NOMENCLATURE OF STAGES OF GROWTH AND DECLINE. 19 



The ephebolic characters are as a rule inherited or homogenous within the 

 special series in which they originated, but are not transmitted from one series 

 to another except through the medium of the nealogic stages of what we have 

 called the tertiary radicals? and they are not, so far as we know, ever concen- 

 trated in the earliest larval or naepionic stages; they occur too late in the 

 history of types. 



We classify in the nealogic and ephebolic stages such characters as follows : 

 the sharply defined ridge-like pilae and tubercles, the channels with their lateral 

 ridges, and keels, and especially the hollow keel, the highly developed rostrum 

 of the higher suborders, especially Ammonitinse, the lateral lappets of the aper- 

 tures, and the branching marginal lobes and saddles of the sutures of suborders 

 above Goniatitinae. Speaking in a general way, we should include in these 

 categories those progressive characters which appear late in the life of the 

 shell among the higher suborders, and at the acme of their development in 

 time, which are not found in the stock of discoidal radical forms. When the 

 shell began to assume the ribs or pike, as we prefer to call them, the nealogic 

 period may be said in a general way to have been entered upon. It has be°en 

 found that these stages of growth indicated genetic relationship with radical 

 forms, which were not infrequently merely different genera or species within 

 the limits of the same family, and often occurred on the same or only slightly 

 different horizons. The nealogic stages of the higher Ammonoids, Ammoni- 

 tinaa and Lytoceratinas, have not the constancy and general importance of the 

 naepionic stages, but are transient in the history of the types, appearing and 

 disappearing in the same limited series of forms. They consist of the less im- 

 portant modifications which first appeared in the adolescent or adult stages at 

 a late period in the history of a type, and were then inherited in the nealogic 

 stages at earlier ages in successive species of the same series, according to the 

 usual action of the law of acceleration. The nealogic category cannot be as 

 definitively separated from the characteristics of adults as from those of the 

 larvae. Their first appearance in adults indicated the establishment of a new 

 species in any given series, since they are invariably differences so far as their 

 predecessors and congeners in the same series are concerned. However much 

 they may represent or reproduce the characters of species in other series, they 

 are essentially differentials as regards the adult stages of ancestral species of 

 the same series. Thus the nealogic characters are as a rule ephebolic, and not 

 nealogic, in origin among the Cephalopoda, and usually become nealogic through 

 inheritance. We shall have frequent occasion farther on to call in the evidence 

 of the ephebolic stages, and to show, as in the Endoceratidaa, that, as a rule, 

 characteristics originated in this stage of growth, as indeed must have been the 

 case with the caecum and the microsiphon. 



At the termination of the progressive stages, which ended with the full 

 development of the ephebolic characters, the first stage of decline, or the gera- 

 tologic period, began to make its appearance, and became more and more appar- 

 ent as the specimens advanced in age. It was found that, as has been observed 



1 See, for secondary and tertiary radicals, p. 22 et seq. 



