Mar. 

 1910 



DEVELOPMENT 



The test ornament of Ammonites is usually considered under two 

 headings : (i) lateral, (2) ventral, areas. The two areas often develop 

 independently ; and so a rather arbitrary separation is a convenience. 



For the lateral test-ornament (or simply, ornament) of Ammonites, 

 development follows a cycle, which may be shown in diagrammatic 

 form, as under : — 



5*, uni-, 5**, bi-, 5***, multi-tuberculate. 



And the development of a carinate periphery may be similarly 

 tabulated, for comparison ; — 



Table IV — Periphery Cycle 



fY 5, carinate and sulcate — >> 



M 



Z 



w 

 (J 

 < 

 z 



< 



o 



w 

 z 



M 



Stages of anagenesis may be marked by plain numbers ; of cata- 

 genesis by numbers with c attached. So a costate anagenetic stage 

 is 4, and a costate catagenetic, 4c. Then 4 denotes a costate stage 

 increasing in strength and pre-spinous ; but 4c a costate stage decreasing 

 in strength and possibly post-spinous : the change from anagenesis 

 to catagenesis (reverse) may take place at any stage. 



After anagenesis, instead of continuous catagenesis of ornament, 

 there may be renewed anagenesis following a certain period of cata- 

 genesis (subsidiary cycle). This may be observed in the development 

 of the A. armatus Ammonites from a bituberculate form like A. birchi.^ 

 The phylogeny reads thus— i, 2, 3, 4, 5*, 5**, 5*, 4c, 3c, 4, 5* ; 



I See also Two Toarcian Amm. ; Quart. Journ. Geol. Soc. lix, 1903, 460. 



