920 NEW YORK STATE MUSEUM 



Summary 



At the horizon of the Tully limestone, but where it has other- 

 wise run out, there occurs a layer of pyrite which spreads over 

 a considerable part of western New York. It is a more or 

 less discontinuous deposit, appearing as lenses, each covering 

 considerable area, but not over 1 foot in thickness. This pyrite 

 contains a fauna of dwarfed forms, consisting of not less than 

 45 species. The sea is one whose waters were polluted with 

 iron in solution, and by decaying vegetation. These two to- 

 gether have made an unfavorable environment, which has 

 dwarfed the forms inhabiting it, till they are on an average 

 only one fifteenth the size of the same species in the normal 

 and preceding Hamilton fauna. The iron in the water as ferrous 

 carbonate, was probably precipitated by the sulfuretted hydrogen 

 (Fe000 2 +H 2 S==FeS+C0 2 +H 2 0) and thus formed pyrite. 

 This impure water acted on several generations of animals, but 

 the succeeding generations are no more dwarfed than the first 

 which was subjected to the peculiar surroundings. In addition 

 to the dwarfing the fauna is strongly modified in form, the 

 species appearing in some cases like those of faunas still earlier 

 than the Hamilton, and like young in all cases. This is an 

 instance of arrested development, in which the brachiopods 

 show the most modification. The species have been described 

 as mutations of the various Hamilton species, though the varia- 

 tion is greater than in many species, but the ancestry is evi- 

 dent, and determined by comparison with the young of the 

 ancestral species. 



