Through- 

 out the Mesozoic the Ammonites are extremely abundant ; 



