CEPIlAiOPODA. 67 



SuB-DrvisioN I — Cephalopoda Sepiaria. 



Animals without any internal or external covering or 

 shell, but with an internal cartilaginous or calcareous 

 plate. This subdivision embraces four genera of Sepiae or 

 Cuttle fish. 



Sub-Division II — Cephalopoda Monothalama. 



Having unilocular shells, entirely external, and envelop- 

 ing the animal. 



Genus 5 — ARGONAUTA — LinncBus. 



Generic Character. — Shell spiral, univalve, unilocular, 

 very thin, doubly oarinated, with the spire entering the 

 aperture. 



Argonauta Argos The Argus Argonauta, or Paper 



Nautilus. — Plate X. fig. 2. Shell white, involute, ex- 

 tremely thin and fragile, sides with undulated ridges, and 

 obsoletely striate ; keel with small tubercles, partly reddish- 

 black. Inhabits the Mediterranean. 



Sub-Division III. — Cephalopoda Polythalama. 



Shell m unilocular, partly or wholly internal, and en- 

 veloped in the posterior part of the body. 



FAMILY I. AMMONACEA. 



With the septa sinuous, lobed and cut at the margin, 

 meeting upon the inner wall of the shell, and articulated by 

 acutely notched sutures. 



Genus 6 BACULITES — Lamarck. 



Generic Character — Shell straight, cylindrical, some- 

 times a little compressed, slightly conical; with the walls 

 articulated by sinuous sutures ; dissepiments transverse ; 

 little distant, imperforate, lobed and subdivided at the 

 margin. 



