THEORY "1 RADII LLS \M' MORPHOLOGICAL EQUIVALENCE. 



I 1 gradation in the ornaments, markings, eta, occurred, l>ut i> less marked 



and rarer <>n account of the frequenl absent r the shell. Prof . James Rail 



has 6gured cases "t senile degradation 1 in Orthoceras, and we have ourselves 

 seen several similar examples, 



have not been :il>l<- to trace any remarkable changes in old age among 

 silurian, devonian, or carboniferous goniatitinss. The dyassic and triassic 

 forms <it Ammonoidea with highly ornamented Bhells have not, as Ear as known, 

 exhibited cases i>t' senile metamorphosis in any noticeable abundance, and there 

 larked absence of these in Mojsisovicss plates, although a few arc figured. 

 There is an easily observed increase in the effects of old age upon the la-t 

 whorls of the shell in the Jura. Ever) group, however, does no! >li<>u the effects 

 ly. There are not onlj less remarkable metamorphoses in the 

 : I genus Psiloceras, bul also leas in the Arietidaa as a whole, than in the Am- 



monitinae of the Upper Jura. This retrogression correlates directly with the 

 prevalenci ous uncoiled shells in the Cretaceous. There 



is, the \ inonoidea a general progress up to the Jura, which is 



• •ly expressed in the life <>f the individual as well as in the life of the 

 ill decline in the later Jura and Cretaceous, which is also defi- 

 nitely expressed in a similar h G itologous types and forms are also less 

 frequenl among the paleozoic and earlier mesozoic than in later mesozoic series. 

 / ■i/i/hii-i nth/ had greater strength as indtriduals in ihesi earlier periods, 



I in their »/il »</< . The phenomena presented 1>\ 



ord with tlii* statement, [fwe pick < >u t those types which 



ili<-\ appear t" have been less affected bj degra- 



- than the more specialized forms which arose from them. This 



fact, however, as we have often Btated, corresponds directly with the more com- 



i inization of derivative forms, as contrasted witli the simpler structures 



of radical forms There are more characters introduced in the adults <>t ppecial- 



aii'l the i sappearance and degradation <>i these mails 



of the individual in such types with more obvious modifications 



\ however, geratologous metamorphoses d ;cur 



n Orthoci N The Lituites of the Phillips- 



1 Fort ( limestones, 1 which we are now Btudying, and the 



Lituites and Trochol Holm, have in their youngest stages forma 



which indicate derivation from nautilian shells, thus proving that they are not 



il forms, Inn c! uncoiled derivatives ol prepaleozoic <>r paleozoic 



tiloids, ol which they are the last survivors. Trocho- 



several different genera, and are all degenerate forms 



lit 



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