220 



J. P. Smith — Periodic Migrations. 



zone of Pseudomonotis ochotica, are known in Alaska, British 

 Columbia, California, Nevada, and as far south as the coast of 

 Peru. Pseudomonotis ochotica is widely distributed in Siberia 

 and Japan, and is probably identical with the American 

 Pseudomonotis sub circular is. This group is distinctly Asiatic 

 in origin, and never •reached the Mediterranean waters. Its 

 appearance in America marks another Asiatic immigration, 

 but this time it came from the north.- We see here a 

 reversion to the conditions of the Lower Trias, but on the 

 Asiatic side the immigrants do not seem to have reached any 

 further than to Japan. The widespread occurrence of beds 

 with Pseudomonotis ochotica around the northern shores of 

 the Pacific and around the Arctic Ocean shows a transgression 

 of the sea on what was formerly a continental border. These 

 forms were endemic in the Boreal region, and made their way 

 southward when the transgression of the sea opened the way, 

 on both sides of the Pacific. Of course, this may not have 

 had anything to do with climate, but at the close of the Karnic 

 epoch the fauna of western America shows a sudden change 

 of facies from the Indian-Mediterranean character to that of 

 Siberia, which shows, at least a change in connections. A 

 passage from the Arctic to the Pacific was reopened between 

 Asia and America, and the Boreal, though not necessarily 

 cold-water, type came through, making its way southward. 



Deep water, cutting off Asia from America, would have 

 separated the two Triassic regions just as effectively as cold 

 water, but since there was free passage for the group of 

 Pseudomonotis ochotica down both sides of the Pacific, it is 

 strange that it did not reach Tropical India, and that the 

 Tropical Indian forms ceased temporarily to come to America. 

 If such changes had occurred in Tertiary time we would say 

 without hesitation that the Boreal invasion marked an influx 

 of cold water from the Arctic Ocean through the open passage 

 between Asia and America. 



The group of Pseudomonotis ochotica has also been cited 

 by Suess* from New Caledonia, but this occurrence is doubt- 

 ful, since Pothpletzf found only Monatis salinaria, a Med- 

 iterranean type, in that region. It may be, however, that in 

 the Indian Ocean the Boreal and the Mediterranean facies 

 were temporarily united, in which case the presence of 

 Pseudomonotis ochotica would not necessarily be a proof of 

 the southward extension of a lower temperature. 



A modern instance of the same restricted dispersion is seen 

 in the occurrence of Purpura lapillus in the North Pacific. 



* La Face de la Terre, ii, 422. 



f Perm-, Trias- und Jura-Formation auf Timor und Rotti im indischen 

 Archipel, p. 90. 



