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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 
Goniatitinz. 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 
Iyke 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 tipofa 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 trellisand 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 withevery 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 
PROC. AMER. PHILOS. SOC. XxxII. 148. 2 U. PRINTED MAY 381, 1894. 
