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 NaiitilidcB include the following well-known 

 genera of fossils, Lituites^ Trochoceras, Gomphoceras^ 

 Orthoceras, Trochoceras, Phragmoceras, Cyrtoceras, 

 Clyine7tia, etc. 



Fig. 305.— Section of Shell of Ammonite, showing the large body-whorl, or 

 last chamber. 



In the AmmonitidcB we have the well-known 

 Goiiiatites, Ceratites, Ttirrilites, Bacidites, Hainites, 

 ScaphiteSy 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 



