414 
Nautilinidz, obliterating the primitive characteristics of the second 
septum and substituting the more advanced characteristics of the 
Nautilinidg as is plainly demonstrated in Fig. 16, Pl. ii, of Avar- 
cestes (Goniatites) datesepfatus and in the Primordialide in Gephuro- 
ceras (Goniat) serratum, Fig. 17 of same plate. In the Ammoni- 
tinee and Lytoceratinz, and probably in the Ceratitinee, as in most 
of the Goniatitinz, 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, Pl. iv, of Lmbryology 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 Nautilinidz in certain of the Goniatitine. 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- 
dialidz, 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 
Branco’s figures, and still later in some other species. 
In the Ceratitine 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 Goniatitinz, ex. Prolecanites. 
In Zrachyceras Munsteri 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 
