3o6 OUR COMMON BRITISH FOSSILS, 



Nautilus family we have — (i) the straight-shelled 

 group called, on this account, Orthoceras ; (2) another 

 with the shell bent on itself, as Ascoceras ; (3) a curved 

 shell, Cyrtoceras ; (4) a spiral form, Trochoceras ; (5) 

 a discoidal kind, Gyroceras ; (6) discoidal and pro- 

 duced, Z/Z/z^V^j"; (7) and the involute, as represented 

 by the Nautilus itself. 



In the Ammonite family all of these external 

 shapes are repeated. Thus the straight ammonital form 



Fig. y^\'— keratites nodosns, 



is represented by (i) Bactilites ; (2) Ptychoceras ; (3) 

 Taxoceras ; (4) Tttrrilites ; (5) Crioceras ; (6) Ancylo- 

 cei'as ; and (7) the ordinary Ammonites. 



The Nautilus family, however, was by far the 

 earliest to appear in the seas of the globe, and many 

 of its genera had died out before the true Ammo- 

 nites appeared, although the Goniatites are found in 

 Silurian rocks. 



