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Silurian of their immediate derivation from Nautiloid ancestors. 

 This is precisely what actually occurred and in the Nautiloidea such 

 evidence is easily obtained as has already been stated above in the 

 pages of the Introduction and other publications. 



It also follows, if the theories advanced by the author are true, 

 that the Nautilinidae among Goniatitinae, as ancestors of the Am- 

 monoidea, and especially the genus Mimoceras as the centre of 

 derivation, should also show more prolonged retention of nautiloid 

 characters in their ontogeny than is usual in their supposed descend- 

 ants. The researches of Sandberger, Barrande, Branco and the 

 author show this to be a fact. The figures of PL ii copied from 

 Barrande and Branco exhibit this in Mimoceras compression, am- 

 bigena and the whole of the Nautilinidae of the Silurian, and the 

 essential distinctive characteristic of this family is the nautiloid 

 form of the septa and lateral sutures. The shells of this genus also 

 do not possess a contact furrow, as noted above, and have no an- 

 nular lobes on the dorsum. 



The first suture of Mimoceras compressum, Figs. 3, 4, PI. ii, and 

 in some other allied species of the Devonian is bent into a slight 

 lobe on the venter, which is a purely nautiloid character, and not 

 to be confounded with the ammonoidal lobe in the same situation 

 in the third suture that follows this. This is shown by the occur- 

 rence of similar lobes in the Endoceratidae and some cyrtoceran 

 forms of Nautiloidea and in figures of sutures of Nautilus deslong- 

 champseanus and clementinus of the Cretaceous, also copied from 

 Branco, which have similar first and second sutures. The aselate 

 first septum is in M. compressum, followed on the second septum by 

 a broad, almost imperceptible saddle, also considered aselate by 

 Branco, but which is obviously a transition to the latisellate, or 

 broad-saddle type of suture in the more specialized forms. The 

 limits of the ananepionic substage in this form, which, as said 

 above, is directly transitional to Bactrites, is therefore that part of 

 the whorl which is represented by these two septa and the living 

 chamber in which the animal rested while constructing the second 

 one. 



The characters of these two septa, however, are not repeated in 

 the closer-coiled forms of the Nautilinidae and Primordialida?. In 

 these the repetition of the outline of the second suture may be 

 entirely omitted, the shell passing immediately in the second sep- 

 tum to the repetition of the peculiar undivided ventral of the 



