the Lower Carboniferous epoch another migration from western Europe to the Mississippi basin took place; this was in the zone of Goniatites striatus, which, as was the case with the preceding zone, did not extend beyond the Mississippian Sea. At this time in the western part of North America, the Great Basin Sea was connected with northern Asia, and through that region with Europe. The Pacific region lacks the fauna of Goniatites striatus, but has, instead, that of Productus giganteus. Here, for the first time, we see a sharp differentiation into Mediterranean and Pacific types of faunas, separated by the land barrier of the Rocky Mountain area.
In the age of the Coal Measures we find further evidence of continued invasion of the Mississippian Sea by immigrants from the western European Tethys, in the faunal zone of Gastrioceras Listen. This gi'oup was common in the shallow epicontinental expansion of the old Mediterranean Sea, and reached America along the shores of the Poseidon Ocean, the Paleozoic ancestor of the modern Atlantic, and into the Mississippian basin through the Gulf of Mexico, up into Arkansas, Texas, Oklahoma, Missouri, Kansas, Illinois, etc. But the western sea of the Great Basin was dominated by a fauna from the boreal waters, which came down through the Bering portal, along the old shore line of northwestern America. The Mediterranean fauna extended eastward through the Tethys to Sumatra, and up into the Ural Mountains, showing that the Iberian, the Bokharan, and the Asia Minor portals were all open. The Malaysian portal was apparently closed, and the Madagascar geosyncline as yet shows no evidence of its existence.
The Artinsk, or Permo-Carboniferous transition, fauna has approximately the same regional distribution as that of the zone of Gastrioceras Listeri, except that it reached further northward, to Nova Zembla. But while the Carboniferous cephalopod fauna may be considered as Mediterranean in origin, the Artinsk fauna probably came from further east; its real home seems to have been on the border between Asia and Europe, for there it is most abundantly developed, and bears the closest resemblance to its predecessors.
At this time the northern Asiatic-Pacific fauna had a different character, but is as yet little known. In America it is known in California, and in the southern embayment that stretched from the Pacific to western Texas. It is called the Guadalupian fauna, from Guadalupe Mountains in Texas, where it was first described. In Texas these two faunas, the Artinsk and the Guadalupian, come within a few hundred miles of each other. The east and the west, the Atlantic and the Pacific, were separated then, as they were during most of the Carboniferous.
On the west, the Guadalupian or Pacific fauna, penetrated the Great Basin Sea, but did not extend far inland on the continent, the high lands of Utah and Idaho still separating it from the Cordilleran extension of the Mississippi sea.