FOSSIL CEPHALOPODS. 307 



Practically, therefore, we may regard the Nautilus 

 family as being essentially Palaeozoic, and the Ammo- 

 nites as characteristic of the Secondary period. 



The Nautilida include the following well-known 

 genera of fossils, Lituites, Trochoceras, Gomphoceras, 

 Orthoceras, Trochoceras, Phragmoceras> Cyrtoceras, 

 Clymenia, etc. 



Fig- 35- Section of Shell of Ammonite, showing the large body- whorl or 

 last chamber. 



In the Ammonitida we have the well-known 

 Goniatites, Ceratites, Turrilites, Baculites, Hamites, 

 Scaphites, Ptychoceras, Ancyloceras^ etc. 



We find these beautiful fossils of all sizes (although 

 some so-called species are doubtless the young stages 

 in the development of the larger kinds), as well as 

 possessing wonderfully numerous kinds of external 



