SUPPLEMENT.— AMMONITN DEVELOPMENT. cci 



transverse ornament is weak; then it shows normal development in the following- 

 stages somewhat analogous to those of transverse ornament : a slight line (periphery 

 angnlate), a rib (periphery carinate), ribs and furrows (periphery carinatisulcate), 

 and when the transverse ornament becomes stronger, knotted ribs and furrows 

 (Paltopleuroceras). If, however, the area be already costate, the development of 

 longitudinal ornament produces tuberculation directly. This would make tuber- 

 culate peripheries analogous to tuberculate lateral areas — both produced by similar 

 development, either transverse costation of a longitudinal rib, or longitudinal costa- 

 tion of a transverse fold — depending on whether the transverse or the longitudinal 

 ornament has progressed the most before the other begins, in regard to any given area. 



Examples of longitudinal ornament in its simple form on the lateral area are 

 the longitudinal strise in Amaltheus, Strigoceras, etc., and longitudinal costae in 

 Strigoceras trifurcatum and Str. Truellii; these developments take place when the 

 transverse ornament is in advanced catagenesis. 



In regard to the radial curve, one of the principal features is the degree of 

 rostration — the greater projection thereof must be regarded as anagenetic and 

 reduction as catagenetic. This rostration is the outward projection of the median 

 portion of the periphery — a part which in Nautilus is curved inwards. In Ammo- 

 nites the variation is from no projection at all (Graphoceras) to a very long forward 

 projection (Amaltheus, Harpoceras, Schloenbachia). In a general way the greater 

 projection is connected with greater development of the keel on the periphery ; but 

 this rule only holds good when allied genera are considered by themselves. There 

 may be considerable development without any keel (Zurcheria) ; when a keel 

 appears the development attained then becomes still more pronounced. 



The projection of the rostrum is often associated with general catagenesis ; and 

 not until catagenesis is in a very advanced stage is reduction of the rostrum usually 

 found. Then it accompanies another extreme catagenetic feature — excentrum- 

 bilication ; Ludwigia ambigua is a good instance. 



Lastly, with regard to umbilication it is difficult to say what is anagenetic and 

 what catagenetic. It may be recognised that there are alternate stages of coiling 

 in and coiling out. The former must, perhaps, be considered as anagenetic, though 

 it usually occurs when the series is in general catagenetic — the character 

 anagenetic when the series is catagenetic — which may be called morph- 

 anagenesis in phylocatagenesis. This is very marked in regard to umbilication. 



From Orthoceras through Gyroceras to Nautilus clausus are the various degrees 

 of incoiling — a cone coiling more and more on itself until no umbilicus is left. The 

 Nautilus style of coiling appears in Gymbites — a very simple Ammonoid ; but from 

 Cymbites to Coronieeras at its acme, or to Echioceras the process is reversed — 

 coiling in passes to coiling out ; the umbilicus constantly enlarges. In catagenetic 

 series of Arietidre, Hildoccratidse, etc., coiling out changes to coiling in — the 



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