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THE AMERICAN NATURALIST [Vol. XLVII 



in palseontological study. It is the stage in which the 

 characters of the class to which the animal belongs are 

 established. This period is represented by the protechi- 

 nus of Echini, the proteguhim of Brachiopoda, the pro- 

 dissoconch of Pelecypoda, the protcecium of Bryozoa, the 

 protoconch of Cephalous Mollusca and the protaspis of 

 the Tritobita. This stage in development is represented 

 in fossil as well as living forms in many types, and the 

 primitive radicle that it represents as a phylogenetic 

 stage has been pointed out as Paterina for the Brachi- 

 opoda by Beecher, as a nuculoid type for certain Pele- 

 cypoda, and as Bothriocidaris for the whole class of 

 Echini by Jackson. 



Of postembryonic stages the first are the nepionic or 

 babyhood stages, abundantly recognizable in fossil as 

 well as recent types. Succeeding these are the neanic, 

 or youthful stages. The ephebic is the adult, or that 

 stage in which the full species characters are evinced. 

 Senescence or old age is expressed in gerontic stages, in 

 which appear the loss of characteristic species features, 

 and by such loss an approach is commonly made to the 

 youthful character before such features are attained. 

 Gerontic stages while in a measure repeating youthful 

 characters, do so in the inverse order to that in which 

 they are acquired in ontogeny. As shown abundantly 

 by Hyatt, senescent features are prophetic of the adult 

 characters in regressive series of the group. 



In studies of ontogeny it often occurs that stages need 

 to be further subdivided. For this purpose Professor 

 Hyatt introduced the prefixes ana, meta and para, so that 

 one can speak of the an a nepionic, metanepionic or para- 

 nepionic stage of Nautilus, etc. By means of this 

 simple nomenclature the life stages of any organism are 

 divisible into ten main or thirty minor periods, which 

 are thus readily and clearly expressed. 



In ontogeny, as shown by overwhelming evidence, the 

 organism passes through stages which repeat the char- 

 acters of adults of more primitive types in serial order, 

 and it is believed that this serial order may be safely 



