MOLLUSCA : CEPHALOPODA. 



333 



istic of the two families Nautilidm and Ammonitida, into which 

 the order Tetrabranchiata is divided. 



In the family Nautilidm (fig. 120), the "septa" of the shell 

 ajre simple, curved, or slightly lobed ; the " sutures " are more 

 or less completely plain ; and the " siphuncle " is central, 

 sub-central, or internal («>., on the concave side of the curved 

 shells). 



In the family Ammonitidm (fig. 120), on the other hand, the 

 septa are folded and complex ; the sutures are angulated, zig- 

 zag, lobed, or foliaceous'; and the siphuncle is external (i.e., on 

 the convex side of the curved shells). 



.0 ,0 '^ .0 



Fig. 120. — Diag;ram to illustrate the position of the siphuncle and the form of the septa 

 m various Tetrabrancliiate Cephalopoda. - The upper row of figures represents 

 transverse sections of the shells, the lower row represents the edges of the septa. 

 a a Avitnoniie or Baculite ; bb Ceratite; cc Goniatite; dd Clytnenia; e e Nau- 

 tilus or Ortkoceras. 



In both these great types of shell, a series of representative 

 forms exists, resembling each other in the manner in which the 

 shell is folded or coiled, but differing in their fundamental 

 structure. All these different forms may be looked upon as 

 produced by the modification of a greatly elongated cone, the 

 structure of which may be in conformity with the type either 

 of the NautilidiB or of the AmmonitidcB. The following table 

 (after Woodward) exhibits the representative forms in the two 

 families : — 



* NautilidcB. 



Shell straight .... Orthoceras . 



Ascoceras . 

 Cyrtoceras . 

 Trochoceras 

 Gyroceras . 



bent on itself . 



curved . . . 



spiral .... 



discoidal ... 



discoidal and produced Lituites 



involute .... Nautilus 



Ammonitida. 



Baculites. 



Ptychbceras. 



Toxoceras. 



Turrilites. 



Crioceras. 



Ancyloceras. 



Ammonites. 



After the Natitilus itself, the most important form of the 

 NautilidcE is the Orthoceras i^g. 121). In structure this was 



