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number of branches composed entirely of close-coiled forms con- 

 tinue the existence of the order. 



The Ammonoids have similar straight radicals, but these are few 

 in number, dying out in the Devonian, leaving in that period a 

 number of branches of closely coiled and involute forms, the 

 Goniatitinae. These immediately manifest a capacity for expansion 

 and become the radicals of other involute and more modified invo- 

 lute series which expand in the Trias and Jura, becoming less 

 numerous and degenerate in the Cretaceous and cease to exist with 

 that period or soon afterwards. The history of the Ammonoidea so 

 far as the succession of different forms is concerned is as a whole 

 like that of a single series of the Nautiloidea which can be traced 

 back to a primary straight radical and which has a complete history 

 of modifications, but which necessarily occupies much less space 

 chronologically, evolving and disappearing within perhaps the limits 

 of a single epoch of geologic time. 



The trunk of the Nautiloidea is in other words a huge cone-like 

 trunk, clothed with branches but topped only by a few straggling 

 persistent survivors shooting up through time and reaching the 

 present surface with the tip of a single twig. The trunk of the Am- 

 monoidea is only a slender short branch, springing from the Nauti- 

 loid trunk, but spreading out and splitting up into many smaller 

 branches. Like a climbing vine of huge proportions it ascends 

 through geologic history, resting upon the level of each age or epoch 

 as upon a horizontal trellis and spreading into great masses of branches 

 at each of these resting places. It shows throughout its evolution 

 less power to resist the action of the surroundings both in the num- 

 ber and high specialization of the forms produced with every change 

 in geologic history, but also in the more rapid and earlier disap- 

 pearance of each type, and finally in the total disappearance of the 

 entire order. 



This comparison fully accords with the true picture of the genetic 

 relations. The remarkably sudden appearance and fully developed 

 structures of these earlier ammonoids finely illustrates the fan-like 

 character of the evolution of forms from centres of distribution, and 

 the quickness with which they must have spread and filled up the 

 unoccupied habitats. 



The contemplation of the wonderful phenomena presented by 

 these series has finally led the author to the conclusion that the 



