TETRABRANCHIATE CEPHALOPODS. 



63 



forms exists, resembling each other in the manner in which 

 the shell is folded or coiled, but differing in their funda- 

 mental structure. All these different forms may be looked 

 upon as produced by the modification of a greatly-elongated 



Fig. 449.— Diagram to illustrate the position of the siphuncle and the form of the septa in 

 various Tetrabranchiate Cephalopoda. The upper row of figures represents transverse sec- 

 tions of the shells, the lower row represents the edges of the septa, a, a. Ammonite or 

 Baaiilite ; b, h, Ceratite ; c, c, Goniatite ; cl, d, Clymenia ; e, e, Nautilus or Ortlioceras. 



cone, the structure of which may be in conformity with the 

 type either of the Nautilidce or of the Ammonitidce. The 

 following table (after Woodward) exliibits some of the repre- 

 sentative forms in the two families : — • 



Shell straight, 



bent on itself, 



curved, 



spiral, . 



discoidal, . 



discoidal and produced, Lituites, 



involute, .... Nautilus 



Xautilidte. 



Ortlioceras, 



Ascoceras, 



Cyrtoceras, 



Trochoceras, 



Gyroceras, 



Ammonitidce. 



Baculites. 



Ptychoceras. 



Toxoceras. 



Turrilites. 



Crioceras. 



Ancyloceras. 



Ammonites. 



DiSTEiBUTiox OF Teteabranchiata IN TiME. — Ecgarded 

 as a whole, the Tetrabranchiate Cephalopods form a group 

 which early attained its maximum, and which is now almost 

 extinct. The greatest development, in point of numbers, 

 took place in the Palaeozoic period ; and the forms then ex- 

 isting belonged to decidedly simpler types than those which 

 followed them. The greatest number of tyiKS existed during 

 the Mesozoic period ; and here the order still maintained 



