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Nautilinidae, obliterating the primitive characteristics of the second 

 septum and substituting the more advanced characteristics of the 

 Nautilinidae as is plainly demonstrated in Fig. 16, PL ii, of Anar- 

 cestes (Goniatites) lateseptatus and in the Primordialidae in Gephuro- 

 ceras (Goniat) s erratum, Fig. 17 of same plate. In the Ammoni- 

 tinae and Lytoceratinae, and probably in the Ceratitinae, as in most 

 of the Goniatitinae, this substage is obviously limited to the first 

 septum and the corresponding living chamber. The limits of this 

 living chamber in one form may possibly be indicated by the trans- 

 verse imbricated line between the third and fourth septa in my 

 Fig. 1, PI. iv, of Embryology of Fossil Cephalopods. This line 

 seems to demonstrate an arrest of growth at this time in the calca- 

 reous deposits corresponding to that indicated in Fig. 11 of the 

 same plate which is probably due to a former aperture. 



The metanepionic substage must obviously begin with the advent 

 of the characteristics of the tubular microsiphuncle and the ventral 

 lobe in sutures, whether this occurs in the second or third septum 

 or later. 



It is limited in duration to the repetition of the characteristics 

 of the Nautilinidse in certain of the Goniatitinse. Thus that family 

 of the Silurian and Devonian is phylo-metanepionic, or corresponds 

 in the phylum in its ephebic characters to the metanepionic sub- 

 stage of its descendants. The closely allied family of the Primor- 

 dialidae, for example, as shown in Fig. 17, PL ii, has several septa 

 with this character appearing in the metanepionic substage, the 

 construction of the divided ventral lobe so characteristic of all 

 normal forms of Ammonoidea not taking place until the shell is 

 nearly or about 3 mm. in diameter in one species, according to 

 Pranco's figures, and still later in some other species. 



In the Ceratitinae of the Trias this substage is in many species, 

 as shown by Branco's drawings, prolonged through several septa 

 and there are decided indications that it is subdivisible into two 

 parts, one characterized by the purely nautilinian ventral -lobe and 

 lateral sutures with only one broad lobe, and a second older portion 

 having the undivided ventral lobes and lateral sutures of other rad- 

 ical forms among Goniatitinae, ex. Prolecanites. 



In Trachyceras Munstei'i the eighth suture, according to Branco, 

 is still undivided or nautilinian, and Tropites, according to the 

 same author's figures, has this substage still more prolonged. In 

 Megaphyllites, Pinnacoceras, etc., all more highly specialized forms 



