C. Schuchert — Russian Carboniferous and Permian. 45 



" If this conclusion is correct, i. e., the fact that a migration of 

 the Ammonite world from north to south took place toward 

 the end of the Permian, then, I ask, is it not conceivable that 

 the Productus-i&xm& accustomed to a cold sea was, in con- 

 sequence of the raising of the general temperature of the 

 water, driven from its original northern habitat toward the 

 south, where shortly before the disappearance of the Permian 

 ice period it found the required normal temperature ? Is it 

 not further conceivable that, with a continued increase in the 

 temperature of the sea, which under some circumstances may 

 have occurred very rapidly, the JBrachiopocl fauna, also, may 

 here have been suddenly exterminated, while the Ammonites, 

 apparently better adapted to warmer water, developed in a 

 most remarkable manner ? Is it not, moreover, conceivable 

 that the end of Paleozoic time was characterized by a probable 

 rapid increase in temperature which spread itself from north 

 to south? If this hypothesis is correct, we must then assume 

 that the dividing line between Paleozoic and Mesozoic time is 

 marked by a change of temperature which spread from the 

 northern hemisphere, thus forcing the Paleozoic Brachiopoda 

 southward, where for a short time in a suitable medium, — a 

 sea cooled by the Permian glaciers, they attain a wonderful 

 development, and then die out when the average temperature 

 exceeded that beneficial to them " (pp. 375-6). 



These suggestions of Noetling's are of the greatest interest. 

 However, as Medlieottia occurs in Texas in association with 

 an undoubted Upper Carboniferous fauna of the type so well 

 known in the Mississippi valley, the question arises — Are not 

 these the oldest Medlicottias ? If so, then the further question 

 is raised — Was not the migration east and west through the 

 great mediterranean Thetys instead of from north to south? 

 In 1901, Noetling (4, p. 457) explains this change of fauna on 

 the basis of change in the depth of the sea. He states : " The 

 Brachiopoda of the deeper waters died out and are replaced 

 by the Ammonites, lovers of shallow waters and flat coasts." 



It is the impression of the reviewer that, until more is 

 known in regard to the extremely interesting Productus- 

 limestone fauna of the El Paso, Texas, region and similar or 

 Artinsk faunas from California and Alaska, no safe deductions 

 as to paths of migration can be made. 



From the third paper by Noetling, on the Otoceras horizon, 

 is gleaned the following : — 



" In a treatise on the divisions of the pelagic Trias, Mojsiso- 

 vics, Waagen, and Diener, supported by the statements of 

 Waagen that there exists a hiatus in the Salt Range between 

 the uppermost Permian and the lowest Trias, have denied the 



