Xlll. 



The times are divided into eras, which have received 

 locality names, as Devonic era from Devonshire, England, 

 old historic names, as Siluric era, from the old tribe of 

 Silures, or names derived from the lithological character of 

 beds deposited during the era, as Carbonic era, from the 

 coal beds deposited at that time. The division is chiefly 

 based on biological changes,. 



The subdivision of eras is not a uniform one. The best is 

 that proposed by H. S. Williams,* who would recognize in 

 general an early, a middle and a later period in each era. 

 The terms Eo, Meso and Neo, proposed by him, form suitable 

 prefixes to which can be added the distinctive era name. 

 Thus Eodexonian, Mesodevonian and Neodevonian are 

 proper names to apply to the early, middle and later periods 

 of the Devonic era. 



Periods are divided into epochs, these latter marking the 

 continuance of a characteristic species and its associates. 

 The name applied to the epoch should be the specific name of 

 the important species, a rule which as yet is not very 

 generally followed. From the nature of the division it 

 follows that it can not hold over very wide areas, and that 

 its length may vary in different regions. The epoch during 

 which the Hamilton shales of New York were deposited — 

 here called the Spiriferoides epoch, from Athyris spiriferoides 

 — can perhaps not be recognized outside of north-eastern 

 United States and portions of Canada. The Stringocephalus 

 epoch of Europe is, in part at least, its foreign equivalent. 

 On the other hand, the Disjunctus and Intumescens epochs 

 ( Table B ) are recognized in Europe, where they are marked 

 by the same species. So too, the Acuminatus epoch, during 

 which the Corniferous limestone of eastern North America 

 was deposited, may be recognized in European geological 

 history as the time during which Spirifer cultrijugatus, 

 the European equivalent of S. acuminatus, existed. 



*Loc. cit. 



