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complicated structure of Ammonoidea. They show that these 

 forms do not retain the tendency to form a caecum 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 caecum 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 Ammonitinae, except, of 

 course, the stock in which they originated, the Goniatitinae. 



In the Ceratitinae, Ammonitinae and Lytoceratinae 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 



