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complicated structure of Ammonoidea. They show that these 
forms do not retain the tendency to form a czcum with double 
walls as in Nautiloidea, and such an example as that figured in 
Nautilus pompilius, in which a misplaced second septum necessarily 
shows a long tubular czecum like that of the living chamber of 
Diphragmoceras, probably does not occur. In other words, one of 
the most persistent of the nepionic characteristics of Nautiloidea 
does not exist in the more specialized shells of Ammonoidea so far 
as known. 
It is obvious from the preceding that the paranepionic substage 
begins in most forms of this order with the first appearance of the 
divided ventral lobe, or what I have called the siphonal saddle and 
it is limited in extent by the duration of the simple entire goniatitic 
outlines of the sutures which accompany all the substages of the 
nepionic stage in all the suborders of Ammonitinz, except, of 
course, the stock in which they originated, the Goniatitine. 
In the Ceratitine, Ammonitine and Lytoceratinz it is gener- 
ally true that this occurs, and the ananeanic substage begins with 
subdivision of the lobes and saddles into minor lobes and saddles or 
digitations, and this is often also accompanied by the advent of a 
minute siphonal lobe in the apex of the siphonal saddle. It is 
not essential here to discuss the limits of the neanic stage and its sub- 
stages. ‘They vary so much with the condition of development and 
the position of each species in its own series or genus and of each 
series or genus in its own group, that it is impracticable to define 
them except in very comprehensive terms. 
Thus one may say the limit of the neanic stage is reached when 
the specific characteristics begin to appear in normal progressive 
forms. But there are exceptions to this in some highly tachygenic 
species, as in Oxynoticeras oxynotum, for example, and many others 
in which certain characteristics are carried back to earlier substages. 
Still, as a rule, this definition does good service if the occurrence 
of exceptions are constantly anticipated. 
The limits of the substages can be obtained in some species of 
each series, and are quite distinct in the external characteristics of 
the form of the whorl and of the ornamentation. The sutures of 
the ananeanic substage are different from those of the metaneanic 
since they are much simpler and less completely digitated, but 
there is, as a rule, but slight, if any, differences between the sutures 
of the metaneanic and paraneanic or ephebic sutures. These 
