THE ORDOVICIAN PERIOD 



99 



Evolution of Ike Chamber-shelled (Tetrabrunch) Cephalopods 



Quaternary 



Tertiary 



Cretaceous 



Jurassic 

 Triassic 



Permian 



Mississippian 

 Pennsylvanian 



Devonian 



Silurian 



Ordovician 



C Chamber-shelled Cephalopods rep- 

 I resented only by a few genera of 

 I close-coiled Nautiloids, e.g. mod- 

 l em Pearly Nautilus (Fig. 13). 



Ammonoids very rare and in lowest ' 

 Tertiary (Eocene) only. 



Ammonoids much like Jurassic 

 though somewhat diminished and 

 with straight forms (e.g. Baculi- 

 tes, Fig. 161f), and curved or 

 open-coiled forms more common. . 



Ammonoids greatly advanced in ' 

 numbers, species, and complexity 

 of septa, and they reach their 

 climax, e.g. Ceratite with scal- 

 loped septa (Fig. 127); Ammonite 

 with highly frilled septa (Fig. 141) ; 

 and some curved and straight 

 Ammonoids. 



C Ammonoids common, some showing } 

 I distinctly increased (highly 

 I curved) complexity of septa (e.g. 

 I Waagenoceras, Fig. 115). 



{ Much like Devonian, but complex- 

 \ ity of septa in Goniatites some- 

 l what increased. 



Ammonoids first appear with only 

 slight (angular) complexity of 

 septa junctions, e.g. Goniatite 

 (Fig. 77). 



Much like Ordovician. 

 noids. 



No Ammo- 



Cambrian 



Close-coiled forms, e.g. Trocholites 



(Fig. 55d). 

 Open-coiled forms, e.g. Trochoc- 



eras (Fig. 55c). 

 Curved forms, e.g. Cyrtoceras 



(Fig. 55b). 

 Straight forms, e.g. Orthoceras 



(Fig. 55a). 



Straight and curved forms only. 



Close coiled Nauti- 

 loids only persist, 

 e.g. Nautilus, but 

 more varied than 

 now. 



Some Nautiloids pre- 

 sent, but Orthoce- 

 ras becomes extinct 

 in Triassic. 



Nautiloids, including 

 Orthoceras, persist, 

 but subordinate. 



Nautiloids still pre- 

 dominate. 



Simpler forms (Nau- 

 tiloids) continue as 

 in Silurian. 



Coiled Nautiloid 

 forms predominate. 



Straight forms pre- 

 dominate. 



