222 • J. P. Smith — Periodic Migrations. 



This fauna is characterized by the genera Gardioceras and 

 Aucella, which had their home in northern Europe and northern 

 Siberia, and appeared in western Europe only sporadically as the 

 result of incursions. Pompeckj* has shown in several papers 

 that a Polar sea existed in the Middle and Upper Jura arid Lower 

 Cretaceous, from which incursions were made from time to 

 time into the more southerly regions, when changes in physi- 

 cal geography made it possible. Pie has also shown that the 

 Aucella fauna had its real home in that region, where it 

 formed a truly genetic series, and that it appeared only spo- 

 radically in other regions, where the species speedily degen- 

 erated and the fauna becomes extinct unless replenished by 

 another migration. 



The suggestion of climatic influence on the dispersion of 

 marine animals in the Upper Jurassic is very strong, for the 

 Aucella did not make its way into the Indian Ocean, although 

 the way was probably open. . It went only where the condi- 

 tions of its own proper habitat existed, if only temporarily. 

 Even so conservative a naturalist as J. D. Danaf admits that 

 in Jurassic time there was a cold current to the southeast along 

 the western coast of North America, making possible the 

 migration of Aucella from the Boreal into warm temperature 

 or even subtropical waters. 



Aucella did, however, make its way into northern India, 

 probably from southern Russia, during a time of extension of 

 the sea in that direction in the Kimmeridge and Tithonian 



epochs.^ 



Lower Cretaceous. — After the Jurassic beds were laid down 

 there was in California a break in sedimentation, and the uplift 

 of the Sierra Nevada took place. But it was orogenic, and 

 although widespread, it did not affect the geographic relations, 

 for with the opening of the Cretaceous the same northern 

 types were still there. Aucella was still the most characteris- 

 tic genus, and along with it were many species of ammonites 

 closely related to Russian species. Aiccella crassicollis was 

 even identical with a characteristic Russian form. These 

 Knoxville species were probably in part immigrants from the 

 Boreal region, although some of them may have been modified 

 descendants from species that were endemic in the American 



* Ueber Aucellen etc. N. J. fur Min. Geol. und Pal. xiv, 343, 1901 and 

 Jura Fossilien aus Alaska, Verh. k. o. Euss. Min. Gesell. (St. Petersburg), 

 xxxviii, 376, 1901. The Jurassic Fauna of Cape Flora, Franz Josef Land; 

 The Norwegian North Polar Expedition 1893-1896, Scientific Results, p. 141, 

 1898. 



+ Manual of Geology, p. 794, 1895. 



X S. Nikitin, Bemerkungen uber die Jura- Ablagerungen des Himalayas und 

 Mittelasiens. Neues Jahrb. fur Min., Geol. und Pal., ii, 124, 1889. 



