218 J. P. Smith — Periodic Migrations. 



Lower Trias. — The writer* has already shown in several 

 papers that the Lower Trias of California and the Great Basin 

 shows an intimate relationship to that of Asia, and none with 

 that of the Mediterranean region. Many genera are repre- 

 sented by closely allied species on both sides of the Pacific 

 Ocean that are wholly unknown in Europe at that time; 

 such are Flemingites, Ophiceras, Proptychites, Lecanites, 

 Aspidites, Clypites, Pseudosageceras, Ussuria, and many 

 others. Some of the species of these may even be identi- 

 cal, but even without this the association of the genera is 

 such that a paleontologist from Asia would feel himself to be 

 perfectly at home while collecting in eastern California or in 

 Idaho. In some respects the relationship of the fauna of the 

 western coast seems to be closer with that of northern Asia 

 than with that of India ; for instance, P seudosageceras and 

 Ussuria, which are not uncommon in the Meekoceras beds of 

 the Inyo Range in California, and the Aspen Mountains of 

 Idaho, have never been found anywhere else except at Ussuri 

 Bay in eastern Siberia, from which place they were first 

 described. These genera are probably not of American origin, 

 and most of them are of unknown antecedents. But, fortu- 

 nately, the geologic history of some of these forms is known. 

 Ophiceras, Xenaspis, Xenodiscus, and Hungarites, which 

 occur in the Trias of Asia and America, chiefly in the lower 

 beds, have also been found in the Permian of southern Asia. 

 To this region, then, we must look for the source of the 

 Lower Triassic ammonites which appear as immigrants in the 

 American waters, marking the first distinct. Asiatic invasion. 



After the deposition of the Meekoceras beds, a few species 

 with Mediterranean affinities begin to make their appearance 

 in western America. But it is noteworthy that, at this time, 

 the Indian still appears to have been cut off from the Mediter- 

 ranean region.* The geographic regions described by E. von 

 Mojsisovicsf for the Trias will hold good only for the Lower 

 Trias, and probably not even for the whole of that period. 



Middle Trias. — In the Middle Trias a certain kinship still 

 persists between the marine faunas of western America and 

 Asia, though this may be due as much to inheritance from 

 similar ancestors, as to immigration. No species are any 

 longer common to the two regions, and many genera, even, are 

 different on opposite sides of the ocean. But at the same time 

 a kinship between the American and the Mediterranean faunas 

 begins to be noticeable, especially in the nodose ceratites and 

 other members of the Ceratitidse. It is possible that during 



*Jour. Geol., vi, 776-786, 1898; Jour. Geol., ix, 512-521, 1901. 

 f Arktische Trias-Faunen, pp. 147-155, 1886, and Beitr. Kennt. obertriad- 

 ischen Cephalopoden-Fatmen des Himalaya, pp. 114-128, 1896. 



