ANCIENT PORTALS OF THE EARTH 395 



teaus and oceanic trouglis dates from the Permo-Carboniferous revolu- 

 tion, although the North Atlantic and the South Atlantic were not 

 occupied by the sea until early Tertiary time. 



During Cambrian time the Bering, Central American, Iberian, and 

 Asia Minor portals were open, probably corresponding to regions of 

 Pre-Cambrian folding. The others had not yet come into existence, or 

 at any rate information is lacking concerning them. In the Lower 

 Silurian we know of the existence of the Bering portal. During the 

 Devonian the Bering and the Asia Minor portals were open all the time, 

 the Crimean part of the time, while the Bokharan portal was merely 

 part of the open Asiatic sea. During the Carboniferous era all the major 

 portals were open at times except that leading down to Madagascar. 

 Somewhere near the border between Coal Measures and Permian the 

 Paleozoic topographic revolution inaugurated centers of distribution and 

 portals connecting them that held sway during nearly the whole of 

 Mesozoic time. 



If diastrophism should be the final arbiter of the division of geo- 

 logical time, then on the basis of physiography the Permian, including 

 the Artinsk stage, should be included in the Mesozoic. For while 

 Permian life is distinctly more closely related to that of the Paleozoic, 

 Permian physiography is like that of the Triassic. 



Interregional Faunal Zones. — It is customary to speak of cosmo- 

 politan faunas in the past ages, but this term is properly applicable 

 only to the Cambrian and Silurian, when local differentiation had not 

 yet caused the extremes of later times. And even in the Cambrian, as 

 the scanty faunas become better known, provincial differences appear. 

 In Devonian time provincial distinctions were already well devel- 

 oped, and the invasion of a region by an exotic fauna is easily recog- 

 nized. The first interregional migration that is definitely known 

 occurred early in the Upper Devonian, when the American waters were 

 invaded by a fauna that could not have sprung from its predecessors in 

 that region, but was endemic in Eurasia. This is the zone of the 

 Cuboides fauna, which was followed by still further immigration from 

 the same center of dispersion, in the zone of Manticoceras intumescens 

 of the Upper Devonian. The connection with Europe was through the 

 back door, through northern Siberia, across ISTorth America, for the 

 continental mass of North Atlantis and Appalachia prevented direct 

 communication. 



With the opening of the Carboniferous age the subsidence of the 

 southern part of Appalachia allowed direct intermigration between the 

 waters of western Europe and the Mississippian Sea through the 

 Poseidon basin. Here we find the faunal zone of Aganides rotatorius 

 common to the two regions, but unknown anywhere else. This is the 

 first direct invasion of the American seas by a population from the 

 western Tethys, or ancient Mediterranean basin. In the latter part of 



