198 



This order is separated into three divisions, viz. Polythalamous Cephalo- 

 poda, Monothalamous Cephalopoda, and Naked Cephalopoda. 



DIVISION I. POLYTHALAMOUS CEPHALOPODA. 



Shell multilocular, completely or partially enveloped, situated on the poste- 

 rior part of the body of the animal, often adhering. 



This division contains seven families, — Orthocerata, Lituolata, Cristata, 

 Sphaerulata, Radiolata, Nautilacea, and Ammoneata. 



I. Shell midtilocular, the septa simple, not showing any divided sinuous sutures 

 on the internal surface of the shell. 



ORTHOCERATA.— Five Genera. 



Shell straight, or nearly so; not spiral. 



Belemnites. Shell straight, an elongated cone, formed of two distinct 

 and separable parts. The external sheath solid, full at the upper part, with 

 a conical cavity; the internal, a conical nucleus, pointed, chambered trans- 

 versely through its whole length, multilocular; the chambers perforated by a 

 syphon in the centre. 



1 fossil species. 

 Orthocera. Shell straight or slightly arched, subconical, striated on the 



