MOLLUSC A. CEPHALOPODA. 175 



Siphuncle sub-central, placed towards the ventral (usually) 

 convex side. Surface smooth or with transverse ribs or 

 striae. Silurian (perhaps also Ordovician, Devonian and 

 Carboniferous). 



Cyrtoceras. Like Orthoceras but always curved, and 

 with the siph uncle usually sub-marginal. Cambrian to 

 Carboniferous. 



Nautilus. Shell spiral, coiled in one plane, whorls 

 few and more or less embracing. Last chamber large, 

 aperture simple, with an external sinus. Septa concave, 

 sutures simple or with slight lobes and saddles. Siphuncle 

 sub-central, septal necks short and directed backwards. 

 Surface of shell smooth or ornamented with striae or ribs. 

 Ordovician to present day. 



Aturia. Shell discoidal, whorls completely embrac- 

 ing ; suture-line zig-zag, with a deep lateral lobe on each 

 side. Siphuncle on the internal margin; septal necks 

 elongated, completely covering the siphuncle. Eocene 

 and Miocene. 



SUB-OBDER. AMMONOIDEA. 



The shell is generally coiled into a plane spiral. The 

 sutures are undulated or bent into lobes and saddles. 

 The siphuncle is marginal and does not contain internal 

 deposits. The septal necks (except in Clymenia and 

 Goniatites) are directed forwards. 



Clymenia. Shell discoidal; whorls numerous, all 

 visible, but each partly embraces the preceding one ; the 

 last chamber is long, generally occupying three-quarters 

 of the last whorl. Aperture with a ventral sinus. Sutures 

 simply lobed. Siphuncle on the internal margin ; septal 



