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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 Ammonitinae, 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 Mimoceras com- 

 pressus and others on PL ii, of this paper, and also in Sandberger's 

 figures of species of Goniatitinae 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 



