104 



these fossils ai-e ]>ractically reliable <iiii(les to the position of 

 these strata, as they are not fonnd above or beloAv them. 



But if we attempt correlation by index species over greater 

 areas we shall have to be less precise. Thus, Stropbeoc]ontci 

 (Iemiss;i and Pleurodictyiun stylopora, together with a num- 

 ber of other species, snch as Sjfirifer <>rRnuJ<)si]s, Stropheo- 

 douta concnvn, etc., are practically confined to, and indica- 

 tive of, the Hamilton stage of Western New York. Bnt 

 while these species are sure guides to the identification of the 

 Hamilton stage over this area, they can not be used in coi're- 

 lating even the greatei' subdivisions of this stage. Thus, in 

 the region about Eighteen Mile Creek the s])ecies menticmed 

 are entirely confined to the Hamilton shales below the 

 Encrinal limestone, not a single specimen having been found 

 in the Moscow shale above that bed. But in the Genesee 

 Valley these s])ecies are entirely confined to the Moscow 

 shales above the Encrinal limestone, none having been 

 observed below that bed. (V)nse(]uently, detailed correlaticm 

 l)y these s])ecies alone is im])ossible. 



In the intercontinental correlation of formations, the 

 greatest caution is necessary in the selection of index 

 species, and it not infrequently happens that a character- 

 istic species of a particulai* horizon in one country may be 

 wholly restricted to a different horizon in another country. 

 Tropidoleptus cnrinatus and Vitulina pustulosn are ex- 

 amples of this. These two brachiopods are eminently 

 characteristic of the Hamilton or Middle Devonian group 

 of North America, occurring in it wherever this group is 

 typically developed. They are again characteristic of beds 

 in the Amazon Kiver district, wdiich by some are placed in 

 the Middle, and by others in the Lower Devonian grou]). 

 In Bolivia they characterize the Ida shales, which are con- 

 sidered Low^er Devonian, and in South Africa these species 

 are reported from beds of the same horizon (Ulrich). Tro- 

 pidoleptus cnrinatus has also been reported from the Lower 

 Devonian of the Bosphorus and the Rhine district. Lepto- 

 coplia flahellites is commonlv found associated with these 



