280 



MOLLUSCA. 



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 Nautilidcz or of the Ammonitidcz. The following table 

 (after Woodward) exhibits some of the representative forms in 

 the two families : 



Nautilida. 



Shell straight .... Orthoceras . 



bent on itself . . Ascoceras . 



curved .... Cyrtoceras . 



spiral Trochoceras 



discoidal .... Gyroceras . 



,, discoidal and produced Lituites . . 



involute Nautilus . 



.0 



Ammonitida. 



Baculites. 



Ptychoceras. 



Toxoceras. 



Turrilites. 



Crioceras. 



Ancyloceras. 



Ammonites. 



Fig. 249. 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 

 sections of the shells, the lower row represents the edges of the septa, a a A mmonite or 

 Baculite ; b b Ceratite ; ccGoniatite; ddClymenia; ee Nautilus or Orthoceras. 



DISTRIBUTION OF TETRABRANCHIATA. IN TIME. Regarded 

 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 existing 

 belonged to decidedly simpler types than those which followed 

 them. The greatest number of types existed during the Meso- 

 zoic period ; and here the order still maintained a great abun- 

 dance of individuals. With the close of the Secondary epoch 

 a large number of complex types disappeared wholly, and the 

 order was left without any representative in the Tertiary Rocks 

 except the simple and ancient genus Nautilus. 



