ccii INFERIOR OOLITE AMMONITES. 



umbilicus decreases ; but sooner or later in catagenesis, depending on the series, 

 the process changes — coiling in gives place to coiling out. When the change 

 supervenes late, after considerable incoiling has produced angustumbilication, then 

 the coiling out is often rapid — it produces the phenomenon which I have called 

 excentrumbilication — Hyperlioceras rudidiscites, for example. When carried to 

 excess, outcoiling produces Ancyloceras — a return to the Gyroceratan form. Carried 

 to an extreme, it produces Baculites — a return to the Orthoceras mode. There is, 

 therefore, a cycle of development — from straightness to extreme involution and 

 back to straightness again ; but in completing this cycle there are many periods of 

 interruption — periods of renewed anagenesis ' — while return may begin long before 

 umbilical closing is obtained, and many Ammonite stocks die out without com- 

 pleting the cycle. 



It will be seen from the foregoing remarks how different are the times of 

 development of the various characters — how different are the morphogenetic 

 acmes. Thus elaboration of a suture-line (septal morphanagenesis) is frequently 

 carried on till late phylocatagenesis. This is in obedience to mechanical 

 necessities, which were discussed pp. 134, 138. Then development of longitudinal 

 ornament on the periphery — the carina — may be early or late in a series, or it may 

 be delayed altogether. And in regard to umbilication the morphanagenesis is so 

 frequently associated with phylocatagenesis that one is inclined to look upon it as 

 really a catagenetic feature. 



From the phases of development of characters which have been detailed it will 

 be seen what changes may be expected in the various genetic stocks, bearing in 

 mind two principles, ontogenetic repetition of phylogeny, and earlier inheritance 

 (tachy genesis). Taking the Hildoceratidce which show phylocatagenesis from 

 tuberculate to costate, to subcostate, to smooth, there will be shown in the ontogeny 

 of each species this sequence of events carried on to a certain degree. The more 

 catagenetic is a species, the further it will carry out the sequence. Correlated with 

 these characters of ornament there will be catagenesis of whorl shape — from infla- 

 tion to compression ; anagenesis of suture-line, with possible catagenesis at last ; 

 anagenesis of rostration, also with possible catagenesis ultimately; catagenesis of 

 such longitudinal ornament as carination, 2 with ultimate failure; anagenesis or 

 closing of umbilicus, with, later, catagenesis or expansion of umbilicus (excentrum- 

 bilication), especially correlated with catagenesis of rostrum and disappearance of 

 carina. 



1 Compare Spheeroceras for stages of umbilical closing, and Morphoceras, a descendant of a 

 Sphaeroceratoid, for stages of a closed umbilicus opening out more and more. In Morph. dimorphum 

 the peculiar form is due to rapid umbilical catagenesis. 



2 Increased prominence of the carina gaining at the expense of compression of periphery can 

 hardly be accounted anagenesis. 



