J. P. Smith — Periodic Migrations. 221 



This species is abundant in the North Atlantic, and has made 

 its way through the Boreal region into the Pacific. On the 

 western coast of North America, where there are no sudden 

 changes in the temperature of the sea water, this species has 

 made its way as far south as Margarita Bay, in lat. 24° N., 

 mean temperature 73° F. On the Asiatic side it has made its 

 way through Bering Sea down the shores of Kamschatka with 

 the cold water, but has been stopped by the sudden change of 

 temperature at Hakodadi, lat. 41° N., Japan, mean tempera- 

 ture 52° F., where the warm Japan current meets the cold 

 current from Bering Sea. That this is not an accident of dis- 

 tribution is shown by the fact that Purpura lapillus has, in 

 the Atlantic, a similar distribution, and for the same reasons. 

 On the African side it reaches lat. 32° N., mean temperature 

 66° F., and on the American side it is barred back by the sud- 

 den change of temperature at lat. 42° N.', mean temperature 

 52° F.* There can be no doubt that the temperature, or 

 rather evenness of change of temperature, controls the distri- 

 bution of Purpura lapillus now, and it would seem only 

 reasonable to suppose that similar conditions in the Trias 

 caused the unequal distribution of Pseudomonotis ochotica. 



Lias. — It is probable that during the Lias the southward 

 migration of the Boreal type of animals was interrupted, for 

 the Arietites group, which was characteristic of that epoch in 

 Europe, is known in California and Nevada, as well as in 

 Mexico and South America. It was, however, practically 

 universal, having been found also in the Indian region, though 

 not as yet from the Jurassic Arctic sea. In the Upper Lias 

 the genus Amaltheus was widely distributed in Europe and in 

 the Boreal region, but has not yet been found in North 

 America. It is known from New Grenada, associated with a 

 typical Mediterranean fauna. It seems probable that the Lias 

 of California and Nevada is merely a northward extension of 

 the South American type. 



Middle Jura. — In California the Middle Jura, like the Lias, 

 appears to have been of Mediterranean type, but in the Black 

 Hills we have a southward extension of a fauna characteristic 

 of northern Europe, and of the region around the northern 

 Pacific Ocean. This boreal type is extensively developed in 

 Alaska and in northern Siberia, and its appearance in the 

 Black Hills marks the beginning of another incursion from 

 the north. 



Upper Jura. — The incursion of the Boreal fauna into 

 America which began in the Middle Jura reached further 

 southward and westward in the Mariposa epoch of the Upper 

 Jura, down through California to San Luis Potosi in Mexico. 



*A. H. Cooke, Cambridge Natural History, iii, p. 363, 1895. 



