104 



these fossils are practically reliable guides to the position of 

 these strata, as they are not found above or below them. 



But if we attempt correlation by index species overgreater 

 areas we shall have to be less precise. Thus. Stropheodonta 

 demissa and Pleurodictyum stylopora, toget her with a num- 

 ber of other species, such as Spinier granulosus, Stropheo- 

 donta concava, etc., are practically confined to, and indica- 

 tive of. the Hamilton stage of Western New York. But 

 while these species are sure guides to the identification of the 

 Hamilton stage over this area, they can not be used in corre- 

 lating even the greater subdivisions of this stage. Thus, in 

 the region about Eighteen Mile Creek the species mentioned 

 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 soecies are entirely confined to the Moscow 

 shales above the Encrinal limestone, none having been 

 observed below that bed. ( Consequently, detailed correlation 

 by these species alone is impossible. 



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 particular horizon in one country may be 

 wholly restricted to a different horizon in another country. 

 Tropidoleptus carinatus and Vitulina pustulosa 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 River district, which by some are placed in 

 the Middle, and by others in the Lower Devonian group. 

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

 sidered Lower Devonian, and in South Africa these species 

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

 pidoleptus carinatus has also been reported from the Lower 

 Devonian of the Bosphorus and the Pvhine district. Lepto- 

 ccelia Habcllites is commonly found associated with these 



