STKATIGRAPHICAI, I A 25 



all over the world. We iu-ver tind a ( 'arlximliTou* fauna 

 antedating a Silurian or Devonian fauna, though we may tin- 1 

 tin- lirst succeeding the second without the presence of the int-i 

 mediate Devonian assemblage. This common order of succession 

 has been termed Homotax-is, and the Silurian System of Britain in 

 spoken of as homotaxial with and equivalent to the Silurian of 

 America ; even the larger subdivisions of the mn- may be paralleled 

 with those of the other, as homotaxial equivalents or representatives, 

 so that we may speak of the Niagara Group as equivalent to the 

 Wenlock Beds, understanding by this that the groups are homotaxial 

 equivalents and not absolutely coeval formations. 



To sum up the preceding observations, the occurrence of tin- 

 same assemblage of species at different localities may be taken as 

 evidence that the beds containing them are of the same age, and 

 were formed at the same time. Again, a knowledge of the general 

 succession of generic forms enables us to fix the relative age of any 

 set of fossils, and, consequently, the homotaxial position of the beds 

 containing them. In other words : 



1. Assemblages having many species in common may be regarded 



as contemporaneous. 



2. Assemblages having many genera in common may be regarded 



as homotaxial equivalents. 



