972 , PRINCIPLES OF STRATIGRAPHY 



Simple corals proto-corallum. 



Brachiopoda protegulum (Beecher). 



Pelecypoda prodissoconch (Jackson). 



Gastropoda : protoconch (protorteconch) (Hyatt an4.Grabau). 



Scaphopoda periconch (Hyatt). 



Cephalopoda protoconch (Owen). 



Trilobites protaspis (Beecher). 



Echinoidea protechinus (Jackson). 



The Nepionic Stage is the babyhood stage of ep-embryonic ex- 

 istence. Its exact hmitation cannot be defined in general terms, 

 as it is different in different classes of organisms. In general, it 

 may be said that, for ammonites, it covers most, if not all, of the 

 morphic stages in which the suture is of the goniatite type. In the 

 case of Schloenbachia, cited above, the morphic stages, up to the 

 point where the young ammonite resembles in its stitures the genus 

 Paralegoceras, are considered by Smith to belong in the nepionic 

 stage. Here the neanic stage begins shortly before the suture has 

 lost its goniatite character, but an ammonite character has made its 

 appearance in the form of a keel. In Fulgur caricum and other 

 advanced species of this group of gastropods the nepionic stage 

 may be regarded as completed with the end of the tubercled condi- 

 tion {F. fusiformis, morphic stage). 



The neanic is the youthful or adolescent stage, which comprises 

 the interval during which the organism acquires all the characteris- 

 tics of maturity. When this condition is reached the organism 

 enters on the ephebic, or adult, stage. Long-lived individuals often 

 show old age or senile characteristics, which consist especially in the 

 loss of ornamentation and a degenerate change in the manner of 

 growth. This is the gerontic stage in the ontogenic cycle, and it is 

 followed by death. 



Gerontic characteristics may appear early in the life of indi- 

 viduals of a specialized race. Thus the loss of characters and the 

 degenerate change in growth may occur while the individual is .still 

 in the adult (ephebic) stage or even earlier. Such races are said 

 to be phylogerontic and are approaching extinction. Thus the late 

 Cretacic cephalopods, which lost tlie power of coiling either partly 

 (Heteroceras, etc.) or wholly in the adult (Baculites), represent 

 the phylogerontic terminals of the degenerating race of ammonoids. 



It must, however, be clearly borne in mind that the existence of 

 phylogerontic lines or races at any time does not indicate that the 

 phylum or class as a whole is gerontic. Even within the same 

 genus there may be found species showing a gerontic tendency. 

 The phylogerontism here applies only to the particular branch in 

 question, while the rest of the evolutional tree of this phylum may 



