HOMOTAXIAL EQUIVALENTS OF THE PERMIAN 327 



change in physical conditions, but in this case nothing more. 

 Similar and even more marked changes occur at a hundred dif- 

 ferent horizons in the Carboniferous lower down. When shallow 

 waters prevailed, lamellibranch and gasteropod faunas occupied 

 the areas. When pelagic conditions occurred, the occupants of 

 the district were chiefly brachiopods. The latter moved in as 

 the former moved out. Comparisons of faunas of different 

 classes avail little ; they must be of the same class if tangible 

 results are to be expected. 



In drawing conclusions regarding the fossils of the beds that 

 have been referred to the Permian and the Permo-Carboniferous, 

 the utmost caution is imperative. The terranes have been only 

 very imperfectly and very unequally explored. Comparisons of 

 faunas have been largely between zoological groups of different 

 classes. Many of the beds in the general vertical section are 

 understood only in a vague way. There are long intervals about 

 which nothing either stratigraphically or faunally is known. 

 With a few isolated exceptions, organic remains have yet been 

 found only in the lower half of the succession. Fossiliferous 

 beds reach, according to our present knowledge, only up to the 

 middle of the Marion. In Texas the "Permian " fossils described 

 by White and Cope were from the Wichita and Lower Clear Fork 

 beds. 



Taking the fossils, the horizons of which are definitely known, 

 and as chiefly determined by Prosser, fifty-two species are recorded 

 from the Wabaunsee. Of these only two new ones occur in the 

 Cottonwood. In the Neosho following, one third of the twenty- 

 one species noted are not reported from the lower beds ; they are 

 lamellibranchs. In the Chase eleven of the thirty-three species 

 appear for the first time. The Marion contains fewer species, but 

 they are forms occurring at lower horizons. The principal 

 brachiopods run through the whole sequence. 



THE ORIGINAL PERMIAN 



Historical statement. — The Upper Paleozoic rocks occurring 

 along the western flanks of the Urals, in eastern Russia, in 



