410 
In general terms transition to the ephebic stage takes place in 
the paraneanic substage or near its termination, and characteristics 
derived from the ephebic stages of immediate ancestors in the same 
phylum, such as the trapezoidal whorl of Temnocheilus mentioned 
above, are completely replaced by characteristics peculiar to the 
genus and species. While there are often marked distinctions be- 
tween this and the ananeanic substage, the differences are much 
less obvious between this and the ephebic stage except in those shells 
in which this period has degenerate characteristics. In these phylo- 
gerontic forms marked distinctions are likely to make their appear- 
ance owing to the disappearance of hereditary external ornaments 
and markings which have been present until near the end of the 
neanic stage. 
The ephebic stage has not been so fully studied among the nauti- 
loids or ammonoids, and in both of these orders it might be 
considered questionable whether any subdivisions were essential. 
But I have found it convenient to subdivide this stage in some of 
the descriptions given in this memoir, and since this stage is much 
prolonged in some forms of Ammonitinz, especially those with 
numerous whorls like the shells of Caloceras, Vermiceras and the 
like, it is probable that when its characteristics have received more 
attention subdivisions will be found to be as necessary as in other 
stages. The gerontic stage has been described above and is neces- 
sarily illustrated in the text which follows, and the subdivision of 
this stage into two or more according to the species is convenient — 
in order to distinguish the well-marked substages of decline. 
The limits of the earlier epembryonic stages are somewhat more 
difficult to define among Ammonoidea than Nautiloidea, because the 
shells of the former are the bearers of a larger number of heredi- 
tary characters, and being more highly specialized descendants of 
the latter, the history of these stages is more complicated by the 
intrusion of new modifications through the action of the law of 
tachygenesis. 
The protoconch, with a large aperture connecting with the open- 
ing of the conch, is plainly seen in the figures of AZimoceras com- 
pressus and others on Pl. ii, of this paper, and also in Sandberger’s 
figures of species of Goniatitinze on the same plate. This is very 
distinct from the aspect of the apex of the conch in Nautiloidea. 
In that order the neck of the protoconch must have been at least 
as narrow from side to side as the scar on the outer surface of the 
